On the Reactions of Diphenyliodonium and Triphenylsulphonium
Salts with Hydroxyl and 2-Hydroxy-2-Propyl Radicals
Evidence for Reduction of Diphenyliodonium Ions to Diphenyliodo Radicals
Yusuf Yagci*, S. Peter Pappas**, and Wolfram Schnabel***
* Istanbul T ech nical U n iversity, D ep artm ent o f C hem istry, Istanbul, T urkey.
** N o rth D a k o ta State U niversity, P olym er and C oatin gs D ep artm ent, F argo, N .D , 58105, U .S.A .
Z. N atu rforsch . 42 a , 1 4 2 5 -1 4 2 7 (1987); received A ugust 21, 1987
H yd roxyl radicals react with d ip h en yliod on iu m ion s, P h 2I + , and trip henylsu lph on iu m ion s,
P h 3S +, in a q u eou s solu tion at room tem perature with rate con stan ts > 5 x 109 l/m o ls . T h ese
reaction s lead to the form ation o f a broad transient ab sorp tion band w ith a m axim u m at
ca. 370 nm , w hich is attributed to O H -ad d u cts with cycloh exad ien yl structures. 2-H yd roxy-2-p rop yl
radicals react with d ip h en yliod on iu m ion s in aq u eou s solu tion at room tem perature with
k z = 6 x 1 0 7 1/m ol s. T h e transient ab sorp tion band form ed in this reaction has a m axim u m at ab ou t
390 nm and is attributed to d ip h en yliod o radicals, P h 2I . In contrast, no d etectab le reaction occu r­
red with 2-h y d roxy-2-p rop yl radical and trip henylsu lph on iu m ion.
1. Introduction
Carbon-centered free radicals produced photochemically or thermally can be oxidized to carbocations by onium salts [1], This reaction has been uti­
lized to initiate the polymerization of tetrahydrofuran,
epoxides and other compounds, that undergo cationic
polymerization. Iodonium salts such as diphenyl­
iodonium hexafluoroarsenate, P h 2IAsF6, and the cor­
responding phosphate, P h 2IPF6, have been success­
fully employed as oxidizing agents [2], Typical
reactions, postulated for the generation of cationic
initiators by irradiation of benzophenone in the pres­
ence of propanol-2 and P h 2IAsF6, are:
It is seen that the reduction of diphenyliodonium ion
leads to diphenyliodo radical, P h 2I'. However, the
absorption spectrum of P h 2I ‘ seems to be unknown,
which has limited the interpretation of flash photoly­
sis studies in which diphenyliodonium salts take
part [3],
In the present work diphenyliodo radicals were pro­
duced in the absence of potential overlapping chromophores such as benzophenone ketyl radical, in or­
der to facilitate the determination of its absorption
spectrum. For this purpose 2-hydroxy-2-propyl radi­
cals were generated by reacting • OH radicals with
propanol-2 in the presence of P h 2I + :
ch
P h 2CO + (CH3)2C H O H
P h 2C —O H + (CH3)2C —O H ,
CH3
(2)
(c h 3)2c - o h + P h 2r
—
(C H3)2C —OH + P h 2I'.
(3)
*** Reprint requests to Prof. Dr. W. Schnabel, H ahnM eitn er-In stitu t Berlin G m b H , Bereich Strahlenchem ie,
P ostfach 3 9 0 1 2 8 , D -1 0 0 0 Berlin 39, F ederal R epublic o f
G erm an y.
ch
3
•OH + H - C - O H -> H 20 + C —O H .
(1)
P h 2C —O H + P h 2I +
— ► (Ph)2C —OH + P h 2I \
3
ch
(4)
3
Reaction (4) occurs with k = 4 x 109 1/mol s. Hy­
droxyl radicals were generated by the radiolysis of
N 20-saturated water. Pulse radiolysis in conjunction
with optical absorption detection was employed: di­
lute aqueous solutions containing both propanol-2
and P h 2IPF6 were irradiated with single 50 ns pulses
of 16 MeV electrons generated by a linear accelerator.
Complementary studies with P h 3SBF4 were also per­
formed.
