Photosolvolysis of (E)-styryl(phenyl)iodonium tetrafluoroborate. Generation and reactivity of a primary vinyl cation

Article abstract of DOI:10.1021/jo0107397

The photochemistry of (E)-styryl(phenyl)iodonium tetrafluoroborate in methanol and 2,2,2-trifluoroethanol as well as in dichloromethane and toluene has been investigated. In all solvents the vinylic C-I bond is more photoreactive than the aromatic C-I bond. Homolysis as well as heterolysis of both bonds occurs, but the latter type of cleavage predominates. In alcoholic solvents, the incipient phenyl cation produces a nucleophilic substitution product. The primary styryl cation gives nucleophilic substitution, elimination, and rearrangement products. The dependence of the photoreaction on the nucleophilicity of the solvent indicates that in the presence of good nucleophiles a 10-I-3 compound is the reactive iodonium species. In this case the reaction proceeds via an S_Ni mechanism. In the absence of good nucleophiles an 8-I-2 species gives photoreaction via an S_N1 mechanism. This is corroborated by the solvent dependence of the UV spectra, and the product composition upon photoreaction with bromide in varying concentration. Photoreaction of the iodonium salt in a chlorinated alkane yields (E)- and (Z)-β-fluorostyrene in a Schiemann-type reaction. Reaction in toluene yields Friedel-Crafts products. The results of the photochemical reactions are compared to those of the thermal ones, and the implications of the differences are discussed.

Full text of DOI:10.1021/jo0107397

J . Org. Chem. 2002, 67, 693-702
693
P h otosolvolysis of (E)-Styr yl(p h en yl)iod on iu m Tetr a flu or obor a te.
Gen er a tion a n d Rea ctivity of a P r im a r y Vin yl Ca tion
Roel Gronheid,† Gerrit Lodder,*,† and Tadashi Okuyama‡
Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502,
2300 RA Leiden, The Netherlands, and Faculty of Science, Himeji Institute of Technology, Kamigori,
Hyogo 678-1297, J apan
lodder@chem.leidenuniv.nl
Received J uly 23, 2001
The photochemistry of (E)-styryl(phenyl)iodonium tetrafluoroborate in methanol and 2,2,2-
trifluoroethanol as well as in dichloromethane and toluene has been investigated. In all solvents
the vinylic C-I bond is more photoreactive than the aromatic C-I bond. Homolysis as well as
heterolysis of both bonds occurs, but the latter type of cleavage predominates. In alcoholic solvents,
the incipient phenyl cation produces a nucleophilic substitution product. The primary styryl cation
gives nucleophilic substitution, elimination, and rearrangement products. The dependence of the
photoreaction on the nucleophilicity of the solvent indicates that in the presence of good nucleophiles
a 10-I-3 compound is the reactive iodonium species. In this case the reaction proceeds via an SNi
mechanism. In the absence of good nucleophiles an 8-I-2 species gives photoreaction via an SN1
mechanism. This is corroborated by the solvent dependence of the UV spectra, and the product
composition upon photoreaction with bromide in varying concentration. Photoreaction of the
iodonium salt in a chlorinated alkane yields (E)- and (Z)-â-fluorostyrene in a Schiemann-type
reaction. Reaction in toluene yields Friedel-Crafts products. The results of the photochemical
reactions are compared to those of the thermal ones, and the implications of the differences are
discussed.
Sch em e 1. Mech a n ism of F or m a tion of P r od u cts
in th e Th er m a l Meth a n olysis of th e
In tr od u ction
Thermal solvolysis of vinyliodonium salts has been the
subject of numerous recent1 and a few older2 studies.
Owing to the superb leaving group ability of the iodonio
group (>105 times better than triflate),3 this class of
compounds gives remarkable chemistry such as in-plane
vinylic SN2 substitution1 and SN1 substitution via highly
unstable (vinylic) carbocations.3,4 Until recently there
were no examples of photochemical solvolysis of vinyl-
iodonium salts.5 This is in stark contrast to diaryl-
iodonium ions the photochemistry of which has been
thoroughly investigated over the past decades.6-8 Here we
present an in-depth study of the photochemical solvolysis
of (E)-styryl(phenyl)iodonium tetrafluoroborate (1). As
solvents were used methanol, trifluoroethanol, dichloro-
(E)-Styr yl(p h en yl)Iod on iu m Ion (1)
† Leiden University.
‡ Himeji Institute of Technology.
(1) For a recent review see: Okuyama, T. Rev. Heteroatom Chem.
1999, 21, 257.
(2) (a) Nesmeyanov, A. N.; Tolstaya, T. P.; Petrakov, A. V.; Gol’tsev,
A. N. Dokl. Akad. Nauk SSSR 1977, 235, 425. (b) Nesmeyanov, A. N.;
Tolstaya, T. P.; Petrakov, A. V.; Leshcheva, I. F. Dokl. Akad. Nauk
SSSR 1978, 238, 70.
(3) Okuyama, T.; Takino, T.; Sueda, T.; Ochiai, M. J . Am. Chem.
Soc. 1995, 117, 3360.
(4) (a) Hinkle, R. J .; Thomas D. B. J . Org. Chem. 1997, 62, 7534.
(b) Hinkle, R. J .; McNeil, A. J .; Thomas, Q. A.; Andrews, M. N. J . Am.
Chem. Soc. 1999, 121, 7437.
(5) Gronheid, R.; Lodder, G.; Ochiai, M.; Sueda, T.; Okuyama, T. J .
Am. Chem. Soc. 2001, 123, 8760.
(6) (a) Dektar, J . L.; Hacker, N. P. J . Org. Chem. 1990, 55, 639. (b)
Dektar, J . L.; Hacker, N. P. J . Org. Chem. 1991, 56, 1838. (c) Hacker,
N. P.; Leff, D. V.; Dektar, J . L. J . Org. Chem. 1991, 56, 2280.
(7) Gu, H.; Zhang, W.; Feng, K.; Neckers, D. J . Org. Chem. 2000,
65, 3484.
methane, and toluene. The results are compared to those
of the corresponding thermal reactions.
Particularly relevant for our investigations is the
thermal solvolysis of 1 in methanol and trifluoroethanol.9
In methanol four products are formed: iodobenzene (the
leaving group), phenylacetylene (the elimination prod-
uct), and (Z)- and (E)-â-methoxystyrene (the substitution
products). The mechanism of formation of the products
was unraveled, using specifically deuterium-labeled 1
(Scheme 1).
(8) DeVoe, R. J .; Olofson, P. M.; Sahyun, M. R. V. In Advances in
Photochemistry; Volman, D. H.; Hammond, G. S.; Neckers, D. C., Eds.;
J ohn Wiley & Sons: New York, 1992; Vol. 17, p 313.
(9) (a) Okuyama, T.; Ochiai, M. J . Am. Chem. Soc. 1997, 119, 4785.
(b) Okuyama, T.; Ishida, Y.; Ochiai, M. Bull. Chem. Soc. J pn. 1999,
72, 163.
10.1021/jo0107397 CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/16/2002

694 J . Org. Chem., Vol. 67, No. 3, 2002
Gronheid et al.
Sch em e 2
Phenylacetylene is formed by solvent-assisted R-elim-
ination, initially yielding a vinylidene carbene (Ic) which
subsequently rearranges to the alkyne. In this case
complete loss of label is observed (route A). Substitution
with inversion of configuration proceeds via an in-plane
SN2 substitution, yielding (Z)-â-methoxystyrene. No scram-
bling of the label occurs (route B). Substitution with
retention of configuration occurs when iodobenzene leaves
with anchimeric assistance of the phenyl ring, yielding
a vinylenebenzenium ion (I1). Nucleophilic attack by the
solvent on this ion gives (E)-â-methoxystyrene with net
retention of configuration and complete scrambling of the
deuterium label (route C). In 2,2,2-trifluoroethanol (TFE),
a solvent which is a much weaker base than methanol,
no elimination occurs. Also no vinylic SN2 substitution
takes place, because TFE is also a very poor nucleophile.
