Photolysis of bromo- and chloro-substituted benzyl derivatives. Competition between ionic and radical pathways

Article abstract of DOI:10.1021/jo0001481

Photolysis of 1-fluoro-2-halo-1,2-diphenylethanes was studied in solutions of tetrahydrofuran, acetonitrile, and cyclohexane. The effect of free radical inhibitor and metal hydrides on products formation as well as their ratio was analyzed to elucidate the reaction pathway. In the first step homolytic C-X bond cleavage occurs from a single excited state, resulting in a biradical pair. Further reaction path depends on the type of the halogen bonded and on the solvent polarity. Electron transfer within the radical pair cage is apparently more rapid for bromides than for chlorides and is opposite as expected on the basis of the relative electronegativities of chlorine and bromine. As radicals approach each other, they fall into ionic or radical product channels. This is influenced by solvent polarity, resulting in the larger yield of ionic products in the case of acetonitrile as in the case of less polar cyclohexane.

Full text of DOI:10.1021/jo0001481

6890
J . Org. Chem. 2000, 65, 6890-6896
P h otolysis of Br om o- a n d Ch lor o-Su bstitu ted Ben zyl Der iva tives.
Com p etition betw een Ion ic a n d Ra d ica l P a th w a ys
Berta Kosˇmrlj and Boris Sˇket*
University of Ljubljana, Faculty of Chemistry and Chemical Technology, Asˇkercˇeva 5,
SI-1000 Ljubljana, Slovenia
berta.kosmrlj@uni-lj.si
Received February 2, 2000
Photolysis of 1-fluoro-2-halo-1,2-diphenylethanes was studied in solutions of tetrahydrofuran,
acetonitrile, and cyclohexane. The effect of free radical inhibitor and metal hydrides on products
formation as well as their ratio was analyzed to elucidate the reaction pathway. In the first step
homolytic C-X bond cleavage occurs from a single excited state, resulting in a biradical pair. Further
reaction path depends on the type of the halogen bonded and on the solvent polarity. Electron
transfer within the radical pair cage is apparently more rapid for bromides than for chlorides and
is opposite as expected on the basis of the relative electronegativities of chlorine and bromine. As
radicals approach each other, they fall into ionic or radical product channels. This is influenced by
solvent polarity, resulting in the larger yield of ionic products in the case of acetonitrile as in the
case of less polar cyclohexane.
In tr od u ction
Product analyses indicated that both radical and ionic
intermediates were involved. We found that in the first
step homolytic benzylic C-Cl bond cleavage occurred
resulting in a radical pair. Further reaction pathway,
however, depended upon the solvent used. In cyclohexane
and tetrahydrofuran (THF), the products obtained were
formed via benzylic radical intermediates. On the other
hand, photolysis in acetonitrile resulted in products,
which could be ascribed to both benzylic cation and
benzylic radical intermediates. The presence of LiAlH4
had a great impact on the photoreaction and accelerated
the reduction of the C-Cl bond in the aromatic ring,
leading to toluene as the main product. The crucial
influence of LiAlH4 on the reaction pathway was also
shown in the photoreduction of various fluoro-substituted
1,2-diphenylethanes, which were practically stable in the
presence of hydride alone, as well as when they were
subjected to direct irradiation.10
Herein, the results of photolysis of bromo- and chloro-
substituted benzyl derivatives, i.e., 1-chloro-2-fluoro-1,2-
diphenylethane (1a ) and 1-bromo-2-fluoro-1,2-diphenyl-
ethane (1b), are presented. The solvent effect was studied
by direct irradiation of substrates in THF, cyclohexane,
and acetonitrile. The influence of free radical inhibitor
(2,6-di-tert-butyl-4-methylphenol (DBPC)) and metal hy-
dride (LiAlH4) on the reaction pathway and product
distribution was investigated as well.
The photochemistry of the heteroatomic benzylic bond
has been extensively studied for over 30 years.1 It results
mainly in C-X cleavage due to the carbon-halogen bond
absorption by n f σ* transition. The discovery that
partitioning between homolytic and heterolytic cleavage
occurs was of great importance in further research.2 The
competition between the pathways for the formation of
ionic and radical intermediates depends on many factors,
including the leaving group and the solvent polarity.
To determine the reaction mechanism different benz-
ylic systems (PhCH2X) were investigated: benzyl acetates
(X ) OAc),1,3 sulfonium salts (X ) (CH3)2S+BF4-),4
ammonium salts (X ) (CH3)3N+Cl-),5 phosphites (X )
OP(OR)3),6 and a group of homobenzylic systems.7 The
most thoroughly studied examples have been benzyl
halides (X ) Cl, Br).8
Previously, we studied the solvent effect on photolysis
of 3-chloro- and 4-chloro-substituted benzyl chlorides.9
(1) (a) Zimmerman, H. E.; Sandel, V. R. J . Am. Chem. Soc. 1963,
85, 915-922. (b) Zimmerman, H. E.; Somasekhara, S. J . Am. Chem.
Soc. 1963, 85, 922-927.
(2) (a) Cristol, S. J .; Mayo, G. O.; Lee, G. A. J . Am. Chem. Soc. 1968,
91, 214-215. (b) Kropp, P. J .; J ones, T. H.; Poindexter, G. S. J . Am.
Chem. Soc. 1973, 95, 5420-5421. (c) Poindexter, G. S.; Kropp, P. J . J .
Am. Chem. Soc. 1974, 96, 7142-7143.