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1426
Y . Yagci et al. ■ O n the R eaction s o f D ip h en yliod on iu m and T rip h en ylsulp h on iu m Salts
Results
Pulse radiolysis of N 20-saturated solutions of
P h 2IPF6 (4.0 x 10~3 — 1.5 x 10~2 mol/1) in the ab­
sence of propanol-2 resulted in the formation of the
transient absorption spectrum (a) shown in Figure 1.
This spectrum, which is attributed to the product of
the reaction of • OH radicals with diphenyliodonium
X (nm)
Fig. 1. Irradiation o f P h 2IP F 6 (4 x 1 0 “ 3 mol/1) (a) and
P h 3SBF4 (3 x 1 0 _3 mol/1) (b) in N 20 -sa tu r a te d aq u eou s
so lu tio n w ith a 50 ns pulse o f 16 M eV electron s at room
tem perature. Dabs = 60 G y. T ransient ab sorp tion spectra
form ed during the pulse.
ions, was formed during the pulse, i.e. with a lifetime
< 50 ns. When the solution contained 1.1 mol/1
propanol-2, almost all (ca. 98%) of the - OH radicals
were scavenged by the alcohol according to reaction
(4). In this case only a small portion of the transient
absorption was formed during the pulse. The major
portion was formed after the pulse. The absorption
spectrum corresponding to the slow mode of forma­
tion is shown in Figure 2. It possesses a maximum at
about 390 nm. The rate of formation of this spectrum
increased with increasing diphenyliodonium salt con­
centration and conformed with 1st order kinetics. Fig­
ure 3 shows a plot of the 1st order rate constant vs. the
diphenyliodonium salt concentration. From the slope
of the straight line one obtains k = 6.0 x 107 1/mol s.
When triphenylsulphonium tetrafluoroborate was
exposed to the attack of • OH radicals in N 20 saturated aqueous solutions at concentrations be­
tween 1.2 x 10~3 and 1.2 x 10” 2 mol/1, the transient
absorption spectrum (b) in Fig. 1 with a broad maxi­
mum at 370 nm was formed during the pulse. Howev­
er, upon irradiating solutions of P h 3SBF4 containing
1.12 mol/1 propanol-2, an additional absorption was
not formed after the pulse as in the case of diphenyl
iodonium salt solutions. With P h 3SBF4 solutions,
only a very weak absorption formed during the pulse
corresponding to the depletion of • OH radicals by
reaction with propanol-2.
Discussion
X (nm)
Fig. 2. Irradiation o f an N 20 -sa tu r a te d aq u eou s so lu ­
tion co n ta in in g P h 2IP F 6 (4 x 1 0 - 3 mol/1) and p rop an ol-2
(1.12 mol/1). T h e spectrum p resented here w as ob tain ed from
the difference in the op tical d en sities recorded at 10 and 1 /is
after the end o f the pulse. D abs = 60 G y.
The spectra shown in Fig. 1 (a) and (b) are attribut­
ed to the products of the reaction of the hydroxyl
radical with P h 2I + and P h 3S +, respectively. These
products are presumably the adducts formed accord­
ing to the reactions.
OH 4- Ph2I +
10
-
0
4
[Ph2IPF6l
Fig. 3. R eaction o f 2 -h yd roxy-2-p rop yl radicals with P h 2I +
ion s. P lot o f the p se u d o -1st order rate co n sta n t vs. the io d o nium salt co n cen tration . F or other co n d itio n s refer to legend
to Figure 2.
Hydroxyl radicals are prone to add to compounds
containing aromatic rings whereas they frequently ab-
Y. Y agci et al. • O n the R ea ctio n s o f D ip h en y lio d o n iu m and T rip h en ylsulp h on iu m Salts
stract hydrogens from aliphatic compounds. Hydrox­
yl adducts with cyclohexadienyl structure are re­
ported to possess absorption maxima at ca. 320 nm
[4].
Taking into account a possible red-shift of the max­
imum due to substitution of a hydrogen by iodine or
sulphur, it seems justified to assign the spectra in
Fig. 1 to the adducts formed according to reactions
(5) and (6), respectively. The rapid build-up of the
products of the reaction of • O H radicals with P h 2I +
and P h 3S +, respectively, during the pulse (r < 50 ns)
indicates high bimolecular rate constant in both cases:
k 2 > 5 x 109 1/mol s.
