Photochemical generation of a primary vinyl cation from (E)-bromostyrene: Mechanisms of formation and reaction

Article abstract of DOI:10.1021/jo0264877

The photochemistry of (E)-bromostyrene was investigated to determine the nature of the product-forming intermediates and to clarify the mechanism of formation of vinylic cations and vinylic radicals. Both a cation- and a radical-derived product are formed, and the ionic origin of the former product is demonstrated by significant scrambling of the label, starting from specifically deuterated (E)-bromostyrene. MO calculations show that the isolated incipient primary vinyl cation is not a metastable species, but that specific interaction with a counterion in combination with a polar environment makes it metastable. The effects of variation of the wavelength of irradiation, solvent polarity, temperature, and isotopic substitution all agree with a mechanism of direct heterolytic C-Br bond cleavage producing an ion pair followed by formation of a radical pair via electron transfer. The vinylic cation is proposed to stem directly from the indirectly populated lowest excited singlet state of bromostyrene with an energy of activation of 6.7 kcal/mol. Branching between proton loss and electron transfer in the resulting ion pair determines the ratio of cation- to radical-derived product. The E/Z-isomerization occurs in a separate process and does not involve C-Br bond cleavage.

Full text of DOI:10.1021/jo0264877

P h otoch em ica l Gen er a tion of a P r im a r y Vin yl Ca tion fr om
(
E)-Br om ostyr en e: Mech a n ism s of F or m a tion a n d Rea ction
†
Roel Gronheid, Han Zuilhof, Mark G. Hellings, J an Cornelisse, and Gerrit Lodder*
Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502,
2
300 RA Leiden, The Netherlands
lodder@chem.leidenuniv.nl
Received September 27, 2002
The photochemistry of (E)-bromostyrene was investigated to determine the nature of the product-
forming intermediates and to clarify the mechanism of formation of vinylic cations and vinylic
radicals. Both a cation- and a radical-derived product are formed, and the ionic origin of the former
product is demonstrated by significant scrambling of the label, starting from specifically deuterated
(E)-bromostyrene. MO calculations show that the isolated incipient primary vinyl cation is not a
metastable species, but that specific interaction with a counterion in combination with a polar
environment makes it metastable. The effects of variation of the wavelength of irradiation, solvent
polarity, temperature, and isotopic substitution all agree with a mechanism of direct heterolytic
C-Br bond cleavage producing an ion pair followed by formation of a radical pair via electron
transfer. The vinylic cation is proposed to stem directly from the indirectly populated lowest excited
singlet state of bromostyrene with an energy of activation of 6.7 kcal/mol. Branching between proton
loss and electron transfer in the resulting ion pair determines the ratio of cation- to radical-derived
product. The E/Z-isomerization occurs in a separate process and does not involve C-Br bond
cleavage.
In tr od u ction
Two aspects are still topics of discussion: the nature
of the photogenerated vinyl cations in the absence of
stabilizing R-substituents and the mechanism of forma-
tion of the C-X bond cleavage products.
The photochemistry of vinylic halides in solution has
been studied for more than two decades.1 A major
impetus has been the fact that many electronically
excited vinylic halides yield vinylic cations in appreciable
yields, next to the corresponding vinyl radicals. Such
cations are difficult to obtain via thermal generation;
prolonged reaction times at elevated temperatures and
use of good leaving groups and/or strongly stabilizing
Thus far, most photochemical studies have employed
vinyl halides with cation-stabilizing groups at the R-posi-
1
tion, such as phenyl and anisyl. The resulting cations
behave in the same way as their corresponding thermally
generated species, which means that the same product-
yielding intermediate is involved in both reactions, i.e.,
2
R-substituents are required. Therefore, the photochemi-
cal production of vinylic cations at ambient temperatures
is worth pursuing. This allows investigation of their
5,6
a linear free vinyl cation. Except with special sub-
strates, primary vinyl cations cannot be thermally gener-
7
ated. Several examples of photochemical generation of
1
reactivity under mild reaction conditions as well as their
8
primary vinyl cations have been reported. Formation of
application in synthesis.3
,4
such destabilized cations is expected to be accompanied
by rearrangements, producing more stable vinyl cat-
ions.2 Indeed, in a combined experimental gas-phase and
theoretical study, Apeloig et al. showed that the primary
,9
*
To whom correspondence should be addressed. Fax: +31-71-
5
274488.
†
Current address: Laboratory of Organic Chemistry, Department
â-phenyl vinyl cation I
1
isomerizes without barrier by a
of Biomolecular Sciences, Wageningen University, Dreijenplein 8, 6703
HG Wageningen, The Netherlands.
1,2-hydride shift across the double bond to the R-phenyl
(1) (a) Kropp, P. J . In CRC Handbook of Organic Photochemistry
10
vinyl cation I
rearrangement from I
silver-assisted solvolysis of (E)- and (Z)-bromostyrene.
Another possibility for I to stabilize itself would be a
,2-phenyl shift, leading to the vinylenebenzenium ion
2
.
In line with this, Lee and Ko proposed
and Photobiology; Horspool, W. M., Song, P.-S., Eds.; CRC Press: Boca
Raton, 1995; p 1142. (b) Kitamura, T. In CRC Handbook of Organic
Photochemistry and Photobiology; Horspool, W. M., Song, P.-S., Eds.;
CRC Press: Boca Raton, 1995; p 1171. (c) Lodder, G.; Cornelisse, J .
In The Chemistry of Functional Groups-Supplement D2: The Chem-
istry of Halides, Pseudohalides and Azides, Part 2; Patai, S., Rappoport,
Z., Eds.; Wiley, Chicester, 1995; p 861. (d) Lodder, G. In Dicoordinated
Carbocations; Rappoport, Z., Stang, P. J ., Eds.; J ohn Wiley & Sons:
Chichester, 1997; p 377.
1
to I in the condensed phase upon
2
1
1
1
9
1
(5) Kitamura, T.; Kobayashi, S.; Taniguchi, H.; Fiakpiu, C. Y.; Lee,
C. C.; Rappoport, Z. J . Org. Chem. 1984, 49, 3167.
(
2) (a) Stang, P. J .; Rappoport, Z.; Hanack, M.; Subramanian, L. R.
(6) (a) Van Ginkel, F. I. M.; Cornelisse, J .; Lodder, G. J . Am. Chem.
Soc. 1991, 113, 4261. (b) Van Ginkel, F. I. M.; Cornelisse, J .; Lodder,
G. J . Photochem. Photobiol. A: Chem. 1991, 61, 301.
(7) Bromoallenes: (a) Lee, C. V.; Hargrove, R. J .; Dueber, T. E.;
Stang, P. J . Tetrahedron Lett. 1971, 2519. (b) Schiavelli, M. D.; Ellis,
D. E. J . Am. Chem. Soc. 1973, 95, 7916. Bromomethylene cyclopro-
pane: (c) Hanack, M.; Baessler, T.; Eymann, W.; Heyd, W. E.; Kopp,
R. J . Am. Chem. Soc. 1974, 96, 6686.
Vinyl Cations; Academic Press: New York, 1979. (b) Kitamura, T.;
Taniguchi, H.; Tsuno, Y. In Dicoordinated Carbocations; Rappoport,
Z., Stang, P. J ., Eds.; J ohn Wiley & Sons: Chichester, 1997; p 321.
(
3) Schiavelli, M. D. In Dicoordinated Carbocations; Rappoport, Z.,
Stang, P. J ., Eds.; J ohn Wiley & Sons: Chichester, 1997; p 433.
4) Kitamura, T.; Kobayashi, S.; Taniguchi, H. J . Org. Chem. 1990,
(
5
5, 1801.
1
0.1021/jo0264877 CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/27/2003
J . Org. Chem. 2003, 68, 3205-3215
3205

Gronheid et al.
CHART 1. Mech a n ism of Dissocia tive P h otobeh a vior of Vin yl Ha lid es, In volvin g Con secu tive F or m a tion
of Ra d ica ls a n d Ion s (Mech a n ism 1)
CHART 2. Mech a n ism of Dissocia tive P h otobeh a vior of Vin yl Ha lid es, In volvin g P a r a llel F or m a tion of
Ion s a n d Ra d ica ls (Mech a n ism 2)
I
3
. The cations I
1
-I
3
, next to the vinyl radical I
4
, can be
hermic to occur for species R lacking such substituents.16
Nevertheless, such vinyl halides do give cation-derived
products. Second, the hypothesis of consecutive forma-
surmised as intermediates in the photochemistry of (E)-
bromostyrene (1E).
