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744
Controlling parameters for radical cation
fragmentation reactions: Origin of the intrinsic
barrier
Deepak Shukla, Guanghua Liu, Joseph P. Dinnocenzo, and Samir Farid
Abstract: C—C bond cleavages of radical cations of 2-substituted benzothiazoline derivatives were investigated to determine the parameters controlling the fragmentation rate constants. In spite of the low oxidation potentials of the compounds, fragmentation rate constants greater than 1 × 106 s–1 could be achieved through weakening of the fragmenting
bond by substituents that stabilize the radical fragment and exert steric crowding. A quantitative assessment of the relative roles of radical stabilization vs. steric effects to weaken the fragmenting C—C bond was achieved through DFT
calculations. The calculated activation enthalpies matched reasonably well with the experimentally determined values. A
thermokinetic analysis revealed that the fragmentations of benzothiazoline radical cations have relatively large intrinsic
kinetic barriers, ascribed to the delocalized nature of the product radical and cation fragments. Interestingly, the same
factors that lead to the large intrinsic barriers led, simultaneously, to large thermodynamic driving forces for the fragmentations, which should lead to lower activation barriers. These effects oppose each other kinetically and provide important insight into the design of fast radical ion fragmentation reactions.
Key words: benzothiazoline, radical cation, fragmentation, steric effects, DFT.
Résumé : Dans le but de déterminer les paramètres qui contrôlent la constante de vitesse de fragmentation, on a étudié
la rupture de la liaison C—C des cations radicalaires des dérivés de la benzothiazoline substituée en position 2. En dépit du faible potentiel d’oxydation de ces composés, on peut obtenir des constantes de vitesse de fragmentation supérieures à 1 × 106 s–1 en affaiblissant la liaison qui subit la fragmentation à l’aide de substituants qui stabilisent le
fragment radicalaire tout en exerçant un encombrement stérique. Faisant appel aux calculs DFT, on a évalué quantitativement le rôle relatif de la stabilisation du radical par rapport aux effets stériques dans l’affaiblissement de la liaison
C—C qui se fragmente. Les enthalpies de liaisons calculées correspondent assez bien aux valeurs expérimentales. Une
analyse thermocinétique a révélé que les fragmentations des cations radicalaires de la benzothiazoline ont une large
barrière cinétique intrinsèque relative, attribuable à une délocalisation du radical et des fragments de cations obtenus. Il
est intéressant de noter que les mêmes facteurs qui conduisent à une large barrière intrinsèque, conduisent simultanément à de très grandes forces motrices thermodynamiques pour la fragmentation, ce qui devrait provoquer une diminution de la barrière d’activation. Cinétiquement, ces effets qui s’opposent l’un à l’autre fournissent une connaissance
plus profonde du processus impliquant les réactions rapides de fragmentation de l’ion radical.
Mots clés : benzothiazoline, cation radical, fragmentation, effets stériques, DFT.
[Traduit par la Rédaction]
Shukla et al.
757
Introduction
One-electron oxidation can decrease bond dissociation energies of otherwise stable molecules to such an extent that
fast fragmentation can occur readily. This effect is best understood in terms of a thermodynamic cycle, pioneered by
Arnold and co-workers (1). According to the cycle, the
higher the oxidation potential of the molecule, the lower the
bond dissociation energy of the radical cation and, in
general, the more likely that fast fragmentation of the corresponding radical cation would be achieved. Conversely, as
the oxidation potential is lowered, the bond dissociation energy (BDE) of the radical cation increases and will eventually reach a point where the fragmentation will be too slow
to be competitive with other reactions of the radical cation.
Fragmentations of C—C bonds of several radical cations
Received 20 March 2003. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 26 June 2003.
Dedicated to Donald R. Arnold, a pioneer in the chemistry of photoinduced electron transfer.
G. Liu and J.P. Dinnocenzo.1 Department of Chemistry, University of Rochester, Rochester, NY 14627-0216, U.S.A.
D. Shukla2 and S. Farid.3 Research Laboratories, Eastman Kodak Company, Rochester, NY 14650-2109, U.S.A.
1
Corresponding author (e-mail: [email protected]).
Corresponding author (e-mail: [email protected]).
3
Corresponding author (e-mail: [email protected]).
2
Can. J. Chem. 81: 744–757 (2003)
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Shukla et al.
have been investigated and it has been confirmed that the
fragmentation rate constants indeed depend strongly on the
oxidation potential of the substrate (2).
It is conceivable that compounds with low oxidation potentials could yield radical cations that rapidly fragment if
the fragmenting bonds are weakened through proper substitution. There are, in principle, two ways to weaken such
bonds: (1) by having substituents on the fragments (the radical and the cation) that stabilize these species; and (2) by
steric crowding in the reactants. There are a number of reported examples where fragmentation of radical cations derived from low oxidation potential compounds have been
achieved by such weakening of the fragmenting bonds (3).
Herein, we report a new class of low oxidation potential
compounds — substituted benzothiazolines — that undergo
rapid fragmentation when oxidized to their radical cations.
Pulsed laser techniques were used to generate and characterize the benzothiazoline radical cations, as well as to measure
their fragmentation kinetics. In addition, DFT calculations
were performed to estimate the BDEs of the radical cations
as well as their activation barriers for fragmentation. Finally,
the relative contributions to bond weakening in the radical
cations from stabilization of the fragments vs. steric crowding were determined.
745
[1]
Compound 1 was used as a model nonfragmenting analogue to determine the absorption spectrum of the main
chromphore of the radical cation.
Five other derivatives (2–6) with varying substitution of R
to promote fragmentation, were synthesized to test the effect
of increasing stabilization of the radical R· and of weakening
of the bond on the fragmentation rate constant.
The benzothiazoline derivatives were prepared from 2methylamino-benzenethiol according to reported procedures
(eq. [2]) (4, 5) through direct condensation with the appropriate ketone (compounds 2 and 3) or through reaction with
an acid chloride to give a thiazolium salt, followed by the
addition of CH3MgBr (compounds 1 and 4–6).
Results and discussion
Structural features and synthesis
The initial aim of this work was to choose an electrophore
with such a low oxidation potential that cleavage of a nonstabilized radical fragment upon one-electron oxidation
would be very slow. Systematic structural changes to increase the stability of the radical fragment would then be
tested with the aim of achieving fragmentation of the radical
cation with a rate constant of 1 × 105 to 1 × 107 s–1, a convenient range for transient kinetics. Substituted benzothiazoline (2,3-dihydrobenzothiazole) derivatives, BT–R (eq. [1])
appeared to be well-suited for this study. They have low oxidation potentials (e.g., 0.77 V vs. SCE for R = Me), the cation fragment (BT+) is highly stabilized, and derivatives with
varying R — the cleaving radical fragment — can be readily
synthesized.
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746
[2]
Experimental approach
Different approaches were considered to generate and
study the fragmentation kinetics of the radical cations of the
benzothiazoline derivatives (BT–R·+), by flash photolysis. In
one approach, an excited sensitizer reacts with a donor (acting as a cosensitizer, C) to form the radical cation of the latter (C·+) in high quantum yield (6). Next, a secondary
electron transfer from BT–R to C·+ generates BT–R·+. To
avoid direct excitation of the reactant, it is desirable for the
sensitizer to absorb at longer wavelength than the BT–R derivative. The absorption of BT–R derivatives below 350 nm
renders sensitizers such as 1,4-dicyanonaphthalene and Nmethylquinolinium of limited use. The dynamics of the radical cations BT–R·+ are most conveniently studied at their absorption maxima, which, as shown below, are near 420 nm.
Thus, sensitizers such as 9,10-dicyanoanthracene that have
strong absorptions in this region are also of limited use.
Because of the lack of a suitable electron transfer sensitizer with absorption in the 370 ± 10 nm window that would
meet the above requirements, we applied a different approach to generate the radical cations BT–R·+. This approach
is based on the chemistry of N-methoxyphenanthridinium
(MeOP+), which is reported to undergo photochemical N—O
bond cleavage to yield phenanthridine radical cation (P·+)
and a methoxy radical (eq. [3]) (7). The radical cation of
phenanthridine is a powerful oxidant (oxidation potential of
phenanthridine is ~1.9 V vs. SCE) that can be used in a subsequent bimolecular reaction to generate the radical cation
of an added donor (eq. [3]).
