5210 J . Org. Chem., Vol. 65, No. 17, 2000
Ferna´ndez-Zertuche et al.
Sch em e 6
same barrier in model A, thus stabilizing intermediate
19 with respect to 16. These energy barriers are large
enough to warrant the observation of the intermediates
by trapping them at low temperature. Although one
quantum effect we did not take into account in the
present study is the inclusion of the zero-point energy
(ZPE) corrections, these will be small and will not modify
the stability of the intermediates.
Str u ctu r es. For the intermediates 16 and 19, C-C
distances for the three linked carbon atoms (a,b,c) in the
allene moiety are typical for double-bonds. Although one
would expect the abc angles to be somewhat smaller than
the 180° found for linear allenes, the abc angles are only
slightly different than those found in benzene. For
example, the abc bend angle is approximately equal to
110°, much closer to an sp2 hybridization than the
corresponding linear sp hybridization for the central
carbon atom. The other bend angles in the ring are
accordingly larger than the 120° expected for a regular
hexagon. The fact that there is only a relatively small
structural change in the ring for the cyclohexatriene to
ciclohexadienone transformation is reflected in the simi-
lar stability of these structures.
Exp er im en ta l Section
Gen er a l P r oced u r e. Melting points are uncorrected. 1H
and 13C spectra were recorded at 200 MHz. IR spectra were
recorded using FTIR apparatus. Mass spectra were obtained
by EI or CI methods. All reactions were followed by TLC using
E. Merck silica gel 60 F-254. Flash column chromatography
was performed with E. Merck silica gel (230-400 mesh). All
reactions were carried out in flame-dried glassware under a
positive pressure of dry argon. Tetrahydrofuran (THF) was
distilled from sodium (benzophenone indicator).
4,5-Dim eth oxy-2-(ph en yleth yn yl)-2-(tr im eth ylsilyloxy)-
3,5-cycloh exa d ien on e (5). To a solution of 0.33 g (3.2 mmol)
of phenylacetylene in 35 mL of dry THF, under argon, at -78
°C, was injected 1.6 mL (3.2 mmol) of n-butyllithium (2.0 M
in hexanes). The mixture was stirred at this temperature for
25 min and then transferred dropwise, via cannula, to a
suspension of 0.50 g (2.9 mmol) of 4,5-dimethoxy-3,5-cyclo-
hexadiene-1,2-dione in 200 mL of dry THF also under argon
at -78 °C. When the addition was complete, the mixture was
stirred for an additional 1 h. The reaction was then quenched
with 0.75 mL of trimethylsilyl chloride and allowed to warm
to room temperature. The solvent was evaporated and the
residue, dissolved in dichloromethane, passed through a flash
column (silica gel, hexanes/ethyl acetate 4:1). Removal of the
solvent afforded a yellow oil which crystallized upon addition
of diisopropyl ether to give 0.95 g (94%) of product as white
crystals: mp 108 °C; 1H NMR 7.25 (m, 5H), 5.44 (s, 1H), 5.41
(s, 1H), 3.84 (s,3H), 3.75 (s, 3H), 0.28 (s, 9H). IR 2980(w), 1682-
(s), 1600(s), 1500(w), 1460(m), 1250(s), 1080 (s). Anal. Calcd
for C19H22O4Si: C, 66.64; H, 6.47. Found: C, 66,66; H, 6.37.
Structure 19 is a symmetric C2v molecule and, as
expected from Hammonds postulate, the transition state
resembles more reactant 19 than product 21.
Con clu sion s. The photolysis of 4,5-dimethoxy-2-(phe-
nylethynyl)-2-(trimethylsilyloxy)-3,5-cyclohexadienone 5
induces the rearrangement of this compound to enones
9a , 9b, and 11 in reasonable yields through a sequence
of steps that involve the electrocyclic ring opening of 5.
This process generates ketenes 6a and 6b which are
immediately captured by nucleophilic species present in
the reaction media to give dienynes 7. We have demon-
strated using HF and MP2 ab initio calculations that the
electrocyclization of 7 can generate intermediates that
can be best described as the cyclic allenes 8 or 10. The
results shown here are also consistent with earlier
literature reports in which the cyclic allene 1,2-cyclo-
hexadiene is favored over the diradical or dipolar forms.