The only product in this case is (E)-â-(2,2,2-trifluoro-
ethoxy)styrene, formed via route C.
Upon irradiation of diaryliodonium salts in various
solvents, heterolysis of the carbon-iodine bond occurs,
predominantly from the first singlet-excited state. Elec-
tron transfer within the ion-molecule pair yields the
corresponding radical and radical ion. From the triplet-
excited state exclusive homolysis is observed.6b Consider-
ing the iodonio group as a pseudohalide, this behavior is
quite different from that of simple halobenzenes in
solution. With such compounds, homolytic cleavage of the
carbon-halogen bond is the only dissociative pathway
for both the singlet and triplet excited state.10 Apparently
the high nucleofugality of the phenyliodonio group also
has a large effect on the photochemistry of phenyliodo-
nium compounds.
Reaction of photogenerated vinyl cations with aromatic
compounds (vinylic photo-Friedel-Crafts reaction) has
only been reported for an intramolecular case.16 Inter-
molecular vinylation has been performed using thermally
generated vinyl cations (Scheme 2)17 and using the parent
vinyl cation C2H3 generated by tritium decay.18 The
+
thermal solvolysis of a cyclohexenyliodonium compound
in benzene also yields Friedel-Crafts type adducts,19 but
in this case the origin of the aromatic moiety is uncer-
tain: is it the leaving group (undergoing ipso substitu-
tion), or the solvent? We wondered whether intermolecu-
lar photochemical vinylation of aromatic compounds
would be feasible, using iodonium precursors.
The results of the investigation of 1 reported here
provide answers to the following questions: (1) Does
photochemistry of vinyl(phenyl)iodonium compounds lead
to (primary) vinyl cation-derived products? (2) How does
the photochemistry of vinyl(phenyl)iodonium compounds
compare with its thermal chemistry? (3) How does 1
photoreact in the absence of external nucleophiles? (4)
Does electrophilic aromatic substitution occur upon ir-
radiation of 1 in the presence of aromatic compounds?
Resu lts a n d Discu ssion
Alcoh olic Solven ts. P h otop r od u cts in Meth a n ol.
Irradiation of (E)-styryl(phenyl)iodonium tetrafluorobo-
rate (1) in methanol (λexc ) 248 nm) results in the reaction
mixture depicted in Scheme 3. In marked contrast to its
thermal behavior, photoexcited 1 does yield a rearranged
vinyl ether (i.e., 7-OMe) next to two unrearranged ones
(i.e., Z-6-OMe and E-6-OMe) as vinylic nucleophilic
substitution products.
At sufficiently low conversion of 1, the rates of forma-
tion of all photoproducts were shown to be constant at
constant light flux, indicating primary photoproducts. In
Figure 1, two typical examples are given for the formation
of phenylacetylene (5) and (Z)-â-methoxystyrene (Z-6-
OMe), respectively. During the photoreaction, (Z)-â-
iodostyrene is formed in trace amounts. The kinetics of
formation of this product show, however, that it is a
secondary photoproduct, presumably formed by E f Z
isomerization of (E)-â-iodostyrene (2).
The effect of the leaving group has earlier been studied
in the photochemistry of vinyl halides.11 The results of
these studies were, however, inconclusive. Both increases
and decreases in cation/radical ratio upon changing the
leaving group (F f Cl f Br f I) have been reported,
but the efficiency of the photochemical reaction was
shown to increase in all cases. The phenyliodonio leaving
group extends this series with a pseudohalide, and
efficient formation of a primary vinyl cation from 1 by
C-I bond heterolysis is anticipated.5,12
The reactivity of vinyl cations has been studied by
varying the nucleophilicity of the solvent. For example,
irradiation of vinyl bromides such as bromostilbene gives
exclusive HBr elimination in diethyl ether and n-hexane,
but elimination as well as nucleophilic substitution in
methanol.11a,b,13,14 The thermal solvolysis of 1 in an apolar
solvent such as chloroform results in reaction of I1 with
its counterion, yielding (E)-â-fluorostyrene via a mech-
anism similar to that in Scheme 1C.15 We wondered
how 1 would react upon photolysis in nonnucleophilic
solvents.
The various primary photoproducts are proposed to be
formed via three carbon-iodine bond cleavage pathways
(Scheme 4): (1) Heterolytic cleavage of the phenylic
(14) Lodder, G. In Dicoordinated Carbocations; Rappoport, Z.; Stang,
P. J ., Eds.; J ohn Wiley & Sons: Chichester, 1997; Ch.8.
(15) Okuyama, T.; Fujita, M.; Gronheid, R.; Lodder, G. Tetrahedron
Lett. 2000, 41, 5125.
(16) (a) Kitamura, T.; Kobayashi, S.; Taniguchi, H. Chem. Lett. 1984,
547. (b) Kitamura, T.; Kobayashi, S.; Taniguchi, H.; Hori, K. J . Am.
Chem. Soc. 1991, 113, 6240.
(17) (a) Roberts, R. M.; Abdel-Basset, M. B. J . Org. Chem. 1976,
41, 1698. (b) Stang, P. J .; Anderson, A. G. Tetrahedron Lett. 1977, 17,
1485. (c) Stang, P. J .; Anderson, A. G. J . Am. Chem. Soc. 1978, 100,
1520. (d) Kitamura, T.; Kobayashi, S.; Taniguchi, H.; Rappoport, Z. J .
Org. Chem. 1982, 47, 5003.
(18) (a) Fornarini, S.; Speranza, M. Tetrahedron Lett. 1984, 869. (b)
Fornarini, S.; Speranza, M. J . Am. Chem. Soc. 1989, 111, 7402.
(19) Ochiai, M.; Takaoka, Y.; Sumi, K.; Nagao, Y. J . Chem. Soc.,
Chem. Commun. 1986, 1382.
(10) Lodder, G.; Cornelisse, J . In The Chemistry of Functional
Groups - Supplement D2: The chemistry of halides, pseudohalides
and azides Part 2; Patai, S; Rappoport, Z., Eds.; J ohn Wiley & Sons:
Chichester, 1995; Ch. 16.
(11) (a) Sˇket, B.; Zupan, M.; Pollak, A. Tetrahedron Lett. 1976, 783.
(b) Sˇket, B.; Zupan, M. J . Chem. Soc., Perkin Trans. 1 1979, 752. (c)
Kitamura, T.; Kobayashi, S.; Taniguchi, H. J . Org. Chem. 1982, 47,
2323. (d) Zupancic, N.; Sˇket, B. J . Photochem. Photobiol. A: Chem.
1991, 60, 361. (e) Verbeek, J . M.; Stapper, M.; Krijnen, E. S.; Van Loon,
J . D.; Lodder, G.; Steenken, S. J . Phys. Chem. 1994, 98, 9526.
(12) Gronheid, R.; Zuilhof, H.; Hellings, M. G.; Cornelisse, J .; Lodder,
G. Manuscript in preparation.
(13) (a) Op den Brouw, P. M.; Laarhoven, W. H. J . Org. Chem. 1982,
47, 1546. (b) Krijnen, E. S.; Zuilhof, H.; Lodder, G. J . Org. Chem. 1994,
59, 8139.