[3]
4
Can. J. Chem. Vol. 81, 2003
Steady state photolysis and reaction products
To correlate the decay of the radical cations to their fragmentations, it was important to first confirm that the products of these reactions are those expected according to
eqs. [1] and [3]. The photoreactions of MeOP+ with the
benzothiazolines 3–6 at equimolar concentrations in CD3CN
were followed by 1H NMR and the products were identified
by comparison with authentic samples (see Experimental
section for details). The products were indeed those expected from the N-O fragmentation of MeOP+ (phenanthridine and methanol) and those of the C-C fragmentation
of the radical cations BT–R·+ (2,3-dimethylbenzothiazolium
and products of the cleaved radical R·). In each case the
main product of the radical R· was the dimer R–R (~70 to
>90%). In some cases the deuterated product R–D (via deuterium abstraction from the solvent), and (or) the corresponding olefin (via loss of hydrogen atom) were formed
(eq. [4]) (see Experimental section).
[4]
An unexpected, minor photolysis product of these reactions was adduct 7 (eq. [4]).4 This compound was formed in
all reactions of 3–6 in ~12–20% yield. The structure of this
compound, which has not yet been isolated in pure form,
was based on 1H NMR spectral analysis. Importantly, the alternative assignment of the isomeric structure 8 to this compound was ruled out by independent synthesis of 8 via the
base-catalyzed reaction of MeOP+ with dimethylbenzothiazolium (eq. [5]). Assignment of the NMR signals of compound 8 was based on several 2D NMR and NOE
experiments, which establish the connectivity of the two
moieties.
Photolysis of MeOP+ in the presence of the benzyl derivative (2) yielded, in addition to phenanthridine and methanol,
2-methyl-3-ethylbenzothiazolium, an expected fragmentation
product of 2·+. However, none of the likely products of benzyl
The mechanism for formation of 7 has not been investigated. One possibility involves initial reaction of MeO· with BT–R by hydrogen
atom abstraction from the N-Me group. The resulting α-amino radical might then add to MeOP+ at C-6 to give a N-alkoxy amine radical
cation. Subsequent intramolecular electron transfer from the benzothiazoline moiety to the N-alkoxy amine radical cation followed by fragmentation of the resulting benzothiazoline radical cation would produce 7. Further experiments are required to test this hypothesis.
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Shukla et al.
Fig. 1. Absorption spectrum of phenanthridine radical cation
(P·+) obtained from photolysis of MeOP+ in acetonitrile (closed
circles) and that obtained in the presence of 1,4-dimethoxybenzene (DMB) (open circles), which matches the spectrum of an
independently generated DMB·+.5
[5]
radical, such as 1,2-diphenylethane or toluene were formed
in this reaction. There is also no indication that either benzyl
methyl ether (a coupling product between benzyl and
methoxy radicals) or N-benzylacetamide (an addition product of benzyl cation to acetonitrie, followed by hydrolysis) is
formed. In spite of the fact that not all fragmentation products have been identified in this case, the clear evidence for
formation of the benzothiazolium salt suggests that fragmentation of 2·+ does take place. Additional support for this conclusion is provided by the fact that the activation enthalpy
calculated for fragmentation of 2·+ agrees well with that
measured experimentally for its disappearance (see Density
functional calculations).
Characterization of the radical cations BT–R·+
The following experiments were carried out to probe the
applicability of MeOP+ photochemistry (eq. [2]) to generate
and study the reactivity of radical cations by flash photo5
747
Fig. 2. Absorption spectra of the radical cations of compounds 1
and 4, obtained from photolysis (380 nm) of MeOP+ in the presence of these electron donors in acetonitrile. The spectra were
obtained after a delay time of >2 µs to avoid contributions owing
to the secondary transient species produced from photolysis of
MeOP+ (see text).
lysis. Irradiation of MeOP+ in acetonitrile gave a transient
(λ max at 670 nm), which was assigned to P·+ based on the
lack of quenching by dioxygen and on its efficient quenching by 1,4-dimethoxybenzene (DMB, Eox = 1.3 V vs. SCE).
In the presence of 0.1 M DMB, the 670 nm absorption is
completely replaced by that shown in Fig. 1 (λ max = 440 and
458 nm), which corresponds to the spectrum of DMB radical
cation generated independently.5 Using the benzothiazoline
derivatives as donors (eq. [3]) this approach provided a convenient method to generate their radical cations and to study
their fragmentation kinetics.
As expected, the phenanthridine radical cation (P·+) is efficiently intercepted by the benzothiazoline derivatives (kq ≈
6 × 109 M–1 s–1). Thus, 355 or 380 nm laser excitation of
MeOP+ in the presence of 5–10 mM of 1 in aerated or in argon-saturated acetonitrile led to a long-lived species (~200–
250 µs) with an absorption maximum at ~410 nm (Fig. 2).
Support for assigning this absorption to 1·+ was obtained
from a number of experiments. Excitation of 1,4-dicyanonaphthalene at 343 nm in the presence of 0.3 M biphenyl as a
cosensitizer in acetonitrile led to the formation of biphenyl
radical cation, characterized by the strong absorption at 670
and 380 nm. Absorption due to the sensitizer radical anion
was removed by purging the samples with dioxygen, where
electron transfer leads to the formation of O2·–, which does
not absorb in the visible region. Addition of 1 (0.5 to
1.5 mM) led to quenching of the biphenyl radical cation
with a bimolecular rate constant of 1.8 × 1010 M–1 s–1 and
concomitant appearance of an absorption identical to that
An authentic spectrum of DMB·+ was obtained in aerated acetonitrile by 355 nm excitation of DCA (OD355 = 0.6) in the presence of 0.15 M
biphenyl (as a cosensitizer) and 0.01 M DMB.
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748
obtained in the above mentioned experiment with MeOP+.6
Similarly, 343 nm excitation of N-methylquinolinium with
toluene (1.5 M) as a cosensitizer in dioxygen-purged 1,2dichloroethane generated the toluene radical cation (λ max =
450 nm), which was efficiently quenched by 1 (~2 mM).
The transient spectrum recorded after complete quenching of
the toluene radical cation was also very similar to that obtained in the other experiments mentioned above.
Assigning the ~410 nm absorbing transient to 1·+ was further confirmed through electron transfer interception by a
lower-oxidation-potential compound. The transient obtained
from the reaction of MeOP+ with 1 in acetonitrile was
readily quenched by tri-p-anisylamine (Eox = 0.52 V vs.
SCE) with the concomitant formation of the radical cation of
the latter, characterized by its absorption maximum at
720 nm (6). Plots of the rate constants for the transient decays and for growth of the trianisylamine radical cation vs.
amine concentration were linear. Slopes of the plots provided a quenching rate constant of 4.1 × 109 M–1 s–1.
Shown also in Fig. 2 is the spectrum of 4·+ obtained using
MeOP+ as described above for 1·+. Clearly the absorption
spectra of the radical cations are not the same in spite of the
fact that the low-oxidation-potential electrophore (the
benzothiazoline moiety) is the same. The weak absorption
that appears as a shoulder at ~470 nm in the spectrum of the
nonfragmenting 1·+ seems to be considerably enhanced in
the spectrum of 4·+. The spectra of 2·+ (not shown) and of 5·+
(Fig. 3) are similar to that of 4·+.
Additional support for the spectral assignment of the benzothiazole radical cations was derived from the transient assigned to 5·+, whose decay was concomitant with formation
of the 9-methylfluorenyl radical. Unlike the radicals resulting from the fragmentation of the other radical cations mentioned in this work, which lack absorptions in the visible
region, the methylfluorenyl radical could be readily identified by its absorption (λ max = 485 nm) (see Fig. 3) (8). This
absorption matches that of an authentic sample, also shown
in Fig. 3, generated from photolysis of the dimer (compound 9)
(eq. [6]). As expected, in aerated solution this absorption
band is suppressed as a result of quenching by dioxygen.
Can. J. Chem. Vol. 81, 2003
Fig. 3. (Top) Transient spectra from photolysis (380 nm) of
MeOP+ in the presence of the methylfluorenyl derivative 5 (0.01
M) in acetonitrile at different delay times after the pulse. The
spectrum after (a) 0.2 µs is essentially that of the radical cation
5·+, with increasing delay times ((b) and (c)) this spectrum is
gradually replaced by that of methylfluorenyl radical. (Bottom)
Spectrum after (d) 13 µs delay and spectrum of methylfluorenyl
radical obtained from direct photolysis of the dimer (9) (e) at
308 nm in acetonitrile. The spectra from photolysis of MeOP+
could not be measured below 400 nm because of absorption by
MeOP+.