(3) (a) Pincock, J . A.; Wedge, P. J . J . Org. Chem. 1994, 59, 5587-
5595. (b) Hilborn, J . W.; MacKnight, E.; Pincock, A. J .; Wedge, P. J . J .
Am. Chem. Soc. 1994, 116, 3337-3346. (c) Cozens, F. L.; Pincock, A.
L.; Pincock, J . A.; Smith R. J . Org. Chem. 1998, 63, 434-435.
(4) Maycock, A. L.; Berchtold, G. A. J . Org. Chem. 1970, 35, 2532-
2538.
Resu lts
(5) Ratcliff, M. A., J r.; Kochi, J . K. J . Org. Chem. 1971, 36, 3112-
THF a s a Solven t. Eight hours irradiation of a 0.01
M solution of 1-chloro-2-fluoro-1,2-diphenylethane (1a )
3120.
(6) Ganapathy, S.; Sekhar, B. B. V. S.; Cairns, S. M.; Akutagawa,
K.; Bentrude, W. G. J . Am. Chem. Soc. 1999, 121, 2085-2096.
(7) (a) Cristol, S. J .; Schloemer, G. C. J . Am. Chem. Soc. 1972, 94,
5916-5917. (b) Slocum, G. H.; Schuster, G. B. J . Org. Chem. 1984,
49, 2177-2185. (c) Arnold, B.; Donald, L.; J urgens, A.; Pincock, J . A.
Can. J . Chem. 1985, 63, 3140-3146. (d) Banks, J . T.; Garc´ıa, H.;
Miranda, M. A.; Pe´rez-Prieto, J .; Scaiano, J . C. J . Am. Chem. Soc. 1995,
117, 5049-5054. (e) Pincock, J . A.; Wedge, P. J . J . Org. Chem. 1995,
60, 4067-4076. (f) Itoh, Y.; Gouki, M.; Goshima, T.; Hachimori, A.;
Kojima, M.; Karatsu, T. J . Photochem. Photobiol., A 1998, 117, 91-
98.
(8) (a) Cristol, S. J .; Greenwald, B. E. Tetrahedron Lett. 1976, 25,
2105-2108. (b) Appleton, D. C.; Brocklehurst, B.; McKenna, J .; Smith,
M. J .; Taylor, P. S.; Thackeray, S.; Walley, A. R. J . Chem. Soc., Chem.
Commun. 1977, 108-109. (c) Cristol, S. J .; Bindel, T. H. J . Org. Chem.
1980, 45, 951-957. (d) Cristol, S. J .; Bindel, T. H. J . Am. Chem. Soc.
1981, 103, 7287-7293.
(9) Kosˇmrlj, B.; Sˇket, B. Acta Chim. Slov. 1998, 45, 463-474.
(10) Kosˇmrlj, B.; Kralj, B.; Sˇket, B. Tetrahedron Lett. 1995, 36,
7921-7924.
10.1021/jo0001481 CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/28/2000

Photolysis of Br- and Cl-Substituted Benzyl Derivatives
Sch em e 1
J . Org. Chem., Vol. 65, No. 21, 2000 6891
Ta ble 1. P r od u ct Distr ibu tion a in P h otolysisb of 1 in
Tetr a h yd r ofu r a n
occurs in the first step, generating an aryl radical and a
halogen atom. In the second step these radicals abstract
hydrogen atom from BH4- giving BH3- •, which initiates
an SRN1-like chain reaction.11a (b) Excited aryl halide
hν
hν + LiAlH4
hν + DBPC
product
1a
1b
1a
1b
1a
1b
-
reacts with BH4 and direct transfer of hydride ions
2
3
4
5
6
8
5
2
3
8
17
28
5
14
15
1
1
9
10
19
9
8
22
23
5
11
6
2
6
10
25
16
6
9
12
occurs.11b,d (c) Single electron transfer occurs in the first
step from BH4 resulting in a complex [ArX- • BH4 ],
which after intramolecular hydrogen atom transfer leads
to reduced products.11g Similar reaction mechanisms were
proposed for photoreduction of organic halides by Li-
AlH4.12 To establish the effect of metal hydride on the
photobehavior of 1a and 1b, their photolyses in the
presence of LiAlH4 were studied. Resulting reaction
mixtures were very complex, and their compositions
differed from those obtained by direct photolysis (Table
1).
-
•
7
3
8
9
35
21
15
21
11
36
22
7
16
5
27
5
10
11
12
13
14
15
16
3
5
9
1
10
2
6
14
11
To distinguish the ionic versus the radical reaction
path, photolyses were run in the presence of the free
radical inhibitor as well. GC/MS analyses revealed the
formation of the same products as in the case of direct
photolysis, however, in different proportions (Table 1).
Photolysis of a 0.01 M solution of 1-bromo-2-fluoro-1,2-
diphenylethane (1b) in THF under the same reaction
conditions led to 100% conversion into cis- (2) and trans-
a
Relative yields in %, determined by GC/MS. b Irradiation time,
8 h; λ ) 253.7 nm; concentration of substrate ) 0.01 M, DBPC )
0.01 M, LiAlH4 ) 0.02 M.
in THF resulted in 100% conversion into a mixture of
cis- (2) and trans-fluorostilbene (3), cis- (4) and trans-
stilbene (5), 1-fluoro-1,2-diphenylethane (6), 9-(2-tetrahy-
drofuryl)phenanthrene (7), 1-fluoro-2-(2-tetrahydrofuryl)-
1,2-diphenylethane (8), 1-(4-chlorobutoxy)-2-fluoro-1,2-
diphenylethane (9a ), and dimeric product 1,4-difluoro-
1,2,3,4-tetraphenylbutane (10) in the relative ratio shown
in Table 1. The products of direct photolysis of 1 in THF
are presented in Scheme 1.