8
tion of vinylic radicals and cations predicts a ratio of
radical to cationic products from excited vinylic bromides
or chlorides, which is determined by the relative reduc-
tion potentials of bromine and chlorine in the solvent
used. This has been investigated in detail for the photo-
1
8
chemistry of â-alkenyl-â-halostyrenes and 1-aryl-2-(2,2′-
biphenyldiyl)vinyl halides19 and found to be counter-
The mechanism of the photochemical formation of
factual: in all these cases, as in the compounds studied
vinylic cations from vinylic halides is still not clear. The
by Kropp,1
a,12
the bromo derivatives rather than the
proposal of Kropp12 envisions the formation of vinylic
chlorides gave rise to the highest efficiency of ion product.
To accommodate all observed phenomena another reac-
tion mechanism has been proposed (mechanism 2, Chart
cations via vinylic radicals. These radicals are formed by
homolytic carbon-halogen bond cleavage and yield cat-
ions by subsequent electron transfer (ET) to the halogen
+
•
2
). in which vinylic cations R and vinylic radicals R are
atom (mechanism 1, Chart 1).
1
9
This mechanism has found widespread acceptance,13-15
formed in parallel from the excited vinylic halide.
but some observations do not easily fit this proposal.
First, calculations indicate that the electron-transfer step
is energetically feasible for species with a good cation-
stabilizing group at the R-position, but it is too endot-
In this paper, we report a study of the photochemistry
of one of the simplest vinyl bromides excitable with 254
nm light, (E)-bromostyrene (1E) and two of its isoto-
pomers: (E)-1-bromo-2-deutero- and (E)-1-bromo-1-deu-
tero-2-phenylethene (1E-2D and 1E-1D). The aim of the
study is 2-fold: (1) to demonstrate the intermediacy of a
vinyl cation and to probe its nature, by monitoring
scrambling of the deuterium label and by ab initio
calculations, and (2) to resolve the mechanism of forma-
(8) (a) Suzuki, T.; Sonada, T.; Kobayashi, S.; Taniguchi, H. J . Chem.
Soc., Chem. Commun. 1976, 180. (b) Sket, B.; Zupan, M.; Pollak, A.
Tetrahedron Lett. 1976, 783. (c) McNeely, S. A.; Kropp, P. J . J . Am.
Chem. Soc. 1976, 98, 4319. (d) Sket, B.; Zupan, M. Tetrahedron Lett.
1
7
977, 257. (e) Sket, B.; Zupan, M. J . Chem. Soc., Perkin Trans. 1 1979,
52. (g) Suzuki, T.; Kitamura, T.; Sonoda, T.; Kobayashi, S.; Taniguchi,
tion of the cationic (I
1
3 4
-I ) and radical (I ) intermediates
H. J . Org. Chem. 1981, 46, 5324. (h) Op den Brouw, P. M.; Laarhoven,
W. H. Recl. Trav. Chim. Pays-Bas 1982, 101, 58. (i) Kitamura, T.; Muta,
T.; Kobayashi, S.; Taniguchi, H. Chem. Lett. 1982, 643. (j) Kropp, P.
J .; McNeely S. A.; Davis, R. D. J . Am. Chem. Soc. 1983, 105, 6907. (k)
Sonawane, H. R.; Nanjundiah, B. S.; Panse M. D. Tetrahedron Lett.
and their stable products upon C-Br bond cleavage, by
measuring the effect of the solvent polarity, activation
energies and isotope effects (IEs) on the product forma-
tion.
1
985, 26, 3507. (l) Kitamura, T.; Kobayashi, S.; Taniguchi, H. J . Am.
Chem. Soc. 1986, 2641. (m) Zupancic, N.; Sket, B. J . Photochem.
Photobiol. A. Chem. 1991, 60, 316. (n) J ohnen, N.; Schnabel, W.;
Kobayashi, S.; Fouassier, J . P. J . Chem. Soc., Faraday Trans. 1992,
8
8, 1385.
(9) (a) Rappoport, Z.; Noy, E.; Houminer, Y. J . Am. Chem. Soc. 1976,
9
8, 2238. (b) J a¨ ckel, K. P.; Hanack, M. Chem. Ber. 1977, 10, 199. (c)
Stang, P. J .; Dueber, T. E. J . Am. Chem. Soc. 1977, 99, 2602. (d) Lee,
C. C.; Paine, A. J .; Ko, E. C. F. J . Am. Chem. Soc. 1977, 99, 7267. (e)
Kitamura, T.; Kobayashi, S.; Taniguchi, H. J . Org. Chem. 1982, 47,
(16) The driving force of the ET reaction R Br- f R• Br• can be
estimated by comparing the driving force for R ) benzyl (for which
experimental data in acetonitrile are available) and R ) styryl. The
+
2
323.
10) Apeloig, Y.; Franke, W.; Rappoport, Z.; Schwarz, H.; Stahl, D.
J . Am. Chem. Soc. 1981, 103, 2770.
11) (a) Lee, C. C.; Ko, E. C. F. J . Org. Chem. 1975, 40, 2132. (b)
(
1
7
(
driving force of the ET reaction for R ) benzyl is +12 kcal/mol. The
gas-phase enthalpy of formation of the vinylic cation from the vinylic
radical is calculated to be 23 kcal/mol more positive than that of the
enthalpy of formation of the benzyl cation from the corresponding
radical (PM3 calculations). Therefore, for the unstabilized vinylic
cation, the ET is calculated to be exothermic by about 11 kcal/mol.
(17) Parker, V. D. Acta Chem. Scand. 1992, 46, 501.
(18) Krijnen, E. S.; Zuilhof, H.; Lodder, G. J . Org. Chem. 1994, 59,
8139.
(19) Verbeek, J .-M.; Stapper, M.; Krijnen, E. S.; Van Loon, J .-D.;
Lodder, G.; Steenken, S. J . Phys. Chem. 1994, 98, 9526.
The intermediacy of vinyl cations in this reaction has been disputed:
J aeckel, K.-P.; Hanack, M. Chem. Ber. 1977, 199.
(
12) Kropp, P. J .; McNeely, S.; Davis, R. D. J . Am. Chem. Soc. 1983,
05, 6907.
13) (a) Kitamura, T.; Kobayashi, S.; Taniguchi, H. J . Am. Chem.
1
(
Soc. 1986, 108, 2641. (b) Kitamura, T.; Kobayashi, S.; Taniguchi, H.;
Hori, K. J . Am. Chem. Soc. 1991, 113, 6240 and references therein.
(
(
14) Gregorcic, A.; Zupan, M. J . Fluorine Chem. 1988, 41, 163.
15) Horspool, W.; Armesto, D. Organic Photochemistry. A Compre-
hensive Treatment; Ellis Horwood Ltd.: Chicester, 1992; p 448.
3
206 J . Org. Chem., Vol. 68, No. 8, 2003

Generation of a Vinyl Cation from (E)-Bromostyrene
Resu lts a n d Discu ssion
SCHEME 1
Rea ctive In ter m ed ia tes. P r od u ct Com p osition . In
a wide range of solvents (methanol, acetonitrile, acetic
acid, 2,2,2-trifluoroethanol, 1,4-dioxane, n-pentane, and
hexane), irradiation of (E)-bromostyrene (1E) at 254 nm
under argon yields three primary photoproducts: (Z)-
bromostyrene (1Z), phenylacetylene (2), and styrene (3)
SCHEME 2. Mech a n ism s of Rea ction of I u p on
P h otoch em ica l F or m a tion fr om 1 (A), fr om 4 in
1
(
Scheme 1).20 Regularly, traces of oxidation products,
such as benzaldehyde and methyl benzoate (in methanol),
are detected; the amount varies per experiment but is
always small (<0.3%). Under an air atmosphere, these
products are formed with significant efficiencies at the
expense of the formation of 3. In the alkane solvents
several secondary products are detected. One of them was
identified as 1,2-dibromo-1-phenylethane, the Markovni-
kov product of HBr addition to 1.