Photosolvolysis of (E)-Styryl(phenyl)iodonium Tetrafluoroborate
J . Org. Chem., Vol. 67, No. 3, 2002 695
from I3 again yields 5. Nucleophilic attack by the solvent
on I3 yields R-methoxystyrene (7-OMe). Acid-catalyzed
hydrolysis of this unstable enol ether yields acetophenone
(8).
The products expected from the fourth possible mode
of C-I bond cleavage, homolytic cleavage of the phenylic
carbon-iodine bond, are benzene, formed by hydrogen
atom abstraction from the solvent by the initially formed
phenyl radical, and 2, formed from the radical cation of
2 by acquisition of an electron. However, benzene, if
present, is obscured by the solvent in the GC-product
analysis. Compound 2 is present, but may also be formed
by the heterolytic pathway. The occurrence of this
homolytic pathway can therefore not be demonstrated,
but is by no means excluded.
The pathways of photoreactivity of 1 are comparable
to those of diaryliodonium salts6 in the sense that both
heterolytic and homolytic cleavage of C-I bonds occur.
The radical-derived products in the photochemistry of
diphenyliodonium salts are all formed by hydrogen atom
abstraction from the solvent, giving net replacement of
the phenyliodonio group by a hydrogen atom. The photo-
generated aryl cations give two types of photoproducts:
Products derived from nucleophilic attack of the solvent
(methanol) or products derived from reaction with the
neutral leaving group (iodobenzene) within the solvent
cage to give Friedel-Crafts type adducts. Addition of the
aryl cation at all positions of iodobenzene is observed:
para, meta, ortho, and ipso. Products derived from nucleo-
philic substitution by the solvent methanol obviously are
also observed in the photochemistry of 1, but Friedel-
Crafts products (i.e., p-, m-, or o-iodostilbenes) are not.
In some experiments traces of (E)-stilbene are formed
(the product derived from Friedel-Crafts addition at the
ipso position).
F igu r e 1. Formation of phenylacetylene (9) and (Z)-â-
methoxystyrene (2) upon irradiation of 1 (λexc ) 248 nm) in
methanol as a function of time.
Sch em e 3
Sen sitiza tion Exp er im en ts. In diphenyliodononium
compounds the multiplicity of the excited state deter-
mines whether homolytic or heterolytic photochemical
bond cleavage prevails.6 It is of interest to see whether
this is also the case for 1. Moreover, upon sensitized
irradiation of E-vinyl halides, only E/Z-isomerization is
observed.12 In general, E/Z-isomerization of alkenes is a
well-known photochemical process, occurring from the
triplet as well as the singlet excited state.22 E/Z-Isomer-
ization of 1 to the corresponding Z-vinyl(phenyl)iodonium
ion is particulary relevant because it presents an alter-
native mechanism of formation of 5. Vinyliodonium
compounds with a hydrogen atom positioned at the trans-
â-position with respect to the phenyliodonio moiety, are
known to rapidly react thermally by trans-elimination
of iodobenzene and a proton,23,24 yielding the correspond-
ing alkyne. Therefore, 5 and 9 are the only expected
products, if in analogy with vinyl halides, E/Z-isomer-
ization of 1 is the only mode of reaction from its triplet
excited state (Scheme 5).
carbon-iodine bond leading to (E)-iodostyrene (2) and the
phenyl cation. Reaction of the latter species with the
solvent results in anisole (3-OMe). This pathway does
not occur in the thermal methanolysis of 1.9,20 (2) Homo-
lytic cleavage of the vinylic carbon-iodine bond yielding
the styryl radical and the radical cation of iodobenzene.
The styryl radical abstracts a hydrogen atom from the
solvent to give styrene (4). When the iodobenzene radical
cation acquires an electron, neutral iodobenzene (9) is
formed. This type of chemical behavior is also not
observed upon thermal solvolysis of 1.9 (3a) Heterolytic
cleavage of the vinylic carbon-iodine bond resulting in
9 and the open primary styryl cation (I2). This species
can undergo nucleophilic attack by the solvent or loss of
a â-hydrogen. These two pathways account for (E)- and
(Z)-â-methoxystyrene (E- and Z-6-OMe) and phenyl-
acetylene (5). Both addition of solvent to I2 and proton
loss from I2 can in principle be preceded by one or more
1,2-phenyl shifts, via the benzenium ion I1. These shifts
are however degenerate, and therefore not included. (3b)
Intermediate I2 can also rearrange by a 1,2-hydride shift,
to the secondary styryl cation I3.21 Loss of a â-hydrogen
Irradiation of benzophenone in methanol, in the pres-
ence of E-1 was performed at λexc ) 366 nm. At this
wavelength no light is absorbed by the iodonium salt
(vide infra), so direct excitation of 1 does not occur. The
major products (>90%) are phenylacetylene (5) and
iodobenzene (9). Also small amounts of 2, 4, and the vinyl
(22) See e.g.: Waldeck, D. H. Chem. Rev. 1991, 91, 415.
(23) Ochiai, M.; Oshima, K.; Masaki, Y. J . Chem. Soc., Chem.
Commun. 1991, 869.
(24) Ochiai, M.; Sumi, K.; Takaoka, Y.; Kunishima, M.; Nagao, Y.;
Shiro, M.; Fujita, E. Tetrahedron 1988, 44, 4095.
(20) Cleavage of the phenylic C-I bond of 1 is observed upon
addition of a strong nucleophile see ref 2a.
(21) Lee, C. C.; Ko, E. C. F. J . Org. Chem. 1975, 40, 2132.

696 J . Org. Chem., Vol. 67, No. 3, 2002
Gronheid et al.
Sch em e 4. Mech a n ism of F or m a tion of P h otop r od u cts fr om 1 in Meth a n ol a n d TF E
Ta ble 1. Qu a n tu m Yield s of F or m a tion of P r od u cts in
th e P h otor ea ction of (E)-Styr yl(p h en yl)iod on iu m
Tetr a flu or obor a te (1) in Meth a n ol a n d
2,2,2-Tr iflu or oeth a n ol
Sch em e 5
Φ (in methanol;
R ) CH3)
Φ (in 2,2,2-trifluoroethanol;
R ) CH2CF3)
2a
3-OR
4
5
Z-6-OR
E-6-OR
7-OR
8
0.133 ( 0.008
0.038 ( 0.003
0.079 ( 0.011
0.325 ( 0.020
0.121 ( 0.011
0.398 ( 0.015
0.038 ( 0.007
0.010 ( 0.002
0.756 ( 0.051
0.168 ( 0.010
0.046 ( 0.007
0.012 ( 0.001
<0.02b
0.0066 ( 0.0005
0.0065 ( 0.0004
- -c
ethers E- and Z-6-OMe are detected (all formed with
about equal efficiency). This indicates that in the sensi-
tized irradiation of E-1, indeed the elimination product
is formed via E/Z-isomerization of the iodonium salt. This
was further tested by sensitized irradiation of selectively
deuterated 1.
0.228 ( 0.037
0.463 ( 0.054
9
a
When benzophenone was irradiated in the presence of
R-deuterated E-1 (E-1-D), 91.3 ((1.1)% of the elimination
product 5 was not scrambled (i.e., 8.7% nondeuterated
phenylacetylene was formed) in line with Scheme 5.
Thus, although E/Z-isomerization is not the only reactive
pathway of triplet excited 1, it is certainly the most
important one. Upon direct irradiation of E-1-D in
methanol 21.6 ((1.7)% of scrambling was observed. This
means that different pathways for elimination are fol-
lowed upon sensitized and direct photolysis. The larger
amount of scrambling in the direct irradiation is in line
with initial formation of I2, followed by rearrangement
via the benzenium ion I1 and proton loss.12 In an
alternative mechanism for isotope scrambling, part of
5 may be formed via loss of the R-hydrogen (or R-
deuterium) from I2, yielding vinylidene carbene Ic which
will rearrange to 5.9 Whereas homolysis is the exclusive
pathway of reaction upon sensitized photolysis of di-
phenyliodonium compounds, pathway 2 is at best a
minor pathway upon sensitized irradiation of 1: only
trace amounts of 4 are formed. The same is true for
the heterolytic pathways 1 and 3, yielding 2 and 6.