It is known that the presence of metal hydrides
(NaBH4, LiAlH4, etc.) has a great impact on the photore-
duction of organic halides. Investigations on photoreduc-
tion of aryl halides11 in the presence of NaBH4 revealed
the following three pathways. (a) In the excited molecule
of aryl halide a homolytic cleavage of the C-X bond
(11) (a) Barltrop, J . A.; Bradbury, D. J . Am. Chem. Soc. 1973, 95,
5085-5086. (b) Tsujimoto, K.; Tasaka, S.; Ohashi, M. J . Chem. Soc.,
Chem. Commun. 1975, 758-759. (c) Epling, G. A.; Florio, E. J . Chem.
Soc., Chem. Commun. 1986, 185-186. (d) Epling, G. A.; Florio, E.
Tetrahedron Lett. 1986, 27, 675-678. (e) Abeywickrema, A. N.;
Beckwith, A. L. J . Tetrahedron Lett. 1986, 27, 109-112. (f) Kropp, M.;
Schuster, G. B. Tetrahedron Lett. 1987, 28, 5295-5298.
(12) (a) Beckwith, A. L. J .; Goh, S. H. J . Chem. Soc., Chem. Commun.
1983, 905-906. (b) Beckwith, A. L. J .; Goh, S. H. J . Chem. Soc., Chem.
Commun. 1983, 907-907. (c) Shimizu, N.; Watanabe, K.; Tsuno, Y.
Bull. Chem. Soc. J pn. 1984, 57, 885-886. (d) Ashby, E. C.; Deshpande,
A. K. J . Org. Chem. 1994, 59, 3798-3805. (e) Ashby, E. C.; Welder, C.
O. J . Org. Chem. 1997, 62, 3542-3551.

6892 J . Org. Chem., Vol. 65, No. 21, 2000
Kosˇmrlj and Sˇket
Ta ble 2. P r od u ct Distr ibu tion a in P h otolysisb of 1 in
Cycloh exa n e a n d Aceton itr ile
Sch em e 2
hν
hν + DBPC
hν + O2
cyclo-
hexane
aceto-
nitrile
cyclo-
hexane
aceto-
nitrile
acetonitrile
product 1a 1b 1a 1b 1a 1b 1a 1b
1b
1
2
3
4
5
28 28 40 21 48 19 15
8
13
2
5
6
1
27
5
4
1
1
8
18
1
4
5
4
3
1
2
7
3
3
24
4
11
9
37 17 13
3
9
3
2
6
3
3
3
2
7
8
6
6
7
4
34
4
1
10
13
17
18
19
20
21
22
23
Photolysis of 1b in acetonitrile under the same reaction
conditions led to 79% conversion with the formation of
almost the same products as described for 1a , in relative
yields shown in Table 2. Since the formation of benzal-
dehyde (18), benzoic acid (19), and benzoyl fluoride (20)
could be ascribed to the presence of dissolved oxygen, we
performed an additional experiment where the reaction
mixture was purged by oxygen during irradiation. As
expected, the relative yields of 18-20 increased, while
the formation of 2-fluoro-1,2-diphenylethanol (22) and
2-fluoro-1,2-diphenylethanone (23) was observed as well
(Table 2).
5
5
3
8
2
7
9
2
3
7
2
5
14
2
1
8
13
8
23
13
9
44
5
5
1
36 33
36 37
3
a
Relative yields in %, determined by GC/MS. b Irradiation time,
3 h; λ ) 253.7 nm; concentration of substrate ) 0.01 M, DBPC )
0.01 M.
fluorostilbene (3), cis- (4) and trans-stilbene (5), 1-fluoro-
1,2-diphenylethane (6), 9-fluorophenanthrene (11), and
1-(4-bromobutoxy)-2-fluoro-1,2-diphenylethane (9b ). In
the presence of the hydride, besides the above-mentioned
products, benzyl alcohol (12), phenanthrene (13), 1,2-
diphenylethane (14), 1,2,3,4-tetraphenyl-1,3-butadiene
(15), and 1-butoxy-2-fluoro-1,2-diphenylethane (16) were
formed as well (Table 1).
Discu ssion
Photolysis of 1a and 1b in THF, a good hydrogen donor
of a nucleophilic character, led to the formation of a
complex reaction mixture. Roughly, the formation of four
main groups of products was observed: (a) stilbene
products, (b) reduction products, (c) products resulting
from reactions with solvent molecules, and (d) dimeriza-
tion products.
Fluorostilbene products 2 and 3 might arise from either
the ion pair or free cation 24 by elimination of a proton
(Scheme 2). Further photoreduction can lead to cis- and
trans-stilbene (4 and 5, respectively). The cis- and trans-
stilbene could be generated by photoelimination of HF
from 1-fluoro-1,2-diphenylethane (6) as well; therefore the
results obtained by direct photolysis were compared to
those obtained by photolysis in the presence of LiAlH4.
The latter should act as a strong base and enhance the
formation of 4 and 5. The results indeed show the
increase in the relative yields of 4 and 5. However, the
overall relative yields of stilbene products 2-5 were
comparable in both cases, thus indicating that only a
minor possibility of photoelimination of HF existed.
