Photoinduced electron-transfer bicyclopropenyl-benzene rearrangements of 2,2′,3,3′-tetraphenylbicyclopropenyls: A new mechanism via Dewar benzene

Article abstract of DOI:10.1021/ol049917j

(Equation presented) The 9,10-dicyanoanthracene-sensitized photoreaction of 1-methyl- and 1.1′-dimethyl-2,2′,3,3′- tetraphenylbicyclopropenyl gives the corresponding benzene and Dewar benzene derivatives, indicating that their photoinduced electron-transfer bicyclopropenyl-benzene rearrangements proceed via Dewar benzenes.

Full text of DOI:10.1021/ol049917j

ORGANIC
LETTERS
2004
Vol. 6, No. 6
1029-1032
Photoinduced Electron-Transfer
Bicyclopropenyl-benzene
Rearrangements of
2,2′,3,3′-Tetraphenylbicyclopropenyls:
A New Mechanism via Dewar Benzene
Hiroshi Ikeda,* Yosuke Hoshi, Yuka Kikuchi, Futoshi Tanaka, and
Tsutomu Miyashi
Department of Chemistry, Graduate School of Science, Tohoku UniVersity,
Sendai 980-8578, Japan
ikeda@org.chem.tohoku.ac.jp
Received January 14, 2004
ABSTRACT
The 9,10-dicyanoanthracene-sensitized photoreaction of 1-methyl- and 1,1′-dimethyl-2,2′,3,3′-tetraphenylbicyclopropenyl gives the corresponding
benzene and Dewar benzene derivatives, indicating that their photoinduced electron-transfer bicyclopropenyl-benzene rearrangements proceed
via Dewar benzenes.
According to theoretical calculations,1 bicyclopropenyl is the
highest in energy among all valence isomers of benzene.
Since the pioneering work of Breslow2a first demonstrated
that, on heating, hexaphenylbicyclopropenyl underwent
isomerizatoin to give hexaphenylbenzene, several reaction
mechanisms were proposed for the bicyclopropenyl-benzene
rearrangements, which were achieved by thermolyses,2 Ag+-
catalyzed reactions,3 and photolyses.4 The rearrangement can
be also triggered by photoinduced electron transfer (PET).
A CIDNP study5 of the p-chloranil-sensitized photoreaction
of 1,1′-dimethylbicyclopropenyl (1) revealed that the initially
formed tricyclohexane radical cation 2•+ rearranges to
dimethylbenzvalene radical cations, which, in turn, rearrange
to o- and m-xylenes. The 9,10-dicyanoanthracene (DCA)-
sensitized photoreaction of 1,1′-dimethyl-2,2′,3,3′-tetraphe-
nylbicyclopropenyl (3a) in acetonitrile was reported6 to form
1,2-dimethyl-3,4,5,6-tetraphenylbenzene (4a) exclusively via
a tricyclohexane radical cation intermediate 5a•+. To gain
further insight into the mechanistic pathway7 for 3, we
reinvestigated the DCA-sensitized photoreactions of 3a as
well as 1-methyl-2,2′,3,3′-tetraphenylbicyclopropenyl (3b).2c
Here, we report that the intrinsic process is the formation of
(3) Ag+-catalyzed reactions: (a) Weiss, A.; Schlierf, C. Angew. Chem.,
Int. Ed. Engl. 1971, 10, 811; Angew. Chem. 1971, 83, 887. (b) Weiss, R.;
Andrae, S. Angew. Chem., Int. Ed. Engl. 1973, 12, 150-152; Angew. Chem.
1973, 85, 145-147. (c) Weiss, R.; Andrae, S. Angew. Chem., Int. Ed. Engl.
1973, 12, 152-153; Angew. Chem. 1973, 85, 147-148. (d) Wolf, W. H.
d.; Landheer, I. J.; Bickelhaupt, F. Tetrahedron Lett. 1975, 179-182. (e)
Landheer, I. J.; Wolf, W. H. d.; Bickelhaupt, F. Tetrahedron Lett. 1975,
349-352.
(1) (a) Schulman, J. M.; Disch, R. L. J. Am. Chem. Soc. 1985, 107, 5059-
5061. (b) Li, Z.; Rogers, D. W.; McLafferty, F. J.; Mandziuk, M.; Podosenin,
A. V. J. Phys. Chem. A 1999, 103, 426-430.
(2) Thermolyses: (a) Breslow, R.; Gal, P. J. Am. Chem. Soc. 1959, 81,
4747-4748. (b) Breslow, R.; Gal, P.; Chang, H. W.; Altman, L. J. J. Am.
