Novel radical cation reactions of bichromophoric systems. Transannular aryl migrations; Mechanistic and exploratory organic photochemistry

Article abstract of DOI:10.1021/jo960467l

Eight aryl-substituted 1,4-pentadienes were subjected to photochemically induced electron transfer using dicyanonaphthalene and dicyanoanthracene. The radical-cations produced underwent a regioselective cyclization, wherein one electron-deficient aryl group of one diarylvinyl moiety bonds to the β-carbon of the second diarylvinyl group. A pattern of regioselectivity and reactivity was encountered. As cyclization proceeds, the odd-electron density becomes localized in the benzhydryl side chain while the positive charge becomes localized in the second portion of the molecule. Substitution in one diarylvinyl branch designed to delocalize odd-electron density but destabilize electron deficiency led to higher reactivity than the unsubstituted parent 1,1,5,5-tetraphenyl-3,3-dimethyl-1,4-pentadiene. The benzhydryldihydronaphthalene photoproducts themselves proved photochemically reactive. On sensitization, the benzhydryldihydronaphthalene reacted with a transannular 1,5-migration of one aryl group of the benzhydryl moiety. Where the two benzhydryl aryl groups were different, the cyanophenyl group migrated in preference to phenyl, and both diastereomers led to the same product stereoisomer. Ab initio and semiempirical computations were in accord with the radical cation and triplet regioselectivity.

Full text of DOI:10.1021/jo960467l

6526
J . Org. Chem. 1996, 61, 6526-6534
Novel Ra d ica l Ca tion Rea ction s of Bich r om op h or ic System s.
Tr a n sa n n u la r Ar yl Migr a tion s; Mech a n istic a n d Exp lor a tor y
Or ga n ic P h otoch em istr y†,1,2
Howard E. Zimmerman* and Kurt D. Hoffacker
Chemistry Department, University of Wisconsin, Madison, Wisconsin 53706
Received March 7, 1996X
Eight aryl-substituted 1,4-pentadienes were subjected to photochemically induced electron transfer
using dicyanonaphthalene and dicyanoanthracene. The radical-cations produced underwent a
regioselective cyclization, wherein one electron-deficient aryl group of one diarylvinyl moiety bonds
to the â-carbon of the second diarylvinyl group. A pattern of regioselectivity and reactivity was
encountered. As cyclization proceeds, the odd-electron density becomes localized in the benzhydryl
side chain while the positive charge becomes localized in the second portion of the molecule.
Substitution in one diarylvinyl branch designed to delocalize odd-electron density but destabilize
electron deficiency led to higher reactivity than the unsubstituted parent 1,1,5,5-tetraphenyl-3,3-
dimethyl-1,4-pentadiene. The benzhydryldihydronaphthalene photoproducts themselves proved
photochemically reactive. On sensitization, the benzhydryldihydronaphthalene reacted with a
transannular 1,5-migration of one aryl group of the benzhydryl moiety. Where the two benzhydryl
aryl groups were different, the cyanophenyl group migrated in preference to phenyl, and both
diastereomers led to the same product stereoisomer. Ab initio and semiempirical computations
were in accord with the radical cation and triplet regioselectivity.
In tr od u ction
However, these bichromophoric systems have not been
studied under radical-cation-forming conditions. This
paper describes our research in this direction.
Our past studies of bichromophoric systems includes
an exhaustive study of the di-π-methane rearrangement.3
Equation 1 gives a typical example.4 One question arose
Resu lts
Syn th esis of Rea cta n ts. Of the di-π-methane reac-
tants of interest, several were known and were prepared
by known methods; these (18-21) are shown in Scheme
1. Further reactants were obtained as depicted in
Scheme 1. One interesting reaction is the formation of
enone 15 from the reaction of 3,3-dimethylglutaric an-
hydride. An initial addition-elimination of phenyl-
lithium to one carbonyl affords a benzoyl carboxylate that
then adds two further phenyllithium molecules to afford
the triphenyl ketocarbinol 14. The sequence provides a
convenient approach to such bifunctional molecules. Our
previous syntheses of similar products were considerably
longer.7
Ra d ica l Ca tion Rea ctivity of th e 1,1,5,5-Tetr a a r yl-
3,3-d im eth yl-1,4-p en ta d ien es. The absorption spec-
trum of the tetraphenyldiene 1 showed relatively little
absorption above 300 nm. Dicyanonaphthalene (DCN)
absorbed nicely in the 300-345 nm region, and the
excited singlet (S1) of DCN has a reduction potential8 of
-2.17 eV, which is sufficient to remove an electron from
a diphenylvinyl group (Eox ) 1.88).9 Effectively, the same
point is made by consideration of the Weller equation10
hν
(1)
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1
2
whether the corresponding radical cation systems would
undergo the same or a different reaction. This seemed
of special interest in view of the increasing role of electron
transfer in organic rearrangements.5 More specifically,
diphenylethylene itself has been the object of a consider-
able number of radical cation investigations.6 The major
reactivity observed has been either nucleophilic addition
by solvent or bimolecular reactivity. We note that the
diarylvinyl moiety has been present in a variety of
reactants undergoing the di-π-methane rearrangement.
† This paper is dedicated to Frederick G. Bordwell in recognition of
his unique and many brilliant contributions to physical-organic
chemistry.
X Abstract published in Advance ACS Abstracts, August 1, 1996.
(1) This is part 244 of our general series and 181 of our photochemi-
cal papers.
(2) Photochemical part 179: Zimmerman, H. E.; Kutateladze, A. G.
J . Am. Chem. Soc. 1996, 117, 249-250.
∆GET ) 23.06[Eox(D/D•+) - Ered(A/A•-) - e/aꢀ - E0,0
]
(3) (a) Zimmerman, H. E.; Grunewald, G. L. J . Am. Chem. Soc. 1966,
88, 183-184. (b) See ref 3c,d for reviews. (c) Zimmerman, H. E. In
Organic Photochemistry; Padwa, A., Ed.; Marcel Dekker, Inc., New
York: 1991; Vol. 11. (d) Zimmerman, H. E. In Handbook of Photo-
chemistry and Photobiology; Horspool, W., Song, P. S., Eds.; CRC
Press: London, 1995.
(2)
(i.e., 2), which leads us to a ∆G of electron transfer of
-8.0 kcal/mol. This combination seemed ideal for pho-
tochemical formation of the radical cation of tetra-
(4) Zimmerman, H. E.; Mariano, P. S. J . Am. Chem. Soc. 1969, 91,
1718-1727.
(5) (a) Kavarnos, G. J .; Turro, N. J . Chem. Rev. 1986, 86, 401-449.
(b) Albrecht, E.; Averdung, J .; Bischof, E. W.; Hiedbreder, A.; Kirsch-
berg, T.; Mu¨ller, F.; Mattay, J . J . Photochem. Photobiol. A: Chem. 1994,
82, 219-232.
(6) (a) Arnold, D. R.; Maroulis, A. J . J . Am. Chem. Soc. 1976, 98,
5931-5937. (b) Mizuno, K.; Ishii, M.; Otsuji, Y. J . Am. Chem. Soc.
1981, 103, 5570-5572. (c) Mattes, S. L.; Farid, S. J . Am. Chem. Soc.
1983, 105, 1387-1389. (d) Mattes, S. L.; Farid, S. J . Am. Chem. Soc.
1986, 108, 7356-7361.
(7) Zimmerman, H. E.; Nuss, J . M. J . Org. Chem. 1986, 51, 4604-
4617.
(8) (a) Mattes, S. L.; Farid, S. In Organic Photochemistry; Padwa,
A., Ed.; Marcel Dekker: New York, 1983; Vol. 6, p 233. (b) Arnold, D.
R.; Maroulis, A. J . J . Am. Chem. Soc. 1976, 98, 5931-5937.
(9) Gollnick, K.; Schnatterer, A.; Utschick, G. J . Org. Chem. 1993,
58, 6049-6056.
(10) Rehm, D.; Weller, A. Isr. J . Chem. 1970, 8, 259-271.
S0022-3263(96)00467-7 CCC: $12.00 © 1996 American Chemical Society

Radical Cation Reactions of Bichromophoric Systems
J . Org. Chem., Vol. 61, No. 19, 1996 6527
Sch em e 1. Syn th esis of Di-π-m eth a n e r ea cta n ts
supra), the cyano substitution was in the benzhydryl
group of the product. Also, with the bis-p-chloro analog
20 the reaction proceeded nicely, and p-chloro appeared
in the benzhydryl group of photoproduct 25. This trans-
formation as well as the cyano examples are shown in
eq 4.
H2SO4
PhMgBr
MeOH,
C6H6
COMe COOMe
O
Ph
O
O
COOMe
ArLi
Ph
Ph
Ph
3
4
5
Ar′
Ar′′
hν
(4)
TsOH
Ar
CuCN
Ph
biphenyl, A
Ph
Ar
Ph
Ar
Ar
Ar
Ar
Ph
Ph
Ph
Ph
12, m-CNPh
Ph
Ph
Ar
Ar
OH
Ar′′
A
Ar′
9, m-ClPh
10, m-Me
11, p-Me
6, m-ClPh
7, m-Me
8, p-Me
21
23
24
25
26a,b
Ph-p-CN Ph-p-CN DCA
Ph-m-CN Ph-m-CN DCA
Ph-p-Cl Ph-p-Cl DCN
12
20
17
Ph
Ph-p-CN DCA
MeOH
H+
PhLi
HO
Ph
O
O
O
O
O
Also of interest was the radical cation photochemistry
of the monocyano di-π-methane reactant 17. Two stere-
oisomers were obtained, and an X-ray structure of one
showed this to be the trans isomer 17t. However, under
reaction conditions these isomers interconverted at a
rapid rate compared to further reaction. Two diastere-
omeric photoproducts were observed in a 2.4:1 ratio. The
structure of the major product 26a was established by
X-ray crystallography, and the NMR spectrum of the
minor isomer was almost identical except for minor peak
shifts. The reaction is included in eq 4.
