9332
J. Am. Chem. Soc. 2000, 122, 9332-9333
Scheme 1a
Generation, Characterization, and Rearrangements
of 4,5-Benzocyclohepta-1,2,4,6-tetraene
Paul A. Bonvallet and Robert J. McMahon*
Department of Chemistry, UniVersity of Wisconsin
1101 UniVersity AVenue, Madison, Wisconsin 53706-1396
ReceiVed April 13, 2000
ReVised Manuscript ReceiVed August 3, 2000
The complex chemistry of the C11H8 potential energy surface
involves several fascinating skeletal rearrangements (Scheme 1).1
1-Naphthylcarbene (1), 4,5-benzobicyclo[4.1.0]hepta-2,4,6-triene
(2), and 2,3-benzocycloheptatrienylidene (3) interconvert ther-
mally in solution and photochemically in low-temperature ma-
trixes, as do the corresponding isomers in the 2-naphthylcarbene
series (8, 9, and 10).2,3 Although the 1- and 2-naphthylcarbene
rearrangement manifolds remain independent at moderate tem-
peratures, a pathway connecting the two manifolds becomes
accessible at higher temperatures. Flash vacuum pyrolysis of either
1- or 2-naphthyldiazomethane4,5 or the highly exothermic reaction
of naphthalene with atomic carbon6 affords cyclobuta[de]naph-
thalene (4), which arises from C-H insertion in 1-naphthylcarbene
(1). 4,5-Benzocyclohepta-1,2,4,6-tetraene (6) has long been
postulated as the key intermediate in the interconversion pathway
connecting the isomeric 1- and 2-naphthylcarbenes (1 and 8), and
recent density functional theory calculations support this inter-
pretation.7 Yet, despite 20 years of study, direct experimental
evidence for allene 6 remains confoundingly elusive,8 and the
mechanism of the naphthylcarbene rearrangements remains the
target of scrutiny.3-5,7
a B3LYP/6-31G* relative energies (kcal mol-1) given in italics (singlet
state + ZPVE; ref 7).
Scheme 2
We report herein the generation, spectroscopic characterization,
and photochemical and thermal reactivity of 4,5-benzocyclohepta-
1,2,4,6-tetraene (6). In devising a suitable route to 6, we adapted
the novel diazo precursor 13 that Chapman and Abelt utilized to
generate the parent cyclohepta-1,2,4,6-tetraene (14) (Scheme 2).9
Synthesis of the R,â-unsaturated tosylhydrazone salt 15 proceeded
from the corresponding saturated ketone, 6,7-benzobicyclo[3.2.0]-
hepta-3,6-dien-2-one,10 following the general protocol of Chapman
and Abelt.9 Isolation of neat diazo compound 11 by the
conventional thermolysis of tosylhydrazone salt 15 is problematic,
as the deep purple diazo compound decomposes rapidly, even
on a -78 °C coldfinger. Matrix isolation of diazo compound 11
is achieved by pyrolysis of salt 15 and direct co-deposition of
the pyrolysate with argon at cryogenic temperatures. The experi-
mental and B3LYP/6-31G*11,12 calculated13 IR spectra for 2-diazo-
6,7-benzobicyclo[3.2.0]-hepta-3,6-diene (11) are in good accord
(Figure 1).
Broadband irradiation (λ >571, >472, or >237 nm) of diazo
compound 11, matrix isolated in argon at 10 K, results in the
complete disappearance of 11 accompanied by the appearance
of 4,5-benzocyclohepta-1,2,4,6-tetraene (6): IR (Ar, 10 K) 3060
(1) (a) Jones, W. M. In Rearrangements in Ground and Excited States;
deMayo, P., Ed.; Academic Press: New York, 1980; Vol. 1, Chapter 3. (b)
Gaspar, P. P.; Hsu, J.-P.; Chari, S.; Jones, M. Tetrahedron 1985, 41, 1479-
1507. (c) Platz, M. S.; Maloney, V. M. In Kinetics and Spectroscopy of
Carbenes and Biradicals; Platz, M. S., Ed.; Plenum Press: New York, 1990;
Chapter 8. (d) Wentrup, C. In Methoden der Organischen Chemie (Houben-
Weyl); Regitz, M., Ed.; G. Thieme Verlag: Stuttgart, 1989; Vol. E19b, pp
824-976. (e) Platz, M. S. Acc. Chem. Res. 1995, 28, 487-492.
(2) Coburn, T. T.; Jones, W. M. J. Am. Chem. Soc. 1974, 96, 5218-5227.
(3) (a) West, P. R.; Mooring, A. M.; McMahon, R. J.; Chapman, O. L. J.
Org. Chem. 1986, 51, 1316-1320. (b) Albrecht, S. W.; McMahon, R. J. J.
Am. Chem. Soc. 1993, 115, 855-859. (c) Bonvallet, P. A.; McMahon, R. J.
J. Am. Chem. Soc. 1999, 121, 10496-10503.
(11) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785-
789.
(12) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(4) (a) Becker, J.; Wentrup, C. Chem. Commun. 1980, 190-191. (b)
Wentrup, C.; Mayor, C.; Becker, J.; Lindner, H. J. Tetrahedron 1985, 41,
1601-1612.
(13) 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.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, ReVision A.6; Gaussian
Inc.: Pittsburgh, PA, 1998.
(5) (a) Engler, T. A.; Shechter, H. Tetrahedron Lett. 1982, 23, 2715-2718.
(b) Engler, T. A.; Shechter, H. J. Org. Chem. 1999, 64, 4247-4254.
(6) Zheng, F.; McKee, M. L.; Shevlin, P. B. J. Am. Chem. Soc. 1999, 121,
11237-11238.
(7) Xie, Y.; Schreiner, P. R.; Schleyer, P. v. R.; Schaefer, H. F. J. Am.
Chem. Soc. 1997, 119, 1370-1377.
(8) Waali, E. E.; Lewis, J. M.; Lee, D. E.; Allen, E. W. I.; Chappell, A. K.
J. Org. Chem. 1977, 42, 3460-3462.
(9) Chapman, O. L.; Abelt, C. J. J. Org. Chem. 1987, 52, 1218-1221.
(10) Ohkita, M.; Nishida, S.; Tsuji, T. J. Am. Chem. Soc. 1999, 121, 4589-
4597.
10.1021/ja001291e CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/09/2000