Moreover, DBPC had an insignificant influence on the
stilbene yields, suggesting that the free radical interme-
diate is not involved in formation of stilbene products
and, additionally, confirming the intermediacy of car-
bocation 24 (Table 1).
The formation of reduction product 6 by direct pho-
tolysis of 1 can be explained via hydrogen atom abstrac-
tion of the out of cage radical. However, the presence of
DBPC as free radical inhibitor (Table 1) was shown to
have no effect on the yield of 6, thus diminishing the
possibility of this pathway. On the other hand, we cannot
rule out the involvement of the caged radical pair.
Alternatively, the formation of 6 can be explained by
protonation of the carboanionic intermediate 25 (Scheme
3). The intermediate 25 can be generated by nucleophilic
attack of tetrahydrofuran oxygen (or by chloride ion in
the case of the cyclohexane solution) on the n,σ* excited
state of 1. A similar mechanism was proposed by Kropp
Cycloh exa n e a s a Solven t. Three hours irradiation
of a 0.1 M solution of 1a in cyclohexane resulted in 72%
conversion into a mixture of cis- (2) and trans-fluorostil-
bene (3), 1-fluoro-1,2-diphenylethane (6), phenanthrene
(13), 1-cyclohexyl-2-fluoro-1,2-diphenylethane (17), dimer
10, benzaldehyde (18), benzoic acid (19), and benzoyl
fluoride (20). When the reaction was carried out in the
presence of free radical inhibitor the yields of the last
three products were lower, thus indicating their forma-
tion via out of cage radicals formed by the cleavage of
the ethanic C-C bond (Table 2).
Photolysis of 1b led under the same reaction conditions
to 72% conversion (Table 2). Similarly as described above,
the presence of the free radical inhibitor diminished the
yields of 19 and 20 (Table 2).
Aceton itr ile a s a Solven t. Three hours irradiation
of 1a in acetonitrile at λ ) 253.7 nm led to 60%
conversion. A mixture of starting compound 1a , cis- (2)
and trans-fluorostilbene (3), cis- (4) and trans-stilbene (5),
phenanthrene (13), benzaldehyde (18), benzoyl fluoride
(20), and 1-(N-acetylamino)-2-fluoro-1,2-diphenylethane
(21) was detected (Table 2).

Photolysis of Br- and Cl-Substituted Benzyl Derivatives
J . Org. Chem., Vol. 65, No. 21, 2000 6893
Sch em e 3
Sch em e 5
Sch em e 4
Ta ble 3. Excess En er gy a fter Bon d Clea va gea
1a
1b
b
λ0,0
462
290
172
450
231
219
D0(R-X)c
D
d
Ea
4.2
55.2
a
b
In kilojoules per mole. Estimated from the onset of absorp-
tion. c Values used are those for benzyl chloride and bromide:
Freedman, A.; Yang, S. C.; Kawasaki, M.; Bersohn, R. J . Chem.
d
Phys. 1980, 72, 1028-1033. Values used are those for hydrogen
abstraction from C2H6 by X•: March, J . Advanced Organic
Chemistry, 4th ed.; Wiley: New York, 1992.
and co-workers in the photolysis of 1-bromo- and
1-iodonorbornane.2c,13 However, when the irradiation of
1a was carried out in the presence of LiAlH4 (Table 1), a
substantial increase of 6 was observed. This can be
explained by attack of hydride on the intermediate
carbocation 24.
transfer to bromine atom in a biradical pair, yielding an
ionic pair, is a far more favorable process.
As shown in Table 1, irradiation of 1a and 1b in THF
afforded a similar mixture of photoproducts. However,
in the case of the chloro analogue the radical derived
products were formed predominantly. To study the
solvent effect, we further selected cyclohexane and ac-
etonitrile.15 Similarly to THF, cyclohexane possesses an
ability to donate the hydrogen atom, but it lacks the
nucleophilicity, while acetonitrile, as nucleophilic solvent,
is able to trap the ion pair. Experiments in the presence
of metal hydride were not performed for the solubility
reasons.
Photolysis of 1a and 1b in cyclohexane (Table 2) gave
similar results as in THF. On the other hand, mainly
1-(N-acetylamino)-2-fluoro-1,2-diphenylethane (21) was
formed in acetonitrile (Table 2). The mechanism proposed
for its formation involves addition of carbocationic inter-
mediate 24 to the polarized CN bond of acetonitrile
followed by hydrolysis. The formation of 18-20 was
assumed to be an outcome of the competing reaction with
oxygen dissolved in solvent; therefore an experiment
where the reaction mixture was purged with O2 was
performed. Indeed, the yields of benzaldehyde (18),
benzoic acid (19), and benzoyl fluoride (20) significantly
increased. The mechanism for their formation as well as
for the origin of 22 and 23 is proposed in Scheme 6.
The mechanism for direct photolysis of 1 in THF is
outlined in Scheme 1. Initial homolytic cleavage from a
single excited state produces a radical pair, which either
In the absence of the hydride, carbocationic intermedi-
ate 24 can be trapped by nucleophilic tetrahydrofuran
oxygen. Subsequent ring opening by hydrogen halide
results in the formation of 1-fluoro-2-(4-halobutoxy)-1,2-
diphenylethane (9) (Scheme 4). It is not surprising that
when experiments were run in the presence of LiAlH4
product 9 is not detected; the photoreduction of the C-X
bond mediated by LiAlH4 occurs easily, resulting in
1-butoxy-2-fluoro-1,2-diphenylethane (16) (Table 1).