Ph
Ph
Ph
Ph
15
Ph
13
14
p-BrPhMgBr
1. TsOH
2. CuCN
Ph-p-CN
OH
Ar
Ph-p-Br
Ph
Ph
Ph
Ph
Ph
Ar
Ph
Ph
Ph
17
16
18, p-MeOPh
19, m-MeOPh
20, p-ClPh
21, p-CNPh
Interestingly, the di-p-tolyl analog 11 and the di-p-
methoxy diene 18 proved unreactive.
phenyldiene 1. It was quickly ascertained that the usual
di-π-methane rearrangement, depicted in eq 1, had not
occurred. Rather, an isomeric product was formed. The
NMR spectrum revealed two nonequivalent methyl groups,
one vinyl, two methines, and 19 aromatic hydrogens.
Several structures seemed possible, and an X-ray analy-
sis led to structure 22. Thus, the reaction can be seen
to involve a cyclization in which the â-carbon of one
diphenylvinyl group bonds to the ortho-carbon of a phenyl
group of the second diphenylvinyl moiety. This is shown
in eq 3.
H
H
PhCN
Ph
Ph
PhCN
H
H
Ph
Ph
26a
26b
Exploratory runs on the di-m-chloro 9, di-m-methyl 10,
and di-m-methoxy 19 dienes indicated that these were
reactive. However, practical problems precluded struc-
ture elucidation of the photoproduct mixtures. While for
each of 9 and 10 NMR evidence indicated two regioiso-
mers of the same basic structure as the parent 22, the
m-methoxy analog led to a multiplicity of products,
possibly from secondary photoreactions.
Ph
Ph
hν
(3)
Ph
Ph
biphenyl,
DCN
Ph
Ph
Ph
Tr ip let Secon d a r y P h otoch em istr y. One curious
observation was the formation of a secondary photoprod-
uct near the end of the photolysis of the bis(p-cyanophe-
nyl) diene 21 to form bicyclic 23. It was confirmed that
this product did derive from secondary photochemistry,
since the same product resulted from independent pho-
tolysis of bicyclic 23. The structure of this photoproduct
was established by X-ray crystallography as the tricyclic
benzonorbornene 27 and thus clearly involved migration
of a p-cyanophenyl group. Note eq 5. This was a reaction
1
22
With this result known, it was of interest to determine
the scope of the reaction. The reaction regioselectivity
was of particular interest in the reaction of unsymmetri-
cally substituted di-π-methane systems. Cyano substitu-
tion was particularly relevant. The photolysis of 1,1-
bis(p-cyanophenyl)-3,3-dimethyl-5,5-diphenyl-1,4-pen-
tadiene (21) with DCA in acetonitrile led to formation of
a crystalline product, mp 283-284 °C. The structure of
this photoproduct 23 was obtained by X-ray analysis and
found to have the same basic skeleton as the product 22
of the unsubstituted diene, however, with cyano substitu-
tion in the benzhydryl moiety rather than in the dihy-
dronaphthalene ring.
Ar′
Ph
Ph
hν
Ar′′
(5)
Ar′
Sens
H
NC
27, Ar′ = p-CNPh, Ar′′ = p-CNPh
28, Ar′ = Ph, Ar′′ = p-CNPh
This led us to investigate the behavior of the corre-
sponding 1,1-bis(m-cyanophenyl)-3,3-dimethyl-5,5-diphe-
nyl-1,4-pentadiene (12). Again, a single photoproduct,
24, was obtained, and the structure was elucidated by
X-ray crystallography. As in the p-cyano case (vide
23, Ar′ = p-CNPh
26, Ar′ = Ph
not encountered with reactants having less effective
electron withdrawing groups. It did not seem reasonable

6528 J . Org. Chem., Vol. 61, No. 19, 1996
Zimmerman and Hoffacker
Sch em e 2. Ra d ica l Ca tion Cycliza tion Mech a n ism
Ar
hν
biphenyl,
DCN or DCA
Ar
Ar
Ar
Ph
Ph
Ph
1, Ar = Ph
Ph
Ar
Ar
Ph
29, Ar = Ph
30, Ar = p-CNPh
12, Ar = p-CNPh
22, Ar = Ph
23, Ar = p-CNPh
Ar
Ar
Ar
Ar
F igu r e 1. Close to nonbonding MO’s. Note six-membered
ring Ar has the same pattern of electron densities as the one
drawn explicitly. MO energies in hartrees (627.5 kcal/mol per
hartree).
H
Ph
Ph
31, Ar = Ph
32, Ar = p-CNPh
33, Ar = Ph
34, Ar = p-CNPh
is largely localized in the “heptatrienyl” and the odd-
electron density is largely concentrated in the benzhydryl
group as depicted in Scheme 2. This was true as well
for the hydrocarbon system lacking the cyano groups and
also the di-p-cyano-substituted one. Both moieties are
of the odd-alternant variety, and it is of consequence that
the lost electron comes from the nonbenzylic portion of
the molecule (vide infra). The subsequent proton loss
leads to the aromatic radical 33. Finally, back-electron
transfer and protonation affords the final photoproduct,
e.g., 22.
that p-cyanophenyl migration would be preferred by
electron deficiency, and it was suspected that the second-
ary reaction was, instead, a triplet process.
That the reaction was a triplet process was confirmed
by the observation of a rapid conversion of bicyclic 23 to
benzonorbornene 27 on irradiation using benzophenone
as a triplet sensitizer.
As a consequence, we studied the triplet photochem-
istry of the monocyano bicyclic 26a ,b stereoisomers.
These also had exhibited secondary photochemistry; but,
in the radical cation photochemistry of the mono-p-
cyanophenyl diene 17 the secondary photochemistry was
somewhat more competitive with the initial cyclization.
In order to ascertain that this monocyanophenyl rear-
rangement also proceeded via the triplet rather than the
radical cation, bicyclic isomers 26 were subjected to
sensitization by benzophenone. The reaction proceeded
to afford the same product. The benzophenone-sensitized
rearrangement was interesting in several respects. First,
both stereoisomers of reactant 26 led to a single diaste-
reomer of the monocyanobenzonorbornene 28, whose
structure was established by X-ray crystallography.
Second, this established that there had been a preferen-
tial cyanophenyl migration, although, a priori, either a
phenyl or a cyanophenyl group might have shifted. The
observation of preferential cyanophenyl migration con-
firmed the triplet nature of the reaction. Both the
reaction regioselectivity and the stereochemistry are
discussed below. Equation 5, however, depicts the nature
of the overall reaction.
The reaction regioselectivity is in accord with this
mechanism, which involves (see Scheme 2) bonding of the
â-vinyl carbon of the bis(p-cyanophenyl)vinyl group to an
ortho carbon of the diphenylvinyl moiety. However, the
source of this regioselectivity is understood from the ab
initio computations on radical cations 31 and 32. With
the positive charge concentrated in the heptatrienyl
group, one would anticipate destablization by an electron-
withdrawing group such as cyano. Additionally, the
p-cyano substituent helps stabilize the benzhydryl radi-
cal-like portion of the molecule.
Inspection of the ab initio computations on the inter-
mediate 31 and 32 derived from reactants 1 and 12,
respectively, showed two nearly degenerate MO’s. The
largest electron densities are as depicted in Figure 1. At
first glance the pairs of MO’s appear to be “benzylic” and
“heptatrienyl” in nature. However, there is weak cou-
pling through the newly formed σ bond, which separates
the two systems and is labeled with an asterisk. The
sign alternation anticipated for the nonbonding hep-
tatrienyl system is, indeed, found in the eigenvector.
However, the benzylic moieties do not show the typical
plus-minus alternation. Both in the parent diene case
1 and the p-cyanophenyl diene 12, the intermediates 31
and 32 have the odd-electron mainly in the benzhydryl
moiety as shown in Scheme 2 and the positive charge
mainly in the heptatrienyl π-system. The regioselectiv-
ity, thus, seems to arise from electron-withdrawing
substituents avoiding the positive heptatrienyl system
but stabilizing the odd-electron benzhydryl portion of the
molecule as the reaction proceeds. In the case of the
p-cyano substitution the cyano group thus avoids the
positive portion of the molecule and stabilizes the odd-
electron density. With m-cyano substitution the electron
deficient part of the system is, again, avoided, despite
little if any odd-electron delocalization in the benzhydryl
group resulting.
Discu ssion
Th e Radical Cation Cyclization Mech an ism . Turn-
ing first to the overall reaction mechanism, we can write
the rearrangement as in Scheme 2. Here, one anticipates
initial formation of the radical cation. Where the aryl
groups bear electron-withdrawing substituents, the posi-
tive charge will be localized in the diphenylvinyl moiety
as shown. Cyclization then leads to cation radical 31.