TF E (B) a n d fr om 4 in Meth a n ol (C)
Product 1Z results from E/Z-isomerization around the
CdC bond, product 2 from the loss of hydrogen bromide,
and product 3 from replacement of the bromine atom by
a hydrogen atom. Production of hydrogen bromide in the
formation of 2 is noticeable from the decrease in pH
during the reaction, as well as from the formation of
products derived from attack of HBr on the starting
material 1E or product 1Z.
iodobenzene as the leaving group instead of bromide.25
In 2,2,2-trifluoroethanol (TFE), 4 mainly yields acetophe-
none as cation-derived product (via rearrangement fol-
lowed by solvent addition) next to traces of the elimina-
tion product 2 and the products of direct nucleophilic
Photochemical dehydrohalogenation of vinyl halides
has previously been shown to proceed via an intermediate
vinyl cation.1
8,21
Product 2 is therefore concluded to be
cation-derived, which is confirmed by the isotope scram-
bling study presented below. No products from nucleo-
philic attack of the solvent to the intermediate vinyl
cation are obtained. This is in contrast with the behavior
addition of the solvent to I
methanol, the main cation-derived photoproducts from
are the â-methoxystyrenes, next to 2 and only a trace
1
(Scheme 2, pathway B). In
4
1
8
of rearranged product. In this case, methanol, I , and
of photogenerated R-aryl and R-alkenyl vinyl cations,
1
2
2
iodobenzene are formed together within a solvent cage
but in line with that of the p-methoxystyryl cation and
of R-cyano- and R-formylstyryl cations.21 Apparently,
due to precomplexation of methanol to 4 (Scheme 2,
pathway C).2
5,26
Methanol is a stronger base than TFE,
â
proton loss from C of vinyl cations, lacking a cation-
thus yielding elimination product 2, but also a stronger
nucleophile yielding the â-methoxystyrenes. The presence
of methanol in the solvent cage thus advances proton
abstraction and nucleophilic substitution at the expense
of rearrangement. In principle, the same is true for the
bromide anion in the ion pair of Scheme 2, path A, so
here also nucleophilic substitution by internal return
within the solvent cage is very well possible. However,
since this will reform â-bromostyrene and largely with
retention of configuration this mode of reactivity goes
unnoticed in the photochemistry of 1. In fact, this may
even explain the relatively low quantum yield of disap-
pearance of 1 (∼0.3) compared to 4 (∼1), because the
internal return is a photon wasting pathway of reaction.
Thus, although the same carbocation is formed in the
photochemistry of 1 and 4 in TFE and methanol, its
reactivity is quite different and is controlled by the nature
of the leaving group.
stabilizing group (i.e., alkenyl or aryl) at the R-position,
is so fast that nucleophilic attack cannot compete. Previ-
ously, the large propensity for loss of a â-proton in the
photochemistry of vinyl halides has been attributed to
the formation of “hot” vinylic cations.1
2,23
We propose that
the exclusive formation of elimination products in the
case of unstabilized and destabilized vinyl cations in the
photochemistry of vinyl halides can be fully explained
by proton abstraction by the leaving halide anion from a
“cold” cation within a tight ion pair.
This proton abstraction within the solvent cage is facile
because of the high kinetic basicity of the bromide anion
(Scheme 2, pathway A). This phenomenon is not uncom-
mon and well documented, even for more stable carboca-
tions.24 This interpretation is corroborated by the results
of the irradiation of (E)-styryl(phenyl)iodonium tetrafluo-
roborate (4), a compound similar to 1 with neutral
1
The styryl cation I , not encumbered by an anion, has
(
20) Cf: Galli, C.; Gentili, P.; Guarnieri, A.; Kobayashi, S.; Rap-
also been prepared in methanol by the nuclear decay of
tritium technique. In that case also, rearrangement and
poport, Z. J . Org. Chem. 1998, 63, 9292.
(
(
21) Krijnen, E. S.; Lodder, G. Tetrahedron Lett. 1993, 34, 729.
22) Kitamura, T.; Kobayashi, S.; Taniguchi, H. J . Am. Chem. Soc.
27
substitution are important processes.
1
986, 108, 2641.
(
23) Kropp, P. J . Acc. Chem. Res. 1984, 17, 313.
(24) (a) Thibblin, A. J . Phys. Org. Chem. 1992, 5, 367. (b) Thibblin,
(25) Gronheid, R.; Lodder, G.; Okuyama, T. J . Org. Chem. 2002,
67, 693.
(26) Gronheid, R.; Lodder G.; Ochiai, M.; Sueda, T.; Okuyama, T.
J . Am. Chem. Soc. 2001, 123, 8760.
(27) (a) Speranza, M. Chem. Rev. 1993, 93, 2933. (b) Nefedov, V.
D.; Sinotova, E. N.; Kuzhelev, L. P.; Lebedev, V. P. Radiokhimiya 1987,
29, 73.
A. Chem. Soc. Rev. 1993, 22, 427. (c) Thibblin, A. In Organic
Reactivity: Physical and Biological Aspects; Golding, B. T., Griffin, R.
J ., Maskill, H., Eds.; The Royal Society of Chemistry: Cambridge, 1995;
p 415. (d) Pirinccioglu, N.; Thibblin, A. J . Am. Chem. Soc. 1998, 120,
512. (e) Chiappe, C.; De Rubertis, A.; Lemmen, P.; Lenoir, D. J . Org.
Chem. 2000, 65, 1273.
6
J . Org. Chem, Vol. 68, No. 8, 2003 3207

Gronheid et al.
SCHEME 3
2-H. Second, partial migration of the phenyl ring across
the carbon-carbon double bond yields the vinylene
benzenium ion I
stable than I . Loss of either hydrogen or deuterium,
yielding 2-D or 2-H, can occur directly from I because a
significant part of the positive charge resides on the
vinylic carbon atoms. Alternatively, I can reopen, giving
either the I -1D or I -2D. Proton or deuterium loss from
these intermediates yields 2-D or 2-H. Third, R-H loss
from I initially yields an vinylidene carbene (route C).
3
(route B), which is energetically more
1
3
3
TABLE 1. P er cen ta ge of Deu ter iu m in P h otop r od u ct 2
u p on Ir r a d ia tion of 1E-2D in Solven ts of Va r yin g
P ola r ity
1
1
ET(30)a,30
1
2-D (%)
Alkylidene carbenes are known to quickly rearrange to
n-pentane
methanol
acetonitrile
acetic acid
TFE
31.0
55.4
45.6
51.7
59.8
8.0 ( 0.2
16.5 ( 0.8
18.1 ( 0.9
18.7 ( 0.8
21.5 ( 1.0
alkynes.32 R-Elimination from I
tion of 2-D.
leads to exclusive forma-
1
Routes A and B predict about equal amounts of 2-H
33
and 2-D to be formed. Route C predicts exclusively 2-D.
a
Since in all of the solvents used the percentage of
In kcal/mol.
+
scrambling is lower than 50%, direct loss of D from I
1
-
2
D must be a significant pathway, in competition with
the operating scrambling pathway.
Cation I (involved in route A) can be independently
Replacement of bromide by hydrogen, forming 3, is a
well-known photochemical reaction of vinyl halides and
was concluded to occur via an intermediate vinyl radi-
cal.19 This is in line with the sensibility of the formation
2
3
4
prepared by photolysis of 1-bromo-1-phenylethene (5).
Upon irradiation of 5 in methanol, we observed 1-meth-
oxy-1-phenylethene (6) as primary nucleophilic substitu-
tion product (Scheme 5). Not even traces of 6 are found
in the photochemistry of 1E in methanol. Formation of
vinyl ether 6 in the photochemistry of 5 in methanol and
its absence in the photochemistry of 1E in methanol
exclude the intermediacy of I2 in the formation of 2-D
and thus route A.
of 3 toward oxygen. Scavenging of the vinyl radical I
4
thus occurs by hydrogen atom transfer from the solvent,
yielding 3, or by oxygen, yielding the vinyl peroxyl radical
•
19,28
I
4
O
2
,
and ultimately oxidation products.
Isotop e Scr a m blin g. Deuterium scrambling studies
were performed using 1-bromo-2-deutero-2-phenylethene
1E-2D). Since rearrangements are only expected in the
(
chemistry of vinyl cations, and not in the chemistry of
vinyl radicals,29 scrambling was monitored soley in the
cation-derived product 2. The extent of deuterium scram-
bling was analyzed using GC-MS. The low detection
limit of this technique (compared to 1H or H NMR,
necessary to study scrambling in 3 and 1Z) allows
scrambling to be studied at low conversion. Upon irradia-
tion of 1E-2D in various, even apolar, solvents, deuterium
scrambling does occur, leading to deuterated 2 (Scheme
If route C were the actual mechanism of formation of
the rearrangement product 2-D, it should be a readily
accessible reaction path for all styryl cations bearing an
R-hydrogen atom. The reaction path should be especially
important for styryl cations with electron-withdrawing
groups attached to the phenyl ring or to the double bond,
which increase the acidity of the R-hydrogen atom.
However, no elimination product is formed in the pho-
2
3
5
tosolvolysis of compound 7 in methanol (Scheme 6).
3
, Table 1). The amount of scrambling is independent of
Moreover, using deuterated methanol, the formation of
the vinyl ethers was shown not to occur via an interme-
diate alkylidene carbene (inserting into the H(D)-O bond
the conversion of the photoreaction.
The occurrence of rearrangement confirms that 2 is
formed via a cationic intermediate. The dependence of
the amount of scrambling on the solvent polarity is also
indicative of the ionic origin of 2. The percentages
3
2
of the solvent ). So, route B remains as the only plausible
mechanism for the formation of 2-D in the photochem-
istry of 1E-2D.