A significant amount of 1-fluoro-2-(2-tetrahydrofuryl)-
1,2-diphenylethane (8) was formed in direct photolysis
of 1a (Table 1). Since the free radical inhibitor displayed
no influence on the reaction pathway, and the possibility
of the formation of 8 via carbocationic intermediate is
scarce, its origin can be explained by the cage substitu-
tion process.14 The initial formed radical pair is inter-
cepted by a scavenging molecule, in our case by solvent
molecule (THF or cyclohexane), thus forming a new pair
of radicals which either combine or diffuse from solvent
cage (Scheme 5). As expected, the product 8 is not formed
by photolysis of bromo derivative 1b. This could be
ascribed to the lower tendency of bromine for hydrogen
abstraction as a result of considerable activation energy
for that process (Table 3). In this case, an electron
(13) Kropp, P. J .; Poindexter, G. S.; Pienta, N. J .; Hamilton, D. C.
J . Am. Chem. Soc. 1976, 98, 8135-8144.
(14) Leffler, J . E. An Introduction to Free Radicals; Wiley: New
York, 1993.
(15) Electric dipole moment in debye units for acetonitrile, cyclo-
hexane, and THF are 3.92, ∼0, and 1.75, respectively. The values are
taken from Handbook of Chemistry and Physics, 76th ed.; CRC Press:
Boca Raton, 1995.

6894 J . Org. Chem., Vol. 65, No. 21, 2000
Kosˇmrlj and Sˇket
Sch em e 6
diffuses from the cage resulting in the observed formation
of radical products or is transformed into an ion pair by
electron transfer. In accordance with this mechanism the
overriding factors that determine the facility of electron
transfer and diffusion of the radical out of cage are
connected with the nature of the carbon-halogen bond
and solvent polarity.
It is known that the balance of radical versus ionic
photobehavior in alkyl halides is clearly related to the
properties of radical pairs and depends on the type of
halogen atom. Kropp and co-workers suggested that
electron transfer to iodine might occur more readily than
expected on the basis of electronegativity because of
either its greater polarizability or the lesser charge point
density compared with bromine.13,16 On the basis of later
results, Kropp and Adkins suggested that the difference
in behavior is due to more rapid separation from the
radical pair for bromides. This might arise from the
difference between bond dissociation energy and energy
of the n,σ* electronic transition between the lowest
vibrational levels of the ground and the excited states,
being greater for bromides than for iodides.17
The presented results of the photobehavior of 1a and
1b show that chloride 1a gives higher yields of products
derived from the out of cage radical intermediates than
the corresponding bromide 1b. The results are in agree-
ment neither with the difference in electronegativity nor
with the difference between bond dissociation energy and
energy of the n,σ* electronic transition between the
lowest vibrational levels of the ground and the excited
states, which is greater for bromide than chloride (Table
3).
ability of bromine and the fact that formation of the
bromide ion involves generation of a lesser point charge
density comparing to chloride ion.18 Another factor might
be the difference in reactivity of the chlorine and bromine
radicals toward hydrogen abstraction. The chlorine atom
is more reactive for efficient hydrogen atom abstraction
than bromine, thus inhibiting the competing electron
transfer (Table 3).
The solvent effect on the ratio between ionic and
radical products produced through the radical pair was
investigated by several authors. Kropp established that
the photobehavior of alkyl halides depends on the viscos-
ity of the solvent.13,16 The more viscous solvent 1,2-
ethanediol resulted in a substantial increase in the ratio
of ionic to radical products. The increased lifetime of the
radical pair cage in a more viscous solvent apparently
allows electron transfer to compete more efficiently with
diffusion from the cage. Cristol and co-workers deter-
mined that polar solvent effects can be reflected in the
following cases: polar solvents (acetonitrile) favor elec-
tron transfer, and the nonpolar solvents (THF and
cyclohexane) are good hydrogen donors, so they could
react with chlorine atoms of the radical pairs to divert
the intermediate from an ion pair formation.19 Walling
and co-workers studied the thermolysis of acyl peroxides,
which leads to products of both radical and ionic pro-
cesses.20 They proposed that first intermediate was a
species that they termed as an “intimate ion-radical pair”.
This is an intermediate in which electronic interaction
between the fragments is still extensive, and ionic and
paired diradical formations represent contributing struc-
(18) Crystal ionic radii of bromide and chloride ions are 0.196 and
0.181 nm, respectively. The values are taken from Handbook of
Chemistry and Physics, 76th ed.; CRC Press: Boca Raton, 1995.
(19) Cristol, S. J .; Stull, D. P.; Daussin, R. D. J . Am. Chem. Soc.
1978, 100, 6674-6678.
(20) Walling, C.; Waits, H. P.; Milovanovic, J .; Pappiaonnou, C. G.
J . Am. Chem. Soc. 1970, 92, 4927-4932.
Apparently, there are prevailing factors that determine
the facility of electron transfer, such as greater polariz-
(16) Kropp, P. J . Acc. Chem. Res. 1984, 17, 131-137.
(17) Kropp, P. J .; Adkins, R. L. J . Am. Chem. Soc. 1991, 113, 2709-
2717 and references therein.