Ab initio computations using GAUSSIAN9411 at the
ROHF/3-21G level and with an NBO hybrid basis set12
revealed the interesting result that the positive charge
(11) Gaussian 94, Revision C.2: Frisch, M. J .; Trucks, G. W.;
Schlegel, H. B.; Gill, P. M. W.; J ohnson, B. G.; Robb, M. A.; Cheeseman,
J . R.; Keith, T.; Petersson, G. A.; Montgomery, J . A.; Raghavachari,
K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J . V.; Foresman, J . B.;
Cioslowski, J .; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.;
Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J . L.;
Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J .; Binkley, J . S.;
Defrees, D. J .; Baker, J .; Stewart, J . P.; Head-Gordon, M.; Gonzalez,
C.; Pople, J . A. Gaussian, Inc., Pittsburgh, PA, 1995.
(12) NBO 4.0. Glendening, E. D.; Badenhoop, J . K.; Reed, A. E.;
Carpenter, J . E.; Weinhold, F. Theoretical Chemistry Institute,
University of Wisconsin, Madison, WI, 1994.

Radical Cation Reactions of Bichromophoric Systems
J . Org. Chem., Vol. 61, No. 19, 1996 6529
Sch em e 3. Rea ction Mech a n ism for th e Lon g-Ra n ge Ar yl Migr a tion
Ar′
Ph
Ar′′
Ph
Ar′
27
28
H
NC
23
26
hν
benzophenone
NC
1
NC
Ph
1
1
Ph
1
Ar′
H
H
Ph
1
Ar′
1
H
Ar′
NC
39
40
43
44
41
42
p-Tol
An
tion to the terminal alkene carbon of the diarylvinyl
moiety. Of the several chromophores present in reac-
tants 23 and 26, the triplet energy of the diphenylvinyl
type moiety should be near the triplet energy of 1,1-
diphenylethylene (59 kcal/mol),14 while that of the cy-
anophenyl group should be near 77 kcal/mol (cf. T1 for
benzonitrile).15 With the diaryl chromophore locally
excited to T1, migration of the benzhydryl aryl group to
the â-carbon of this chromophore is reasonable. We note
that the free-valence of T1 styrene is maximal at the
â-carbon (0.82 vs 0.41 R),16 accounting for migration to
this site. The preference of cyanophenyl over phenyl is
understood on the basis of the enhanced delocalization
of the bridged triplet diradical 42 relative to the coun-
terpart lacking a cyano group at an odd-electron center.
This has analogy in the greater migratory aptitude of
p-cyanophenyl17 relative to phenyl as seen in the triplet
rearrangement of T1 of 4-(p-cyanophenyl)-4-phenylcyclo-
hexenone.
p-Tol
An
H
H
Ph
Ph
35
36
Ph
Ph
Ph
Ph
H
H
p-Tol
An
OMe
37
38
F igu r e 2. Four unreactive radical cation intermediate species.
If the cyclized radical-cation species, 31 or 32, is close
to the transition state for cyclization of the original
radical cation, then it is understandable why the cyano
substitution prefers the benzhydryl moiety in the reac-
tion.
The second aspect of interest is the lack of stereospeci-
ficity, giving rise to monocyano and dicyano triplet
photoproducts 28 and 27, respectively, independent of the
reactant stereochemistry. The lack of stereospecificity
signifies that T1 species 44 has a long enough lifetime to
permit free-rotation and equilibration of rotamers.
Another point of interest is the inhibited reactivity of
the p-anisyl diene 18 and the di-p-tolyl diene 11. In these
cases there is selective stabilization of these radical cation
reactants relative to the cyclized intermediates 35-38
(Figure 2). For one regioselectivity, 37 and 38, the
substituent, being originally located para, appears on the
heptatrienyl cationic group not bearing a positive charge.
For the other regioselectivity, the initial radical cation
stabilization by p-methyl and p-anisyl leads to species
35 and 36 in which these groups interact only with an
odd-electron moiety, odd-electron stabilization being less
helpful energetically than the cationic counterpart.
In agreement with the preceding discussion, we note
that the oxidation potentials9 for 1,1-dianisylethylene,
1,1-ditolylethylene, and diphenylethylene are 1.32, 1.66,
and 1.88 eV, respectively, with the reactive tetraphenyl-
diene 1, with a diphenylvinyl moiety, thus being of higher
energy than the ditolyl analog by 0.2 eV or ca. 6.4 kcal/
mol.
It is the third facet that is most remarkable, namely
the total stereoselectivity.18 Thus, it is the thermody-
namically less stable and more sterically encumbered
diastereomers in which aryl group substituted on car-
bon-7 of the benzonorbornenyl becomes anti to the benzo
ring and proximate to the sterically bulky gem-dimethyl
group. Note Figure 3. Both MM3 and AM1 computa-
tions were carried out on the diastereomeric photoprod-
ucts 27 (anti) vs 45 (syn) as well as 28 (anti) vs 46 (syn).
(13) Zimmerman, H. E.; Little, R. D. J . Am. Chem. Soc. 1974, 96,
5143-5152.
(14) Go¨rner, H. J . Phys. Chem. 1982, 86, 2028-2035.
(15) Wagner, P. J .; May, M. L. J . Phys. Chem. 1991, 95, 10317-
10321;
(16) Dictionary of π-Electron Calculations; Coulson, C. A., Streit-
wieser, A., Eds.; W. H. Freeman and Co.: San Francisco, 1965.
(17) Cf. Zimmerman, H. E.; Rieke, R. D.; Scheffer, J . R. J . Am. Chem.
Soc. 1967, 89, 2033-2047.
(18) “Stereoselectivity” and “stereospecificity” are used here in the
context suggested originally: Zimmerman, H. E.; Singer, L.; Thyaga-
rajan, B. S. J . Am. Chem. Soc. 1959, 81, 108-116.
Secon d a r y P h otoch em istr y Du e to T1. The triplet
aryl migration shown in eq 5 has precedent in some of
our previous studies.7,13 The reaction mechanism for this
rearrangement is given in Scheme 3. Three aspects are
particularly worthy of discussion. First, as noted above,
there is a preference for cyanophenyl over phenyl migra-

6530 J . Org. Chem., Vol. 61, No. 19, 1996
Zimmerman and Hoffacker
Ar′
Ph
Ar′
Ph
Ar′′
Ar′′
27, anti (84.50 kcal/mol)
28, anti (70.86 kcal/mol)
45, syn (81.48 kcal/mol)
46, syn (67.95 kcal/mol)
higher energy
kinetically preferred
lower energy
unobserved
F igu r e 3. Possible diastereoisomers. “Syn” and “anti” refer
to proximity to the benzo moiety. AM1 energies.
F igu r e 5. Anti conformer of open diradical 43 minus cyano.
Overlap between top lobe of p-orbital a and back lobe of
p-orbital b.
chair geometry. Independent of the starting conforma-
tion, convergence led to a rotational minimum in which
the back lobe of the orbital at center b (Figure 5) overlaps
more strongly with the p-orbital at center a than the front
lobe at center b does. This “anti” conformer has the aryl
F igu r e 4. Conformations of the triplet diradicals 47.
•
group of the Ar′CH moiety aiming away from the benzo
It was possible that the anti conformers of 43 (and 44)
of a precursor triplet of the general structure 47 might
exhibit a much larger rate of intersystem crossing to the
S0 diradical than the syn isomer. This singlet anti
diradical then would be constrained to close to the anti
product. Intersystem crossing (ISC) in such diradicals
is controlled mainly by spin-orbit coupling of the odd-
electrons with the angular orbital momentum of the
hybrids of the system. Spin-orbit coupling is a phenom-
enon we have studied from the theoretical viewpoint,2,19
and this would have been interesting experimentally. The
truncated diradical system in which the basic carbon
skeleton, minus substituents, is depicted in several
conformations in Figure 4.
ring, while the hydrogen at this center aims more toward
the benzo moiety. The effect is steric due to repulsion
•
between the Ar′CH hydrogen and the axial phenyl
group and also with repulsion between the aryl group
and the benzo ortho-hydrogen. This effect is seen only
relatively early in the reaction mechanism before the six-
membered ring has assumed the final boat conformation.
Nevertheless, there is a second mechanistic rationale.
This involves 1,3 bridging along with intersystem cross-
ing to afford the spiro intermediate 48. This strained
species may undergo a rearomatization via a ground-
state-allowed 1,3-sigmatropic rearrangement as shown
in eq 6.
However, spin-orbit coupling (SOC) computations on
this basic carbon skeleton as a function of relative
orientation of the two p-orbitals of the triplet diradical
indicated that SOC coupling was close to symmetric
relative to the rotational angle about bond 1,7 (note
Figure 4); here, we take a zero degree dihedral for the
conformer D in which the H-C-H is in the plane
containing the two odd-electron centers and perpendicu-
lar to the boat-shaped ring. However, spin-orbit cou-
pling for diradical conformers B and C in which the
p-orbital at C-7 is directed 45° away from the π-bond and
toward the π-bond, respectively, revealed little preference
NC
H
Ph
Ar′
Ar′′
Ar′
Ar′′
Ph
Ph
1
H
(6)
Ar′
1
48
27
43
Con clu sion . Organic photochemistry has many fac-
ets, e.g., solution photochemistry, solid state reactivity,
and electron transfer reactivity. This study has focused
on the last and demonstrated that, just as singlet and
triplet reactivity differ, radical cation reactivity provides
still another avenue for finding unique molecular behav-
ior.
with values of 2.83 and 3.07 cm-1
. If anything, the
slightly greater SOC for conformer C would predict
twisting of the aryl group at C-7 toward the benzo ring
in the full molecule.