MO Ca lcu la tion s on C H7+. Having proven the
T
qualitatively correlate with the E (30) values, with more
8
deuterium scrambling in the more polar solvents. This
is probably caused by the increased lifetime of the ion
pair (Scheme 2A) in such solvents, giving more time for
the rearrangement to occur.
presence of I1 and the absence of I2 in the condensed-
phase photochemistry of 1E, it is of interest to investigate
why upon formation of I in the gas phase only products
1
1
0
derived from I are formed. Medium effects may well
2
Three different mechanisms for deuterium scrambling
in the styryl cation should be considered. First, a 1,2-
stabilize the cation I1 and make it a metastable inter-
mediate in solution, similar to the situation of the parent
hydride shift converts I
1 2
into another styryl cation I
(
Scheme 4, route A). The driving force for this reaction
(30) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry,
2
nd ed.; VCH: Weinheim, 1988; p 365.
31) Kobayashi, S.; Hori, Y.; Hasako, T.; Koga, K.; Yamataka, H. J .
is the well-known stabilizing effect of a phenyl group at
the R-position of a carbocation, as well as the (small)
destabilizing effect of a phenyl group at the â-position of
(
Org. Chem. 1996, 61, 5274.
(32) (a) Hartzler, H. D. In Carbenes; Noss, R. A., J ones, M., Eds.;
J ohn Wiley & Sons: New York, 1975; Vol. 2, p 43. (b) Kirmse, W.
Angew. Chem., Int. Ed. Engl. 1997, 36, 1164.
a vinyl cation.31 Loss of H or D from I
+
+
2
yields 2-D or
(
33) This statement is only valid if the isotope effect for H+ loss
(
28) (a) Carpenter, B. K. J . Am Chem. Soc, 1993, 115, 9806. (b)
Mertens, R.; von Sonntag, C. Angew. Chem. 1994, 33, 1262.
29) (a) Haynes, J . K.; Kampmeier, J . A. J . Org. Chem. 1972, 37,
equals one, which is unlikely. However, the primary IE will be normal
(larger than one), preferentially forming 2-D, whereas our experiments
give more 2-H (Table 1). So, the conclusion that a considerable part of
(
•
2 is formed by direct â-D+ loss from I
4
167. (b) B3LYP/6-31G(d,p) calculations on the PES of C
8
H
7
show that
1
-2D still holds.
rearrangements of the incipient styryl radical either yield less stable
products, or have too large activation barriers (>43 kcal/mol) to be
overcome at room temperature: Gronheid, R. Ph.D. Thesis, Leiden
University, 2001.
(34) Andraos, J .; Kresge, A. J .; Obraztsov, P. A. J . Phys. Org. Chem.
1992, 5, 322.
(35) Van Alem, K. Ph.D. Thesis, Leiden University, 2001, Chapter
2.
3
208 J . Org. Chem., Vol. 68, No. 8, 2003

Generation of a Vinyl Cation from (E)-Bromostyrene
1
SCHEME 4. Mech a n ism s of Deu ter iu m Scr a m blin g of th e Styr yl Ca tion I , In volvin g a 1,2 D-Sh ift (A), a 1,2
P h en yl-Sh ift (B), or r-Elim in a tion (C)
SCHEME 5
Our results using the SCRF-IPCM model to account
for solvent polarity are in line with a recent study of the
parent vinyl cation.39 In that case, the dielectric was
shown to influence the relative stabilities of the open and
bridged species, but the order of stabilities does not
change. The stabilization by the dielectric correlates
vinyl cation, where the more delocalized bridged struc-
ture is observed in the gas phase36 and the charge-
localized open stucture in the condensed phase.37 The
qualitatively with the extent of charge delocalization. For
+
C
8
H
7
the largest changes in relative stabilities upon
, the cation with a localized
charge (Table 1, column 6). The cations with a π-delo-
calized positive charge (I and I ) only benefit to a minor
extent from the dielectric.
solvation are calculated for I
1
relative stabilities of the selected isomeric geometries I -
1
+
I
3
of C
of theory in the gas phase and in a dielectric continuum
with ꢀ ) 78 (Table 2).
Intermediate I with its vinylic hydrogen atoms or-
8 7
H are calculated at the B3LYP/6-31G(d,p) level
2
3
r
Computation of the two extreme pathways for the
hydride shift (within the plane of the phenyl ring, or
perpendicular to this plane) is performed by means of a
2
thogonal to the plane of the phenyl ring is calculated to
be the global minimum, and all energies are reported
with respect to its energy. In accordance with the
previous study on C
is found for isolated I
theory. Inclusion of electron correlation is necessary for
this result. At the HF/6-31G(d,p) level of theory, I
local minimum. A B3LYP/6-31G(d,p) geometry of I
obtained from the HF/6-31G(d,p)-optimized geometry by
freezing the H-C dC bond angle and reoptimization of
the geometry at the B3LYP/6-31G(d,p) level. A frequency
calculation of the thus-obtained geometry has real fre-
quencies for all optimized coordinates. This indicates that
the 1,2-hydride shift, yielding I
pathway to stabilize I . Rotation of the phenyl ring out
relaxed PES scan, by stepwise variation of the H-C
bond angle (Figure 1).
Both pathways have no barrier. Hopping from the
perpendicular” to the “in-plane” path occurs somewhere
â
d
C
R
+
10
8
H
1
7
,
no minimum energy structure
at the B3LYP/6-31G(d,p) level of
“
in the region where the two cross, as indicated by the
dashed line in Figure 1. It is important to note that
initially the energy gain for the 1,2-hydride shift is only
small and that the energy gain becomes larger toward
the end of the reaction path.
A transition state on the reaction path from I to I is
1 2
expected to be reagent-like because of the large exother-
micity of the reaction (even in a dielectric). To study
1
is a
was
1
â
R
2
, is the only barrierless
whether the presence of a dielectric makes cation I
1
a
1
metastable species, the I -like part of the lowest energy
1
of the plane, and subsequent anchimeric interaction of
the phenyl ring with the positively charged carbon atom,
path was recalculated with incorporation of the SCRF-
IPCM method (Figure 1). Clearly, the PES for the 1,2-
hydride shift is flatter in the presence of a dielectric, but
the rearrangement from I yielding I still occurs without
1 2
barrier.
yielding I
barrier. I
kcal/mol less stable than I
3
, therefore must occur with an activation
is 16.0 kcal/mol more stable than I , but 24.0
. This is in line with results
3
1
2
for triarylvinyl cations, where the open structure usually
Specific interactions between a carbocation and its
counterion have previously been shown to be crucial for
is more stable and the phenonium ion is either a
transition state or a local minimum, as in this case.38
4
0,75
a correct description of alkyl carbocations in solution.
(
36) (a) Kanter, E. P.; Vager, Z.; Both, G.; Zajfman, D. J . Chem. Phys.
1
2
986, 85, 7487. (b) Vager, Z.; Naaman, R.; Kanter, E. P. Science 1989,
44, 426.
(39) Bagno, A.; Modena, G. Eur. J . Org. Chem. 1999, 2893.
(40) (a) F aˇ rca s¸ iu, D.; H aˆ ncu, D.; Haw, J . F. J . Phys. Chem. A 1998,
102, 2493. (b) F aˇ rca s¸ iu, D.; Lukinskas, P. J . Phys. Chem. A 1998, 102,
10436. (c) F aˇ rca s¸ iu, D.; Norton, S. H.; H aˆ ncu, D. J . Am. Chem. Soc.
2000, 122, 668.
(
(
37) Lucchini, V.; Modena, G. J . Am. Chem. Soc. 1990, 112, 6291.
38) Yamataka, H.; Biali, S. E.; Rappoport, Z. J . Org. Chem. 1998,
6
3, 9105.
J . Org. Chem, Vol. 68, No. 8, 2003 3209

Gronheid et al.
SCHEME 6
TABLE 2. Rela tive Sta bilities (in k ca l/m ol) of Sp ecies
I1-I3 a t th e B3LYP /6-31G(d ,p ) Com p u ta tion a l Level in th e
Ga s P h a se, w ith Th eir Nu m ber of Im a gin a r y
F r equ en cies, a n d in a Dielectr ic Con tin u u m Usin g th e
SCRF -IP CM Mod el
energy no. of imaginary energy in energy
species symmetry + ZPE
frequencies
dielectric change
I1
I2
I3
Cs
C2v
C2v
40.8
0
24.0
a
0
0
32.6
0
22.6
8.2
0
1.4
b
b
b
a
Not a stationary point (see text). b By definition.
-
F IGURE 2. Geometry of the I
distance.