Photolysis of Br- and Cl-Substituted Benzyl Derivatives
J . Org. Chem., Vol. 65, No. 21, 2000 6895
Ta ble 4. Dep en d en ce of th e Ra tio of Ion ic a n d Ra d ica l
P r od u cts on Solven t in Dir ect P h otolysis of 1
prepared according to literature. Cyclohexane (LiAlH4), THF
(LiAlH4), and acetonitrile (CaH2) were refluxed over the
mentioned drying agents and distilled in an inert atmosphere
before use.
ionic/radical ratio
compound
C6H12
THF
MeCN
The reaction mixtures were analyzed by GC and GC/MS and
by comparison of the spectral data of products to those of the
authentic samples: cis- (2) and trans-fluorostilbene24 (3), cis-
(4) and trans-stilbene (5), 1-fluoro-1,2-diphenylethane25 (6),
9-fluorophenanthrene (11), benzyl alcohol (12), phenanthrene
(13), 1,2-diphenylethane (14), 1,2,3,4-tetraphenyl-1,3-buta-
diene (15), benzaldehyde (18), benzoic acid (19), benzoyl
fluoride (20), 1-(N-acetylamino)-2-fluoro-1,2-diphenylethane26
(21), 2-fluoro-1,2-diphenylethanol27 (22), and 2-fluoro-1,2-
diphenylethanon28 (23). Compounds 1,2-diphenyl-1-fluoro-2-
(2-tetrahydrofuryl)ethane (8), 1-(4-chlorobutoxy)-2-fluoro-1,2-
1a
1b
0.3
2.0
0.7
5.5
7.0
12.0
tures of a resonance hybrid. The fragments become
separated by solvent. Leffler and More reported the
results of decomposition of bis(bicyclo[2.2.2]octane-1-
formyl) peroxide and pivaloyl peroxide in a series of
solvents of different dielectric constants.21 The rate
constant for the radical reaction is relatively insensitive
to polar effect, while the rate constant for the ion pair
reaction undergoes large changes. Less polar solvents
favor a less polarized canonical structure, while a polar
solvent favor a canonical structure, which provides
conversion to the ion pair.
Our results show the possible existence of an intimate
ion-radical pair, which in a polar solvent (acetonitrile)
leads to substantially higher yields of ionic products.
Namely, the ionic/radical ratio (I/R) increased with
increasing solvent polarity from 0.3 in cyclohexane to 7.0
in acetonitrile for chloro derivative 1a , while in the case
of bromo derivative 1b the I/R ratio increased from 2.0
in cyclohexane to 12.0 in acetonitrile (Table 4).15 The
considerable increase in ionic products in acetonitrile in
comparison with THF reveals the importance of the
solvent’s ability to donate a hydrogen atom. The reaction
between a halogen atom in a radical pair and a hydrogen
atom from solvent is much faster with a chlorine than a
bromine atom, which was already observed by Cristol and
co-workers.18
diphenylethane
(9a ),
1-(4-bromobutoxy)-2-fluoro-1,2-
diphenylethane (9b), 1,4-difluoro-1,2,3,4-tetraphenylbutan (10),
1-butoxy-2-fluoro-1,2-diphenylethane (16), and 1-cyclohexyl-
2-fluoro-1,2-diphenylethane (17) were unknown and were
isolated and characterized as reported.
Typ ica l Exp er im en ta l P r oced u r e. First, 0.05 mmol of
substrate was dissolved in 5 mL of solvent, and the reaction
mixture was irradiated at 253.7 nm for the time listed in the
Tables. Whenever stated (see Tables), 0.05 mmol of inhibitor
was added prior to the reaction. The reaction mixtures were
analyzed by GC and GC/MS. When the metal hydride was
added, the substrate was dissolved in 5 mL of a clear solution
29
of LiAlH4 (0.10 mmol, c ) 0.02 M) in THF, and after
irradiation the excess of hydride was destroyed by pouring of
the reaction mixture into water. The organic phase was dried
over anhydrous Na2SO4 and concentrated under reduced
pressure prior to the analyses.
9-(2-Tetr a h yd r ofu r yl)p h en a n th r en e (7) was formed in
too small quantities for isolation (Table 1). It was characterized
on the basis of its mass spectrum. GC/MS: 248(M+, 100),
177(30), 71(12).
1-F lu or o-2-(2-tetr a h yd r ofu r yl)-1,2-d ip h en yleth a n e (8).
Four isomers of compound 8 were isolated by HPLC (semi-
preparative silica gel column, 5 µm; mobile phase, hexane/
diethyl ether (93:7); flow rate, 2 mL/min; UV detection at 254
nm) in the elution order as cited. Isom er A. 1H NMR: δ 1.75-
1.88 (m, 4H), 3.38 (ddd, J ) 5.4, 9.1, 14.0 Hz, 1H), 3.79-3.88
(m, 1H), 3.91-3.99 (m, 1H), 4.07-4.16 (m, 1H), 6.05 (dd, J )
5.4, 45.5 Hz, 1H), 6.81-6.86 (m, 2H), 7.00-7.06 (m, 2H), 7.12-
7.20 (m, 3H), 7.20-7.25 (m, 3H). 19F NMR: δ -180.68 (dd, J
) 14.0, 45.5 Hz). EIMS: 180 (100), 109 (16), 71 (35). HR EIMS
calcd for fragment C11H13FO: 180.0950; found: 180.0952.