[6]
Fragmentation rate constants and activation energies
The fragmentation rate constants of the radical cations
BT–R·+, obtained via 355 or 380 nm excitation of MeOP+,
were determined from the decay kinetics in acetonitrile. The
use of MeOP+ in these flash photolysis experiments allowed
for a high enough concentration (10–15 mM) of the reactants to ensure fast interception of the phenanthridine radical
cation. Under these conditions the growth rates of the radical
6
cations are faster than their decay. For all samples the
radical cation decays were somewhat complicated by the
presence of a second transient species. The decay rate constant for this latter species was the same in all samples and
had the same temperature dependency. The nature of this
second transient has not been established, but it is likely to
Compound 1 absorbs at the excitation wavelength and concentrations higher than 1.5 mM and could not be used in these experiments.
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Shukla et al.
749
Table 1. Comparison of fragmentation rate constants (kfr at
25°C) and activation enthalpies for substituted benzothiazoline
radical cations 2·+–6·+ and R-H bond dissociation energies.
Compound (R-)
2
3
4
5
6
(PhCH2-)
(Ph2CH-)
(PhCMe2-)
(9-MeFl-)
(Ph2CMe-)
kfr (s–1)
1
9.8
1.9
5.0
6.1
∆H‡ (BT–R·+)a
c
×
×
×
×
104
105
106
106
~16
9.5
9.6
3.9
4.4
±
±
±
±
0.9
0.5
0.7
0.6
BDE (R-H)b
89.8
85.8
87.3
79.7
82.8
Fig. 4. Plot of the logarithm of fragmentation rate constants
divided by the absolute temperature (ln (kfr/T)) for the radical
cations of compounds 3–6 vs. the reciprocal of the absolute temperature (1000/T) in acetonitrile. The negative slopes of the linear plots obtained by least-squares fitting are: 4.82, 4.84, 2.00,
and 2.15, respectively. The dashed line (negative slope of 2.98)
corresponds to the decay rate constant of another intermediate
from the photolysis of MeOP+ detected in all cases (see text).
Note: All energies in kcal/mol.
a
From nonlinear least-squares fit of the temperature-dependent rate data;
reported errors are one standard deviation.
b
Reference (9).
c
This value was estimated from the room temperature fragmentation rate
constant for 2·+ assuming that the activation entropy was the same as for 3·+.
be due to a species resulting from MeOP+ chemistry because
of its independence on the added BT–R derivative. Thus, although this experimental approach suffered from the need to
analyze the data as a sum of two exponentials, it offered the
best option to obtain the desired kinetic data.
The fragmentation rate constants of 3·+–6·+ were in the
range that could be readily determined by laser flash
photolysis. The rate constants of these reactions were also
measured as a function of temperature (see Fig. 4). Listed in
Table 1 are the fragmentation rate constants at 25°C and the
activation enthalpies, which were derived from nonlinear
least-squares fitting of the kinetic data in Fig. 4.
Unlike the fragmentations of 3·+–6·+, which could be
readily investigated by flash photolysis, the radical cation 2·+
had a lifetime of ~50–60 µs and, importantly, its decay
showed only a slight temperature dependence. This behavior
suggests that the decay may be largely due to other deactivation processes rather than being a measure of the fragmentation rate constant. Indeed, based on cyclic voltammetry, a
rate constant of only 1 ± 0.2 s–1 was obtained for the fragmentation of 2·+ (see Experimental section).
From the above mentioned data it is evident that the fragmentation rate constants for 2·+–6·+ fall into three groups.
Cleavage resulting in formation of the primary, benzyl radical (from 2·+) is particularly slow (~1 s–1). The cleavages to
form both the secondary, diphenylmethyl radical (from 3·+)
and of the tertiary, cumyl radical (from 4·+) are in the range
of ~1 × 105 s–1. The cleavages to form the tertiary,
methylfluorenyl, and diphenylethyl radicals (from 5·+ and
6·+) are in the range of 5 × 106 s–1. The increased substitution on the radical fragment clearly increases the fragmentation rate constant.
Although the activation energies for the fragmentation of
2·+–6·+ roughly correlate with increasing stability of the radical fragment, further analysis reveals that other factors must
also play a role in controlling the fragmentation barriers.
The relative radical (R·) stabilities can be estimated from the
R—H bond dissociation energies (BDE (R-H)) of the corresponding hydrocarbons, as shown in Table 1. The data show
clear discontinuities in the correlation of BDE (R-H) and
∆H‡. For example, ∆H‡ for 3·+ and 4·+ are identical within
experimental error yet, as judged by BDE (R-H), the radical
formed from 4·+ is less stabilized than from 3·+ by
1.5 kcal/mol. Similarly, ∆H‡ (5·+) is 0.5 kcal/mol lower than
∆H‡ (6·+), however, based on radical stability one would ex-
pect the radical formed from 5·+ to be ca. 3 kcal/mol more
stablized than that formed from 6·+. These data clearly reveal
that radical stability is not the sole determinant of the relative energetic barriers for radical cation fragmentation; it
seems likely that differential steric effects also play a crucial
role. To gain insight into the relative contributions of these
two factors, we decided to use density functional theory to
calculate both the bond dissociation energies and the activation energies for C—C bond fragmentation of 2·+–6·+.
Density functional calculations
The thermodynamic and kinetic properties of the substituted benzothiazoline radical cations were modeled with
density functional calculations. The B3LYP functional was
chosen because it has been shown to be particularly wellsuited to radical cation calculations (10). Computations were
performed with the Gaussian 98 series of programs (11). All
geometries were fully optimized using a 6-31G* basis set
and all optimized species were determined to be either minima or saddle points by frequency calculations. The radical
cation carbon–carbon BDEs for 2·+–6·+ were determined as
the differences between the energies of the benzothiazolium
cation (BT+) and the radical fragments (R·) and that of the
most stable radical cation conformer, unless otherwise noted.
The energy differences include electronic energies, zero
point energy corrections, and thermal corrections (at
298.15 K). In several cases, BDEs from higher energy radical cation conformers were calculated to compare with the
transition state energies for conformational interconversion.
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750
Activation energies
The activation energies for fragmentation of 2·+–6·+ were
sought for comparison with the experimentally determined
values. The relative energies of the conformational isomers
of the radical cations were first computed to determine the
lowest energy conformer for each compound. The energetic
barriers for conformational interconversion were also computed. The results are graphically illustrated in Fig. 5.
The activation energies for fragmentation were computed
from the lowest energy conformer for each radical cation.
For the radical cations with the lowest activation energies for
fragmentation, activation energies were also computed for
the higher energy conformers. The results are shown in
Fig. 5.
The highest computed barrier for fragmentation
(14.4 kcal/mol) was found for 2·+. This value is in line with
the observed fragmentation rate constant for 2·+ of ~1 s–1,
which corresponds to an activation barrier of ~16 kcal/mol,
~2 kcal/mol higher than the computed value. The computed
barriers for 3·+ (7.8 kcal/mol) and 4·+ (7.0 kcal/mol) are substantially lower. Gratifyingly, the computed barriers for 3·+
and 4·+ are also consistent with those determined experimentally (9.5 and 9.6 kcal/mol). Again, as in the case of 2·+, the
measured barriers for 3·+ and 4·+ are higher than the computed values by ca. 2 to 3 kcal/mol.
As shown in Fig. 5, the computed activation energies for
fragmentation of 3·+ and 4·+ begin to become comparable to
the activation energies for conformational interconversion.
This trend continues as one progresses toward the most reactive radical cations (5·+ and 6·+). In these latter cases, the activation energies computed for fragmentations from the
lowest energy conformers are both lower than for conformational interconversion, particularly for 6·+. The computed
activation energy for fragmentation of 6·+ (4.7 kcal/mol) is
in excellent agreement with the experimental value
(4.4 kcal/mol) and is ca. 3 kcal/mol less than for 3·+ and 4·+,
in reasonable agreement with the experimental results (∆∆H‡ ≈
5 kcal/mol).
Interestingly, the computed fragmentation barrier from the
lowest energy conformer of 5·+ (6.4 kcal/mol) is intermediate between those calculated for 4·+ and 6·+. This contrasts
with the measured activation enthalpy for fragmentation of
5·+ (3.9 kcal/mol), which is more similar to that of 6·+
(4.4 kcal/mol) than to that of 4·+ (9.6 kcal/mol). A possible
origin of this apparent discrepancy was discovered when
comparing the relative energies of the conformers for 2·+–6·+
with those for the corresponding neutral molecules (2–6).