Proceeding to other sources of stereoselectivity, we note
that this unusual behavior seems likely to arise from one
of two mechanistic possibilities. The first mechanism
Exp er im en ta l Section
•
involves a sequence of events in which the Ar′CH
Gen er a l P r oced u r es. All reactions were performed under
an atmosphere of anhydrous nitrogen unless otherwise indi-
cated. Melting points were determined in open capillaries with
a Meltemp heating block and are uncorrected. Anhydrous
magnesium sulfate was used as the drying agent. Neutral
workup refers to quenching the reaction with saturated
ammonium chloride, extracting with ether unless otherwise
specified, washing the organic layer with water and brine,
drying, filtering, and concentration in vacuo. Acidic workup
included a 2 M aqueous hydrochloric acid wash after ether
extraction. Basic workup included a saturated aqueous so-
dium bicarbonate wash after ether extraction. Column chro-
matography was performed with silica gel (Matheson, Coleman
and Bell, grade 62, 60-200 mesh) mixed with 1.0% (v/v)
Silvania 2282 phosphor and slurry-packed with hexane into
moiety becomes axial, initially in a chair conformation
(note Figure 5), followed by eventual formation of the boat
conformation required for benzonorbornene product for-
mation. At some point in this process, intersystem
crossing must occur. Simple MM320 and AM1 computa-
tions were implemented starting with axial conformers
•
of the Ar′CH group but with the six-membered ring
(19) Zimmerman, H. E.; Kutateladze, A. G. J . Org. Chem. 1995, 60,
6008-6009.
(20) (a) Allinger, N. L. MM3, University of Georgia, Dec 1990. (b)
Dewar, M. J . S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J . J . P. J . Am.
Chem. Soc. 1985, 107, 3902-3909.

Radical Cation Reactions of Bichromophoric Systems
J . Org. Chem., Vol. 61, No. 19, 1996 6531
quartz columns that were monitored with a hand-held UV
lamp. Plates (20 × 20 cm) for preparative thick-layer chro-
matography (TLC) were prepared with MN-Kieselgel G/UV
254 silica gel. Exploratory photolyses were carried out with
a Hanovia 450-W medium-pressure mercury lamp equipped
with a Pyrex (λ > 280 nm) 2-mm filter or with a 5-mm
recirculating filter solution of 0.07 M sodium metavanadate21
solution unless otherwise indicated. All photolysis experi-
ments were purged with deoxygenated and dried nitrogen for
1 h before and during photolysis.22 Acetonitrile was distilled
from calcium hydride prior to use. Tetrahydrofuran (THF) was
purified by successive distillation, under a nitrogen atmo-
sphere, from calcium hydride, lithium aluminum hydride, and
sodium benzophenone ketyl. Diethyl ether was freshly dis-
tilled from sodium benzophenone ketyl.
Gen er a l P r oced u r e for X-r a y Cr ysta llogr a p h y An a ly-
sis. X-ray diffraction data were collected on a Nicolet (Syntax)
P-1 diffractometer for single crystals of each compound. Unit
cell parameters were determined by least-squares refinement
of 25 reflections. Data were collected with three check
reflections monitored after every 97 reflections. Data having
F > 3σ(F) were rejected. Lorentz and polarization corrections
were applied, and each structure was solved under the
appropriate space group symmetry by direct methods using
SHELXSTL23 or SHELX93.24 Hydrogen atoms were calcu-
lated, and full-matrix least-squares refinement was carried out
employing anisotropic thermal parameters for all non-hydro-
gen atoms and isotropic thermal parameters for all hydrogen
atoms.28
of methyl 3,3-dimethyl-5,5-diphenyl-4-pentenoate in 10 mL of
THF. After 2 h at reflux, neutral workup afforded a slightly
yellow oil which was charged onto a 48 × 3.0 cm column. The
following fractions were collected: fraction 1, 500 mL of
hexane; fraction 2, 500 mL of 1% ether/hexane; fraction 3, 500
mL of 2% ether/hexane, 2.76 g (83%) of 1,1-bis(m-chlorophe-
nyl)-3,3-dimethyl-5,5-diphenyl-4-penten-1-ol (6) as a colorless
oil that was used without further purification.
The spectral data were the following: 1H NMR (CDCl3, 300
MHz) δ 7.5-7.0 (m, 18 H), 6.11 (s, 1 H), 3.12 (s, 1 H), 2.53 (s,
2 H), 0.81 (s, 6 H); IR (neat) 3510, 3060, 3020, 2955, 2920,
1590, 1575, 1470, 1445, 1420, 1165, 1155, 1075 cm-1; MS m/e
486.1517 (calcd for C31H28Cl2, 486.1521).
To 2.76 g (5.65 mmol) of 1,1-bis(m-chlorophenyl)-3,3-dim-
ethyl-5,5-diphenyl-4-penten-1-ol in 50 mL of refluxing pyridine
was added 4.0 mL (43 mmol) of phosphorus oxychloride. After
being refluxed for 16 h, the solution was poured into ice and
acidic workup afforded a yellow oil that was charged onto a
25 × 2.5 cm column eluted with 1% ether/hexane, and the
following fractions were collected: fraction 1, 750 mL, nil;
fraction 2, 350 mL, 2.17 g (82%) of 1,1-bis(m-chlorophenyl)-
3,3-dimethyl-5,5-diphenyl-1,4-pentadiene as an oil.
The spectral data were the following: 1H NMR (CDCl3, 300
MHz) δ 7.50-6.75 (m, 18 H), 5.94 (s, 1 H), 5.93 (s, 1 H), 1.04
(s, 6 H); IR (neat) 3055, 3020, 2960, 2925, 2865, 1590, 1560,
1490, 1470, 1445, 1420, 1415, 1275, 1155, 1130, 1095, 1075,
1030 cm-1; MS m/e 468.1427 (calcd for C31H26Cl2, 468.1412);
UV (CH3CN) λ(max) ) 252 (ꢀ ) 19 000).
1,1-Di-(m -tolyl)-3,3-d im eth yl-5,5-d ip h en yl-1,4-p en ta d i-
en e (10). To a solution of m-tolylmagnesium bromide pre-
pared from 3.93 g (23.0 mmol) of m-bromotoluene in 50 mL of
THF and 0.419 g (17.3 mmol) of magnesium turnings was
added dropwise 2.00 g (6.79 mmol) of methyl 3,3-dimethyl-
5,5-diphenyl-4-pentenoate in 20 mL of THF. After the mixture
was stirred for 30 min, neutral workup afforded a slightly
yellow oil that was charged onto a 45 × 3.0 cm column. The
following fractions were collected: fraction 1, 650 mL of
hexane, 29 mg of m-bromotoluene; fraction 2, 500 mL of 2%
ether/hexane, nil; fraction 3, 650 mL of 4% ether/hexane, 2.31
g of a yellow oil. Fraction 3 and 5 mg (3 µmol) of p-
toluenesulfonic acid monohydrate were dissolved in 60 mL of
benzene and refluxed with azeotropic removal of water for 10
h. Basic workup afforded 2.2 g of an oil that was charged onto
Meth yl 3,3-Dim eth yl-5,5-diph en yl-4-pen ten oate (5). On
the basis of the method of Zimmerman,25 to 34.0 g (0.18 mol)
of methyl 3,3-dimethylglutarate26 in 250 mL of THF was added
over 2 h a solution of phenyl Grignard prepared from 22.0 g
(0.90 mol) of magnesium turnings and 97 mL (0.92 mol) of
bromobenzene in 200 mL of THF. After the addition was
complete, the solution was refluxed for 2.5 h, cooled to rt,
quenched with saturated ammonium chloride, dried, filtered
through Celite, and concentrated in vacuo to give 53 g of 3,3-
dimethyl-5,5-diphenyl-5-hydroxypentanoic acid lactone (4) as
a white solid in quantitative yield and was used without
further purification.
The spectral data were the following: 1H NMR (CDCl3, 200
MHz) δ 7.28 (m, 10 H), 2.60 (s, 2 H), 2.19 (s, 2 H), 1.06 (s, 6
H); IR (thin film) 2960, 1745, 1440, 1240, 1145, 1000, 990 cm-1
;
a
49 × 3 cm column, and the following fractions were
MS m/e 280.1461 (calcd for C19H20O2, 280.1463).
collected: fraction 1, 1150 mL of hexane; fraction 2, 1100 mL
of 5% ether in hexanes, 1.75 g (60%) of 1,1-di-m-tolyl-3,3-
dimethyl-5,5-diphenyl-1,4-pentadiene as an oil.
