3882 J. Am. Chem. Soc., Vol. 123, No. 17, 2001
Lewis et al.
Table 3. Calculated Ground State (S0), Lowest Singlet State (S1),
Excitation (∆E0,1) Energies, and Observed Absorption Maxima
(λmax
)
S0,a
kcal/mol
S1,b
kcal/mol
∆E0,1
,
λmax,
nm
kcal/mol (nm)
c-2a
HP-2a
i-2a
P-2a
i-2b
79.4
92.5
69.1
61.8
75.7
174.8
161.4
154.8
149.2
149.8
95.4 (300)
68.9 (415)
85.7 (333)
87.4 (327)
74.1 (386)
270
470c
378
348
a Calculated using MOPAC/AM1. b Calculated using ZINDO. c Val-
ue for HP-6.
This intermediate does not undergo further reaction and reverts
to c-6 either thermally or upon long-wavelength irradiation. The
4b-methyl group in HP-6 presumably prevents rearrangement
to a dihydropyridine intermediate analogous to i-2a. Irradiation
of 2,4,6-trimethylstilbene is reported to result in highly inef-
ficient conversion to 1,3-dimethylphenanthrene, a process that
requires the loss of methane.3,30 The absorption maximum for
HP-6 is at substantially longer wavelength than that for the
intermediate formed upon anaerobic irradiation of 2 (Figure 1),
providing additional evidence that the latter species is not the
primary photoproduct HP-2a (Scheme 2).
Rearrangements of 4a,4b-dihydrophenanthrene intermediates
to more stable dihydrophenanthrene products have been ob-
served to occur via both unimolecular and catalyzed mecha-
nisms. Ho and co-workers9,31,32 have observed the occurrence
of 1,9-hydrogen shifts following anaerobic photocyclization of
a number of stilbenes and related diarylethylenes. The primary
dihydro intermediates formed upon photocyclization of aryl
vinyl sulfides and 2-vinylbiphenyls are reported to undergo 1,4-
and 1,5-hydrogen shifts, respectively.33,34 Thus there is ample
precedent for the occurrence of intramolecular 1,n-hydrogen
shifts, including those which seemingly violate the selection
rules for thermal suprafacial sigmatropic rearrangements.35
Amine-catalyzed rearrangements of 4a,4b-dihydrophenanthrenes
to 1,4-dihydrophenanthrenes are believed to occur via sequential
1,3-hydrogen transfer steps.7,8,36 The formation of 9,10-dihydro-
9,10-dicyanostilbene upon irradiation of R,R′-dicyanostilbene
6,37 requires two 1,3-hydrogen transfer steps and is proposed to
occur via a radical chain mechanism.3 We observe that the
formation of i-2a upon irradiation of c-2 is not influenced by
the presence of added propylamine and conclude that it is
formed by a unimolecular hydrogen shift. Whether the hydrogen
shift is a concerted 1,7-process or occurs via two or more
sequential shifts remains to be established.
Figure 4. Calculated S0 and S1 energies for c-2, HP-2a, i-2a, and P-2a.
in Table 3 are the absorption maxima of these species, where
available. The lines connecting the points in Figure 4 show their
energetic relationship and do not imply the absence of barriers
between ground or excited-state species. As expected, the initial
cyclization process is exergonic in the excited state but
endergonic in the ground state. The calculated S0-S1 energy
gaps for c-2a, HP-2a, and i-2a are 95.4, 68.9, and 85.7 kcal/
mol, in accord with the larger red-shift for the absorption
maxima of HP-6 vs i-2a (470 and 378 nm, respectively).
Conversion of ground-state HP-2a to i-2a is calculated to be
more exergonic than ring opening to c-2a. However, the
activation energy for the latter process which is symmetry
allowed may be lower, accounting in part for the low quantum
yield for formation of P-2a.
Aromatization of the Dihydropyridine Intermediate. Aro-
matization of 4a,4b-dihydrophenanthrene in the presence of
oxygen is known to be a free radical chain process that can be
inhibited by additives such as 2,6-di-tert-butyl-4-methylphenol
(BHT).2,38 We find that irradiation of t-2 in the presence of a
100-fold excess of BHT in degassed solution has no effect on
the formation of P-2a. The formation of P-2a under anaerobic
conditions is also insensitive to added propylamine, water, or
traces of oxygen present in nitrogen-purged solutions. Conver-
sion of i-2a to P-2a is calculated to be only ca. 7 kcal/mol
exergonic (Table 3), plausibly accounting for the moderately
long lifetime of i-2a at room temperature (Figures 1 and 3).
The spontaneous loss of hydrogen has not previously been
observed for 4a,4b-dihydrophenanthrene intermediates.3 How-
ever, irradiation of N-methyldiphenylamine in degassed solution
results in the formation of the aromatized photocyclization
product N-methylcarbazole.39-43 Bowen and Eland39 reported
the formation of molecular hydrogen in this reaction; however,
Fo¨rster et al.42 disputed this report. Grellman et al.43 proposed
that the dihydro intermediate disproportionates to yield a mixture
of carbazole and the unstable tetrahydrocarbazole. Dispropor-
tionation of i-2a to yield a mixture of P-2a and an unstable
tetrahydrophenanthrene could account for both the formation
of P-2a with a maximum yield of ca. 40% and the presence of
The energetics of the sequential conversion of c-2a to HP-
2a and i-2a (Scheme 2) has been explored by using semiem-
pirical AM1 geometry optimized calculations to obtain the
ground state energies and ZINDO calculations to obtain the
vertical excited state energies for each of the ground state
geometries (see Experimental Section). The gas-phase results
are reported in Table 3 and shown in Figure 4. Also reported
(30) Mallory, F. B.; Mallory, C. W. J. Am. Chem. Soc. 1972, 94, 6041.
(31) Ho, T.-I.; Wu, J.-Y.; Wang, S.-L. Angew. Chem., Int. Ed. 1999, 38,
2558.
(32) Wu, J.-Y.; Ho, J.-H.; Shih, S.-M.; Hsieh, T.-L.; Ho, T.-I. Org. Lett.
1999, 1, 1039.
(33) Schultz, A. G.; DeTar, M. B. J. Am. Chem. Soc. 1976, 98, 3564.
(34) Horgan, S. W.; Morgan, D. D.; Orchin, M. J. Org. Chem. 1973,
38, 3801.
(35) Woodward, R. B.; Hoffmann, R. The ConserVation of Orbital
Symmetry; Verlag Chemie: Weinheim, Germany, 1970.
(36) Somers, J. B. M.; Laarhoven, W. H. J. Photochem. Photobiol. A:
Chem. 1989, 48, 353.
(38) Bromberg, A.; Muszkat, K. A. J. Am. Chem. Soc. 1969, 91, 2860.
(39) Bowen, E. J.; Eland, J. H. D. Proc. Chem. Soc., London 1963, 202.
(40) Grellmann, K.-H.; Sherman, G. M.; Linschitz, H. J. Am. Chem. Soc.
1963, 85, 1881.
(41) Linschitz, H.; Grellmann, K.-H. J. Am. Chem. Soc. 1964, 86, 303.
(42) Fo¨rster, E. W.; Grellmann, K.-H.; Linschitz, H. J. Am. Chem. Soc.
1973, 95, 3108.
(37) Wismontski-Knittel, T.; Muszkat, K. A.; Fischer, E. Mol. Photochem.
1979, 9, 217.
(43) Grellmann, K.-H.; Ku¨hnle, W.; Weller, H.; Wolff, T. J. Am. Chem.
Soc. 1981, 103, 6889.