Apparently E/Z-isomerization followed by trans-elimina-
tion is the most efficient reaction path from the triplet-
excited state.
Z-2 was shown to be a secondary photoproduct in the irradia-
b
tion of E-1 and is added to E-2. Upper limit estimate, because 1
is contaminated with small amounts of 5 and 9. c Under the
workup conditions, R-trifluoroethoxystyrene is not stable and only
8 is observed from path 3b.
Qu a n tu m Yield s of P r od u ct F or m a tion in Meth a -
n ol a n d 2,2,2-Tr iflu or oeth a n ol. In the thermal sol-
volysis of 1 a large effect on the product distribution is
observed upon changing the solvent from methanol to
TFE. The photochemical solvolysis has therefore also
been performed in TFE. The same products are obtained
as described for the irradiation in methanol, with 2,2,2-
trifluoroethoxy groups instead of methoxy groups, but the
product distribution is quite different. For a quantitative
comparison the quantum yields of product formation in
the two solvents have been measured (Table 1).
In both solvents the photosolvolysis of 1 is quite
efficient. In methanol, the sum of the quantum efficien-
cies of formation of the leaving groups (iodobenzene and
(E)-iodostyrene) is 0.89. The total quantum yield of
formation of all other photoproducts in methanol equals
unity within experimental error. This may indicate that
the homolytic C-I bond cleavage pathways producing the
radical cations of the leaving groups yield unidentified
products. In TFE the quantum yield of the photoreaction,
based on the leaving groups, is ∼0.6, substantially higher
than that based on the other products (∼0.3). Cationic
polymerization in this excellent cation-stabilizing solvent
may well be responsible for the unaccounted products.
Clearly, the lowest triplet-excited state is not the origin
of most of the products in the direct irradiation of 1, as
shown by the different amounts of isotope scrambling and
the relative difference in efficiency in formation of photo-
products (vide infra). The triplet-excited state may be
active in the formation of part of 5 and 9. The singlet-
excited state (or a higher triplet state) is responsible
for the majority of the products observed upon direct
excitation.
In the thermal solvolysis of 1 exclusive cleavage of the
vinylic C-I bond is observed. Although this bond is still
the most reactive one in the photochemical solvolysis, also
considerable cleavage of the phenylic C-I bond is ob-

Photosolvolysis of (E)-Styryl(phenyl)iodonium Tetrafluoroborate
J . Org. Chem., Vol. 67, No. 3, 2002 697
Hypervalent iodine (8-I-2)27 compounds are known to
coordinate readily with a wide variety of nucleophiles
such as halide anions, to give the corresponding tricoor-
dinated 10-I-3 (λ3-iodane) species.28 When the solvent is
nucleophilic, the overall photochemical behavior of 1 will
be a mixture of the photochemistry of the free and the
solvent-coordinated iodonium ion.29 Excitation of the free
iodonium ion yields I2 in an SN1 mechanism, which gives
substitution products in ∼1:1 E/Z ratio, as well as
elimination and rearrangement products as depicted in
Scheme 4. Excitation of the λ3-iodane results in formation
of I2, 9 and a solvent molecule within the solvent cage
(Scheme 6). Nucleophilic attack of the solvent molecule
within the solvent cage will preferentially give retention
of configuration and lead to E-6-OR (path I), by an SNi
mechanism.30 The phenomenon is more important in
methanol than in TFE, since methanol is a stronger
nucleophile than TFE. The E/ Z stereoselectivities for the
thermal and the photochemical reaction in methanol and
TFE thus have different origins. In the thermal reaction
the ratio is determined by the relative importance of SN1
and SN2 pathways, and in the photochemical reaction by
the occurrence of λ3-iodanes.
In addition, the mechanism in Scheme 6 (path II)
clarifies the higher efficiency of formation of elimination
product 5 in methanol than in TFE (Table 1). Upon
photolysis of 1 in methanol, I2 is formed with methanol
inside the solvent cage, a cation-molecule pair. The
methanol not only acts as a nucleophile, but also as a
base. Abstraction of a proton from I2 yields 5. In the case
of TFE, the proton has to be abstracted by the leaving
group (9), or by external TFE. Both agents are less basic
than methanol, causing a lower efficiency of formation
of 5.
This situation is reminiscent of the product formation
upon excitation of (E)-â-bromostyrene in methanol.12 In
this case bromide, a stronger base than methanol, is
formed with I2 within a solvent cage, as a cation-anion
pair, and exclusive elimination is observed. Complete
suppression of the formation of nucleophilic substitution
products due to the high kinetic basicity of a halide anion
is not uncommon and well documented.31
The present interpretation is in line with our recent
results on the photochemistry of two E/ Z-isomeric
methyl-substituted analogues of 1.5 In those cases also
preferential retention of configuration was observed in
methanol, but not in TFE. Also more rearrangement of
the initially formed vinyl cation occurred in TFE, and
Sch em e 6. Mech a n ism s of F or m a tion of En ol
Eth er s 6 a n d Acetylen e 5
served. The ratio of cleavage of the vinylic and phenylic
C-I bond changes from 5.7:1 in methanol to 2.8:1 in TFE.
In the photochemical nucleophilic substitution path-
ways of 1 in methanol and TFE a marked stereochemical
difference with its thermal solvolysis is encountered. In
the thermal solvolysis the stereoselectivity gets higher
on going to weaker nucleophilic solvents.9 In sharp
contrast, in the photochemical reaction the E/Z selectivity
in 6, observed in methanol, is completely lost in TFE.
An explanation for this observation will be presented
later in this section.
The photochemical formation of ion-derived products
from 1 in methanol and in TFE shows two remarkable
differences. First, the relative importance of paths 3a and
3b (see Scheme 4): In methanol, path b (products 5, 7,
and 8) is a minor path of reaction, while it is the major
one in TFE. For path a (products 5 and 6), the situation
is reversed. Since a much smaller amount of 5 is formed
in TFE than in MeOH, path b may be a pathway to form
5, but it is not as important as path a.
The relative importance of paths a and b is determined
by the lifetime of I2. The primary styryl cation I2 is known
to be a very unstable species.21,25 On the basis of quantum
chemical calculations, rapid rearrangement to I3 is
predicted.12 Fluorine-substituted alcohols are known to
be good cation-stabilizing solvents,26 so the lifetime of I2
is expected to be larger in TFE than in methanol. This
gives I2 more time to rearrange to the more stable I3 in
TFE, and this makes path b more important.
In the thermal reaction of 1 no products derived from
I3 (i.e., 7-OR and 8) are formed. In this case, the initially
formed cationic species is the bridged benzenium ion I1.
This structure already corresponds to a local minimum,
and it will react with the solvent before rearrangement
to I3 can occur.
(27) The N-X-L notation conveniently describes the nature of a
hypervalent compound by the number (N) of formally assignable
electrons of more than the octet in a valence shell directly associated
with the central atom (X) in directly bonding a number (L) of ligands
(substituents): Akiba, K.-y. In Chemistry of Hypervalent Compounds;
Akiba, K.-y., Ed.; Wiley-VCH: New York, 1999; Ch. 1.