A solution of 53 g of crude 3,3-dimethyl-5,5-diphenyl-5-
hydroxypentanoic acid lactone, 275 mL of methanol, 275 mL
of benzene, and 43.5 mL (0.783 mol) of concd sulfuric acid was
stirred mechanically for 4 d. Basic workup afforded 59 g of a
yellow oil that was passed through a 8 × 12 cm pad of silica
gel eluted with ether/hexane to give 50.00 g (94% based on
starting ester) of methyl 3,3-dimethyl-5,5-diphenyl-4-pen-
tenoate as a slightly yellow oil.
The spectral data were the following: 1H NMR (CDCl3, 300
MHz) δ 7.3-6.75 (m, 18 H), 5.95 (s, 1 H), 5.94 (s, 1 H), 2.27 (s,
3 H), 2.19 (s, 3 H), 1.02 (s, 6 H); IR (neat) 3055, 3020, 2960,
2920, 1600, 1490, 1445, 1375, 1360, 1130, 1075, 1035, 910, 885,
765, 700 cm-1; MS m/e 428.2503 (calcd for C33H32, 428.2504);
UV λ(max) ) 252 (ꢀ ) 21 000). Anal. Calcd for C33H32: C,
92.47; H, 7.53. Found: C, 91.93; H, 7.66.
1,1-Di-p -t olyl-3,3-d im et h yl-5,5-d ip h en yl-1,4-p en t a d i-
en e (11). To a solution of p-tolyllithium prepared from 4.07
g (23.8 mmol) of p-bromotoluene in 100 mL of diethyl ether
and 9.6 mL (20 mmol) of 2.1 M butyllithium in hexanes was
added dropwise 1.60 g (5.42 mmol) of methyl 3,3-dimethyl-
5,5-diphenyl-4-pentenoate in 50 mL of diethyl ether. After the
mixture was stirred for 3 h, neutral workup afforded 2.66 g of
an oil identified as 1,1-di-p-tolyl-3,3-dimethyl-5,5-diphenyl-4-
penten-1-ol, which was used without further purification.
The spectral data were the following: 1H NMR (CDCl3, 200
MHz) δ 7.4-7.1 (m, 10 H), 6.13 (s, 1 H), 3.63 (s, 3 H), 2.30 (s,
2 H), 1.04 (s, 6 H); IR (neat) 3055, 3020, 2955, 1950, 1735,
1595, 1445, 1325, 1165, 1200, 1230, 1030, 885, 760, 725, 705
cm-1; MS m/e 294.1594 (calcd for C20H22O2, 294.1620).
1,1-Bis(m -ch lor op h en yl)-3,3-d im eth yl-5,5-d ip h en yl-1,4-
p en ta d ien e (9). To a solution of (m-chlorophenyl)magnesium
bromide prepared from 3.39 g (17.8 mmol) of m-bromochlo-
robenzene in 50 mL of THF and 0.421 g (17.3 mmol) of
magnesium turnings was added dropwise 2.00 g (6.80 mmol)
The spectral data were the following: 1H NMR (CDCl3, 200
MHz) δ 7.5-7.0 (m, 18 H), 6.09 (s, 1 H), 2.75 (s, 1 H), 2.57 (s,
2 H), 2.25 (s, 6 H), 0.80 (s, 6 H); MS m/e 446.2604 (calcd for
C33H34O, 446.2610).
(21) Sodium metavanadate solution was prepared by dissolving the
appropriate amount of sodium metavanadate in 5% sodium hydroxide.
A 1 cm thick solution of 0.07 M sodium metavanadate cuts off more
than 99.9% of the light intensity with wavelengths shorter than 324
nm.
To the crude 1,1-di-p-tolyl-3,3-dimethyl-5,5-diphenyl-4-
penten-1-ol obtained above were added 50 mL of dry benzene
and 0.01 g (50 µmol) of p-toluenesulfonic acid monohydrate,
and the mixture was refluxed for 3 h with azeotropic removal
of water using a Dean-Stark trap. Basic workup afforded 2.32
g of a yellow oil that was charged onto a 32 × 2.5 cm column
and eluted with hexane to give a colorless oil that was
recrystallized from benzene to give 1.40 g (60%) of 1,1-di-p-
(22) Meites, L.; Meites, T. Anal. Chem. 1948, 20, 984-985.
(23) Sheldrick, G. M. SHELXTL PLUS, VERSION 4.2, Siemens
Analytical X-Ray Instrumentations, Inc., Madison, WI.
(24) Sheldrick, G. M. J . Appl. Crystallogr. Manuscript in prepara-
tion.
(25) Zimmerman, H. E.; Gruenbaum, W. T. J . Org. Chem. 1978, 43,
1997-2005.
(26) Babler, J . H.; Haack, R. A. Synth. Commun. 1983, 13, 905-
911.

6532 J . Org. Chem., Vol. 61, No. 19, 1996
Zimmerman and Hoffacker
tolyl-3,3-dimethyl-5,5-diphenyl-1,4-pentadiene as a white solid,
mp 67-70 °C.
workup afforded an oil that was charged onto a 19 × 3.5 cm
column and eluted with ether/hexane to give 5.13 g of a
mixture of (E)- and (Z)-3,3-dimethyl-1-(p-bromophenyl)-1,5,5-
triphenyl-1,4-pentadienes as a tacky solid that was used
without further purification.
The spectral data were the following: 1H NMR (CDCl3, 300
MHz) δ 7.7-7.4 (m, 38 H), 5.96 (s, 1 H), 5.95 (s, 1 H), 5.93 (s,
1 H), 5.91 (s, 1 H), 1.03 (s, 6 H), 1.02 (s, 6 H); IR (neat) 2960,
1735, 1595, 1490, 1445, 1070, 1015, 825, 760 cm-1; MS m/e
480.1282 (calcd for C31H27Br, 480.1274).
To 5.1 g (11 mmol) of (E)- and (Z)-3,3-dimethyl-1-(p-bromo-
phenyl)-1,5,5-triphenyl-1,4-pentadienes in 70 mL of N-meth-
ylpyrolidinone was added 1.698 g (19.0 mmol) of cuprous
cyanide, and the mixture was refluxed for 20 h. The cooled
solution was poured into 10 mL of ethylenediamine and
allowed to sit for 1 h. It was then extracted into chloroform
and washed with 10% ammonium hydroxide until the aqueous
layer was no longer blue, water, and brine, dried, filtered, and
concentrated in vacuo to give 4.83 g of a red oil. The oil was
chromatographed on a 19 × 3.5 cm column, and the following
fractions were collected: fraction 1, 400 mL of hexane and 500
mL of 5% ether/hexane, 2.03 g of an oily mixture; fraction 2,
250 mL of 8% ether/hexane, 2.75 g of an oil. Crystallization
of fractions 1 and 2 with pentane gave 1.23 g (27%) of (E)-3,3-
dimethyl-1-(p-cyanophenyl)-1,5,5-triphenyl-1,4-pentadiene, mp
136 °C. The structure of diene 17 was established by X-ray
crystallography.
The spectral data were the following: 1H NMR (CDCl3, 300
MHz) δ 7.50-6.90 (m, 19 H), 6.00 (s, 1 H), 5.96 (s, 1 H), 1.05
(s, 6 H); IR (thin film) 2915, 2225, 1600, 1495, 1445, 1155,
1120, 1070, 1030, 835, 760, 700 cm-1; MS m/e 425.2150 (calcd
for C32H27N, 425.2143). Anal. Calcd for C32H27N: C, 90.31;
H, 6.39. Found: C, 90.16; H, 6.60.
DCN-Sen sitized Ir r a d ia tion of 3,3-Dim eth yl-1,1,5,5-
tetr a p h en yl-1,4-p en ta d ien e (1). A solution of 0.189 g (0.472
mmol) of 3,3-dimethyl-1,1,5,5-tetraphenyl-1,4-pentadiene4 and
0.124 g (0.693 mmol) of 1,4-dicyanonaphthalene in 50 mL of
acetonitrile was irradiated through a Pyrex filter with a 100
W Hanovia lamp for a total of 9.5 h. The solvent was removed
in vacuo, and the crude photolysate was charged onto a 32 ×
2.5 cm column and eluted with hexanes. The following
fractions were collected: fraction 1, 1650 mL, nil; fraction 2,
500 mL, 85 mg of a yellow oil that contained starting material
and a product. The oil from fraction 2 was recrystallized from
hexanes to give 20 mg (11%) of 1-benzhydryl-1,2-dihydro-2,2-
dimethyl-4-phenylnaphthalene (22) as colorless crystals (mp
172.5-173 °C). The structure of dihydronaphthalene 22 was
established by X-ray crystallography.
The spectral data were the following: 1H NMR (CDCl3, 200
MHz) δ 7.50-6.70 (m, 19 H), 6.56 (d, J ) 1 Hz, 1 H), 4.55 (d,
J ) 8.6 Hz, 1 H), 3.43 (dd, J ) 8.6, 1 Hz, 1 H), 1.03 (s, 3 H),
0.97 (s, 3 H); MS m/e 400.2184 (calcd for C31H28, 400.2191).
Anal. Calcd for C31H28: C, 92.95; H, 7.05. Found: C, 93.38;
H, 7.27.
Sh or t DCA-Sen sit ized Ir r a d ia t ion of 1,1-Bis(p -cy-
an oph en yl)-3,3-dim eth yl-5,5-diph en yl-1,4-pen tadien e (21).
