Indian Journal of Chemistry
Vol. 43A, December 2004, pp. 2497-2502
A quantum chemical investigation of electrophilic addition reaction of
bromine to exo-tricyclo[3.2.1.0 2.4]oct-6-ene
Rtza Abbasoglu* & Sevil
Sava~kan
Ytlmaz
Karadeniz Technical University, Chemistry Department 61080 Trabzon, Turkey
Email: rabbas@ktu .edu.tr;[email protected]
Received 19 January 2004; revised 16 September 2004
Full geometric optimization of exo-tricyclo[3.2.1.0 2.4]oct-6-ene(exoTCO) has been done by the semiempirical methods
and the structure of the molecule investigated. The double bond of molecule is endo-pyramidalized and the two face s of
double bond are no longer equivalent. Exo face of the double bond of the molecule has regions having high electron
den sity(qi.HoMo) and bigger negative potential. The exoTCO ... Br2 system has been investigated by AM I method and
exoTCO ... Br2(exo) molecular complex has been found to be relatively more stable than the exo ... Br2(endo) complex. The
cationic intermediates of the reaction have been studied by semiempirical methods. Exo-bromonium cation is found to be
more stable than endo bromonium cation. Exo-facial selectivity has been observed in the addition reaction to exoTCO of
bromine which is caused by electronic and steric effects. Exo-classical bromocarbonium cation(lIl) is more stable than
rearrangament cation(V) which is formed with Wagner-Meerwein rearrangament. Bromocarbonium cation(III) is the most
stable ion among the cationic intermediates and the ionic addition occurs via the formation of this cation. The mechani sm of
the addition reaction has also been di sc ussed.
IPC Code: Int.Cl. 7 C07B 37/02
Halogenation of organic compounds is an important
step in the preparation of various synthetic
intermediates or products. Addition of bromine to the
carbon-carbon double bond with molecular bromine is
formally one of the simplest reactions typical of
unsaturated compounds. The nature of the
intermediates of the addition depends on temperature,
steric factors, torsional effects, 11:- and <J-participation
in the transition state and the formation of nonclasical ions or a fast equilibrium of classical ions 1.2.
The bromination of unsaturated bicyclic systems with
molecular bromine leads to reaITangements of the
molecular skeleton 3 . 8 . In order to analyze the reaction
mechanism and stereochemistry details, some data
about the structures and stabilities of the intermediates
such as olefin-halogen molecular complexes and
cations formed during the reactions are needed. Since
the intermediates possess low stabilities and high
reactivities, it is difficult to obtain such data
experimentally.
Nevertheless,
quantum-chemical
calculations provide a reliable source of information
about the structure and stability of intermediates
without the aid of experimental measurements.
We have been interested for some time in the
regiochemistry and stereochemistry of the addition
reactions of halogens to unsaturated strained
molecules5.6.9. 15. We report herein the results obtained
for the investigation of the addition bromine to exotricyclo[3.2.1.0 2.4] oct-6-ene. Reaction of exo2
tricyclo[3.2.1.0 .4]oct-6-ene(exoTCO) with bromine
gave two non-rearranged products; 6-exo-7 -exodibromo-exo-tricyclo[3.2 . 1.02 .4]octane(2),
6-exo-7and
endo-dibromo-exo-tricyclo[3.2.1.02 .4]octane( I)
three reaITanged
products; 5-exo-bromo-3-exobromomethyltricyclo [2.2. 1.02,6]heptane(3), 6-exobromotri cyclo [3,2 ,1,02.7] oct -3-ene(4), 4-exo-6-exodibromotricyclo [3,2, I ,02.7] octane(5)16. In order to
carry out the detailed analysis of the formation
mechanism and stereochemistry of the product of this
reaction, a quantum chemical investigation of the
structures and stabilities of the reaction intermediates
seem to be very important. On the other hand, the
of
5-exo-bromo-3-exo-bromomethylformation
tricyclo[2.2.1.02,6]heptane(3) product from the cation
through the Wagner-Meerwein reaITangament or exo
classical is still a subject of discussion 16. In general,
the stereochemical regularities of addition reaction s of
halogens to unsaturated strained systems are the
subjects of detailed investigation. Stereoselectivity of
these reactions depends the electron structure of th e
double bonds of strained olefins to a large extent. The
most important factors that affect the structure and the
stability of olefin-halogen molecular complexes are
the structure and the properties of olefins.
INDI AN J C HEM, SEC A, DECEMBER 2004
2498
Methodology
The geometry and the electronic structure of the
exo-tricyclo[3.2. l.0 2,4]oct-6-ene(exoTCO)
were
calcu lated by the semiempirical methods MNDO/d
(ref. 17), AM I (ref. 18) and PM3 (ref. 19). ExoTCO .. .