(28) (a) Ochiai, M.; Oshima, K.; Masaki, Y. J . Am. Chem. Soc. 1991,
113, 7095. (b) Okuyama, T.; Takino, T.; Sato, K.; Ochiai, M. Chem.
Lett. 1997, 955. (c) Okuyama, T.; Takino, T.; Oshima, K.; Imamura,
S.; Yamataka, H.; Asano, T.; Ochiai, M. Bull. Chem. Soc. J pn. 1998,
71, 243. (d) Okuyama, T.; Takino, T.; Sato, K.; Ochiai, M. J . Am. Chem.
Soc. 1998, 120, 2275.
The second remarkable difference between the two
solvents is the large difference in the stereoselectivity of
the vinyl ethers 6. In methanol considerable E/Z-selectiv-
ity is observed (E/Z ) 3.3), i.e., mainly retention of
configuration, however, in TFE the E/Z-ratio in 6 equals
1. A refinement of the mechanism of photoreaction of 1
(proposed in Scheme 4) can explain the solvent depen-
dence of the stereoselectivity on the formation of the vinyl
ethers invoking ground-state complexation (Scheme 6).
(29) Complexation with a nucleophile has previously been proposed
to occur photochemically in the nucleophilic photosubstitution of
diaryliodonium ions with sulfur nucleophiles: Spyroudis, S. P. Liebigs
Ann. Chem. 1986, 947.
(30) For previous examples of the SNi mechanism see: (a) March,
J . In Advanced Organic Chemistry, 4th ed.; J ohn Wiley & Sons: New
York, 1992; p 326. (b) Hori, K.; Nishiguchi, T.; Nabeya, A. J . Org. Chem.
1997, 62, 3081 and references therein.
(31) (a) Thibblin, A. J . Phys. Org. Chem. 1992, 5, 367. (b) Thibblin,
A. Chem. Soc. Rev. 1993, 22, 427.
(25) Apeloig, Y.; Franke, W.; Rappoport, Z.; Schwarz, H.; Stahl, D.
J . Am. Chem. Soc. 1981, 103, 2770.
(26) Steenken, S.; Ashokkumar, M.; Maruthamuthu, P.; McClelland,
R. A. J . Am. Chem. Soc. 1998, 120, 11925.

698 J . Org. Chem., Vol. 67, No. 3, 2002
Gronheid et al.
Sch em e 7
of 1 in these solvents do not stem from differences in
electronic excitations.
Effect of Ad d ed Br om id e on th e UV Sp ectr a a n d
th e P h otoch em istr y in Meth a n ol. The validity of the
reaction mechanism depicted in Scheme 6, involving λ3-
iodanes in the photochemistry of 1, was further examined
by adding varying amounts of tetrabutylammonium
bromide to the irradiation mixtures. The effect of added
bromide on the thermal chemistry of 1 in acetonitrile has
been studied in great detail.32 The association constants
of bromide ions with iodonium compounds in acetonitrile
are quite high, resulting in almost complete association
to a 10-I-3 bromide-iodonium complex at low concentra-
tions (K1 ) 2.8 × 103 M-1; Scheme 7). At higher bromide
concentrations a 12-I-4 complex is formed as well, albeit
F igu r e 2. UV absorption spectra of (E)-styryl(phenyl)iodo-
nium tetrafluoroborate in methanol ([1] ) 4.57 × 10-5 M; solid
line) and TFE ([1] ) 4.83 × 10-5 M; dashed line).
elimination was found to be far more important in
methanol than in TFE.
with a far lower association constant (K2 ) 20 M-1
;
Scheme 7).32 The mechanism presented in Scheme 6
predicts preferential attack of the coordinated nucleophile
at the intermediate styryl cation I2. Added bromide can
compete with methanol and give a λ3-iodane complex with
1, if K1 × [Br-] is comparable or greater than the
association constant of methanol with 1 times the metha-
nol concentration. Photolysis of the 1-bromide complex,
according to Scheme 6, should result in bromide addition
to I2 preferentially giving the E-vinyl bromide.
UV Sp ectr a in Meth a n ol a n d 2,2,2-Tr iflu or oeth a -
n ol. Coordination of nucleophiles to iodonium salts leads
to significant changes in the iodonium UV absorption
spectra.28c,d,32 The absorption spectra of 1 in methanol and
TFE are therefore expected to differ significantly, if
formation of λ3-iodanes of 1 plays a more important role
in methanol.
Indeed, the absorption spectra in methanol and TFE
are quite different (Figure 2). The extinction coefficient
of the large and broad absorption band at λmax ) 270 nm
is smaller in methanol (ꢀ ) 12400 M-1 cm-1) than in TFE
(ꢀ ) 19500 M-1 cm-1), and in methanol two extra, narrow
To study the aptitude of bromide to complex with 1 in
methanol, UV spectra of 1 at various bromide concentra-
tions were recorded. Other than in acetonitrile,32 the
changes in absorbance in methanol unfortunately are too
small to accurately measure the equilibrium constants
K1 and K2 of Scheme 7.
Evidence that the 10-I-3 association complex is indeed
already formed efficiently in methanol at low [Br-] comes
from the photochemistry of 1 in the presence of added
bromide ions. Upon irradiation of 1 in a 5 mM Bu4NBr
solution in methanol, (E)- and (Z)-â-bromostyrene are
formed efficiently. Also bromobenzene (formed via path
1; Scheme 4), trace amounts of R-bromostyrene (formed
via path 3b; Scheme 4), and the products described earlier
in the irradiation of 1 in methanol in the absence of
bromide salt (Scheme 3) are produced.
(E)- and (Z)-â-bromostyrene are formed at the expense
of (E)- and (Z)-â-methoxystyrene. The ratio ΦBr/ΦOMe rises
from 2.2 at [Br-] ) 5 mM to ∼44 at [Br-] ) 30 mM.
Already at [Br-] ) 30 mM, the formation of the meth-
oxystyrenes is suppressed to the extent that only traces
of the vinyl ethers are observed. Obviously, bromide ion
competes efficiently with methanol in the association to
the iodonium ion. From the value of 2.2 at [Br-] ) 5 mM,
and the assumption that all â-bromostyrene originates
from a 1-bromide complex, K1 can be estimated to be
∼4 × 102 M-1. This high association constant of Br- with
1 in methanol results in complete suppression of the
formation of vinyl ethers at already relatively low [Br-].
bands are present at λmax ) 245 nm (ꢀ ) 13300 M-1 cm-1
)
and λmax ) 223 nm (ꢀ ) 18800 M-1 cm-1). The absorption
spectrum of 1 in TFE is similar to its spectrum in
acetonitrile, both in shape and intensity (in acetonitrile
ꢀ270 = 19000 M-1 cm-1).33 The iodonium ion is not
expected to be complexed by these solvents, because of
their low nucleophilicity. A ground-state complex is
therefore the probable origin for the blue-shifted absorp-
tion bands in methanol. The new absorption bands are
narrow, which suggests that the 1-methanol ground
state complex is structurally well defined.
The wavelength of irradiation employed (λexc ) 248 nm)
for the photochemical experiments allows direct excita-
tion of the ground-state complex (λmax ) 245 nm).
Selective excitation of the noncomplexed species by
irradiation at longer wavelengths may result in a lower
E/Z-ratio of the methoxystyrenes 6-OMe and a larger
importance of path 3b. This is however not observed. The
E/Z-ratio and the relative importances of paths 3a and
3b are identical within experimental error, irrespective
of employment of λexc ) 248 nm, or λexc ) 296 nm. Thus,
the solvent dependence of the UV-absorption spectra
supports the formation of λ3-iodanes in methanol and
their absence in TFE in line with the mechanism of
Scheme 6, but the differences in photochemical behavior
Variation of the bromide concentration affects the ratio
of the stereoisomers of â-bromostyrene as well as of
â-methoxystyrene (Figure 3).