1
-LiH
2
ion-pair at 3.0 Å
corresponding radical I
4
: the NBO charge on the C
8 7
H
entity is +0.60 and the C
â
-C -H bond angle is 142°,
R
indicating hybridization of the formally charged carbon
2
atom intermediate between sp and sp .
Specific interactions between an anion and I
1
are thus
capable of stabilizing I enough to make it a metastable
1
species. The presence of a dielectric is expected to
stabilize it even more, since this has been shown to
flatten the PES for hydride migration for the bare ion
(
cf. Figure 1). Moreover, as charge separation increases
upon addition of a dielectric continuum, I will have more
ionic character in the ion pair, which will result in a more
linear C dC -H bond and a better description of I in
1
â
R
1
F IGURE 1. Energetics for 1,2-hydride shift from I
1
2
to I ,
the condensed phase. Excessive computational costs of
such calculations refrained us from verifying this.
Mech a n ism of P h otolysis. Na tu r e of th e Rea ctive
perpendicular to (9) and in the plane of (b) the phenyl ring,
dC bond angle. The I -like part of
by variation of the H-C
â
R
1
the lowest energy path is recalculated with inclusion of a
4
4
Excited Sta te. Upon irradiation at λexc ) 313 nm of a
solution of (E)-bromostyrene (1E; 2 mM) in methanol,
using xanthone as sensitizer, 1Z is the only product up
to a conversion of 40% of 1E. This result is in line with
our previous results in the study of R-alkenyl vinyl
halides which give exclusive E/Z-isomerization from their
triplet-excited state as well.18 Therefore, the reactive
state for C-Br bond dissociation is an excited singlet
state.45
dielectric continuum (2).
Of a series of anions tested, lithium dihydride (LiH -
2
)
efficiently describes anion-carbocation interactions, pro-
vided the distance between the ions is sufficiently large
to prevent spontaneous proton abstraction.75 Unfortu-
nately, DFT methods do not give satisfactory results upon
inclusion of counterions because of preferential proton
abstraction.75 Thus, electron correlation was accounted
for using MP2 corrections.41,42
The UV spectrum of 1E exhibits an absorption maxi-
4
-1
-1
mum at 257 nm (ꢀ ) 1.92 × 10 M cm ). Its position
and extinction coefficient are equal within experimental
error in methanol, acetonitrile, and n-pentane. The
solvent-polarity independence of the λmax and the high
extinction coefficient indicate that the initially excited
state is most likely of ππ* type. In any case, both
observations rule out a π f σ* transition, since (a) a πσ*
state has a substantial dipole moment, due to the shift
Addition of a lithium dihydride counterion at 3.0 Å
distance43 results in a metastable species for I
1
! Optimi-
zation of all coordinates, except for the interionic dis-
tance, gives the geometry depicted in Figure 2. The cation
-
I
1
in the ion-pair I
1
-LiH
2
has characteristics intermedi-
ate between those expected for the free ion and the
(41) Electron correlation by MP2 was successfully applied in previ-
ous studies of counterion effects on the structure of carbocations (see
ref 40).
(44) At this wavelength no light is absorbed by the vinyl halide.
(45) Alternatively, the reactive state may be an upper triplet state.
See: (a) Scaiano, J . C.; Arnold, B. R.; McGympsey, W. G. J . Phys.
Chem. 1994, 98, 5431-5434. (b) Elisei, F.; Latterini, L.; Aloisi, G. G.;
D’Auria, M. J . Phys. Chem. 1995, 99, 5365-5372. (c) Latterini, L.;
Elisei, F.; Aloisi, G. G.; D′Auria, M. Phys. Chem. Chem. Phys. 2001, 3,
2765-2770.
(
42) Calculations of the bare cation I
of theory give essentially the same features as the DFT calculations.
No local minimum is calculated for I , which isomerizes barrierless to
1
on the MP2(fc)/6-31G(d,p) level
1
2
I .
(
R
43) Defined as the distance between C and the nearest hydrogen
-
2
.
atom of LiH
3
210 J . Org. Chem., Vol. 68, No. 8, 2003

Generation of a Vinyl Cation from (E)-Bromostyrene
TABLE 3. Effect on th e P r od u ct Ra tio 2/3 of th e
Wa velen gth of Ir r a d ia tion of 1E (2.24 m M) in n -P en ta n e
a t Room Tem p er a tu r e
TABLE 4. Qu a n tu m Yield s of Disa p p ea r a n ce (Φd is) of
1E (1 m M) a n d th e P r od u ct Ra tio 2/3 in Differ en t
Solven ts a t Room Tem p er a tu r e
λ (in nm)
2/3
solvent
Φdis
2/3
2
2
2
2
40
54
70
95
0.38 ( 0.06
0.32 ( 0.08
0.36 ( 0.02
0.42 ( 0.01
methanol
acetonitrile
hexane
0.33 ( 0.01
0.30 ( 0.02
0.22 ( 0.02
0.25 ( 0.01
0.52 ( 0.08
0.59 ( 0.05
0.65 ( 0.08
0.59 ( 0.14
1,4-dioxane
TABLE 5. Activa tion En er gies (in k ca l/m ol) for th e
of charge from the π system to the C-Br bond, and would
be stabilized in polar solvents, and (b) πσ* transitions
are local symmetry-forbidden in the planar equilibrium
geometry of 1E (the overlap between the π-system and
the orthogonal σ* C-Br orbital is near-zero),46 and so the
πσ* absorption intensity will be extremely small in the
solution UV spectra.47 Likewise, the high ꢀmax value is
also strong evidence against an n f σ* transition, which
is the lowest energy transition in alkyl halides. In these
compounds, n f σ* transitions have substantially lower
F or m a tion of P h otop r od u cts fr om 1E in n -P en ta n e
1
Z
2
3
Ea
3.0 ( 0.4
6.7 ( 0.2
6.1 ( 0.3
starts with formation a radical pair (as in mechanism 1,
Chart 1), but is in line with predictions for a reaction
involving initial formation of an ion pair next to a radical
pair (as in mechanism 2, Chart 2).
-
1
-1
ꢀ
values (ꢀmax(CH
wavelengths (λmax(CH
3
Br) ) 265 M cm ) at much shorter
48
Mechanism 1 (Chart 1) predicts a substantial solvent
effect on the relative efficiency of formation of cation- and
radical-derived products. A simple relation is expected:
the more polar the solvent, the more ionic products.
However, no clear relation between the ratio 2/3 and the
polarity of the solvent is observed. This implies that if
ET is one of the reaction steps of the initially formed
intermediate, it cannot be the only one next to diffusion
apart (as in mechanism 1). For mechanism 1, no obvious
other reaction is available for the radical pair. The vinylic
radicals are stable enough to diffuse in competition with
ET.
3
Br) ) 202 nm). Since no signifi-
cant change in the strength of the C-Br bond is expected
upon a π f π* transtition, the reactive state is presum-
ably not the one which is initially excited in the direct
irradiation experiments with 254 nm light. We propose
the reactive state to be a πσ* state, in analogy with vinyl
bromide,49 and in contrast with the alkyl bromides.
Since homolytic photocleavage of vinyl halides as in
mechanism 1 (Chart 1) was proposed in analogy with the
photochemical behavior of alkyl halides, the fact that
these two classes of species have different lowest excited
singlet states may result in different photocleavage
modes.
15
Effect of Va r ia tion of th e Rea ction Tem p er a tu r e.
The amounts of 1Z, 2, and 3 formed were measured by
means of product analysis of the reaction mixture after
fixed irradiation times at reaction temperatures varying
from -80 to +20 °C. Lowering of the temperature yields
lower amounts of all products. At -60 to -80 °C, 1Z is
the only observed product. Analysis of Arrhenius plots
using the data between -40 and +20 °C yields the data
collected in Table 5.
Effect of Wa velen gth of Excita tion . The photo-
chemistry of 1E in n-pentane at room temperature was
studied at wavelengths of excitation ranging from 240
to 295 nm (the absorption onset is at 303 nm; Table 3).
The lack of sensitivity of the photoreaction to the
wavelength of irradiation contrasts with the photochem-
istry of other vinyl halides.1
8,19,21
Apparently, only one
excited state plays a role in the photochemical C-Br bond
cleavage from 1E, yielding cation-derived (2) as well as
radical-derived (3) products. Mechanism 2 suggests
parallel formation of both vinylic radicals and vinylic
cations from electronically excited vinylic halides. Since
the formation of radicals and ions requires very different
charge distributions in the reactive excited state, it is
unlikely that they will simultaneously be formed from a
single excited state. The absence of wavelength depen-
dence therefore argues against mechanism 2 (Chart 2).