Br2 molecular complexes have been studied through
the semi empirical AMI method and their stab le
configurations have been determined. The structure
and the stabi lity of cations formed by hetereo lytic
sp littin g of the molecular complexes and their isomers
have also been investi gated through the MNDO/d ,
AM1 and PM3 methods. Full geometry optimization
was carried out emp loying the Polak-Ribiere
(conjugate gradient) algorithm (convergence of
0 .00001 kcallmol) and RMS gradient at 0 .0001
kcall(A mol) . All the calcu lations have been carried
out using the HYPERCHEM 6.0 software 20 on an
IBM Pentium IV computer.
Results and Discussion
Full geometric optimization of exoTCO molecule
was done by MNDO/d, AM I and PM3 semiempirical
methods and the structure of the molecu le was also
investigated in detail. In the light of the results of each
.
met h 0d, tIle pyraml'd a l'1zatIOn
parameters 2122
'
0f
molecule were determined with the aim of
determining the structural deformation of doub le
bond. The calculated values of the pyramidalization
21
angle 21 (<»), twisting(torsion) angle (<»D) and out-of2
plane bending angle \x) are given in Table l.
According to the determined results, the double bond
of exoTCO molecu le is endo pyramidalized and the
two faces of double bond are no longer equ ivalent.
The electron de nsity in exo direction o f endo
pyramidalized double bond of the mo lecule must be
larger than in endo direction . This extraordinary
geo metrical feature causes a very noticeabl e IT-fac ial
stereoselectivity in addition reacti ons to doubl e
bond 23 . Then, the addition of bromine to exoTCO
molecu le in which the doubl e bo nd is elldo
pyramidalized, should show the exo-selectivity.
In general, the facial selec ti vity of attack on
a pyracnidalized olefin pan-allels the pyra ml dalization 24 .25 . When the pyramidalization degree of
the double bond of olefins increases, their che mi ca l
reacti vi tes also increase23 .
The anal ys is of frontier orbital (HOMO) of
exoTCO molecule showed that this orbital IS
principally localized in the double bond (Fig. I).
As seen in Fig 1 exo and endo faces of endo
pyramidalized double bond of the molecu le are not
equal. The electron density in exo face of double bond
is high. Therefore, the bromination reaction of the
exoTCO molecule shou ld show the stereoselec tivity
property and the addition of bromine should be
realized from exo direction which is hav ing higher
electron density.
One of the most accurate method" in determining
the direction of the electrophilic attac k of halogen to
the double bonds of strained olefins is the molecular
Tab le ) -The calculated heat of formation energies (kca l/mol), energies of frontier mo lec ular orbita ls (e V),
doubl e bond lengths (1\) and pyramidali zation parameters (degrees) of exo-tricyclo[3 .2.1 .0 2.4]oct-6-e ne
Method
/)'H o
EHOMO
ELUM O
rc;c
<1>
<1>D
X
MNOO/d
65.905
70.890
61.122
-9.638
-9.6 13
-9.794
0.999
1.21 6
1.027
1.360
1.3 55
1.348
0.21 3
0.792
0.923
0.0
0.0
0.0
0.343
0.902
1.059
AMI
PM 3
f
.--~.~}
-"-.
2D Contour
3D isos urface
Fig. I- E lectron densi ty di stribution (HOMO) of the exo-tricycloI3,2. 1.02,4Joct-6-ene molec ule
ABBASOGLU et at.: SEMIEMPIRICAL CALCULATIONS FOR ADDITION OF BROMINE TO exo-TCO
electrostatic potential (MESP) calculations. The
MESP surfaces show considerable topographical
variation, with many minima, saddle points, and
maxima. Every rr-bond of olefin has a local minimum
of electrostatic potential on either face. Because the
regions with large negative potentials shou ld direct
the initial approach of an electrophile, the relative
depths of the two minima can be used to predict the
preferred facial selectivity . Alternatively, integrated
volumes of a certain negative potential can be
obtained for the two faces. Electrophilic attack is
predicted to be larger on the face with larger
integrated volume. The approaches have been used
effectively in a number of systems, qualitatively as
well as rigorously26-28. For understanding from which
direction the double bond of exoTCO molecule will
be attacked by bromine, the molecular electrostatic
potential (MESP) of the molecule was calculated by
AM I method (Fig. 2). The electrostatic potential
contour maps of the molecule indicate that the
electrophi li c attack of bromine predominantly occur
on exo face of double bond.