A solution of 0.104 g (0.232 mmol) of 1,1-bis(p-cyanophenyl)-
3,3-dimethyl-5,5-diphenyl-1,4-pentadiene,27 0.015 g (65 µmol)
of 9,10-dicyanoanthracene, and 42 mg (0.27 mmol) of biphenyl
in 200 mL of acetonitrile was irradiated through a solution
filter for a total of 45 min. The solvent was removed in vacuo,
the crude photolysate was charged onto a 39 × 3 cm column,
and the following fractions were collected: fraction 1, 475 mL
of 5% ether in hexane, 37 mg of biphenyl; fraction 2, 1 L of
20% ether in hexane, 98 mg of an oil. Fraction 2 was
crystallized from chloroform in hexane to give 90 mg (86%) of
1-[bis(p-cyanophenyl)methyl]-1,2-dihydro-2,2-dimethyl-4-phe-
nylnaphthalene (23) as colorless crystals, mp 283-284 °C. The
The spectral data were the following: 1H NMR (CDCl3, 200
MHz) δ 7.3-6.9 (m, 18 H), 5.93 (s, 1 H), 5.91 (s, 1 H), 2.28 (s,
3 H), 2.27 (s, 3 H), 1.02 (s, 6 H); IR (neat) 3050, 3020, 2990,
2920, 1510, 1495, 1445, 1180, 1230, 1110, 1075, 1030, 820, 700
cm-1; MS m/e 428.2509 (calcd for C33H32, 428.2504); UV λ(max)
) 252 (ꢀ ) 26 000).
1,1-Bis(m -cya n op h en yl)-3,3-d im eth yl-5,5-d ip h en yl-1,4-
p en ta d ien e (12). To 2.16 g (4.60 mmol) of 3,3-dimethyl-1,1-
bis(m-chlorophenyl)-5,5-diphenyl-1,4-pentadiene (9) in 25 mL
of N-methylpyrrolidinone was added 1.48 g (16.5 mmol) of
cuprous cyanide, and the mixture was refluxed for 24 h. The
cooled solution was poured into 20 mL of ethylenediamine and
allowed to sit for 30 min. It was then extracted into chloroform
and washed with 10% ammonium hydroxide until the aqueous
layer was no longer blue, water, and brine, dried, filtered, and
concentrated in vacuo to give 4.83 g of red oil. The oil was
charged onto a 37 × 3 cm column with the top 5 cm composed
of 1:1 silica gel/Norite and eluted with 2% ether in hexanes,
which yielded 256 mg (12%) of 1,1-bis(m-cyanophenyl)-3,3-
dimethyl-5,5-diphenyl-1,4-pentadiene as a colorless oil.
The spectral data were the following: 1H NMR (CDCl3, 300
MHz) δ 7.6-7.0 (m, 18 H), 6.02 (s, 1 H), 5.90 (s, 1 H), 1.06 (s,
6 H); IR (neat) 3060, 3025, 2925, 2865, 2230, 1595, 1575, 1170,
800, 760, 700 cm-1; MS m/e 450.2100 (calcd for C33H26N2,
450.2096); UV λ(max) ) 222 (ꢀ ) 65 000). Anal. Calcd for
C33H26N2: C, 87.97; H, 5.82. Found: C, 86.34; H, 6.15.
3,3-Dim eth yl-1,5,5-tr ip h en yl-4-p en ten -1-on e (15). To a
0 °C solution of phenylmagnesium bromide prepared from 13.0
mL (0.123 mol) of bromobenzene and 1.70 g (0.246 mol) of
lithium pieces in 150 mL of diethyl ether was added, via
cannula, a solution of 5.01 g (35.2 mmol) of 3,3-dimethylglu-
taric anhydride in 250 mL of diethyl ether, and the solution
was stirred at rt for 7 h. It was quenched by transferring the
solution via cannula to beneath the surface of 200 mL of a
vigorously shaken saturated ammonium chloride solution.
Neutral workup afforded a yellow oil that was refluxed for 20
h with 5.5 mg (3 µmol) of p-toluenesulfonic acid monohydrate
in 150 mL of benzene with azeotropic removal of water using
a Dean-Stark trap. Basic workup afforded an oil that was
charged onto a 18 × 3.5 cm column with the top 3 cm being
composed of 50/50 Norite/silica gel and eluted with 2% ether
in hexanes, which yielded 7.19 g (60%) of 3,3-dimethyl-1,5,5-
triphenyl-4-penten-1-one as a colorless oil.7
The spectral data were the following: 1H NMR (CDCl3, 300
MHz) δ 7.80-7.10 (m, 15 H), 6.23 (s, 1 H), 2.92 (s, 2 H), 1.14
(s, 6 H); IR (CHCl3) 3055, 3025, 2960, 2930, 2870, 1690, 1675,
1595, 1490, 1465, 1455, 1355, 1240, 1220, 1180, 1075, 1030,
1010; MS m/e 340.1834 (calcd for C25H24O, 340.1827).
3,3-Dim eth yl-1-(p-br om oph en yl)-1,5,5-tr iph en yl-4-pen t-
en -1-ol (16). To 7.8 mL (17.9 mmol) of 2.3 M butyllithium in
hexane and 50 mL of diethyl ether at 0 °C was added 4.21 g
(17.8 mmol) of p-dibromobenzene in 50 mL of ether via a
cannula. After 30 min, 5.13 g (15.1 mmol) of 3,3-dimethyl-
1,5,5-triphenyl-4-penten-1-one in 40 mL of diethyl ether was
added dropwise via cannula. After the mixture was stirred 2
h, neutral workup afforded a yellow oil that was charged onto
a 17 × 3.5 cm column, and the following fractions were
collected: fraction 1, 500 mL of 2% ether/hexane; fraction 2,
600 mL of 4% ether/hexane, 6.90 g of an oil. The oil was
crystallized from pentane and recrystallized from benzene/
hexane to give 5.44 g (93%) of 3,3-dimethyl-1-(p-bromophenyl)-
1,5,5-triphenyl-4-penten-1-ol as a white solid, mp 102-105 °C.
The spectral data were the following: 1H NMR (CDCl3, 300
MHz) δ 7.5-7.0 (m, 19 H), 6.12 (s, 1 H), 2.96 (s, 1 H), 2.57 (s,
2 H), 0.81 (s, 3 H), 0.79 (s, 3 H); IR (thin film) 3520, 3055,
2960, 1485, 1070, 1010, 910, 820, 760, 700 cm-1; MS m/e
496.1410 (calcd for C31H29OBr, 496.1401). Anal. Calcd for
C31H29OBr: C, 74.85; H, 5.88. Found: C, 75.30; H, 6.11.
(E)-3,3-Dim eth yl-1-(p-cya n op h en yl)-1,5,5-tr ip h en yl-1,4-
p en ta d ien e (17). To 5.62 g (11.7 mmol) of 3,3-dimethyl-1-
(p-bromophenyl)-1,5,5-triphenyl-4-penten-1-ol were added 100
mL of dry benzene and 0.1 g (0.5 mmol) of p-toluenesulfonic
acid monohydrate and the mixture refluxed for 24 h with
azeotropic removal of water using a Dean-Stark trap. Basic
(27) Zimmerman, H. E.; Cotter, B. R. J . Am. Chem. Soc. 1974, 96,
7445-7453.
(28) The author has deposited atomic coordinates for 17, 22-25, 26a ,
27, and 28 with the Cambridge Crystallographic Data Centre. The
coordinates can be obtained, on request, from the Director, Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ,
UK.

Radical Cation Reactions of Bichromophoric Systems
J . Org. Chem., Vol. 61, No. 19, 1996 6533
structure of dihydronaphthalene 23 was established by X-ray
crystallography.
following fractions were collected: fraction 1, 400 mL of hexane
and 900 mL of 1% ether in hexane, 90 mg of biphenyl; fraction
2, 300 mL of 1% ether in hexane, 316 mg of a yellow oil. The
oil from fraction 3 was placed on a silica gel TLC plate and
eluted twice with 4% ether in hexanes. The fastest moving
band (band 1) contained 63 mg of starting material. The
slower band (band 2) contained an oil that was crystallized
from pentane to give 241 mg (66%) of 1-[bis(p-chlorophenyl)-
methyl]-1,2-dihydro-2,2-dimethyl-4-phenylnaphthalene (25) as
colorless crystals, mp 142-143 °C. The structure of dihy-
dronaphthalene 25 was established by X-ray crystallography.
The spectral data were the following: 1H NMR (CDCl3, 200
MHz) δ 7.80-6.70 (m, 16 H), 6.52 (d, J ) 7 Hz, 1 H), 5.71 (d,
J ) 1.2 Hz, 1 H), 4.68 (d, J ) 8.7 Hz, 1 H), 3.41 (dd, J ) 8.8,
1 Hz, 1 H), 1.05 (s, 3 H), 0.93 (s, 3 H); IR (thin film) 2980,
2950, 2220, 1605, 1500, 1445, 1415, 1350, 840, 820, 770, 755,
700 cm-1; MS m/e 450.2098 (calcd for C33H26N2, 450.2096); UV
λ(max) ) 236 (ꢀ ) 25 000). Anal. Calcd for C33H26N2: C,
87.97; H, 5.82. Found: C, 88.12; H, 5.82.