Effect of Solven t Va r ia tion . The photoreactivity of
The activation energies of the formation of photoprod-
ucts from 1E should give important information. Espe-
cially in an apolar solvent, such as n-pentane, mecha-
nisms 1 and 2 (Charts 1 and 2) are expected to give very
different activation energies for the formation of cation-
derived 2 and radical-derived 3.
The activation barriers for formation of 2 and 3 are
equal within experimental error. This observation dis-
agrees with mechanisms 1 and 2 (Charts 1 and 2). In
the case of mechanism 1, an endothermic ET step (this
step is especially endothermic in n-pentane) forms the
cation-derived product 2, after intial homolysis. In this
mechanism, a significiantly higher activation energy on
the formation of 2 than on the formation of 3 is thus
expected. It is extremely unlikely that the formation of
an ion pair and a radical pair from a single excited state,
as in the case of mechanism 2, have equal activation
energies.
1
E changes only slightly upon variation of the solvent
(Table 4). The quantum yields of disappearance (Φdis) are
somewhat higher in the polar solvents methanol and
acetonitrile than in the nonpolar solvents hexane and 1,4-
dioxane. This trend is not expected for a reaction which
(46) Klessinger, M.; Michl, J . Lichtabsorption und Photochemie
Organischer Molek u¨ le; VCH: Weinheim/New York, 1989; p 30.
(
47) πσ* states are generally hard to observe for organic com-
pounds: Michl, J .; Bonaci c´ -Kouteck y´ , V. Electronic Aspects of Organic
Photochemistry; Wiley: New York, 1990; p 44.
The activation barrier for E/Z-isomerization is mea-
sured to be only 3.0 kcal/mol. The value of the activation
energy for E/Z-isomerization is clearly different from
those leading to products following from C-Br bond
(
48) Gilbert, A.; Baggott, J . Essentials of Molecular Photochemistry;
Blackwell: London, 1991; p 462.
49) Reference 47, pp 380-386.
(
J . Org. Chem, Vol. 68, No. 8, 2003 3211

Gronheid et al.
CHART 3. Mech a n ism of Dissocia tive P h otobeh a vior of Vin yl Ha lid es In volvin g Con secu tive F or m a tion of
Ion s a n d Ra d ica ls (Mech a n ism 3)
TABLE 6. r- a n d â-Deu ter iu m IEs on th e F or m a tion of
P r od u cts u p on Ir r a d ia tion (λexc ) 254 n m ) of
E-â-Br om ostyr en e (1E) in Meth a n ol a t Room
Tem p er a tu r e
heterolysis of the carbon-bromine bond. In this process,
2
the hybridization of the reaction center changes from sp
51
to sp, which leads to an R-secondary deuterium IE > 1.
The difference in hyperconjugative stabilization of the
1
Z
2
3
incipient vinyl cation by â-hydrogen and â-deuterium
a
51
R-secondary IE
â-secondary IE
1.03 ( 0.02
1.03 ( 0.01
1.27 ( 0.11
1.11 ( 0.02
1.39 ( 0.13
0.88 ( 0.05
leads to a â-secondary IE > 1. The magnitudes of the
b
effects compare well with literature values for the
thermal formation of vinyl cations. In solvolytic genera-
tion of primary allenyl cations R-IEs ranging from 1.20
a
Relative rates of formation of products from 1E and 1E-1D.
Relative rates of formation of products from 1E and 1E-2D.
b
7
b,52
to 1.28 (Table 6, 1.27) have been measured.
â-deuterium IE of 1.25 (Table 6, 1.11) is reported for the
thermal formation of a vinyl cation from CH (L)Cd
C(OTf)CH (L ) H or D) with the â-deuterium and the
A
cleavage. This demonstrates that E/Z-isomerization oc-
curs via a separate process, not involving C-Br bond
cleavage.
On the basis of the above results, we propose mecha-
nism 3 (Chart 3) for the photodissociation of the C-Br
bond in 1E. Upon excitation with light of 254 nm followed
3
3
leaving group in a cis-orientation (as is the case for 1E-
2
D).53
The IEs on the formation of 2 can be understood within
by internal conversion, the S (πσ*) state is populated.
1
the framework of all three mechanisms. For mechanism
(Chart 2) as well as mechanism 3 (Chart 3), the
In this state a direct heterolysis of the C-Br bond occurs,
2
yielding an ion pair consisting of a vinylic cation and a
bromide anion.50 The ion pair can undergo one of three
observed IEs are explained by initial heterolysis of the
carbon-bromine bond using the same arguments as used
for the IEs on the thermal formation of vinyl cations. In
mechanism 1, the formation of the cation from the radical
in a secondary, ET step, may be accompanied by a
substantial normal IE on the formation of the cation-
derived product.
processes: (1) proton abstraction from the vinylic cation
to yield phenylacetylene (2), (2) rearrangement of the
vinylic cation to a more stable species (see labeling
experiments), and (3) electron transfer from the bromide
anion to the electron-deficient vinyl cation. The compo-
nents of the resulting radical pair diffuse apart and the
vinyl radical yields 3 as stable product by abstraction of
a hydrogen atom from the solvent.
Mechanism 3 (Chart 3) agrees with the solvent effects.
The formation of ionic species in the first reaction step
accounts for the higher quantum yields of disappearance
in polar solvents. Since no clear relation between the
ratio 2/3 and the polarity of the solvent is observed, the
vinylic cation in the tight ion pair must be so unstable
that diffusion cannot compete with ET. Branching be-
tween proton loss and ET occurs, which is hardly affected
by the solvent polarity. This conclusion is in line with
the absence of nucleophilic substitution products. Mech-
anism 3 also agrees with the activation energies for 2
and 3. Formation of the ion pair is the rate-determining
step and the subsequent ET step leading to formation of
The R- and â-deuterium IE on the appearance of the
radical-derived product 3 are both large, but the first one
is normal, while the second one is inverse. These IEs
cannot be understood within the framework of mecha-
nism 1 or 2, which involve vinyl radical formation via
direct homolysis of the C-Br bond. Upon formation of
the styryl radical from (E)-bromostyrene, there is hardly
5
4
any change in hybridization of the reaction center. Also
hyperconjugative stabilization of the radical is not pos-
sible, due to the spatial orientation of the C -H bond
â
and the orbital in which the unpaired electron resides.
Therefore, both the R- and â-secondary IE on the forma-
•
tion of the vinyl radical 1 are expected to be close to
unity. Since the IEs on the formation of 3 are significantly
different from 1, mechanisms involving direct formation
of the vinyl radical (i.e., mechanisms 1 and 2) can be
discarded. The question now is: Can mechanism 3,
involving formation of the vinyl radical via ET from the
intially produced vinyl cation, explain the observed IEs?
If we assume that all steps in mechanism 3 are
irreversible, the IEs on the formation of 2 as well as 3
will originate from the ET step and the proton loss step.
Although this is less correct these two steps will be
considered separately, and only the effect of the ET step
3
is almost without barrier. Therefore, cation-derived 2
and radical-derived 3 are formed with equal activation
energies, even in n-pentane.
Isotop e Effects. Deuterium isotope effects on the
formation of the E f Z isomerization product 1Z, the
cation-derived product 2 and the radical-derived pro-
duct 3 in the photolysis 1E, 1E-1D, and 1E-2D in
methanol are measured as the relative rates of formation
of these products from 1E and its deuterated analogues
(
Table 6).
The R- and â-deuterium IE on the appearance of 2 are
(51) Melander, L.; Saunders, W. H., J r. Reaction Rates of Isotopic
Molecules; J ohn Wiley & Sons: New York, 1980; Chapter 6.
both normal, as expected for its formation via initial
(52) Schiavelli, M. D.; Germroth, T. C.; Stubbs, J . W. J . Org. Chem.
1
976, 41, 681.
(
50) Direct heterolytic C-X cleavage has been earlier proposed for
(53) Stang, P. J .; Hargrove, R. J .; Dueber, T. E. J . Chem. Soc., Perkin
the photochemical formation of triphenylmethyl cations from triph-
Trans. 2 1974, 843.
enylmethyl chloride: (a) Manring, L. E.; Peters, K. S. J . Phys. Chem.
(54) Galli, C.; Guarnieri, A.; Koch, H.; Mecarelli, P.; Rappoport, Z.
J . Org. Chem. 1997, 62, 4072.
1
984, 88, 3516. (b) Peters, K. Annu. Rev. Phys. Chem. 1987, 38, 253.
3
212 J . Org. Chem., Vol. 68, No. 8, 2003

Generation of a Vinyl Cation from (E)-Bromostyrene
SCHEME 7. Sch em a tic Rep r esen ta tion of th e
In flu en ce of IEs of th e Sep a r a te Step s on th e
Obser ved IE in a Rea ction In volvin g Tw o
Con secu tive Ra te-Deter m in in g Step s, Accor d in g to
Mech a n ism 3
SCHEME 8
the cation. Therefore, the ET step will be energetically
more favorable for the deuterium than for the hydrogen
compound, and the rate will be higher for the first than
for the latter: an inverse IE!