(32) Okuyama, T.; Oka, H.; Ochiai, M. Bull. Chem. Soc. J pn. 1998,
71, 1915.
(33) The ꢀ-value was calculated from data in ref 32.

Photosolvolysis of (E)-Styryl(phenyl)iodonium Tetrafluoroborate
J . Org. Chem., Vol. 67, No. 3, 2002 699
formed is too small (Figure 3). The value for the E/Z ratio
for methoxystyrenes of 3.3 at [Br-] ) 0 mM is included
as well. The drop in the E/Z ratio of the vinyl ethers upon
addition of Br- indicates that external methanol com-
petes with external bromide in the addition to the
photolyzed λ3-iodane complex (depicted as path ii for Br-
addition in Scheme 8).
Ch lor in a ted Alk a n e a s Solven t. Irradiation (λexc
)
248 nm) of the tetrafluoroborate salt of 1 in dichloro-
methane yields the reaction mixture depicted in Scheme
9. The numbers in parentheses are the quantum yields
of formation of the primary photoproducts.
The reaction paths are similar to those in the photo-
chemistry of 1 in alcoholic solvents (Scheme 4). (1)
Heterolytic cleavage of the phenylic C-I bond yields 2
and the phenyl cation. Probably from the latter, chloro-
benzene (3-Cl) is formed by chloride abstraction from the
solvent. This kind of reactivity has previously been
observed, for instance, in the formation of 1-chlorocyclo-
alkenes from 1-cyclovinyl cations in dichloromethane.35
Alternatively, the phenyl radical, formed by homolytic
cleavage of the phenylic C-I bond, can abstract a chlorine
atom from the solvent.36 (2) Homolytic cleavage of the
vinylic C-I bond yields the styryl radical and the
iodobenzene radical-cation. The latter species can acquire
an electron and yield neutral iodobenzene (9). The styryl
radical can abstract a hydrogen atom from the solvent
and yield styrene (4). (3a) Heterolytic cleavage of the
vinylic C-I bond yields 9 and the primary styryl cation
I2. The latter species can lose a proton to give 5. It can
also produce products 6-F by fluoride abstraction from
the, nearby, tetrafluoroborate anion. In poor ion-solvating
solvents, such as dichloromethane, 1 is known not to be
free from its tetrafluoroborate counterion.15,37 The styryl
cation I2 can also acquire chloride from the solvent to
yield (E)- or (Z)-â-chlorostyrene (6-Cl). Alternatively, 6-Cl
may be formed by chlorine atom abstraction from dichloro-
methane36 by the styryl radical formed in path 2. (3b)
Rearrangement of I2 by a 1,2-hydride shift gives the
styryl cation I3. Reaction of this far more stable cation
with fortuitous water yields acetophenone (8). (3c) Hetero-
lytic cleavage via path 3 yields I2 and iodobenzene in a
solvent cage (not depicted in Scheme 4). In-cage recom-
bination at the ipso-position of 9 and loss of I+ gives
stilbene (10). An analogous reaction has been observed
in the photochemistry of diaryliodonium salts.6 In that
case biaryls are produced by heterolysis of the C-I bond
and subsequent in-cage recombination of the cation and
the neutral leaving group. The E-isomer of 10 is formed
preferentially, because it is the product of attack at the
side from which 9 is leaving.38 In alcoholic solvents no
10 is formed, since in that medium more efficient
pathways of reaction are opened. Moreover, in a medium
of low polarity, 1 gives a tighter molecule-ion pair than
in, for instance, methanol, leading to efficient Friedel-
Crafts addition of I2 to iodobenzene.
F igu r e 3. Variation of the E/ Z-ratios of â-bromostyrene (9)
and â-methoxystyrene (2) upon irradiation of 1 ([1] ) 5 mM)
in methanol as a function of [Br-].
The E/Z-ratio of â-bromostyrene drops from 4.5 at [Br-]
) 5 mM to 1.4 at [Br-] ) 200 mM. Extrapolation to [Br-]
) 0 M shows that the E/Z-ratio does not go to infinity
(Figure 3), but to 5.0. This indicates that nucleophilic
attack of the bromide anion within the solvent cage
(Scheme 8; path i) does not exclusively lead to (E)-â-
bromostyrene. Enough initial structural information is
lost in the solvent cage for the bromide anion to be able
to attack the primary styryl cation on the opposite side
from where it left. The extrapolated E/Z ratio of 5.0 at
infinitely low [Br-] is close to the E/Z-ratio for â-meth-
oxystyrene in pure methanol (3.3). This further consoli-
dates the assumption that irradiation of the 1-bromide
and the 1-methanol complex give equivalent photochem-
istry.
Obviously, at higher bromide concentrations a second
pathway for formation of â-bromostyrene is available
which leads to a lower E/Z-ratio. Nucleophilic attack of
uncoordinated bromide at the opposite side of the leaving
iodobenzene-bromide moiety is proposed as the mech-
anism for this pathway (Scheme 8; path ii). At infinitely
high bromide concentration the E/Z-ratio does not drop
to zero, but goes to 0.7. This is caused by free bromide
anions attacking at the side from which the iodobenzene-
bromide moiety is leaving (not depicted in Scheme 8).
On the basis of the mechanism in Scheme 8 and the
assumption that the nucleophilic attack of the external
bromide ion only occurs at the photolyzed 1-Br- complex
and not at uncomplexed photolyzed 1,34 kinetic eq 1 is
derived and fitted to the data of Figure 3. (R is the E/Z
ratio of â-bromostyrenes formed; kE,i is the rate of
formation of the E-isomer by path i, etc.) From the fit,
kE,i/kZ,i and kE,ii/kZ,ii (i.e., the ratio of (E)- and (Z)-â-
bromostyrene at infinitely low (path i) and infinitely high
(path ii) [Br-]) are determined to be 5.0 and 0.7, respec-
tively (as depicted in Scheme 8).
The E/Z-ratio of â-methoxystyrene could only be de-
termined at the two lowest bromide concentrations used,
because at higher [Br-] the amount of â-methoxystyrene
Fluorobenzene, the expected product upon fluoride
abstraction by the phenyl cation, generated in path 1 of
Scheme 9, is not observed upon photolysis of the tetra-
(35) Kropp, P. J .; McNeely, S. A.; Davis, R. D. J . Am. Chem. Soc.
1983, 105, 6907.
(34) The bromostyrenes being already formed in significant amounts
at low [Br-] favors this assumption, because under those conditions
bromide ions cannot compete with the much more abundant methanol
as external nucleophile. At higher [Br-], bromide does compete
efficiently, but in this case all 1 is complexed with bromide.
(36) Afanas’ev, I. B.; Mamontova, I. V.; Samokhavlov, G. I. J . Org.
Chem. USSR 1971, 7, 462.
(37) Swain, C. G.; Roger, R. J . J . Am. Chem. Soc. 1975, 97, 799.
(38) Cf. the SNi mechanism of nucleophilic photosubstitution towards
1.

700 J . Org. Chem., Vol. 67, No. 3, 2002
Sch em e 8. Mech a n ism of F or m a tion of th e â-Br om ostyr en es
Gronheid et al.
Sch em e 9
Sch em e 10. Mech a n ism s of F or m a tion of
fluoroborate salt of 1 in dichloromethane. This may well
be caused by the solvent obscuring fluorobenzene in the
GC analysis.