Exten d ed DCA-Sen sitized Ir r a d ia tion of 1,1-Bis(p-
cya n op h e n yl)-3,3-d im e t h yl-5,5-d ip h e n yl-1,4-p e n t a d i-
en e (21). A solution of 162 mg (0.358 mmol) of 1,1-bis(p-
cyanophenyl)-3,3-dimethyl-5,5-diphenyl-1,4-pentadiene,27 20
mg (0.089 mmol) of 9,10-dicyanoanthracene, and 61 mg (0.396
mmol) of biphenyl in 200 mL of acetonitrile was irradiated
through a solution filter for a total of 2.5 h. The solvent was
removed in vacuo, the crude photolysate was charged onto a
33 × 3 cm column, and the following fractions were collected:
fraction 1, 500 mL of 1% ether in hexane, 60 mg of biphenyl;
fraction 2, 950 mL of 1% ether in hexane, nil; fraction 3, 400
mL of 50% ether in hexane, 140 mg of an oil. Fraction 3 was
placed on a silica gel TLC plate and eluted twice with 10%
ether in hexane, once with 15% ether in hexane, and once with
50% ether in hexane. The fastest moving band (band 1)
contained an oil that was crystallized from chloroform in
hexane to give 55 mg (34%) of 2,3-benzo-6-anti-7-anti-bis(p-
cyanophenyl)-5,5-dimethyl-1-phenylbicyclo[2.2.1]heptene (27)
as colorless crystals (mp 259-260 °C). The slower band (band
2) contained an oil that was crystallized from chloroform in
hexane to give 75 mg (46%) of 1-[bis(p-cyanophenyl)methyl]-
1,2-dihydro-2,2-dimethyl-4-phenylnaphthalene (23) as colorless
crystals, mp 283-284 °C. The structure of heptene 27 was
established by X-ray crystallography.
The spectral data were the following: 1H NMR (CDCl3, 200
MHz) δ 7.5-6.5 (m, 17 H), 5.68 (d, J ) 1.3 Hz, 1 H), 4.50 (d,
J ) 8.0 Hz, 1 H), 3.33 (dd, J ) 8.0, 1.3 Hz, 1 H), 1.03 (s, 3 H),
0.97 (s, 3 H); IR (thin film) 3020, 2960, 1490, 1090, 1015, 830,
760, 700 cm-1; MS m/e 468.1415 (calcd for C31H26Cl2, 468.1429).
Anal. Calcd for C31H26Cl2: C, 79.31; H, 5.58. Found: C, 79.33;
H, 5.81.
DCA-Sen sitized Ir r a d ia tion of 3,3-Dim eth yl-1-(p-cy-
a n op h en yl)-1,5,5-tr ip h en yl-1,4-p en ta d ien e (17). A solu-
tion of 250 mg (0.587 mmol) of 3,3-dimethyl-1-(p-cyanophenyl)-
1,5,5-triphenyl-1,4-pentadiene, 8.5 mg (37 µmol) of 9,10-
dicyanoanthracene, and 72 mg (0.47 mmol) of biphenyl in 180
mL of acetonitrile was irradiated through a solution filter for
a total of 9 h. The solvent was removed in vacuo, the crude
photolysate was charged onto a 42 × 3 cm column, and the
following fractions were collected: fraction 1, 1000 mL of
hexane and 800 mL of 2% ether in hexane, 37 mg of biphenyl;
fraction 2, 200 mL of 2% ether in hexane, 27 mg of a yellow
oil; fraction 3, 500 mL of 2% ether in hexane, 55 mg of an oil;
fraction 4, 1650 mL of 2% ether in hexane, 127 mg of DCA
and other unidentified materials. Fraction 2 was crystallized
from chloroform in hexane to give 25 mg (10%) of 2,3-benzo-
6-a nti-(p-cya nophenyl)-1,7-a nti-diphenyl-5,5-dimethylbi-
cyclo[2.2.1]heptene (28) as colorless crystals (mp 232-234.5
°C). Fraction 3 was dissolved in pentane and allowed to
crystallize. The first crop gave 16 mg (6%) of (RS/SR)-1-(p-
cyanophenyl)phenylmethyl]-1,2-dihydro-2,2-dimethyl-4-phe-
nylnaphthalene (26b) as a white solid (mp 190-192 °C), and
a second crop gave 35 mg (14%) of (RR/SS)-1-[(p-cyanophenyl)-
phenylmethyl]-1,2-dihydro-2,2-dimethyl-4-phenylnaphtha-
lene (26a ) as colorless crystals (mp 132-133 °C). The struc-
tures of heptene 28 and dihydronaphthalene 26a were
established by X-ray crystallography.
The spectral data for 2,3-benzo-6-anti-7-anti-bis(p-cyanophe-
nyl)-5,5-dimethyl-1-phenylbicyclo[2.2.1]heptene (27) were the
following: 1H NMR (CDCl3, 200 MHz) d 7.8-6.7 (m, 16 H),
6.25 (d, J ) 7.2 Hz, 1 H), 4.33 (s, 1 H), 3.87 (s, 1 H), 3.16 (s, 1
H), 0.85 (s, 3 H), 0.46 (s, 3 H); IR (thin film) 3055, 3040, 3005,
2965, 2225, 1605, 1505, 1465, 1460, 1180, 1120, 1035, 910, 835
cm-1; MS m/e 450.2058 (calcd for C31H26N2, 450.2096). Anal.
Calcd for C31H26N2: C, 87.97; H, 5.82. Found: C, 86.07; H,
5.92.
DCA-Sen sitized Ir r a d ia tion of 1,1-Bis(m -cya n op h e-
n yl)-3,3-d im eth yl-5,5-d ip h en yl-1,4-p en ta d ien e (12). A so-
lution of 0.125 g (0.278 mmol) of 1,1-bis(m-cyanophenyl)-3,3-
dimethyl-5,5-diphenyl-1,4-pentadiene, 3.4 mg (15 µmol) of 9,10-
dicyanoanthracene, and 32 mg (0.21 mmol) of biphenyl in 200
mL of acetonitrile was irradiated through a filter solution for
70 min. Solvent was removed in vacuo to give a yellow oil
that was charged onto a 31 × 3 cm column. The following
fractions were collected: fraction 1, 250 mL of 10% ether in
hexanes and 250 mL of 16% ether in hexanes, 20 mg of
biphenyl; fraction 2, 500 mL of 16% ether in hexanes, 35 mg
of starting material; fraction 3, 150 mL of 16% ether in
hexanes, 29 mg of a yellow oil. The oil was recrystallized from
chloroform in hexanes to give 24 mg (19%) of 1-[bis(m-
cyanophenyl)methyl]-1,2-dihydro-2,2-dimethyl-4-phenylnaph-
thalene (24) as colorless crystals, mp 246 °C. The structure
of dihydronaphthalene 24 was established by X-ray crystal-
lography.
The spectral data were the following: 1H NMR (CDCl3, 300
MHz) δ 7.8-6.7 (m, 16 H), 6.52 (d, J ) 7.5 Hz, 1 H), 5.73 (d,
J ) 1.1 Hz, 1 H), 4.65 (d, J ) 8.5 Hz, 1 H), 3.36 (dd, J ) 8.4,
1 Hz, 1 H), 1.06 (s, 3 H), 0.94 (s, 3H); IR (neat) 2230, 1485,
1440, 1350, 1075, 805, 765, 725, 700 cm-1; MS m/e 450.2091
(calcd for C33H26N2, 450.2096).
DCN-Sen sitized Ir r a d ia tion of 1,1-Bis(p-ch lor op h e-
n yl)-3,3-d im eth yl-5,5-d ip h en yl-1,4-p en ta d ien e (20). A so-
lution of 366 mg (0.777 mmol) of 1,1-bis(p-chlorophenyl)-3,3-
dimethyl-5,5-diphenyl-1,4-pentadiene,27 144 mg (0.550 mmol)
of 1,4-dicyanonaphthalene, and 0.098 g (0.64 mmol) of biphenyl
in 200 mL of acetonitrile was irradiated through Pyrex for a
total of 90 min. The solvent was removed in vacuo, the crude
photolysate was charged onto a 32 × 2.5 cm column, and the
The spectral data for 2,3-benzo-6-anti-(p-cyanophenyl)-1,7-
anti-diphenyl-5,5-dimethylbicyclo[2.2.1]heptene (28) were the
following: 1H NMR (CDCl3, 300 MHz) δ 7.8-6.8 (m, 17 H),
6.24 (d, J ) 7.2 Hz, 1 H), 4.36 (s, 1 H), 3.87 (s, 1 H), 3.13 (s, 1
H), 0.83 (s, 3 H), 0.50 (s, 3 H); IR (neat) 2225, 1735, 1455,
1270, 1155, 1035, 800 cm-1; MS m/e 425.2164 (calcd for
C32H27N, 425.2143). Anal. Calcd for C32H27N: C, 90.31; H,
6.39. Found: C, 90.44; H, 6.48.
The spectral data for (RS/SR)-1-(p-cyanophenyl)phenyl-
methyl)-1,2-dihydro-2,2-dimethyl-4-phenylnaphthalene (26b)
were the following: 1H NMR (CDCl3, 300 MHz) δ 7.8-6.65
(m, 17 H), 6.53 (d, J ) 7.2 Hz, 1 H), 5.68 (d, J ) 1.3 Hz, 1 H),
4.60 (d, J ) 8.9 Hz, 1 H), 3.39 (dd, J ) 8.7, 1 Hz, 1 H), 1.05 (s,
3 H), 0.93 (s, 3 H); IR (neat) 3020, 2960, 2225, 1605, 1495,
1445, 1215, 835, 755, 700 cm-1; MS m/e 425.2136 (calcd for
C33H32, 425.2143). Anal. Calcd for C32H27N: C, 90.31; H, 6.39.