The differences in ZPEs on the ET step can also be
calculated using quantum chemical methods. For this
purpose, semiempirical MO programs using the PM3
on the IEs will be discussed here because we find this
treatment more instructive. The effect of the proton loss
step on the IEs is discussed in the Supporting Informa-
tion. There also an analytical solution of the problem is
presented which describes the likely situation that the
overall measured IEs originate from the IEs on the ET
step as well as the IEs on proton loss. All reasonings lead
to the same conclusions.
In general, IEs on the formation of products in a
reaction involving subsequent reaction steps will be the
product of the IEs of all steps. So, for a reaction involving
two consecutive rate-determining steps, the observed IE
parametrization set have proven to give adequate quali-
tative and semiquantitative results.5
7,58
The changes in
ZPEs for R- and â-deuterium substitution on the ET step
were calculated as depicted in the isodesmic reaction in
Scheme 8.
The changes in the calculated ZPEs are qualitatively
in agreement with the predicted IEs on the ET step as
discussed above. The change in ZPE in the isodesmic
equation for the R-IE is positive, indicating that I
stabilized relative to I . On the other hand, the change
in ZPE in the isodesmic equation for the â-IE is negative,
1
-1D is
(IEobs) on the first product equals the IE of the first
reaction step, and IEobs for the second product equals the
product of the IEs of the first and the second step
1
5
9
1 1
indicating that I -2D is destabilized with respect to I .
(Scheme 7).
In conclusion, the IEs on the formation of the radical-
derived compound 3 can be interpreted in terms of
mechanism 3!
The IEs on E/Z-isomerization (Table 6, column 2)
provide information about the pathways for E/Z-isomer-
ization of vinyl halides. Although the lowest triplet
excited state of 1E exclusively yields E/Z-isomerization
products, it is not necessarily an intermediate in the
formation of 1Z from 1E upon direct excitation. Next to
If we accept mechanism 3 to be correct, we predict both
the R- and â-IE on the formation of the cation (IEobs
IE ) to be normal. Concerning the R-deuterium IE on the
formation of the radical, for IEobs to be larger than that
of the cation, IE must be >1. Similarly for the â-IE on
the formation of the radical, for IEobs to be smaller than
IEobs on formation of the cation IE must be <1. In other
)
1
2
2
words, the IEs on the formation of the radical-derived
product, are explained by mechanism 3 provided that the
direct E/Z isomerization from the lowest excited singlet
IE on the ET step which leads from I
1
to its corresponding
-
state, internal return within the cation-anion (I
1
-Br )
radical (I
species.
4
) is normal for the R-D- and inverse for the â-D-
•
pair or the radical-radical pair (I -Br ) can, in principle,
4
yield 1Z as well. Both the R- and the â-secondary IE for
E/Z isomerization of 1E are close to unity, and very
different from the respective IEs for the formation of the
cation- and radical-derived product. This indicates that
Deuterium is an inductive electron donating substitu-
ent as is demonstrated by its negative Hammett σ
constant (-0.0011).55 This means that I
is slightly better
stabilized by an R-deuterium than by an R-hydrogen
atom. Because this stabilization will be less important
in the radical, the ET step is predicted to be more
exothermic for the hydrogen compound than for the
I
1
1
Z is neither a cation- nor a radical-derived product.
Thus, internal return from either the ion pair or the
radical pair is not an important pathway for E/Z-
isomerization. This leaves direct isomerization from the
triplet or the singlet excited state as options.
deuterium compound. Since rates of ET increase with
increased exothermicity,56 the rate of ET is predicted to
Secondary IEs on E/Z-isomerization have been studied
be higher for the hydrogen compound than for the
deuterium compound: a normal IE!
both experimentally6
0,61
and theoretically. In most of
61
For the â-IE the distance between the position of
deuteration and the positive carbon center is too large,
for the inductive effect to play a role. The â-C-H bond is
(
57) B3LYP or MP2 ab intio data cannot be calculated in this way,
because at these levels of theory the styryl cation does not correspond
to a local minimum. Also see the calculations section.
(58) (a) Zuilhof, H.; Lodder, G. J . Phys. Chem. 1992, 96, 6957. (b)
1
however weakened in I , due to the hyperconjugative
Zuilhof, H.; Lodder, G. J . Phys. Chem. 1995, 99, 8033. (c) Hammerich,
O.; Nielsen, M. F.; Zuilhof, H.; Mulder, P. P. J .; Lodder, G.; Reiter, R.
C.; Kage, D. E.; Rice, C. V.; Stevenson, C. D. J . Phys. Chem. 1996,
stabilization of the cation by this bond. The potential
energy curve for this carbon-hydrogen bond will there-
fore be shallow. Because hyperconjugative stabilization
is absent in I , the potential energy curve will be steeper
4
in this case. This causes a larger difference between the
ZPEs for hydrogen and deuterium in the radical than in
1
00, 3454.
59) This conclusion holds as long as the stability of I
(
4
is not
significantly affected by deuteration. This is a plausible assumption
as previously argued.
(
60) (a) Caldwell, R. A.; Misawa, H. J . Am. Chem. Soc. 1987, 109,
6
869. (b) Courtney, S. H.; Balk, M. W.; Philips, L. A.; Webb, S. P.; Yang,
D.; Levy, D. H.; Fleming, G. R. J . Chem. Phys. 1988, 89, 6697. (c)
Brouwer, A. M.; Cornelisse, J .; J acobs, H. J . C. J . Photochem. Photobiol.
A: Chem. 1988, 42, 313.
(
(
55) Young, W. R.; Yannoni, C. S. J . Am. Chem. Soc. 1969, 91, 4581.
56) Marcus, R. A. Int. J . Chem. Kinet. 1981, 13, 865.
J . Org. Chem, Vol. 68, No. 8, 2003 3213

Gronheid et al.
removal of solvent.63 Yield: 99%, with >98% deuterium
incorporation. Phenylacetylene-d (2.5 g; 42 mmol) in hexane
these studies large normal secondary IEs (1.4-2.0) have
been measured for rotation around the CdC bond and
significant activation barriers (up to 12.8 kcal/mol) have
to be overcome. In one case a low activation barrier (0.33
kcal/mol) for rotation (for the photoisomerization of cis-
stilbene in hexane), was observed, as well as an IE equal
to unity.62 It was concluded that the magnitude of the
IE is related to the activation energy for isomerization.
Our small value for the IE on E/Z-isomerization thus is
in agreement with the small observed activation energy
(10 mL), to which benzoyl peroxide (0.18 g; 0.85 mmol) was
added was converted into 1E-1D by addition of hydrogen
bromide at 60 °C.64 The HBr gas, produced by addition of
bromine to tetralin, was continuously bubbled through the
solution under vigorous stirring. The reaction was stopped
when GC analysis showed no 2-D left, and the reaction
mixture was worked up by extraction with water/diethyl ether,
4
drying of the organic layer over MgSO and vacuum distilla-
tion: yield 18%; purity 97.8% (2.0% 1Z-1D); deuterium
incorporation >96%.
(3.0 kcal/mol) for this reaction.
(E)-1-Bromo-2-deutero-2-phenylethene (1E-2D) was syn-
thesized from benzaldehyde-d in THF at -78 °C according to
a literature procedure.65 An 11:1 Z/E-mixture was obtained
in 70% yield. Irradiation at λ ) 313 nm in acetone afforded a
3:2 Z/E-mixture, from which 1E-2D was isolated by prepara-
tive GC; deuterium incorporation 96%.
Con clu sion s
Upon UV-photolysis of (E)-bromostyrene in various
solvents, a cation- and a radical-derived product are
formed. Photolysis of specifically deuterated (E)-bro-
mostyrene results in scrambling of the deuterium label
in the elimination product. Together with the solvent
dependence of the amount of scrambling this demon-
strates unequivocally that a cationic intermediate is
involved in the formation of this product. The scrambling
occurs via a vinylene benzenium ion.
Although the primary vinyl cation, formed upon C-Br
heterolysis, does not exist in the gas phase, calculations
show stabilization by the counterion and by the solvent.
It is therefore a metastable species in solution, in
agreement with our experiments.
1
-Bromo-1-phenylethene (5) was prepared by bromination
4
of styrene in CCl , followed by elimination of HBr using NaOH
in ethanol.66 Column chromatography, on silica gel using
petroleum ether as eluent, yielded 5 in >95% purity (remain-
ders: 1-bromo-2-phenylethene and phenylacetylene).