Chem. Soc. 1965, 87, 5139-5144. (c) Weiss, R.; Ko¨bl, H. J. Am. Chem.
Soc. 1975, 97, 3224-3225. (d) Turro, N. J.; Schuster, G. B.; Bergman, R.
G.; Shea, K. J.; Davis, J. H. J. Am. Chem. Soc. 1975, 97, 4758-4760. (e)
Davis, J. H.; Shea, K. J.; Bergman, R. G. Angew. Chem., Int. Ed. Engl.
1976, 15, 232-234; Angew. Chem. 1976, 88, 254-255. (f) Davis, J. H.;
Shea, K. J.; Bergman, R. G. J. Am. Chem. Soc. 1977, 99, 1499-1507. See
also refs 3b,c.
(4) Photolyses: Weiss, R.; Ko¨bl, H. J. Am. Chem. Soc. 1975, 97, 3222-
3224. See also refs 2a,b.
(5) Abelt, C. J.; Roth, H. D. J. Am. Chem. Soc. 1985, 107, 3840-3843.
(6) Padwa, A.; Goldstein, S. I.; Rosenthal, R. J. J. Org. Chem. 1987,
52, 3278-3285.
10.1021/ol049917j CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/20/2004

two types of Dewar benzenes9 6 and 7 (Scheme 1), which
cannot be accounted for by a tricyclohexane radical cation
intermediate like 5•+.
Figure 1. Time-dependent changes in the product ratios in the
DCA-sensitized PET reaction of 3b (0.1 M) in degassed dichlo-
romethane-d2 at 20 °C. The product yields were determined using
1H NMR. The material balance is 100%.
Scheme 1. DCA-Sensitized PET Reaction of 3
Actually, the independent PET reactions of 6b and 7b result
in the quantitative recovery of 6b and formation of 4b,
respectively. Note that 7b rearranges to 4b slowly, even at
20 °C in the dark. Therefore, the formation of Dewar
benzenes 6 and 7 excludes the intermediacy of the tricyclo-
hexane radical cation 5•+.
A plausible mechanism for formation of 3b via Dewar
benzene derivatives is depicted in Scheme 2. The ring
cleavage of the cyclopropene C1′-C2′ (or C1′-C3′) bond
The anodic peak oxidation potentials (Epa)10 of 3a (+1.24
V vs SCE in dichloromethane) and 3b (+1.22 V) are low
enough to quench the excited singlet state of DCA exer-
gonically, as suggested by their respective calculated free
energy changes for electron transfer,11 ∆Get ) -1.00 and
-1.02 eV, in dichloromethane. In agreement with the
estimate, 3a and 3b quench the fluorescence of DCA
efficiently with respective rate constants of kq ) 1.9 and 1.8
× 1010 M-1 s-1 in dichloromethane.
Scheme 2. Plausible Mechanism for the PET
Bicyclopropenyl-benzene Rearrangement of 3ba
The DCA-sensitized photoreactions of 3a (λ > 410 nm)
in degassed dichloromethane-d2 at 20 °C differ from previ-
ously reported results6 in acetonitrile. As shown in Scheme
1, Dewar benzenes9 6a and 7a are formed together with 4a
in a product ratio that significantly depends on irradiation
time. At 86% conversion (10 h), 4a, 6a, and 7a are formed
in 46, 24, and 14% yields, respectively, but prolonged
irradiation (30 h) results in the formation of a mixture of 4a
(72%) and 6a (28%).14 The fact that the yield of 4a increases
while that of 7a decreases and that of 6a does not change
suggests that 7a readily rearranges to 4a, while 6a is stable
under the conditions employed. Similarly, the DCA-
sensitized photoreaction of 3b in dichloromethane-d2 at 20
°C gives 4b (>+2.0 V), 6b3b (+1.48 V), and 7b (+1.38 V)
in 35, 18, and 41% yields, respectively, at 94% conversion
of 3b. Figure 1 shows the time-dependent changes of the
product ratios observed during irradiation, suggesting that
the immediate precursor of 4b is Dewar benzene 7b.