Found: C, 90.00; H, 6.62.
The spectral data for (RR/SS)-1-[(p-cyanophenyl)phenyl-
methyl]-1,2-dihydro-2,2-dimethyl-4-phenylnaphthalene (26a )
were the following: 1H NMR (CDCl3, 300 MHz) δ 7.65-6.80
(m, 17 H), 6.55 (d, J ) 6.9 Hz, 1 H), 5.73 (d, J ) 1 Hz, 1 H),
4.63 (d, J ) 8.6 Hz, 1 H), 3.45 (dd, J ) 8.6, 1 Hz, 1 H), 1.10 (s,
3 H), 0.97 (s, 3 H); IR (neat) 3020, 2955, 2225, 1600, 1495,
1445, 1215, 835, 765, 700 cm-1; MS m/e 425.2141 (calcd for
C33H32, 425.2143). Anal. Calcd for C32H27N: C, 90.31; H, 6.39.
Found: 89.83; H, 6.44.
DCN-Sen sitized Ir r a d ia tion of 3,3-Dim eth yl-1-(p-cy-
a n op h en yl)-1,5,5-tr ip h en yl-1,4-p en ta d ien e (17). A solu-
tion of 0.430 g (1.01 mmol) of 3,3-dimethyl-1-(p-cyanophenyl)-
1,5,5-triphenyl-1,4-pentadiene, 182 mg (1.02 mmol) of 1,4-
dicyanonaphthalene, and 0.127 g (0.82 mmol) of biphenyl
in 200 mL of acetonitrile was irradiated through Pyrex for a

6534 J . Org. Chem., Vol. 61, No. 19, 1996
Zimmerman and Hoffacker
Ta ble 1. Resu lts of Op tim ized ROHF /3-21G Ca lcu la tion
In ter m ed ia te 31
total of 5.6 h. The solvent was removed in vacuo, the crude
photolysate was charged onto a 41 × 3 cm column, and the
following fractions were collected: fraction 1, 500 mL of hexane
and 200 mL of 2% ether in hexane, 101 mg of biphenyl; fraction
2, 500 mL of 2% ether in hexane, 159 mg (37%) of starting
material; fraction 3, 300 mL of 2% ether in hexane, 200 mg
(47%) of a 1:1 mixture of dihydronaphthalenes 26a and 26b
E(ROHF)
E(MCSCF)
SOC(RMS)
A
B
C
D
-267.461 21
-267.451 08
-267.448 32
-267.436 41
-267.510 15
-267.494 30
-267.494 26
-267.477 23
2.03
2.83
3.07
0.17
1
by H NMR.
Ben zop h en on e-Sen sitized Ir r a d ia tion of (RR/SS)-1-
[(p-Cyan oph en yl)ph en ylm eth yl]-1,2-dih ydr o-2,2-dim eth yl-
4-p h en yln a p h th a len e (26a ). A solution of 2.7 mg (6.0 µmol)
of (RR/SS)-1-[(p-cyanophenyl)phenylmethyl]-1,2-dihydro-2,2-
dimethyl-4-phenylnaphthalene and 2.5 mg (14 µmol) of ben-
zophenone in 2 mL of acetonitrile in an NMR tube was
irradiated through a filter solution for 4 h. After concentration
in vacuo the 1H NMR showed complete conversion to 2,3-benzo-
6-anti-(p-cyanophenyl)-1,7-anti-diphenyl-5,5-dimethylbicyclo-
[2.2.1]heptene (28) as the only observable product.
DCN-Sen sitized Ir r a d ia tion of 1,1-Bis(m -ch lor op h e-
n yl)-3,3-d im eth yl-5,5-d ip h en yl-1,4-p en ta d ien e (9). A so-
lution of 0.571 g (1.22 mmol) of 1,1-bis(m-chlorophenyl)-3,3-
dimethyl-5,5-diphenyl-1,4-pentadiene, 0.228 g (1.28 mmol) of
1,4-dicyanonaphthalene, and 186 mg (0.59 mmol) of biphenyl
in 200 mL of acetonitrile was irradiated through a Pyrex filter
for 2.5 h. Solvent was removed to give a yellow oil that was
charged onto a 47 × 2.5 cm column. The following fractions
were collected: fraction 1, 900 mL of hexanes, nil; fraction 2,
500 mL of hexanes and 150 mL of 2% ether in hexanes, 92 mg
of biphenyl; fraction 3, 600 mL of 2% ether in hexanes, 439
mg of a yellow oil. 1H NMR showed that fraction 3 contained
starting material and two products.
DCN-Sen sitized Ir r a d ia tion of 1,1-Di-m -tolyl-3,3-d im -
eth yl-5,5-d ip h en yl-1,4-p en ta d ien e (10). A solution of 305
mg (0.71 mmol) of 1,1-di-m-tolyl-3,3-dimethyl-5,5-diphenyl-1,4-
pentadiene, 131 mg (0.74 mmol) of 1,4-dicyanonaphthalene,
and 91 mg (0.59 mmol) of biphenyl in 220 mL of acetonitrile
was irradiated through a Pyrex filter for 1 h. Solvent was
removed to give a yellow oil that was charged onto a 40 × 3
cm column. The following fractions were collected: fraction
1, 750 mL of hexane and 500 mL of 1% ether in hexane, 75
mg of biphenyl; fraction 2, 300 mL of 1% ether in hexane, 196
mg of an oil. 1H NMR showed that fraction 3 contained
starting material and two products.
Ben zop h en on e-Sen sitized Ir r a d ia tion of (RS/SR)-1-
[(p-Cyan oph en yl)ph en ylm eth yl]-1,2-dih ydr o-2,2-dim eth yl-
4-p h en yln a p h th a len e (26b). A solution of 2.1 mg (4.9 µmol)
of (RS/SR)-1-[(p-cyanophenyl)phenylmethyl]-1,2-dihydro-2,2-
dimethyl-4-phenylnaphthalene and 2.1 mg (12 µmol) of ben-
zophenone in 2 mL of acetonitrile in an NMR tube was
irradiated through a filter solution for 2 h. After concentration
1
in vacuo the H NMR showed a 40% conversion to 2,3-benzo-
6-anti-(p-cyanophenyl)-1,7-anti-diphenyl-5,5-dimethylbicyclo-
[2.2.1]heptene (28) as the only observable product.
Com p u ta tion s. Structures for intermediates 31 and 32
were obtained with full geometry optimization with the
Gaussian94 package of programs11 using ROHF/3-21G (Table
1). Atomic charges and eigenvectors were obtained using
Weinhold’s NBO analysis.12 1-Electron spin-orbit coupling
constants for diradical 47 were obtained by first geometry
optimizing the diradical while holding the diradical centers
approximately 2 Å apart using Gamess with ROHF/sto-3g.
Single point calculations were then performed on structures
obtained from the optimized structure by rotating the exo-
methylene group. Eigenvectors from ROHF/sto-3g calculations
were used as starting points for mcscf calculations. Spin-
orbit coupling for each conformation was then calculated using
the energies and spin-orbit coupling for four conformations
of diradical 47 (cf. Table 1).
DCN-Sen sitized Ir r a d ia tion of 1,1-Bis(p-m eth oxyp h e-
n yl)-3,3-d im eth yl-5,5-d ip h en yl-1,4-p en ta d ien e (18). A so-
lution of 0.504 g (1.10 mmol) of 1,1-bis(p-methoxyphenyl)-3,3-
dimethyl-5,5-diphenyl-1,4-pentadiene24 and 0.391 g (2.19 mmol)
of 1,4-dicyanonaphthalene in 200 mL of acetonitrile was
irradiated through a Pyrex filter for a total of 19 h. 1H NMR
showed no products. The crude photolysate was charged onto
a 59 × 2 cm column, and elution with 5.5 L of 0.5% ether in
hexanes yielded 382 mg (76%) of starting material.
Ben zop h en on e-Sen sitized Ir r a d ia tion of 1-[Bis(p-cy-
a n op h en yl)m et h yl]-1,2-d ih yd r o-2,2-d im et h yl-4-p h en yl-
n a p h th a len e (23). A solution of 4.3 mg (9.5 µmol) of 1-[bis(p-
cya nophenyl)methyl]-1,2-dihydro-2,2-dimethyl-4-phenyl-
naphthalene and 4.2 mg (23 µmol) of benzophenone in 2 mL
of acetonitrile in an NMR tube was irradiated through a filter
Ack n ow led gm en t. Support of this research by the
National Science Foundation is acknowledged with
appreciation especially for its concern in supporting
basic studies. Assistance and advice in X-ray structure
elucidation by Dr. Douglas Powell were of real help and
are acknowledged.
1
solution for 2.3 h. After concentration in vacuo the H NMR
showed a complete conversion to 2,3-benzo-6-anti-7-anti-bis-
(p-cyanophenyl)-5,5-dimethyl-1-phenylbicyclo[2.2.1]heptene (27)
as the only observable product.
J O960467L