As known, olefin-halogen molecular complex is
formed in the first step of electrophilic addition to
olefins
of
halogens 29 -34 .
According
to
the
thermodynamic stability of the molecular complexes,
it is possible to determine from what direction the
halogen attacks the double bond. Therefore, the
of
stability
and
the
electron
structure
exoTCO ... Briexo)
and
exoTCO .. . Briendo)
molecular complexes formed with the addition from
exo and endo directions to double bond of exoTCO
molecule of bromine are investigated using AM I
method. The electrophi li c attack of bromine to double
bond of exoTCO molecule is possible either from exo
or endo side. Moreover, a bromine molecule may
approach the carbon-carbon double bond in either
axial (the C1) axis of bromine molecule is
perpendicular to the double bond plane) or equatorial
(CX) axis of bromine molecule is parallel to the double
bond plane) position. By considering these, the full
geometric optimization of the various configurations
of exoTCO ... Br2 system has been performed and the
stable configuration corresponding to the minimu m
energy levels have been determined. In thi s
connection, two configurations corresponded to th e
local minima of the exoTCO-Br2 system have been
These
configurations
correspond
to
found .
exoTCO ... Br2(exo) and exoTCO .. .Br2(elldo) molecular
complexes which are formed by the exo and endo
orientation of Br2 molecule to the double bond of
exoTCO in axial position, respectively (Fig. 3) .
The stabilization energies (!1E=(E"xoT("0+EBr2)EexoTCO Br2} of the molecular complexes, the heat of
formation (!1H ~ ), the equi librium distance R X. Br (X
is midpoint of the C=C bond of exoTCO) and the
other calculated properties are given in Table 2 . The
exo comp lex is 0.759 kJ/mol relatively more stable
than the other and has 0 .8 kJ/mol lower heat o f
formation than that of the endo complex (Table 2).
The results obtained indicate that an exo selectivity
must be considered in the electrophilic addition of
bromine molecule to exo-tricyclo[3.2 . 1.0 2 .4 Joct-6-ene .
The rr-facial selectivity observed in the [2 .2. 1]
systems parallels the double bond pyramidalization
and also results in the minimization of steric
interactions by approaching from the same face as the
methano bridge rather than the ethano bridge. On th e
other hand , as we pointed out, the electron density
(qi.HOMO) in exo face of endo pyramidalized doubl e
bond of TCO molecule is higher (Fig. I). That IS,
,
\.
\\
'-..."
....
-._-
2D contour
2499
3D isosurface
Fig. 2- Electrostatic potential contour map of exo-tricyc\o[3.2. 1.02 •4 ] oct-6-enG (A M I)
INDIAN J CHEM, SEC A, DECEMBER 2004
2500
!
exo
endo
Fig 3-The optimized geomet ries of th e exoTCO .. .. Br2(exo) and exoTCO . .. Br2(endo) molecular complexes (A M I).
Table 2- The properties of TCO .. .Briexo) and TCO ... Br2 (endo) molecular complexes (AM 1) (the pyramidalization
parameters are in degree unit)
Equilibrium distance
Stabilization energy
(kllmol)
Heat formation
(kllmol)
Exo
2.89
271.57
3.045
0.022
1.358 2.187 2. 170
0.0
2.429
Endo
2.09
272.37
3. 125
0.0 16
1.355 2.186 0.457
0.0
0.419
R(A)
HOMOTCO-LUMObrom interaction realized from exo
face of the double bond in the formation of exo
molecular complex is more effective than that of endo
face and is optimal. According to the frontier
molecular orbital theory , HOMOolf.LUMOhalogen
interaction is the decisive factor in the formation of
olefin-halogen complex 35 . So, because of electronic
and steric factors, exo molecular complex is more
stable than endo molecular complex (Table 2).
On the other hand, the formation of olefin-halogen
complex
is
realized
with
the
molecular
pyramidalization of the double bond 13.34. The stability
of
complex
increases
by
increasing
the
pyramidalization of o lefin double bond. The
calculations done using AM 1 method showed that the
values of the pyramidalization parameters (<», <»D, X) of
the double bond in TCO ... Br2(exo) complex are higher
than those of TCO ... Br2(endo) complex (Table 2).
Thus, exo-facial stereoselectivity of electrophilic
addition reaction of bromine to exoTCO is caused by
electronic and steric effects. It is obvious that exoselectivity mu st be taken into consideration in this
addition.