(7) The real structures of Dewar benzene derivatives and their mechanism
of formation in the electron-transfer reactions of bicyclopropenyl or
cyclopropenyl cation remain controversial. See ref 8.
(8) Komatsu, K.; Kitagawa, T. Chem. ReV. 2003, 103, 1371-1427.
(9) Weiss and co-workers were the first to isolate Dewar benzene
derivatives and demonstrated their contribution in thermal, Ag+-catalyzed,
and photolytic bicyclopropenyl-benzene rearrangements. See refs 2c, 3a-
c, and 4.
a Conditions: (i) hν/DCA, (ii) back-electron-transfer from DCA•-,
(iii) ∆ at 20 °C, (iv) hole transfer.
1030
Org. Lett., Vol. 6, No. 6, 2004

associated with the ring expansion of the other cyclopropene
ring of 3b•+ forms a σ radical-allyl cation intermediate 8b•+.
Whether this Wagner-Meerwein-type rearrangement occurs
stepwise15 via an intermediate 11b•+ or concertedly via the
transition state [3b•+]TS, the two-bond cleavage process to
form 8b•+ is essentially the same as those in the Ag+-
catalyzed reactions and thermolyses of bicyclopropenyls
proposed by Weiss3b,c,9 and Bergman.2e,f Dewar benzene 6b
is formed via the ring closure A in 8b•+ to 6b•+ followed by
back electron transfer (BET) from DCA•-. Radical cation
7b•+ formed by competitive ring closure B rearranges to 4b•+.
Products 7b and 4b are formed after BET in 7b•+ and 4b•+,
respectively. Dewar benzene 7b with the phenyl group at
the sp3 carbon rearranges readily to 4b under the PET
conditions employed.
Table 1. Sum of the Calculated Partial Spin (F) and Charge
(q) Densities of 3a•+ (C2h) and 3b•+ (Cs)
MDPC
ΣF Σq
MDPC′ or DPC
ΣF
Σq
3a •+ AM1/ROHF
AM1/UHF
+0.50 +0.50 +0.50
+0.50 +0.50 +0.50
+0.50 +0.50 +0.50
+0.50
+0.50
+0.50
+0.50
+0.83
+0.88
+0.89
+0.83
PM3/UHF
ROB3LYP/6-31G(p) +0.50 +0.50 +0.50
3b•+ AM1/ROHF
+0.12 +0.17 +0.88
+0.05 +0.12 +0.95
+0.05 +0.11 +0.95
AM1/UHF
PM3/UHF
ROB3LYP/6-31G(p) +0.06 +0.17 +0.94
Radical cation 3b•+ can form formally alternative σ radical-
allyl cation 9b•+, which would give 6b, 4b, and the
unobserved Dewar benzene 10b. Because 10b with the
phenyl and methyl groups at the sp3 carbons would be
probably more unstable than 7b under the condition em-
ployed, the fact that 10b is not found during the reaction
does not necessarily indicate that 9b•+ is not involved with
the reaction of 3b. However, a preferential formation of 8b•+
over 9b•+ is expected from the point of view of thermody-
namic stability: 8b•+ with a tertiary allylic cation is more
stable than 9b•+ containing a secondary allylic cation.
The formation of 6b and 7b from the unsymmetrical
bicyclopropenyl 3b apparently requires the regiospecific C1′-
C2′ (or C1′-C3′) cleavage of 3b•+, in which the unpaired
electron and positive charge are predominantly localized on
the diphenylcyclopropyl (DPC) subunit, the cyclopropene
ring without a methyl group (Figure 2). To gain further
insight into the electronic structure of 3a•+ and 3b•+, we
conducted semiempirical and DFT calculations.17,22
diphenylcyclopropyl (MDPC) subunits of 3a•+ and for the
MDPC and DPC subunits in 3b•+. Both ΣF and Σq values
for the MDPC subunit of 3a•+ are +0.50, regardless of the
method of calculation. Those for another MDPC subunit
(MDPC′) are accordingly +0.50. In sharp contrast, an
unsymmetrical electronic structure is suggested for 3b•+. The
ΣF and Σq values for the DPC subunits of 3b•+ are
+0.88∼0.95 and +0.83∼0.89, respectively: the residual spin
and charge are suggested to locate in the MDPC subunit.
Results of the calculation indicate that the unpaired electron
and positive charge of 3b•+ are delocalized predominantly
on the DPC, the less electron-donating subunit, while they
are distributed evenly over two MDPC subunits of 3a•+.