Con clu sion s
The first step in photolysis of chloro- and bromo-
substituted 1-fluoro-1,2-diphenylethanes (1) is the ho-
molytic cleavage of the C-X bond from the single excited
state and the formation of the biradical pair. This can
be subsequently divided into either two separate radicals
(out of cage radicals) or into an ionic pair via an electron-
transfer process (within the caged radicals), which de-
pends on both the halogen atom bonded and the solvent
polarity. The larger I/R ratio for bromo than for chloro
compounds are a consequence of the greater polarizability
and the lesser charge point density of bromine, as well
as the efficiency of the chlorine atom for hydrogen atom
abstraction. Increasing solvent polarity increases the
yields of ionic products, which might be an outcome of
the intimate ion-radical pair where the polar solvent
favors the ionic pathway, while in the less polar solvent
(THF, cyclohexane) the ability of solvent to donate a
hydrogen atom must be considered.
1
Isom er B. H NMR: δ 1.73-2.05 (m, 4H), 3.14 (dt, J ) 3.8,
9.9 Hz, 1H), 3.72-3.79 (m, 2H), 4.59-4.67 (m, 1H), 5.84 (dd,
J ) 10.0, 46.7 Hz, 1H), 7.05-7.20 (m, 10H). 19F NMR:
δ
-173.55 (dd, J ) 9.9, 46.7 Hz). EIMS: 180 (96), 109 (14), 71
(100). HR EIMS calcd for fragment C11H13FO: 180.0950;
1
found: 180.0955. Isom er C. H NMR: δ 1.60-1.85 (m, 3H),
3.01 (ddd, J ) 4.4, 7.9, 17.6 Hz, 1H), 3.68-3.78 (m, 3H), 3.85-
3.92 (m, 1H), 5.91 (dd, J ) 7.8, 47.0 Hz, 1H), 7.20-7.35 (m,
10H). 19F NMR: δ -178.55 (dd, J ) 17.6, 47.0 Hz). EIMS: 180
(94), 109 (11), 71 (100). HR EIMS calcd for fragment C11H13
-
FO: 180.0950; found: 180.0950. Isom er D. 1H NMR: δ 1.71-
1.89 (m, 4H), 2.76 (ddd, J ) 2.4, 10.1, 34.3 Hz, 1H), 3.90 (dt,
J ) 6.8, 8.4 Hz, 1H), 3.99 (dt, J ) 6.8, 8.4 Hz, 1H), 4.53 (ddd,
J ) 7.5, 10.1, 14.0 Hz, 1H), 6.17 (dd, J ) 2.4, 46.7 Hz, 1H),
6.90-7.02 (m, 4H), 7.13-7.18 (m, 6H). 19F NMR: δ -195.6
(dd, J ) 34.3, 46.7 Hz). EIMS: 180 (92), 109 (22), 71 (100).
HR EIMS calcd for fragment C11H13FO: 180.0950; found:
180.0954.
Exp er im en ta l Section
The photochemical reactions were performed at λ ) 253.7
nm. The NMR spectra were recorded in CDCl3 at 302 K.
Chemical shifts are given on the δ scale (ppm) and are
referenced to internal TMS for H and 13C spectra and to CCl3F
1
(24) Zupan, M.; Pollak, A. J . Org. Chem. 1977, 42, 1559-1562.
(25) Weigert, F. J . J . Org. Chem. 1980, 45, 3476-3483.
(26) Bensadat, A.; Bodennec, G. Nouv. J . Chim. 1981, 5, 127-133.
(27) Andrews, L. E.; Bonnet, R.; Appelman, E. H. Tetrahedron, 1985,
41, 781-784.
(28) Purrington, S. T.; Lazaridis, N. V.; Bumgardner, C. L. Tetra-
hedron Lett. 1986, 27, 2715-2716.
(29) Krishnamurthy, S.; Brown, H. C. J . Org. Chem. 1982, 47, 276-
280.
for 19F spectra. The starting compounds 1a 22 and 1b23 were
(21) Leffler, J . E.; More, A. A. J . Am. Chem. Soc. 1972, 94, 2483-
2487.
(22) Sˇket, B.; Zupancˇicˇ, N. Vestn. Slov. Kem. Drusˇ. 1992, 39, 399-
403.
(23) Baciocchi, E.; Ruzziconi, R. J . Org. Chem. 1984, 49, 3395-3398.

6896 J . Org. Chem., Vol. 65, No. 21, 2000
Kosˇmrlj and Sˇket
1-(4-Ch lor obu toxy)-2-flu or o-1,2-d ip h en yleth a n e (9a ).
(dd, J ) 5.7, 13.0 Hz, 1H), 5.46 (dd, J ) 5.7, 46.1 Hz, 1H),
7.15-7.38 (m, 10H). 19F NMR: δ -184.25 (dd, J ) 13.0, 46.0
Hz). 13C NMR: δ 13.78, 19.18, 31.73, 69.25, 84.20 (d, J ) 26.3
Hz), 95.39 (d, J ) 177.6 Hz), 126.82 (d, J ) 6.9 Hz), 127.82,
127.94, 127.95, 128.06, 128.27, 128.29, 137.27 (d, J ) 20.4 Hz),
137.75 (d, J ) 2.7 Hz). EIMS: 252(M+ - HF, 0.2), 163(62),
107(100). HR EIMS calcd for fragment C18H20O: 252.1514;
Compound 9a was isolated by HPLC as described for com-
1
pound 8. H NMR: δ 1.67-1.91 (m, 4H), 3.43 (dt, J ) 6.8, 8.4
Hz, 2H), 3.52 (t, J ) 6.4 Hz, 2H), 4.52 (dd, J ) 6.7, 14.0 Hz,
1H), 5.47 (dd, J ) 6.7, 46.8 Hz, 1H), 6.03-7.07 (m, 4H), 7.19-
7.24 (m, 6H). 19F NMR: δ -182.47 (dd, J ) 14.0, 46.8 Hz).