The identity of the deuterated starting materials and the
1
photoproducts was established by comparison ( H NMR, GC,
GC/MS) with reference compounds. All reference compounds
were commercially available, except for 1-methoxy-1-phe-
nylethene (6), which was prepared by a literature proce-
dure.2
5,67
P h otoch em ica l Exp er im en ts. The photochemical reac-
tions were carried out under argon, in three different setups.
For the quantum yield determinations and measurements of
the isotope effects a 65 mL cylindrical Pyrex reaction vessel
was used in which a quartz tube was immersed. This tube
contained a Hanau TNN-15/32 low-pressure mercury arc (254
nm light), which was placed in a filter solution, consisting of
The mechanism of the dissociative photochemistry of
(E)-bromostyrene involves direct heterolytic carbon-
bromine bond cleavage from the lowest excited πσ*
singlet state. Radical-derived products are formed by
electron transfer within the initially formed ion pair. This
is shown by (1) the higher Φdis for polar than for apolar
solvents, (2) the equal activation energies for the forma-
tion of the radical- and the cation-derived product, and
0
4 2 4
.190 M NiSO and 0.4 mM K CrO in water. For irradiations
at λ ) 313 nm (sensitization experiments) the lamp used was
a Hanau TQ-81 high-pressure mercury arc with a quartz cover
glass, surrounded by a glass spiral tube through which cooling
water was circulated. As filter solution in the quartz tube a
solution consisting of 0.025 M potassium biphthalate, 1.0 M
(3) the R- and â-deuterium isotope effects on the forma-
4 4
NiSO , and 0.25 M CoSO in water was used.
tion of styrene. Previously proposed mechanisms starting
either with homolytic cleavage followed by electron
transfer or with parallel homolytic and heterolytic cleav-
age cannot explain these observations.
For actinometry at λ ) 254 nm the photohydrolysis of
3
1
-nitroanisole (3.5 mM) in a 0.1 M NaOH water/methanol (9:
68
) solution, which occurs with a quantum yield of 0.22, was
performed under the same conditions, except the atmosphere
(air instead of argon). The disappearance of starting material
1E (conversions up to 20%) and appearance of products 1Z, 2,
and 3 were studied as a function of time, using an internal
standard (n-decane or 1-dodecanol) and the kinetics described
E/Z-isomerization occurs in a separate process and does
not involve C-Br bond cleavage. This reaction may
proceed via the singlet excited state, the triplet excited
state, or both.
6
earlier. This was done by taking aliquots (0.50-1.00 mL) from
the reaction mixture at appropriate times, followed by analysis
by means of GC.
Exp er im en ta l Section
For the measurements of the effects of the temperature a
somewhat different device was used. The volume of the
reaction vessel was larger (180 mL), and an evacuated double-
walled quartz tube was used to isolate the cooling water for
the lamp from the cold reaction mixture (down to -40 °C).
Low solution temperatures were achieved by placing the whole
setup in a Dewar flask filled with diethyl ether. Solid carbon
dioxide was added to the ether until the desired temperature
was reached. Reaction times were up to 6 min, over which
Ch em ica ls. (E)-Bromostyrene (1E) was purchased as a 13:1
E/Z-mixture. The Z-isomer was removed by repetitive crystal-
lization from hexane (mixed isomers) at -60 °C, which yielded
1
E in a purity >99.3% (0.6% (Z)-bromostyrene; 1Z). To obtain
UV transparent and gas-liquid chromatography (GC) pure
solvents, they were distilled before use.
(
E)-1-Bromo-1-deutero-2-phenylethene (1E-1D) was syn-
thesized in two steps from phenylacetylene (2). First, 2 (11 g;
.11 mol) was converted into phenylacetylene-d (2-D) by
exchanging twice with a suspension of CaO (1.5 g; 27 mmol)
and D O (10 mL; 0.5 mol) at 40 °C for 20-24 h, followed by
filtration, extraction with ether, drying over MgSO , and
0
(
63) Hassner, A.; Boerwinkle, F. P. J . Am. Chem. Soc. 1970, 92,
4879.
(64) Stacey, F. W.; Harris, J . F., J r. Org. React. 1963, 13, 150.
2
4
(
(
65) Matsumoto, M.; Keiko, K. Tetrahedron Lett. 1980, 21, 4021.
66) Ashworth, F.; Burkhardt, G. N. J . Chem. Soc. 1928, 1791.
(
61) (a) Negri, F.; Orlandi, G. J . Phys. Chem. 1991, 95, 748. (b)
(67) Willmore, N. D.; Hoic, D. A.; Katz, T. J . J . Org. Chem. 1994,
59, 1889.
Olson, L. P.; Niwayama, S.; Yoo, H.-Y.; Houk, K. N.; Harris, N. J .;
Gajewski, J . J . J . Am. Chem. Soc. 1996, 118, 886.
(68) (a) Havinga, E.; De J ongh, R. O. Bull. Soc. Chim. Belg. 1962,
71, 803. (b) De J ongh, R. O.; Havinga, E. Recl. Trav. Chim. Pays-Bas
1966, 85, 275.
(62) Saltiel, J .; Waller, A. S.; Sears, D. F., J r. J . Am. Chem. Soc.
1
993, 115, 2453.
3
214 J . Org. Chem., Vol. 68, No. 8, 2003

Generation of a Vinyl Cation from (E)-Bromostyrene
period no noticeable variation in the solution temperature
occurred, with conversions up to 10%.
The effect of wavelength variation was studied using the
light from a light source consisting of a 1000 W Hanovia Hg/
Xe high-pressure short-arc lamp and an Oriel monochromator
ring strain energy for the parent vinyl cation.71 Higher order
electron correlation is computationally too costly. Moreover,
MP types of post-HF calculations frequently give dubious
results for open shell systems, due to significant spin contami-
nation. Therefore density functional theory (DFT), which has
implicit electron correlation and usually gives little spin
contamination with unrestricted calculations of open shell
systems is the method of choice. The B3LYP method was
selected because of its proven accuracy in the prediction of
vinyl cation energetics.72 The 6-31G(d,p) basis set was chosen,
because it contains polarization functions on both carbon and
hydrogen atoms to accurately describe bridging involving both
these atoms.
Charge densities and bond orders were calculated using the
NBO 3.1 program,73 as implemented in GAUSSIAN 98, and
for the reported charges the hydrogens are summed into the
heavy atoms.
All calculations using a dielectric to mimic the solvent were
performed with the SCRF-IPCM method,74 using the solvent
parameters for water. Single-point calculations were per-
formed on the gas-phase B3LYP/6-31G(d,p)-optimized geom-
etries.
(5 nm bandwidth), impinging on a 3 mL quartz cuvette.
Conversions were up to 10%, as determined using n-decane
as internal standard. All experiments were performed in
triplicate, unless noted otherwise. GC analyses were at least
in duplicate.
Equ ip m en t. The GC analyses were carried out using a gas
chromatograph fitted with a capillary CP SIL5-CB column (50
m, L ) 0.25 mm) and a flame ionization detector. Hydrogen
was used as the carrier gas at an inlet-pressure of 72 kPaLow
resolution mass spectra were obtained with a GC/MS setup
consisting of a gas chromatograph, fitted with a CP-SIL 19
CB column (25m, L ) 0.32 mm) using helium as carrier gas,
coupled with a mass spectrometer using electron impact and
chemical ionization with methanol.
Ca lcu la tion s. All ab initio calculations were performed
6
9
using the GAUSSIAN 94 (Rev. D4) or GAUSSIAN 98 (Rev.
7
0
A5) set of programs on an IBM 6000/RISC computer. On all
calculated structures, except for those from the coordinate
driving calculations, frequency calculations were performed
to check whether they are minimum energy structures, transi-
tion states, or higher-order saddle points. The calculations
were performed on the B3LYP/6-31G(d,p) level of theory. This
method was chosen because the Hartree-Fock (HF) method
is known to overestimate ring strain energies (due to the
absence of electron correlation in this method). However,
explicit inclusion of electron correlation by second-order
Møller-Plesset perturbation theory (MP2) underestimates the
As B3LYP calculations have been shown to give erroneous
results for calculations involving counterions, the MP2(fc)
method was used in this case.75
The calculations on the equilibrium IEs were performed on
an IBM RS/6000 platform, using the PM3 parametrization set
as implemented in MOPAC 93.76 After pre-optimization, the
final structures were obtained by optimization with the
eigenvector following method. With the thus obtained geom-
etries, frequency calculations were performed, and the effect
of isotopic substitution on the zero-point energies was deter-
mined.
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Stewart, J . P.; Head-Gordon, M.; Gonzalez, C.; Pople, J . A. Gaussian
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J . Org. Chem, Vol. 68, No. 8, 2003 3215