â-F lu or ostyr en es in th e Th er m a l a n d
P h otoch em ica l Rea ction of 1
The most interesting products in Scheme 9 are the
vinyl fluorides 6-F . Their formation has a thermal
counterpart in the solvolysis of 1 in chloroform, where
exclusively E-6-F is formed, next to the elimination
product 5 and 9.15 No (Z)-â-fluorostyrene is formed, due
to the intermediacy of the vinylenebenzenium ion I1
(Scheme 10).9 The stereoselectivity in the photochemical
formation of 6-F is remarkable: in contrast with the
thermal mode of reactivity, Z-6-F is formed more ef-
ficiently than E-6-F . Preferential formation of the Z-
isomer may indicate that the tight ion-molecule pair of
I2 and 9, formed upon photolysis of 1, hampers the
approach of the fluoride donor to I2 (Scheme 10).
Attack of a dichloromethane solvent molecule on I2 will
also be hampered by 9 in the tight ion-molecule pair.
Thus, preferential formation of (Z)-6-Cl is expected as
well, contrary to the observation. This means that 6-F
and 6-Cl do not have a common precursor in the
photochemical reaction. This is corroborated by the fact
that in the thermal reaction, the products 6-F and 6-Cl
are formed in equal amounts, while in the photochemical
reaction 6-F is formed about three times more efficiently
(Scheme 9). A viable candidate via which (part of) 6-Cl
may be formed is the styryl radical from path 2.36 Steric
hindrance by the phenyl group, in the approach of a

Photosolvolysis of (E)-Styryl(phenyl)iodonium Tetrafluoroborate
J . Org. Chem., Vol. 67, No. 3, 2002 701
Sch em e 11
solvents and in chloroform (cf. Schemes 1 and 10).
Addition of this ion to toluene and subsequent proton
abstraction from the σ-complex yields (E)-methylstil-
benes. Formation of small amounts of the (Z)-methyl-
stilbenes next to the E-isomers may be due to a secondary
(acid-catalyzed) isomerization reaction.
The preferential formation of the Z-isomers of the
methylstilbenes 11 upon irradiation of 1 in the presence
of toluene, resembles the preferential formation of (Z)-
fluoroalkenes upon its irradiation in pure dichloromethane
and can also be explained by assuming the formation of
a tight ion-molecule pair, hampering the approach of the
nucleophile (i.e., toluene).
In the photochemical as well as in the thermal reaction,
products derived from addition to three ring positions of
toluene (ortho: 11o, meta: 11m , and para: 11p ) are
formed. The relative rates of formation of these products
is almost identical for both reactions. In the photochemi-
cal reaction the ratio is 11o:11m :11p ) 2.0:1:1,44 in the
thermal reaction 11o:11m :11p ) 1.7:1:1.2, in both cases
invariant to the E/Z-stereochemistry. These ratios are
remarkably close to the relative rates of formation of
the regioisomers in the condensed-phase vinylation of
toluene by C2H3+ (o:m:p ) 1.3:1:1.0).18 The low positional
selectivity indicates kinetic control of formation of the
σ-complexes, with low activation barriers by the highly
unstable carbocations I1 and I2.45 Alternatively, it may
be the result of facile migration of the unsaturated styryl
moiety over the arene ring in the σ-complex.18 Evidence
against the latter argument is provided by the Friedel-
Crafts reaction of R-anisyl substituted vinyl cations with
toluene.17d These relatively stable vinyl cations show a
significantly higher regioselectivity (o:m:p ) 3:2:9.3).
dichloromethane molecule to this species, explains the
preferential formation of (E)-6-Cl.
The photochemical and thermal15 formation of vinyl
fluorides from vinyl iodonium tetrafluoroborates is un-
precedented. Fluoride abstraction from tetrafluoroborates
has been reported to occur, however, as a side-reaction in
the AgBF4-assisted vinylation of aromatic compounds,17d,39
in the thermal decomposition of diaryliodonium tetra-
fluoroborates40 and in the Balz-Schiemann reaction.41
The occurrence of cationic intermediates in the latter
reaction is widely accepted.37,42 The vinylic counter-
part of this reaction, however, has never been pursued,
because of the difficult accessibility of vinylic diazonium
salts.43
Ar om a t ic Solven t . Upon photolysis of the tetra-
fluoroborate salt of 1 in dichloromethane, one of the
pathways is a Friedel-Crafts reaction, as demonstrated
by the formation of 10 (Scheme 9, path 3c). The leaving
group is the source of the aromatic moiety introduced.
To investigate whether an aromatic medium competes
with the leaving group in the addition step to I2, the
tetrafluoroborate salt of 1 was irradiated in toluene
(λexc ) 296 nm; at this wavelength toluene is not excited).
Dichloromethane (dichloromethane/toluene ) 1/4 (v/v))
was used as a cosolvent.
Con clu sion s
Styryl(phenyl)iodonium salt 1 is an excellent precursor
for the photochemical generation of the primary styryl
cation and allows the study of the reactivity of this ion.
The iodonium salt is extremely photoreactive as demon-
strated by quantum yields of product formation ranging
from 0.65 in 2,2,2-trifluoroethanol to 1.0 in methanol. The
yields of ion-derived products are far higher than in the
corresponding styryl halides. Moreover, the iodonium salt
is more prone to give nucleophilic substitution instead
of elimination. The difference in reactivity is due to the
formation of a cation-molecule pair in the first case and
a cation-anion pair in the latter. The basicity of the
anion is high enough to give exclusive elimination. The
basicity of the molecule is not, resulting in both elimina-
tion and nucleophilic substitution.
There are significant differences between the photo-
chemical and the thermal behavior of 1. First, in the
photochemistry both C-I bonds are cleaved, whereas in
the thermal chemistry only the vinylic C-I bond is
reactive. Second, the ratios of the products formed upon
thermal and photochemical cleavage of the vinylic C-I
bond are quite different. In the photochemical reaction
rearranged substitution products are formed, which is
not the case in the thermal reaction. Also, in the
Indeed Friedel-Crafts type adducts derived from
toluene (methylstilbenes 11) are formed (Scheme 11a)
albeit in low yields (∼10% of total products). Next to 11
all products are observed that are formed upon photolysis
of the tetrafluoroborate salt of 1 in dichloromethane,
including stilbene (10). The percentage of reaction of I2
with the aromatic solvent is larger than with the leaving
group (10/11 ) 0.05). Both E- and Z-isomers of 11 are
formed, the Z-isomers somewhat more efficiently (E/Z )
∼0.4). For the sake of comparison the reaction of 1 was
also carried out thermally in the same solvent mixture
(Scheme 11b). Now mainly (E)-methylstilbenes are pro-
duced (E/Z ) ∼3.7).
For the thermal production of 11, formation of ion I1
is proposed as the first step in the reaction mechanism,
in analogy with the thermal reaction of 1 in alcoholic
(39) Hammen, G.; Hanack, M. Angew. Chem., Int. Ed. Engl. 1979,
18, 614.
(40) Van der Puy, M. J . Fluorine Chem. 1982, 21, 385.
(41) Balz, G.; Schiemann, G. Berichte 1927, 60, 1186.
(42) For a review see: Suschitzky, H. In Advances in Fluorine
Chemistry; Stacey, M.; Tatlow, J . C.; Sharpe, A. G., Eds.; 1965; Vol. 4,
p 1.
(43) Bott, K. In Methoden der Organischen Chemie (Houben-Weyl),
4th ed.; Verlag: Stuttgart, 1993; E 15, p 1101. Formation of a vinyl
chloride from its vinyldiazonium hexachloroantimonate derivative in
dichloromethane or 1,2-dichloroethane has been reported, but the
origin of the chloride (from the counterion or the solvent) is uncertain.
See: Bott, K. Chem. Ber. 1994, 127, 933.