Calculations revealed that the lowest energy conformers for
radical cations 2·+–6·+ also corresponded to the lowest energy conformers for the neutral molecules 2–6, except for
7
Can. J. Chem. Vol. 81, 2003
the 5·+/5 pair.7 The lowest energy conformer for 5·+ is
predicted to be n (see Fig. 5), whereas the lowest energy
conformer of 5 is predicted to be c (Erel (5n) = 2.3; Erel (5s) =
1.4 kcal/mol). The relative energies of the calculated conformers for 5 suggest that over the temperature range used
to measure the fragmentation kinetics of 5·+ (–35 to +21°C),
>90% of 5 should be present as 5c. Making the reasonable
assumption that one-electron oxidation of 5 does not result
in simultaneous oxidation and conformational interconversion, one would expect 5 to be oxidized to initially form
5c·+, i.e., not the lowest energy conformer of the radical cation (5n·+). Consequently, if the energy barrier for fragmentation from 5c·+ is less than the barrier for conformational
interconversion to the lower energy 5n·+ conformer, then the
experimental activation barrier for fragmentation of 5·+
should be compared to the computed fragmentation barrier
predicted from 5c·+ not from 5n·+.
Figure 5 shows that the computed activation enthalpy for
fragmentation of 5c·+ (4.4 kcal/mol) is in good agreement
with the experimental value (3.9 kcal/mol). Interestingly, the
computations also show that the energy of the fragmentation
transition state from 5c·+ is equal to that for conformational
interconversion to 5n·+. Thus, the calculational results predict
that a significant fraction of 5c·+ should partition to the 5n·+
conformer, which is predicted to have a significantly larger
barrier to fragmentation (6.4 kcal/mol). As illustrated in
Fig. 5, if 5c·+ partitions between fragmentation and conformational interconversion, one would expect the experimental decay of 5·+ to show biexponential behavior, i.e., a
fast component due to fragmentation from 5c·+ and a slow
component due to fragmentation from 5n·+. Unfortunately,
the presence of the unknown species resulting from MeOP+
chemistry masked our ability to establish whether the fragmentation of 5·+ was biexponential. Finally, it is worth noting that all of the calculated transition state energies refer to
activation enthalpies, whereas the kinetics are obviously determined by the lowest activation free energies. It seems
plausible that the activation entropies for the radical cation
fragmentations might be significantly lower than ∆S‡ for the
conformational interconversions. If so, the activation free energy for fragmentation of 5c·+ might be lower than for
conformational interconversion to 5n·+. This also would be
consistent with our kinetic data.
Bond dissociation energies
Gas phase and solution phase bond dissociation energies
were calculated for the benzothiazoline radical cations 2·+–6·+.
The gas phase bond dissociation energies (BDE (BT-R·+)g),
were determined using B3LYP/6-31G* calculations as the
sum of the enthalpies of the benzothiazolium cation (BT+)
The origin of the reversal in conformer stability for 5 vs. 5·+ can be traced to two factors: (i) a change in the geometry at nitrogen upon oneelectron oxidation; and (ii) the conformational inflexibility of the fluorenyl ring. The calculations on neutral molecules 1–6 showed that the
nitrogen atom is strongly pyramidalized. In contrast, the nitrogen is much less pyramidalized in the corresponding radical cations and is actually planar for 1·+. The planarization of the nitrogen in radical cations causes the N-CH3 group to move closer to the R groups at C-2, resulting in increased, destabilizing nonbonded interactions. For 5/5·+, planarization of the nitrogen upon one-electron oxidation resulted in a
severe steric interaction between the N-CH3 group and syn-C-1 hydrogen on the fluorenyl ring for the 5c·+ conformer, which caused a significant bow in the ring. In 5 this interaction is greatly diminished by pyramidalization at nitrogen which moves the N-CH3 group away from
the fluorenyl ring. We attribute the interchange in the relative stabilities of the c and n conformers of 5 vs. 5·+ to this structural reorganization. Interestingly, in the otherwise structurally similar pair 6/6·+, a similar steric interaction between the N-CH3 group and one of the hydrogens in the syn-phenyl group of 6c·+ is relieved by rotation of the phenyl group. A comparable structural relaxation for 5c·+ is not possible
because rotation of the syn-phenyl ring is constrained by its linkage to the other phenyl group that makes up the fluorenyl ring.
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Shukla et al.
Fig. 5. Results of density functional calculations showing the relative potential energies (kcal/mol) for the ground state conformers of 2·+–6·+ and for the transition states that interconnect them.
Also shown are activation enthalpies for fragmentation from selected radical cation conformers (vertical lines). Shown in bold
are the ground and transition states for the proposed, reacting
conformers.
and the radical fragments (R·) minus the enthalpy of the
most stable radical cation conformer. For 5·+, the enthalpy of
the 5c·+ conformer was used for the reasons described above.
751
The results are shown in Table 2, along with the
experimental and calculated activation energies for radical
cation fragmentation. The data show that there is a general
correlation of ∆H‡ with the radical cation BDEs.
Table 2 also lists estimates of the solution phase bond dissociation energies for 2·+–6·+ (BDE (BT-R·+)s) which were
obtained by using Arnold’s thermodynamic cycle method
(1), as shown below. For these estimates we have used the
approximation that the oxidations of 2–6 are similar to the
reversible oxidation potential measured of 1 (0.77 V vs. SCE
in CH3CN). The oxidation of the 2,3-dimethylbenzothiazolium radical was estimated from the (irreversible) reduction
of the corresponding cation (~–1.3 V vs. SCE in CH3CN).
Finally, the bond dissociation energies of the neutral benzothiazolines 2–6 (BDE (BT-R)g), were estimated by DFT calculations. The results shown in Table 2 reveal that the
estimated solution phase, radical cation bond dissociation
energies are ca. 7–9 kcal/mol lower than those calculated for
the gas phase. The lower solution phase BDEs would be
consistent with greater solvation of the benzothiazolium cation than that of the radical cations.
The relative BT–R bond dissociation energies for 2·+–6·+
presumably reflect contributions from differences in both the
stabilization of the radicals and the steric crowding present
in the radical cations. We sought to estimate the comparative
contributions of these factors to the relative BDEs. Relative
radical stabilization energies of R· can be defined in a number of ways. One of the most general definitions has been pioneered by Rüchardt, who has compared the difference in
stability between a pure hydrocarbon radical and its substituted analogue in which one or more alkyl groups are replaced by substituents (12). The advantage of this approach
is that it provides radical stabilization increments for substituents that are transferable between different radicals because the intrinsic differences in stabilities for primary,
secondary, and tertiary radicals are factored out. To assess
the relative effects of radical stabilities on the BDEs for 2·+–
6·+, however, we do want to include the differences in stabilities between primary, secondary, and tertiary radicals.
Therefore, we have adopted the more traditional approach to
assessing radical stabilities by comparing the relative R-H
BDEs to a reference compound. We are primarily interested
in only the relative contributions to radical stability along
the series, therefore, we have chosen the least stabilized radical in this series (PhCH2·) as our reference. Thus, we define
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752
Can. J. Chem. Vol. 81, 2003
Table 2. Experimental (∆H‡exp) and calculated (∆H‡calcd) activation energies for fragmentation of 2·+–6·+, gas phase C-R bond dissociation energies calculated for 2–6 (BDE (BT-R)g), gas phase (BDE (BT-R·+)s), and solution phase (BDE (BT-R·+)s) bond dissociation energies calculated for 2·+–6·+.
Compound (R-)
∆H‡exp
∆H‡calcda
BDE (BT-R)ga
2
3
4
5
6
~16
9.6
9.6
3.9
4.3
14.4
7.8
7.0
4.4
4.7
52.2
37.8
38.4
32.1
28.3
(PhCH2-)
(Ph2CH-)
(PhCMe2-)
(9-MeFl-)
(Ph2CMe-)
BDE (BT-R·+)ga
11.6
–0.6
–0.7
–8.5
–10.3
BDE (BT-R·+)sb
~5
~–10
~–9
~–16
~–19
Note: All energies in kcal/mol.
a
From density functional calculations.
b
BDE (BT-R)g – Eox (BT-R) + Eox (BT·); based on the approximation that Eox (BT-R) is the same for 2–6.