Bromine molecule is partly polarized In
exoTCO ...Brz(exo) molecular complex and the
bromine atom nearer to the double bond of exoTCO
possesses a partial positive charge while the other
bromine has a partial negative charge. The bond
length between the bromine atoms in the molecular
complex is relatively longer than that of the neutral
bromine molecLtle. Also, the length of the double
Charge transferred
from TeO to Br2, e
x
Molec ular
complex
<1>0
bond of the molecular complex is 0.003,A. longer than
that of the exoTCO molecule. The results obtained
reveal that exoTCO ... Br2(exo) molecular complex
plays an important part in the heterolytic splitting of
bromine molecule leading to an ionic addition. The
investigation of the molcular complexes have been
shown to be very important on the overall reaction
coordinate of brominations due to autocatalytic action
of bromine.
It is known that an olefin-halogen molecular
complex is more stable in a solvent than in a gas
phase medium and the stabilisation energy of the
complex becomes higher as the solvent polarity
increases 36 . As a result, in the first step of the addition
of bromine to exoTCO, exoTCO . .. Br2 molecular
complex millst be formed either in a gas or a solvent
medium. Subsequently, the splitting of exoTCO .. .Br2
molecular complex is predicted to occur. The cations
and their isomers shown in Scheme 1 are formed as
the possibk cationic intermediates of the reaction.
The structures and relative stability of these cations
have been determined by carrying out geometrical
optimization using MNDO/d , AMI and PM3 methods
and then the standard heat of formation (11H~) have
been also calculated (Table 3) .
According to the semi empirical methods, exo
bridged bromonium cation(l) is more stable than endo
cation(II). This confirms that Br2 prefe rs to attack the
exo side, rather than endo side, hence exo-facial
selectivity in the addition reaction. The results
ABBASOGLU et al. : SEMIEMPIRICAL CALCULATIONS FOR ADDITION OF BROMINE TO exo-TCO
I
II
250 1
ill
v
IV
Scheme 1
Table 3- The calcu lated heats of formation of cations
Cation
II
III
IV
V
Mi~
(kcal/mol)
MNDO/d
AMI
PM3
276.44
277.19
274.76
275 . 11
275 .85
263.61
265 .97
260.40
261.66
261.65
265.80
266.27
260.34
261.55
260.93
obtained from the three semi empirical methods
indicate that the exo bridged bromonium cation (I) is
higher in energy than the classical bromocarbonium
cation(III). The formation of rearranged cation(V) can
be expected by a Wagner-Meerwen rearrangement
from cation(III). But, according to the three
semiempirical methods, the classical bromocarbonium
cation(lII) is more stable than cation(V). In other
words, the conversion of the cation(III) to ion(V) is
not easy. As shown in Table 3, the most stable cation
of cationic
intermediates
is
the
classical
bromocarbonium(III) and the ionic addition reaction
occurs via this cation. According to the theoretical
results obtained in the study, a plausible mechanism
of the ionic addition of Br2 to exoTCO molecule can
be considered as shown in Scheme 2.
It is known that the trans-adducts are formed via
bridged-halogenium ions in the addition of halogens
to olefins 37 . As seen in Scheme 1, the non-rearranged
trans-adduct 1 is also formed via the bridged exobromonium ion(I), and the bromine atoms of the
adduct 1 are in exo and endo configurations,
respectively. On the other hand, trans product can
also be formed by the endo attack of bromide ion to
exoTCO ... Br2
Scheme 2
cation(III). Furthermore, the non-rearranged cisadduct 2 (an exo, exo-dibromide) is formed by the exo
attack of bromide ion to cation(III). C2C3 and C2C4
cyclopropyl bonds (Scheme 2) become weak and the
2502
INDIAN J CHEM , SEC A, DECEMBER 2004
interaction results in electron cloud of cyclopropane
ring of cationic centre in classical bromocarbonium
cation(lII). In other words, carbocation(IIJ) can occur
with participation of the C2C3 cyclopropyl bond to
give rna, when captured by bromide ion giving
rearrangement product (3) (Scheme 2). With
participation of the internal C2C4, cyclopropyl bond,
carbocation(llI) can rearrange to give IIIb that may
underg o a proton loss to give product (4). Bromide
ion attack to cation(IIIb) results in the rearrangement
product (5) (Scheme 2).
To conclude the double bond of exoTCO molecule
is endo pyramidalized. The electron densities (qi.HOMO)
in exo and endo faces of the double bond are not equal
and it is more in endo face. Exo face of the double
bond of the molecule has larger negative
potential.The exo molecular complex is more stable
than the endo complex. The bridged exo-bromonium
cationO) is relatively more stable than the endobromonium cation(II). Exo-facial selectivity should be
observed in the addition reaction to exoTCO molecule
of bromine and it is caused by electronic and steric
factors. Exo-classical bromocarbonium cation(III) IS
more stable than rearrangement cation(V)
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
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