It is also of interest to note this preference for the cyclic
allene form in our study since our molecules contain an
additional double bond in the ring system. We are now
extending our calculations and analysis to similar ex-
amples reported in the literature that involve the genera-
tion and electrocyclization of conjugated alkynyl ketenes
that can also give rise to the formation of cyclic allenes
or the corresponding diradical forms.
4,5-Dim eth oxy-3-p h en yl-4-(2-eth a n oic)-2-(tr im eth ylsi-
lyl)-2,5-cycloh exa d ien on e (9a ). Photolysis using a 450 W
medium-pressure Hanovia mercury lamp and a Pyrex filter
was conducted on 0.40 g (1.17 mmol) of 4,5-dimethoxy-2-
(phenylethynyl)-2-(trimethylsilyloxy)-3,5-cyclohexadienone 5 in
500 mL of wet THF, at -78 °C and under argon for 2 h. After
warming the solution to room temperature, the solvent was
evaporated and the solid obtained recrystallized from diiso-
propyl ether to give 0.16 g (40%) of the product as white
1
crystals: mp 151-152 °C; H NMR 7.4-7.2 (m, 3H), 6.92 (m,
2H), 5.73 (s,3H), 3.91 (s, 3H), 3.17 (s, 3H), 3.17(s, 3H, 2.77 (d,
1H, J ) 13.5 Hz), 2.35 (d, 1H, J ) 15.5 Hz), 0.083 (s, 9H); IR
3000 (br), 1735 (s), 1662 (s), 1600 (s), 1358 (s), 1240 (s), 1200
(s), 1100 (s). Anal. Calcd for C19H22O5Si: C, 63.66; H, 6.18.
Found: C, 63.47; H, 6.09.
4,5-Dim eth oxy-3-ph en yl-4-(2-m eth ylen ecar bom eth oxy)-
2-(t r im et h ylsilyl)-2,5-cycloh exa d ien on e (9b ). Photolysis
using a 450 W medium-pressure Hanovia mercury lamp and
a Pyrex filter was conducted on 0.35 g (1.02 mmol) of 5 in 500
mL of a 1:1 mixture of THF/methanol. The solution was
irradiated under an argon atmosphere at -78 °C for 1.5 h.
The mixture was allowed to warm to room temperature and
the solvent removed. Preparative thin-layer chromatography
(silica gel, hexanes/ethyl acetate 1:1) gave an oil which upon
addition of diisopropyl ether afforded 0.18 g (48%) of the
(14) Gaussian 94, Revision C.3, M. J . Frisch, G. W. Trucks, H. B.
Schlegel, P. M. W. Gill, B. G. J ohnson, M. A. Robb, J . R. Cheeseman,
T. Keith, G. A. Petersson, J . A. Montgomery, K. Raghavachari, M. A.
Al-Laham, V. G. Zakrzewski, J . V. Ortiz, J . B. Foresman, J . Cioslowski,
B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y.
Ayala, W. Chen, M. W. Wong, J . L. Andres, E. S. Replogle, R. Gomperts,
R. L. Martin, D. J . Fox, J . S. Binkley, D. J . Defrees, J . Baker, J . P.
Stewart, M. Head-Gordon, C. Gonzalez, and J . A. Pople, Gaussian, Inc.,
Pittsburgh, PA, 1995.
1
product as white crystals: mp 96-97 °C (dec); H NMR 7.27
(m, 3H), 6.87 (t, 2H, J ) 7.63 Hz), 5.5 (s, 1H), 3.89 (s,3H),
3.61 (s, 3H), 3.20 (s, 3H), 3.19 (q, 2H, J ) 6.51 Hz), 0.15 (s,
9H); 13CNMR 194.35, 189.35, 179.38, 179.20, 169.30, 169.25,
151.11, 143.63, 127.79, 126.48, 125.88, 109.86, 58.61, 58.58,
52.00, 51.33, 42.58. IR 1750 (s), 1702 (s), 1638 (s), 1355 (s),