Table 3. Estimates of the contributions of radical stability and steric effects on the gas phase and solution BT-R bond dissociation energies for 3·+–6·+ relative to those for 2·+.
Radical Cation
R-
2·+
3·+
4·+
5·+
6·+
PhCH2Ph2CHPhCMe29-MeFlPh2CMe-
Relative radical
stabilizationa
(0)
–4
–3
–10
–7
Relative steric
effect (gas phase)b
Relative steric
effect (solution)c
(0)
–8
–10
–10
–15
(0)
–11
–11
–11
–17
Note: All energies in kcal/mol.
a
BDE (PhCH2-H) – BDE (R-H); see Table 1 for values.
b
{[BDE (BT-CH2Ph·+)g – BDE (BT-R·+)g] – [BDE (PhCH2-H) – BDE (R-H)]}.
c
{[BDE (BT-CH2Ph·+)s – BDE (BT-R·+)s] – [BDE (PhCH2-H) – BDE (R-H)]}.
relative radical stabilities for the present purpose as BDE
(PhCH2-H) – BDE (R-H).8 For the relative steric contributions to the BDEs for 2·+–6·+, we also use the benzyl substituted radical cation as a reference. Thus, the relative steric
effects are estimated by subtracting the radical stabilization
contribution from the difference in radical cation BDEs, i.e.,
{[BDE (BT-CH2Ph·+) – BDE (BT-R·+)] – [BDE (PhCH2-H) –
BDE (R-H)]}. The results of this analysis are shown in Table 3. Note that the relative steric effects estimated from the
gas phase and solution BDEs for 2·+–6·+ are in good agreement.
The results in Table 3 show that, relative to 2·+, the BDEs
for 3·+–6·+ roughly fall into two classes. For 5·+, the relative
contributions owing to radical stabilization and steric effects
are nearly equal, 10 and 11 kcal/mol, respectively. For radical cations 3·+, 4·+, and 6·+, the relative radical stabilization
and steric contributions to bond weakening are 3–7 kcal/mol
and 11–17 kcal/mol, respectively, i.e., the steric effects dominate.
Origin of the radical cation fragmentation barriers
Estimates of the BDEs for radical cations 3·+–6·+ in Table 2 show that all are predicted to be exothermic in solution. Nonetheless, the radical cations have substantial
activation barriers to fragmentation. It is of interest to compare this behavior to that observed for the fragmentations of
substituted bibenzyl radical cations (10·+) reported by
Maslak et al. (3a) and to the alkylated NADH radical cations
(11·+) reported by Savéant and co-workers (13). These radical cation fragmentations were found to have very small in8
trinsic barriers for fragmentation (≤4 kcal/mol), showing that
the internal and solvent reorganization energies for the reactions are quite small. In this respect the fragmentations of
3·+–6·+ are clearly different. Based on the fact that 3·+ and 4·+
have substantial activation energies for fragmentation
(∆H‡ ≈ 10 kcal/mol) despite the reactions being exothermic
by 9 to 10 kcal/mol in solution, one can conclude that the intrinsic barriers for fragmentation of the substituted benzothiazline radicals studied here are >10 kcal/mol.
What is the origin of the difference in the intrinsic barriers
for fragmentation of 10·+ and 11·+ vs. 2·+–6·+? We propose
that the difference is related to the relative internal reorganization energies of the fragmentations. It is perhaps easier to
think about the problem by considering the origin of the barrier for the reverse reaction, i.e., cation + radical recombination. The barrier for the recombination reaction will be
directly related to the extent of delocalization of the cation
and radical fragments. The greater the degree of delocali-
Although it would have been desirable to use BDE (R-CH3) rather than BDE (R-H) to estimate relative radical stabilities, the former values
are not well determined for the systems studied here.
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Shukla et al.
zation, the greater will be the degree of electronic, internal
reorganization, and, therefore, the greater will be the reaction barrier. When comparing the cation and radical fragments produced from fragmentation of 10·+ vs. 2·+–6·+, the
primary difference is likely related to the extent of delocalization of the cation, i.e., the benzyl cation (in the case of
10·+) vs. the benzothiazolium cation (in the case of 2·+–6·+).
The benzothiazolium cation is expected to have significantly
less carbocation character at the recombination site than a
benzyl cation, which would lead to a larger internal reorganization energy for the benzothiazolium cation + radical recombination. In addition, for benzothiazoline radical cations
3·+–6·+, the radicals produced by fragmentation are more
delocalized than a simple benzyl radical, which will also
lead to greater internal reorganization energies for recombination and, consequently, for fragmentation too. For the
fragmentation of 11·+ vs. 2·+–6·+, we presume that delocalization of the cation fragments for the two systems is somewhat similar. However, the relative extent of radical
delocalization is clearly different. For 11·+, the radical fragment — ·t-Bu — is clearly much more localized than any of
the radicals formed from fragmentation of 2·+–6·+. Thus, cation + radical recombination to form 11·+ should have a
lower internal reorganization energy than the recombinations
to form 2·+–6·+.
The proposition that the cation + radical recombination
reactions to form 2·+–6·+ should have significant barriers is
supported by the fact that even addition of a localized, primary radical to benzothiazolium cation 12 has a rate constant of only 1 × 106 M–1 s–1 at 25°C (14). It should be clear
that addition of the delocalized radicals, such as those derived from fragmentation of 2·+–6·+, would have significantly
lower rate constants. Consequently, the fragmentations of
2·+–6·+ are expected to have significant intrinsic barriers.
Conclusions
There are three main conclusions from this work. First,
we have demonstrated that benzothiazoline derivatives,
which are among the lowest oxidation potential compounds
yet investigated as candidates for radical cation fragmentation, can undergo rapid fragmentation upon one-electron oxidation if the fragmenting bond is sufficiently weakened by a
combination of stabilizing the radical fragment and by steric
crowding in the reactants. These two factors allow the rate
constants for fragmentation of benzothiazoline radical cations to be tuned by over 6 orders of magnitude. Second,
benzothiazoline radical cations were found to have considerably large intrinsic activation barriers (>10 kcal/mol) for
fragmentation. The barriers were attributed to large reorganization energies of the product fragments, the benzothiazolium cation and the resonance-delocalized radicals.
Consequently, the same factors that lead to the thermodynamic driving forces for the fragmentations — the stabilities
of the benzothiazolium cation and the resonance-delocalized
radicals — simultaneously result in large intrinsic barriers.
753
These factors lead to opposing kinetic effects. On the one
hand, the natural exothermicities of the reactions should decrease the activation barriers for fragmentation according to
the Bell–Evans–Polanyi principle. On the other hand, the
large intrinsic barriers increase the activation energies. This
analysis reveals a fundamental limitation of using delocalized cation and radical products in the design of fast radical
cation fragmentation reactions. As shown in the present
work, steric effects are the more attractive structural feature
to utilize for designing fast radical ion fragmentations because they do not increase the intrinsic barrier. Third, for
relatively fast radical ion fragmentations of sterically congested systems such as those described in this work, one
should keep in mind that the energetic barriers for fragmentation can be lower than the barriers for conformational
interconversion. In these cases, the possibility exists that the
most stable radical ion conformer may not necessarily be the
reactive conformer.
Experimental section
General method
1
H NMR spectra were recorded with either a General
Electric/Nicolet QE-300 spectrometer or a Brüker Avance400 spectrometer. 13C NMR spectra were recorded with an
Avance-400 spectrometer. Proton chemical shifts (δ) are reported in parts per million (ppm) downfield from tetramethylsilane or in ppm relative to the singlet at 7.24 ppm for
the residual CHCl3 in the chloroform-d or the multiplet at
1.93 ppm for the residual CHD2CN in the acetonitrile-d3.
Reported proton–proton coupling constants assume firstorder behavior. Splitting patterns are designated as singlet
(s), doublet (d), triplet (t), quartet (q), multiplet (m), and
broad (br). The meta- and para-couplings are ignored.
Ortho-, meta-, and para-hydrogens of phenyl groups are designated o-, m-, and p-; for magnetically nonequivalent
phenyl groups the signals of the second phenyl group are
designated o′-, m′-, and p′-. The aromatic hydrogens of the
benzothiazoline and benzothiazolium derivatives are numbered 4–7 from the α position to N. Carbon chemical shifts
are reported in ppm relative to internal acetonitrile-d3
(117.61 and 0.60 ppm) or chloroform-d (77.34 (t) ppm).