Figure 3 represents contrasting SOMOs of 3a•+ and 3b•+
Figure 3. Representation of the SOMOs of 3a•+ (left, C2h) and
3b•+ (right, Cs) calculated by ROB3LYP/6-31G(p).
calculated by ROB3LYP/6-31G(p). Thus, the semiempirical
and DFT calculations support the characteristic electronic
structure of 3b•+. If this estimate is appropriate, the fact that
the isomeric Dewar benzene 10b expected from the alterna-
tive radical cation intermediate 9b•+ is not observed during
Figure 2. Definition of two subunits in 3•+.
Table 1 shows the sum of the calculated partial spin and
charge density, ΣF and Σq, respectively, for two methyl-
(14) We observed also that a similar photoreaction (1.5 h) of 3a in
acetonitrile at 20 °C gave 4a and 6a in 83 and 17% yields, respectively.
(15) For the bond cleavage of cyclopropene radical cations, see ref 16.
(16) Padwa, A.; Chou, C. S.; Rieker, W. F. J. Org. Chem. 1980, 45,
4555-4564.
(10) Values of Epa were measured by cyclic voltammetry (Pt electrode,
-
scan rate 100 mV/s) in dry dichloromethane with 0.1 M n-Bu4N+BF4 as
a supporting electrolyte. All substrates gave irreversible waves.
(17) Semiempirical and DFT calculation was carried out at C2h (3a•+
)
(11) Values of ∆Get were estimated by using the Rehm-Weller equation:
or Cs (3b•+) symmetry using PC GAMESS ver. 6.3,18 WinMOPAC2002,19
and Gaussian9820 softwares. Figure 3 was drawn using MOLEKEL
software.21
∆Get ) Eox1/2(3) - Ered1/2(DCA) - E0-0(DCA) - e2/ꢀr, where
12
Ered1/2(DCA) ) -0.89 V,13 E0-0(DCA) ) 2.87 eV, and e2/ꢀr ) 0.23 eV in
dichloromethane. The halfwave oxidation potentials (Eox1/2) were obtained
as Epa - 0.03 V, assuming a one-electron oxidation process.
(12) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259-271.
(13) Ikeda, H.; Minegishi, T.; Takahashi, Y.; Miyashi, T. Tetrahedron
Lett. 1996, 37, 4377-4380.
(18) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.;
Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.;
Su, S. J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem.
1993, 14, 1347-1363.
(19) MOPAC2002; Stewart, J. J. P. Fujitsu, Ltd.: Tokyo, Japan, 2002.
Org. Lett., Vol. 6, No. 6, 2004
1031

irradiation is not inconsistent with the unique electronic
structure of 3b•+. According to orbital interaction theory,24
the characteristic electronic structure of 3b•+ is probably due
to the electronic coupling of two cyclopropene rings through
space and through two C-C σ-bonds (C1′-C1-Me). Similar
electronic couplings were suggested by theoretical calcula-
tions for the neutral forms of the parent bicyclopropenyl and
related compounds.25
The high efficiency of the PET reaction is another feature
of 3b. The corrected quantum efficiency (Φc)26 per ion radical
pair [3b•+/DCA•-] for the formation of 4b, 6b, and 7b at a
10 mM concentration of 3b is 1.4, indicating a chain
mechanism for the photoreaction of 3b. The hole transfer
(HT) from 4b•+ to 3b with a negative free energy change
(∆Ght < -0.81 eV) is a key process. If the rate of HT (kht)
is close to the diffusion control rate (2.2 × 1010 M-1 s-1 at
20 °C), the pseudo-first-order rate constant (kht[3b]) at the
initial stage is calculated to be 2.2 × 108 s-1, which is faster
than kbet < 7.6 × 107 s-1 for the BET27 from DCA•- to 4b•+.
Thus, 4b•+ acts as a carrier and undergoes HT much faster
than 6b•+ and 7b•+, which suffer BET30 from DCA•- much
faster than 4b•+. Interestingly, similar thermodynamics and
kinetics are expected for 4a•+, but the Φc of 3a is as low as
0.13. The remarkable difference in Φc between 3a and 3b
is probably due to the extreme distribution of the unpaired
electron and positive charge in 3b•+, which facilitates the
Wagner-Meerwein-type rearrangement of 3b•+ to 8b•+.