EIMS: 199 (6), 109 (13), 107 (43), 91 (100). HR EIMS calcd
for fragment C11H14ClO: 197.0733; found: 197.0734.
1
found: 252.1520. Isom er b. H NMR: δ 0.87 (t, J ) 7.3 Hz,
1-(4-Br om obu t oxy)-2-flu or o-1,2-d ip h en ylet h a n e (9b).
Compound 9b was isolated by column chromatography (silica
gel, Fluka, mesh 220-440) with hexane as eluent (Rf ) 0.1).
3H), 1.30-1.42 (m, 2H), 1.52-1.63 (m, 2H), 3.34-3.44 (m, 2H),
4.53 (dd, J ) 6.8, 13.6 Hz, 1H), 5.47 (dd, J ) 6.8, 46.8 Hz,
1H), 7.00-7.30 (m, 10H). 19F NMR: δ -181.88 (dd, J ) 13.5,
46.5 Hz). 13C NMR: δ 13.88, 19.30, 31.88, 69.38, 84.83 (d, J )
23.2 Hz), 96.30 (d, J ) 181.0 Hz), 126.73 (d, J ) 6.9 Hz),
127.81, 127.97, 128.02, 128.24, 128.27, 136.71 (d, J ) 20.3 Hz),
137.44 (d, J ) 5.3 Hz). EIMS: 252 (M+ - HF, 0.2), 163 (42),
107 (100). HR EIMS calcd for fragment C18H20O: 252.1514;
found: 252.1521.
1-Cycloh exyl-2-flu or o-1,2-d ip h en yleth a n e (17). Purifi-
cation was done by thin-layer chromatography using hexane
(Rf ) 0.2). The mixture of two isomers was isolated in the ratio
1:1, as determined by 1H NMR. 1H NMR: δ 0.73-2.08 (m,
22H), 2.61 (ddd, J ) 4.2, 8.8, 31.9 Hz, 1H), 3.07 (ddd, J ) 5.7,
8.7, 13.3 Hz, 1H), 5.82 (dd, J ) 8.7, 46.9 Hz, 1H), 6.00 (dd, J
) 4.2, 46.9 Hz, 1H), 6.92-7.04 (m, 6H), 7.08-7.21 (m, 14H).
19F NMR: δ -174.05 (dd, J ) 13.4, 47 Hz), -190.48 (dd, J )
32, 47 Hz). EIMS: 282 (M+,1), 173 (83). HR EIMS calcd for
1
The H NMR showed the presence of two isomers in the ratio
1
of 5:2, which were not separated. H NMR: δ 1.63-2.00 (m,
5.6H), 3.31-3.46 (m, 5.6H), 4.47 (dd, J ) 5.9, 12.3 Hz, 0.4H),
4.52 (dd, J ) 6.7, 14.1 Hz, 1H), 5.44 (dd, J ) 5.9, 46.0 Hz,
0.4H), 5.47 (dd, J ) 6.7, 46.7 Hz, 1H), 7.00-7.40 (m, 14H).
19F NMR: δ -182.49 (dd, J ) 14.0, 46.7 Hz, 5F), -183.61 (dd,
J ) 12.3, 46.0 Hz, 2F). EIMS: 332/330 (M+ - HF, 0.5/0.5),
243/241 (20/20), 137/135 (97/100). HR EIMS calcd for fragment
C
11H14BrO: 241.0228; found: 241.0230.
1,4-Diflu or o-1,2,3,4-tetr a p h en ylbu ta n (10). Compound
10 was isolated by HPLC under conditions described for 8. 1H
NMR: δ 3.48 (dddd, J ) 1.2, 5.2, 9.5, 10.5 Hz, 1H), 4.3 (ddd,
J ) 5.2, 8.1, 17.2 Hz, 1H), 5.60 (dd, J ) 9.5, 46.9 Hz, 1H),
5.83 (dd, J ) 8.1, 46.8 Hz, 1H), 6.46-6.53 (m, 2H), 6.92-7.04
(m, 5H), 7.04-7.15 (m, 5H), 7.18-7.28 (m, 3H), 7.33-7.45 (m,
5H). 19F NMR: δ -171.05 (dd, J ) 10.5, 46.9 Hz), -173.33
(ddd, J ) 1.2, 17.2, 46.8 Hz). EIMS: 398 (M+, 6). HR EIMS
calcd for C28H24F2: 398.1846; found: 398.1856.
C
20H23F: 282.1784; found: 282.1790.
Ack n ow led gm en t. We thank Dr. Bogdan Kralj and
1-Bu toxy-2-flu or o-1,2-d ip h en yleth a n e (16). Two isomers
of compound 16 were isolated by column chromatography
(silica gel, Fluka, mesh 220-440) with hexane/ethyl acetate
(20:1; Rf ) 0.5) as eluent in the elution order as cited. Isom er
a . 1H NMR: δ 0.81 (t, J ) 7.3 Hz, 3H), 1.15-1.31 (m, 2H),
1.41-1.51 (m, 2H), 3.17-3.27 (m, 1H), 3.31-3.41 (m, 1H), 4.47
Dr. Dusˇan Zˇigon at the J ozˇef Stefan Institute for mass
spectral measurements. Financial support from the
Ministry of Science and Technology of Slovenia is
acknowledged.
J O0001481