Acetonitrile was refluxed over CaH2 for 24 h and distilled
fresh prior to use under an atmosphere of nitrogen. Anhydrous diethyl ether and tetrahydrofuran were freshly distilled
from benzophenone ketyl under nitrogen.
Electrochemical measurements
Cyclic voltammetric measurements were carried out on a
glassy carbon disk electrode using a CHI660 electrochemical analyzer (CH Instruments, Inc.) in acetonitrile with
tetrabutylammonium perchlorate as electrolyte. 2,3-Dimethylbenzothiazolium showed irreversible reduction with a
peak potential of –1.30 V vs. SCE. A reversible oxidation
potential was obtained for compound 1 (0.77 V vs. SCE,
3 mm electrode at 0.5 V/s). Compounds 3 and 6 showed irreversible oxidation with peak potentials at 0.63 and 0.65 V
vs. SCE, respectively.
Using a 33 µm carbon electrode at a scan rate ≥25 V/s,
compound 2 showed a quasi-reversible oxidation of 0.70 V
vs. SCE. Scanning at slower rates (2.5–10 V/s) and with
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754
varying delays, a rate constant of 1 ± 0.2 s–1 was obtained
for the follow-up chemical reaction, presumably the fragmentation of 2·+.
Laser flash photolysis
Nanosecond laser flash photolysis experiments were carried out using either 380 or 355 nm excitations. For 380 nm
excitation, a Lambda Physik Lextra 50 excimer laser (XeCL,
308 nm; <180 mJ/pulse; 10 ns pulses) and a Lambda Physik
LPD 3002 pumped-dye laser (Exciton-384 dye for 380 nm;
<10 mJ/pulse; 7 ns pulses) were used. For 355 nm excitation, the frequency-tripled output from an OPOTEK Vibrant
Q-switched Nd:YAG laser system (pulse width 4 ns, 2–8 mJ)
was used. The excitation pulses were attenuated, when necessary, using neutral density filters.
Transient absorption was monitored at right angles to the
excitation. A pulsed Oriel 150 W xenon lamp (Model
66007) was used as the monitoring beam. The analyzing
beam was collected and focused on the entrance slit (2 nm)
of an Instrument S. A. H-20 monochromator. A Hamamatsu
R-446 photomultiplier tube (PMT) in a custom housing
(Products for Research) was attached to the exit slit of the
monochromator. A computer-controlled Stanford Research
Systems high voltage power supply (model PS310) was used
with the PMT. The signals from the PMT were digitized using a Tektronix TDS 620 oscilloscope and transferred to a
PC, via a GPIB interface, for data storage and processing. A
Quantum Composer pulse generator (Model 9318) provided
TTL trigger pulses to control the timing for the laser, lamp,
and oscilloscope.
Appropriate long pass filters were placed on either side of
the sample to prevent photolysis by the analyzing light.
Spectral measurements
To acetonitrile solutions of N-methoxyphenanthridinium
hexafluorphosphate (MeOP+) with an OD of 0.5–0.77 at the
excitation wavelength (380 or 355 nm) were added benzothiazolines 2–6 (5–15 mM). The samples were thoroughly
purged with either dioxygen or argon (~10 min) before passing them through a 1 × 1 cm2 quartz flow cell. The flow rate
was adjusted so that fresh sample was available for each laser shot.
Kinetics measurements
The samples were purged with either dioxygen or nitrogen
prior to, and during, laser irradiation. Typically, a solution of
MeOP+ in acetonitrile (OD at 380 nm = 0.6–0.8) was exposed to 380 nm laser excitation (0.5–0.6 mJ/pulse) in the
presence of the benzothiazoline derivative (6 to 7 mM), and
the decay of the radical cation was followed between 420–
450 nm on the ns to µs time scale. For temperature dependence studies, fresh samples were equilibrated at each temperature for 15–20 min prior to the measurement. Each set
of experiments was repeated at least three times.
Materials
2-Methylamino-benzenethiol was prepared according to a
literature procedure by reductive ring opening of 2,3dihydro-1,3-benzothiazole (Aldrich) using lithium aluminum
hydride (15). Methyl iodide, trimethyloxonium tetrafluoroborate, acetophenone, 1-phenyl-2-butanone, 1,1-diphenyl-
Can. J. Chem. Vol. 81, 2003
acetone, cumene, α-methylstyrene, 1,1-diphenylethylene,
and fluorene were all obtained from Aldrich Chemical Company and used without further purification. 1,1,2,2-Tetraphenylethane (16), 2,2,3,3-tetraphenylbutane (17), 2,3dimethyl-2,3-diphenylbutane (18), and compound 9 (19)
were prepared following previously reported methods. 2Phenyl-2-methylpropionoyl chloride was prepared from the
corresponding acid (20a) by reaction with PCl5 using a procedure analogous to that described in ref. (20b). Products of
the cleaved radicals were identified by comparison with 1H
NMR spectra of authentic samples.
2,3-Dimethylbenzothiazolium hexafluorophosphate
This material was prepared by modification of a literature
procedure (4). To a stirred solution of acetyl chloride (5.0 g,
63.7 mmol) in benzene (dry, 200 mL), a solution of 2methylamino-benzenethiol (8.84 g, 63.7 mmol) in benzene
(20 mL) was added dropwise at room temperature. The reaction mixture was refluxed for 1 h. Solvent removal provided
a pale yellow solid that was dissolved in water (200 mL) followed by addition of potassium hexafluorophosphate (7.0 g
in 100 mL H2O) to afford a white precipitate (19 g, 79%).
1
H NMR (CD3CN) δ: 8.24 (d, J = 8 Hz, 1H), 8.05 (d, J =
9 Hz, 1H), 7.92 (t, J = 8 Hz, 1H), 7.82 (t, J = 8 Hz, 1H),
4.15 (s, 3H), 3.09 (s, 3H).
N-Methoxyphenanthridinum hexafluorophosphate
N-Methoxyphenanthridinum tetrafluorboate was prepared
from the reaction of phenanthridine-N-oxide with trimethyloxonium tetrafluorborate in dichloromethane (7). A solution
of the BF4 salt (1 g) in ~15 mL water was added to an aqueous solution of potassium hexafluorophosphate (2 g in
~10 mL). The mixture was cooled and the precipitate was
recrystallized from water as a crystalline pale yellow solid.
1
H NMR (CD3CN) δ: 10.09 (s, 1H), 9.06 (d, 1H), 9.02 (d, 1H),
8.57 (t, 2H), 8.43 (t, 1H), 8.24–8.13 (m, 3H), 4.61 (s, 3H).
2,2,3-Trimethylbenzothiazoline (1)
This compound was prepared according to a literature
procedure (5). To a stirred suspension of 2,3-dimethyl-benzothiazolium toluene-4-sulfonate (5.0 g, 14.32 mmol) in anhydrous diethyl ether (125 mL) at –40°C, methylmagnesium
bromide (2.87 M solution in diethyl ether, 5.3 mL, 15 mmol)
was added dropwise over 5 min under an atmosphere of N2.
The reaction mixture was refluxed for 2 h and then
quenched with aqueous ammonium chloride solution
(70 mL). The organic layer was separated and the aqueous
layer was extracted with ether (3 × 50 mL). The organic extracts were combined, washed successively with water (2 ×
100 mL) and with brine solution (2 × 100 mL), and dried
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Shukla et al.
over anhydrous magnesium sulfate. Filtration and removal
of the solvent yielded a red oil (2.2 g, 77%). The crude
product was distilled under reduced pressure, bp 80°C at
10 mmHg (1 mmHg = 133.322 Pa) (lit. (5c) bp 75–80°C at
10 mmHg (1 mmHg = 133.322 Pa)) to yield a colorless oil
(2.0 g, 76%). MS m/z: 179 (M+, calculated for C11H15NS).
1
H NMR (CDCl3) δ: 7.05 (d, J = 7.4 Hz, H-7), 6.99 (t, J =
7.6 and 7.8 Hz, H-5), 6.64 (t, J = 7.4 and 7.6 Hz), 6.31 (d,
J = 7.8 Hz, H-4), 2.72 (s, NCH3), 1.65 (s, C(CH3)2). 13C
NMR (CDCl3) δ: 146.64, 124.17, 121.70, 117.55, 106.31,
61.03, 37.28, 28.58, 28.40.