(44) Formation of the para-isomer could not be accurately studied
due to overlap of its GLC peak with that of another (unidentified)
photoproduct and is estimated.
(45) In the reactions of more stable carbocations, such as the
p-methoxybenzyl cation, towards toluene, far less meta-substitution
is observed: o:m:p ) 19:1:47, see: Olah, G. A.; Tashiro, M.; Kobayashi,
S. J . Am. Chem. Soc. 1970, 92, 6369.

702 J . Org. Chem., Vol. 67, No. 3, 2002
Gronheid et al.
was prepared via the method of Bellucci et al.53 The o- and
m-methylstilbene isomers were prepared accordingly.54 In all
cases where pure E- or Z-isomers were obtained, an E/ Z-
mixture of the reference compound was prepared by sensitized
irradiation in argon purged acetone solutions with a medium-
pressure mercury arc through Pyrex.
P h otoch em istr y. For all product studies (both sensitized
and direct excitation) and quantum yield experiments the
same photochemical setup was used. A high-pressure Hg/Xe
arc, from which the IR output was removed by a water filter
was used as the light source. The light beam was processed
through a model 77250 Oriel monochromator, to select the
desired wavelength and aimed at a 3 mL quartz cell, contain-
ing the sample, equipped with a glass stopper with Teflon
septum. All samples were saturated with argon prior to
irradiation, and argon bubbling was continued during the
irradiations.
photochemical reaction the ratio substitution/elimination
is higher than in the thermal reaction. Third, in the
thermal as well as the photochemical reactions there is
a large solvent dependence on the product composition.
However, decreasing the solvent nucleophilicity leads to
more nucleophilic substitution in the thermal and more
rearrangement in the photochemical reaction. This is all
in line with mechanisms, where assisted cleavage (either
by the solvent or the phenyl ring) of the C-I bond occurs
in the thermal reaction and unassisted cleavage in the
photochemical reaction.
In the poorly nucleophilic solvent TFE, the reaction
proceeds from the bare 8-I-2 iodonium species by an SN1
mechanism. Use of nucleophilic solvents or addition of
nucleophiles leads to the formation of 10-I-3 species,
which yield nucleophilic photosubstitution products with
predominant retention of configuration via SNi substitu-
tion.
Upon irradiation of 1 in the nonnucleophilic solvent
dichloromethane significant reactivity with the counter-
ion is observed, indicating the involvement of ion pairs
in such solvents. The vinyl fluoride products are ion-
derived, and the stereoselectivity of the addition step is
controlled by steric hindrance exerted by the leaving
group. Irradiation of 1 in toluene yields photo-Friedel-
Crafts products, via reaction of the primary styryl cation
with the solvent. The regioselectivity resembles that of
the parent vinyl cation C2H3+, indicating kinetic control
in the addition step.
In a typical experiment 3 mL of ∼5 mM solution of the
iodonium salt, containing ∼1 mM n-pentadecanol (in case of
TFE) or n-hexadecane (in the other solvents) as internal
standard, was irradiated at the desired wavelength. At ap-
propriate time intervals 50 µL samples were taken, using a
syringe, through the septum and injected in a test tube
containing ∼1 mL of water and 100 µL of n-hexane. The two
layers were mixed vigorously, and the n-hexane layer was
removed with a pipet and analyzed by GC and/or GC-MS. For
all kinetic experiments at least six samples per irradiation
were taken to study the appearance of photoproduct as a func-
tion of the irradiation time. Conversions of starting material,
as determined by means of product appearance, were kept
below 15%. All quantum yields were obtained as the average
of data from three independent irradiations and calculated
from the kinetic data using least-squares treatment and the
intensity of the light employed. The intensity of the light used
was determined using the chemical actinometer: Actino
Chromo 1R (248/334) from Photon Technology International.
Equ ip m en t. UV spectra were recorded at room tempera-
ture on a double beam Varian DMS 200 spectrophotometer,
Exp er im en ta l Section
Ma ter ia ls. Methanol (Rathburn) and dichloromethane (p.a.
grade; Carlo Erba) were checked for UV transparency and used
as received. 2,2,2-Trifluoroethanol (Aldrich) was boiled with
decolorizing charcoal and subsequently distilled. Toluene
and n-hexane (Baker) were used as received, as were tetra-
butylammonium bromide (Aldrich) and tetrabutylammonium
perchlorate (Fluka). All reference compounds and starting
materials for the syntheses were obtained from Aldrich or
Acros.
1
with pure solvent in the reference cell. H NMR spectra were
recorded on a J EOL 200, using CDCl3 as solvent. As analytical
GC a Hewlett-Packard 6890 series was used, fitted with a CP
SIL5-CB column (30 m, L ) 0.25 mm) using hydrogen as
carrier gas. The flame-ionization detector (FID) was calibrated,
using synthesized or commercially available materials. Isomers
were assumed to have equal response factors. HP Chemstation
was used for analysis of the analytical GC data. Mass spectra
were measured using a GC-MS setup consisting of a Packard
model 438A GC, fitted with a CP SIL5-CB (30 m, L ) 0.25
mm) using helium as carrier gas, coupled with a Finnigan Mat
ITD 700 mass spectrometer. Electron impact was used as the
ionization method.
The preparation of (E)-styryl(phenyl)iodonium tetrafluoro-
borate (1) and its deuterated counterpart E-1-D have been
described in refs 24 and 32, respectively.
All reaction products were characterized by comparison of
retention times on analytical GLC and comparison of mass
spectra (by GC-MS) with those of authentic samples. For
reference compounds, commercial samples were used, when
available. The remaining reference compounds were prepared
by literature procedures: (Z)-â-methoxystyrene (6-OMe)46
(contaminated with small amounts of R-methoxystyrene (7-
OMe)47), (E)-â-iodostyrene (2),48 2,2,2-trifluoroethoxybenzene
(3-OCH2CF 3),49 (E)-â-trifluoroethoxystyrene (6-OCH2CF 3),9
(E)- and (Z)-â-fluorostyrene (6-F ) as a mixture (E:Z ) 13:87),50
and (E)-â-chlorostyrene (6-Cl).51,52 (E)-p-Methylstilbene (11p )
Ack n ow led gm en t. The authors thank Professor
J an Cornelisse (Leiden University) for continuous inter-
est and helpful discussions.
J O0107397
(51) Chowdhurry, S.; Roy, S. J . Org. Chem. 1997, 62, 199.
(52) For previous characterizations of E- and Z-6-Cl see: Dolby, L.
J .; Wilkins, C.; Freg, T. G. J . Org. Chem. 1966, 31, 1110.
(53) Bellucci, G.; Bianchini, R.; Chiappe, C.; Brown, R. S.; Slebocka-
Tilk, H. J . Am. Chem. Soc. 1991, 113, 8012.
(54) For previous characterizations of E- and Z-11o see: Lawrence,
N. J .; Muhammad, F. Tetrahedron 1998, 54, 15361. For E- and
Z-11m : Ito, Y.; Uozu, Y.; Doke, T.; Ueda, M.; Matsuura, T. J . J . Am.
Chem. Soc. 1988, 110, 189.
(46) Miller, S. I. J . Am. Chem. Soc. 1956, 78, 6091.
(47) For previous characterization of 7-OMe see: Willmore, N. D.;
Hoic, D. A.; Katz, T. J . J . Org. Chem. 1994, 59, 1889.
(48) Wright, G. F J . Org. Chem. 1936, 1, 457.
(49) Canning, P. S. J .; McCrudden, K.; Maskill; H.; Sexton, B. Chem.
Commun. 1998, 1971.
(50) Cox, D. G.; Gurusamy, N.; Burton, D. J . J . Am. Chem. Soc. 1985,
107, 2811.