In conclusion, our experimental results first demonstrate
a new mechanism via Dewar benzene for the PET bicyclo-
propenyl-benzene rearrangements of tetraphenyl-substituted
bicyclopropenyl derivatives. This is in line with the mech-
anisms proposed by Weiss and Bergman for the Ag+-
catalyzed reactions and thermolyses. Consequently, this work
proposes a topologically common reaction pathway via
Dewar benzene that is applicable to thermolyses, Ag+-
catalyzed reactions, and PET reactions of tetraphenyl-
substituted bicyclopropenyl derivatives.
Acknowledgment. We gratefully acknowledge the fi-
nancial support of a Grant-in-Aid for Scientific Research on
Priority Areas (417) and others (Nos. 12440173 and 14050008)
from the Ministry of Education, Culture, Sports, Science,
and Technology (MEXT) of Japan.
Supporting Information Available: Experimental details
including physical data for 6 and 7 and Cartesian coordinates
and ΣF and Σq values for 3a•+ and 3b•+ (ROB3LYP/6-31G-
(p)). This material is available free of charge via the Internet
at http://pubs.acs.org.
OL049917J
(20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.11.4; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(21) MOLEKEL 4.3; Flukiger, P.; Luthi, H. P.; Portmann, S.; Weber, J.;
Swiss Center for Scientific Computing: Manno, Switzerland, 2002.
(22) For the ab initio calculation of the related interaction complex
between cyclopropenyl cation and cyclopropenenyl radical, see ref 23.
(23) Raghavachari, K.; Roth, H. D. J. Am. Chem. Soc. 1989, 111, 7132-
7136.
(27) Rate constant (kbet) for the BET from DCA•- to 4b•+ at 20 °C in
dichloromethane was calculated using the following equations (1,28 2,28 and
3) and parameters reported by Kikuchi and co-workers:29
(λs + ∆Gbet + whV)2
∞
1/2
4π3
e-sSw
2
kbet
)
|V|
exp
-
(1)
∑
2
(
)
(
)
{
}
W!
4λskbT
h λskbT
w)0
S ) λv/hν
(2)
(3)
∆Gbet ) -[Eox1/2(4b) - Ered1/2(DCA) - e2/ꢀr]
where parameters V, λs, λv, ν, and ∆Gbet are, respectively, the electronic
coupling matrix element (18 cm-1), solvent reorganization energy (1.0 eV),
vibration reorganization energy (0.3 eV), single average frequency (1500
cm-1), and free energy change for electron-transfer process. In addition, h,
kb, and T, are Planck’s constant, Boltzmann’s constant, and the temperature
(293 K), respectively.
(28) (a) Miller, J. R.; Beitz, J. V.; Huddleston, R. K. J. Am. Chem. Soc.
1984, 106, 5057-5068. (b) Siders, P.; Marcus, R. A. J. Am. Chem. Soc.
1981, 103, 741-747. (c) Siders, P.; Marcus, R. A. J. Am. Chem. Soc. 1981,
103, 748-752. (d) Van Duyne, R. P.; Fischer, S. F. Chem. Phys. 1974, 5,
183-197. (e) Ulstrup, J.; Jortner, J. J. Chem. Phys. 1975, 63, 4358-4368.
(29) Niwa, T.; Kikuchi, K.; Matsushita, N.; Hayashi, M.; Katagiri, T.;
Takahashi, Y.; Miyashi, T. J. Phys. Chem. 1993, 97, 11960-11964.
(30) Values of kbet for the BET from DCA•- to 6b•+ and 7b•+ were
calculated to be 3.5 and 6.2 × 109 s-1, respectively.
(24) Rauk, A. Orbital Interaction Theory of Organic Chemistry; Wiley:
New York 1994; Chapter 3, pp 57-93.
(25) (a) Greenberg, A.; Liebman, J. F. J. Am. Chem. Soc. 1981, 103,
44-48. (b) Spanget-Larsen, J.; Korswagen, C. d.; Eckert-Maksic, M.;
Gleiter, R. HelV. Chim. Acta 1982, 65, 968-982.
(26) Ikeda, H.; Minegishi, T.; Abe, H.; Konno, A.; Goodman, J. L.;
Miyashi, T. J. Am. Chem. Soc. 1998, 120, 87-95.
1032
Org. Lett., Vol. 6, No. 6, 2004