2-Benzyl-2-ethyl-3-methyl-benzothiazoline (2)
A solution of 2-methylamino-benzenethiol (1.0 g,
7.2 mmol) and 1-phenyl-2-butanone in absolute ethanol (7 mL)
was refluxed for 48 h under an atmosphere of N2. The reaction mixture was cooled and kept at –40°C for 3 h. A precipitate came out of solution, which rapidly melted at room
temperature to afford a yellow oil. The latter was distilled
under reduced pressure (165–167°C at 0.04 mmHg
(1mmHg = 133.322 Pa)) to obtain a very pale yellow oil
(1.2 g, 63%). MS m/z: 269 (M+, calculated for C17H19NS).
1
H NMR (CDCl3) δ: 7.36–7.26 (m, 5H), 7.04 (d, J = 7.4 Hz,
1H), 7.00 (t, J = 7.6 and 7.8 Hz, 1H), 6.64 (t, J = 7.4 and
7.6 Hz, 1H), 6.30 (d, J = 7.8 Hz, 1 H), 3.28 (d, J = 13.4 Hz,
1H), 3.10 (d, J = 13.4 Hz, 1H), 2.86 (s, 3H), 2.10 (sextet,
J = 14.9 and 7.2 Hz, 1H), 1.72 (sextet, J = 14.9 and 7.2 Hz,
1H), 1.05 (t, J = 7.2 Hz, 3H). 13C NMR (CDCl3) δ: 148.17,
136.36, 131.10, 127.74, 126.62, 125.17, 124.17, 121.09, 117.60,
105.48, 85.78, 45.46, 30.19, 29.04, 9.15. 13C DEPT135
NMR (CDCl3) δ: 131.10, 127.74, 125.17, 121.09, 117.60,
105.48, 45.46 (negative peak), 30.19 (negative peak), 29.04,
9.15.
2,3-Dimethyl-2-diphenylmethyl-benzothiazoline (3)
A solution of 2-methylamino-benzenethiol (2.0 g,
14.38 mmol) and 1,1-diphenylacetone (3.02 g, 14.38 mmol)
in absolute ethanol (15 mL) was refluxed for 52 h under an
atmosphere of N2. Upon cooling to –40°C, a colorless solid
(2.3 g, 48%) formed which was recrystallized twice from
ethanol to yield colorless plates (2.0 g, 42%), mp 82°C. MS
m/z: 331.0 (M+, calculated for C22H21NS). 1H NMR (CDCl3)
δ: 7.67 (o-H, 2H), 7.53 (o′-H, 2H), 7.33 (m′-H, 2H), 7.27
(p′-H, 1H), 7.20 (m-H, 2H), 7.14 (p-H, 1H), 6.99 (d, J =
7.4 Hz, 1H), 6.92 (t, J = 7.5 and 7.8 Hz, 1H), 6.59 (t, J = 7.4
and 7.5 Hz, 1H), 6.14 (d, J = 7.8 Hz, 1H), 4.51 (s, CHPh2),
2.69 (s, N-CH3), 1.75 (s, CH3). 13C NMR (CDCl3) δ: 147.71,
140.69, 129.89, 128.23, 127.70, 127.03, 126.31, 125.25,
124.06, 121.00, 117.51, 105.99, 83.77, 59.53, 29.14, 27.81.
13
C DEPT45 NMR (CDCl3) (only CH and CH3 carbons) δ:
129.89, 129.83, 128.23, 127.70, 127.03, 126.31, 125.25,
121.00, 117.51, 105.99, 59.53, 29.14, 27.81.
2-Cumyl-2,3-dimethyl-benzothiazoline (4)
To a stirred solution of 2-phenyl-2-methylpropionoyl
chloride (4.8 g, 26.3 mmol) in benzene (dry, 80 mL), a solution of 2-methylamino-benzenethiol (3.7 g, 26.4 mmol) in
benzene (20 mL) was added dropwise at room temperature.
The reaction mixture was refluxed for 1.5 h. Solvent removal gave an oil that was dissolved in CH2Cl2 (100 mL).
The resulting solution was added dropwise to rapidly stirred
755
diethyl ether (dry, 850 mL) to form a yellow precipitate. The
precipitate was dissolved in water (400 mL) at 70–80°C and
an aqueous solution of potassium hexafluorophosphate
(4.8 g in 100 mL H2O) was added. Upon cooling the solution, a pale yellow precipitate formed. Recrystallization
from acetonitrile – diethyl ether (1:10) gave a colorless solid
of the 2-cumyl-3-methyl-benzothiazolium salt (9.0 g, 83%).
1
H NMR (CD3CN) δ: 8.35 (d, J = 8.0 Hz, H-7), 8.02 (d, J =
8.3 Hz, H-4), 7.94 (t, J = 7.3 and 8.3 Hz, H-5), 7.88 (t, J =
7.3 and 8.0 Hz, H-6), 7.5–7.42 (m, m- and p-H, 3H), 7.37–
7.35 (m, o-H, 2H), 3.75 (s, NCH3), 2.04 (s, C(CH3)2). 13C
NMR (CD3CN) δ: 142.10, 130.10, 129.62, 128.25, 125.82,
123.79, 117.30, 116.75, 45.72, 37.92, 28.73. 13C DEPT45
NMR (CD3CN) δ: 130.10, 129.62, 128.91, 128.25, 125.82,
123.79, 116.75, 37.92, 28.73.
To a stirred suspension of 2-cumyl-3-methyl-benzothiazolium hexafluorophosphate (1.0 g, 2.42 mmol) in tetrahydrofuran (80 mL) under an atmosphere of N2, methylmagnesium
bromide (2.73 M solution in ether, 7.5 mL, 22 mmol) was
added dropwise over 15 min at –70°C. The reaction was
stirred at –70°C for an additional 30 min, followed by 4 h at
room temperature, and then refluxed for 15 min. The reaction was quenched with aqueous ammonium chloride solution (50 mL, pH ~ 4) and extracted with diethyl ether (3 ×
50 mL). The ether extracts were washed with brine solution
(2 × 50 mL), dried over anhydrous magnesium sulfate, filtered, and the solvent removed to afford a pale yellow oil
(0.54 g, ~79%). The crude material was recrystallized from
aqueous methanol to yield a pale yellow solid, mp 70–71°C.
MS m/z: 283 (M+, calculated for C18H21NS). 1H NMR
(CDCl3) δ: 7.56 (d, o-H, 2H), 7.34 (t, m-H, 2H), 7.27 (t, pH, 1H), 6.99–6.95 (m, 2H), 6.62 (t, 1H), 6.31 (d, 1H), 2.54
(s, 3H), 1.78 (s, 3H), 1.62 (s, 3H), 1.54 (s, 3H). 13C (CDCl3)
δ: 149.95, 144.51, 128.81, 127.23, 126.31, 125.66, 124.90,
119.81, 117.74, 107.0, 87.03, 49.99, 34.06, 24.76, 24.64,
23.49. 13C DEPT135 (CDCl3) (only CH and CH3 carbons
observed) δ: 128.81, 127.23, 126.31, 124.90, 119.81, 117.74,
106.99, 34.07, 24.76, 24.64, 23.49.
2-(1,1-Diphenylethyl)-2,3-dimethyl-benzothiazoline (5)
Reaction of 2,2-diphenylpropionyl chloride with 2methylamino-benzenethiol, as described above for compound 4 (reflux time of 5 h), gave the corresponding
benzothiazolium chloride in 70% yield as colorless cubes
from acetonitrile – diethyl ether (1:10). 1H NMR (CDCl3) δ:
8.57 (d, 1H), 8.12 (d, 1H), 7.87 (t, 1H), 7.72 (t, 1H), 7.48–
7.44 (m, 6H), 7.25–7.21 (m, 4H), 4.29 (s, 3H), 2.66 (s, 3H).
13
C NMR (CDCl3) δ: 186.79, 144.05, 143.36, 141.55,
130.76, 129.17, 127.79, 123.25, 118.27, 53.64, 40.54, 27.68.
Similarly, compound 5 was prepared from the reaction of
2-(1,1-diphenylethyl)-3-methylbenzothiazolium chloride with
MeMgBr as described above for compound 4. Compound 5
was obtained in 46% yield as colorless crystals from ethanol, mp 58–62°C. MS m/z: 345 (M+, calculated for
C23H23NS). 1H NMR (CDCl3) δ: 7.37–7.17 (m, 10H), 6.98
(d, J = 7.4 Hz, 1H), 6.97 (t, J = 7.5 and 7.8 Hz, 1H), 6.63 (t,
J = 7.4 and 7.5 Hz, 1H), 6.31 (d, J = 7.8 Hz, 1H), 2.20 (br. s,
3H), 2.06 (s, 3H), 1.98 (s, 3H). 13C NMR (CDCl3) δ: 153.83,
149.34, 133.55, 130.81, 129.35, 127.39, 127.09, 126.51,
125.68, 125.05, 119.85, 107.62, 34.32, 27.58, 25.65.