The addition of propanol-2 to the solution at a
sufficiently high concentration resulted in suppression
of the OH adduct formation. In the case of P h 2IPF6,
a new transient absorption was formed relatively
slowly after the pulse. These results are interpreted in
support of reaction (3), and the new absorption is
attributed to P h 2I radicals. It follows that the mea­
sured rate constant, k —6.0 x 107 1/mol s, applies to
reaction (3). In this connection it is interesting to note
that a similar value, k = 3.0 x 107 1/mol s was re­
ported by Timpe [5] for the reaction of benzophenone
ketyl radicals, P h 2COH, with P h2I + ions. The slightly
higher value found in our case might be due to a
higher electron density at the radical site in the
2-hydroxy-2-propyl radicals.
A different situation was encountered in the case of
the triphenylsulphonium salt. Whereas P h 3S + ions
behaved similarly to P h 2I + ions in the reaction with
hydroxyl radicals, a significant difference in the reac­
tivity of these ions towards 2-hydroxy-2-propyl radi­
cals was found. In contrast to P h 2I + ions reaction of
P h 3S + ions with 2-hydroxy-2-propyl radicals was not
detected.
This finding is in accordance with the finding that
triphenylsulphonium salts are not capable of initiating
[1] A. L edw ith, P o ly m er 19, 1217 (1978).
[2] F. A. M . A bd u l R asoul, A. L edw ith, and Y. Y agci, P o ly ­
m er 19, 1219 (1978); Y. Y agci, A. C. A yd ogan , E. Sizgek,
J. P olym . Sei. P olym . Lett Ed. 22, 103 (1984).
[3] S. P. P appas, B. C. P app as, L. R. G atechair, and W.
S ch n ab el, J. P o ly m . Sei. 22, 69 (1984).
[4] E. J. Land, and M . Ebert, Trans. F arad ay Soc. 63, 1183
(1967).
[5] H.-J. T im p e, S.B. A k ad . W iss. D D R , 13 N , A kadem ieV erlag, Berlin 1986.
[6] A. L edw ith, S. A l-K a ss, and A. H u lm e L ow e, C ation ic
P o ly m eriza tio n s and R elated P rocesses, E. J. G oeth als
(ed.), A ca d em ic Press, L o n d o n 1984, p. 275.
1427
free radical promoted cationic polymerization [6, 7],
which was explained in terms of the quite different
redox potentials (relative to SCE) of the two ions:
- 1.2 V (Ph3S + ) [8] and - 0.2 V (Ph2I + ) [9], Similar­
ly, Baumann et al. [10] were unable to detect a reaction
of benzophenone ketyl radicals with triphenylsulpho­
nium ions in flash photolysis and spin trapping experi­
ments.
Experimental Part
M a te ria ls
Propanol-2 (E. Merck, pro analysis) was distilled
prior to use. Diphenyliodonium hexafluorophosphate
and triphenylsulphonium tetrafluoroborate were pre­
pared as previously described [11],
Irra d ia tio n s
The solutions were irradiated with 16M eV elec­
trons in rectangular quartz cells after bubbling with
N 20 for 30 minutes. Fast electrons were generated by
a linear accelerator (Vickers, L-band type). The ab­
sorbed dose was determined with the aid of the Fricke
dosimeter employing 0 2-saturated F e2+ solutions
(G(Fe3+) = 13.0).
A cknow ledgem ents
Dr. Yusuf Yagci is indebted for a grant from
Alexander von Humboldt-Stiftung. We thank Mr.
Martin Wilhelm who assisted in running the linear
accelerator and Mr. Mark Tilley for preparing the
onium salts.
[7] S. P. P appas, L. R. G atech air, and J. H. Jilek, J. P olym .
Sei. C hem . Ed. 22, 77 (1984).
[8] P. S. M cK in n ey and S. R osen th al, J. E lectroanal. Chem .
Soc. 16, 261 (1968).
[9] H. E. B achofner, F. M . Beringer, and L. M eites, J. Amer.
C hem . Soc. 80, 4269 (1958).
[10] H. B aum ann, H.-J. T im pe, V. E. Z ubarev, N . V. F ok,
and M . J. M eln ik ov, Z. C hem . 25, 181 (1985).
[11] J. V. C rivello and J. H. W. Lam , J. P olym . Sei., P olym .
C hem . Ed. 17, 977 (1979).