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756
2,3-Dimethyl-2-(9-methylfluoren-9-yl)-benzothiazoline (6)
9-Methylfluorene-9-carbonyl chloride (prepared in an
analogous manner to 2-phenyl-2-methylpropionoyl chloride,
see above) was reacted with 2-methylamino-benzenethiol, as
described above under compound 4 (1 h reflux time), to give
the corresponding benzothiazolium hexafluorophosphate salt
as colorless crystals in 96% yield. 1H NMR (CDCl3) δ: 8.89
(d, J ≈ 8 Hz, 1H), 8.30 (d, J ≈ 8 Hz, 1H), 7.91 (d, J ≈
7.6 Hz, 2H), 7.79 (t, J ≈ 7 and 8 Hz, 1H), 7.71 (t, J ≈ 7 and
8 Hz, 1H), 7.54 (t, 2H), 7.41–7.36 (m, 4H), 3.30 (s, 3H),
2.22 (s, 3H). 13C NMR (CDCl3) δ: 181.55, 144.38, 143.32,
139.78, 130.70, 130.07, 129.51, 128.87, 128.73, 125.57,
123.88, 121.70, 117.01, 54.21, 36.37, 29.06.
The benzothiazolium salt was treated as described above
for compound 4 to give compound 6 as a colorless viscous
oil in 40% yield after flash chromatography (silica gel, 60%
hexane – diethyl ether). MS m/z: 343 (M+, calculated for
C23H21NS). 1H NMR (CDCl3) δ: 7.89 (d, J = 7.6 Hz, 1H),
7.78 (d, J = 7.7 Hz, 1H), 7.74 (d, J ≈ 7.5 Hz, 1H), 7.72 (d,
J ≈ 7.5, 1H), 7.41 (t, J = 7.5 Hz, 1H), 7.36 (t, J = 7.5 Hz,
1H), 7.30 (t, J = 7.4 Hz, 1H), 7.22 (t, J = 7.5 Hz, 1H), 7.01
(d, J = 7.4 Hz, H-4), 6.95 (t, J = 7.4 and 7.9 Hz, H-6), 6.66
(t, J = 7.4, 7.4 Hz, H-5), 6.32 (d, J = 7.9 Hz, H-7), 2.61 (s,
3H), 1.79 (s, 3H), 1.62 (s, 3H). 13C NMR (CDCl3) δ: 150.21,
148.91, 148.58, 141.215, 140.97, 127.68, 127.52, 126.85,
126.77, 126.45, 126.01, 125.02, 125.98, 119.84, 119.61,
119.17, 118.49, 108.36, 85.96, 61.87, 34.58, 23.35, 20.75.
General procedure for photolysis of benzothiazolines in
acetontrile-d3
An equimolar (~0.01 M) solution of the benzothiazoline
and MeOP+ in acetonitrile-d3 was purged with nitrogen for
5–10 min and then irradiated for 5–10 min in a Rayonet reactor equipped with 400 nm lamps. The color of the reaction
mixture changed from pale yellow to slightly darker yellow
during the photolysis. After photolysis, a known amount of
internal standard, viz., tetrachloroethane was added into the
reaction mixture and 1H NMR recorded. Products were identified by comparison with 1H NMR spectra of authentic samples prepared independently. The yields of products were
determined by 1H NMR integration of diagnostic proton signals of products relative to the area of peak due to internal
standard tetrachloroethane (δ 6.3, s, 2H).
Photolysis products of 3
2,3-Dimethybenzothiazolium (82%, 8.25 (d, J = 8.2 Hz,
H-7), 8.09 (d, J = 8.5 Hz, H-4), 7.92 (t, J = 7.2 and 8.5 Hz,
H-5), 7.83 (t, J = 7.2 and 8.2 Hz, H-6), 4.16 (s, 3H), 3.10 (s,
3H)), phenanthridine, 1,1,2,2,-tetraphenylethane (93%),
adduct 7 (27%), CH3OH(D) (54%).
Can. J. Chem. Vol. 81, 2003
Photolysis products of 6
2,3-Dimethybenzothiazolium (88%), phenanthridine (65%),
2,2,3,3-tetraphenylbutane (82%), 1,1-diphenylethylene (12%),
adduct 7 (22%), CH3OH(D) (63%).
Attempted isolation of adduct 7
An equimolar (0.01 M) solution of benzothiazoline 3 and
MeOP+ in acetonitrile (25 mL) was irradiated under nitrogen
for 20 min in a Rayonet reactor equipped with 400 nm
lamps. The solution was concentrated to ~2 mL and diluted
with diethyl ether (~50 mL). A precipitate formed which
was collected and identified by 1H NMR as 2,3-dimethylbenzothiazolium. Upon addition of hexane to the ethereal
solution the mixture turned turbid and, upon standing, resulted in formation of an oil. The supernatant liquid was decanted and the oil was repeatedly washed with hot hexane to
further remove phenanthridine and 1,1,2,2,-tetraphenylethane. Analysis of the hexane-washed solid by 1H NMR
showed that it was a mixture of primarily 2,3-dimethybenzothiazolium salt and adduct 7, along with traces of 1,1,2,2,tetraphenylethane and phenanthridine.
Compound 7
1
H NMR (CD3CN) δ: 8.18 (d, J = 6.3 Hz, 1H), 7.85–7.77
(m, 5H), 7.48 (t, J = 7.7 Hz, 1H), 7.37 (t, J = 7.7 Hz, 1H),
7.24 (d, J ≈ 8 Hz, 1H), 7.21 (t, J = 7.5 Hz, 1H), 7.10 (t, J =
7.4 Hz, 1H), 7.89 (br d, 6.4, 1H), 5.30 (dd, J = 4.8 and
7.4 Hz, 1H), 4.96 (dd, J = 4.8 and 14.6, 1H), 4.75 (dd, J =
7.4 and 14.6 Hz, 1H), 3.80 (s, 3H), 2.53 (s, 3H).
Preparation of adduct 8
To an equimolar solution of N-methoxyphenanthridinium
hexafluorophosphate (35.5 mg, 0.1 mmol) and 2,3-dimethylbenzothiazolium hexafluorphosphate (30.9 mg, 0.1 mmol) in
4 mL CD3CN, ~30 mg anhydrous sodium carbonate was
added. The solution was stirred at room temperature and the
reaction was monitored by 1H NMR. After ~8 h, the conversion was nearly complete yielding almost exclusively compound 8, which was identified by its NMR spectrum
(CD3CN). The chemical shifts of the 1H NMR were assigned
based on NOE, COSY, and TOCSY experiments using a
500 MHz spectrometer; those of strongly overlapping signals were further confirmed by comparison with simulated
spectra. The NOE experiment indicated the proximity of H-6
(methine triplet at 5.299 ppm) to H-7 (aromatic hydrogen at
7.217 ppm) and of H-3′ (methyl singlet at 3.881 ppm) to
H-4′ (aromatic hydrogen at 7.912 ppm).
Photolysis products of 4
2,3-Dimethybenzothiazolium (87%), phenanthridine (85%),
2,3-dimethyl-2,3-diphenylbutane (70%), α-methylstyrene
(5%), cumene (18%), adduct 7 (13%), CH3OH(D) (63%).
Photolysis products of 5
2,3-Dimethybenzothiazolium (88%), phenanthridine (83%),
bis-(9-methyl-9-fluorenyl) (9) ((85%) δ: 7.49 (d, J = 7.5 Hz,
4H), 7.23 (t, average J = 7.4 Hz, 4H), 7.06 (t, average J =
7.4 Hz, 4H), 6.88 (very broad, 4H), 1.91 (s, 6H)), 9-methylfluorene (11%), adduct 7 (13%), CH3OH(D) (66%).
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Shukla et al.
Acknowledgments
Financial support was provided by the National Science
Foundation (CHE-9812719). We are grateful to Ralph Young
(University of Rochester) for help in analyzing the kinetic
data. We also thank Shihua S. Chen (Eastman Kodak Company) for the electrochemical measurements and James M.
Hewitt (Eastman Kodak Company) for the 2D NMR measurements.
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