4460
J . Org. Chem. 1996, 61, 4460-4461
(Hyd r oxyp h en yl)ca r ben es1,2
in many phenylcarbene reactions, net loss of carbon. We
suspect that reactions of phenylcarbenes with walls or
other molecules, followed by cleavage to give phenyl
radicals, is the major source of this compound. It is the
other two products, tropone and o-cresol, that are of most
interest.
Adam H. Golden and Maitland J ones, J r.*
Department of Chemistry, Princeton University,
Princeton New J ersey 08544
The mechanism of the phenylcarbene rearrangement
is still a subject of some controversy, but there is general
agreement on the intermediacy of seven-membered rings
and some evidence that cycloheptatetraenes are the vital
ingredients in this process.3,6 We regard it as not quite
certain that cycloheptatrienylidenes are not the produc-
tive species in this reaction, but in any event, there is
very little difference between the slightly nonplanar
cycloheptatetraenes and the planar cycloheptatrien-
ylidenes. The ring-expansion/ring-contraction mecha-
nism shown below for the interconversion of the para,
meta, and ortho carbenes uses cycloheptatetraenes.
Received November 13, 1995
Although the phenylcarbene rearrangement still poses
vexing mechanistic questions,3 it nonetheless provides
synthetic opportunities,4 as the benzene ring can operate
to transport divalency from one position on the ring to
another. In this way, one might generate carbenes
unavailable from conventional syntheses. For example,
we hoped that (m-hydroxyphenyl)carbene (1) and
(p-hydroxyphenyl)carbene (2) would rearrange to their
ortho isomer (3) and, perhaps, hydroxyphenylcarbene (4).
The carbenes were generated through pyrolysis of the
corresponding tetrazoles,5 themselves produced from the
commercially available cyano compounds. The tetrazoles
were dropped bit by bit from a solids addition funnel onto
a hot Pyrex surface and the products collected down-
stream in a liquid nitrogen cooled trap. Under these
conditions at 530 °C/0.025 Torr, both the meta and para
isomers gave very similar products in comparable yields
of about 50%.
At any point in this mechanism, ketonization of one of
the seven-membered rings, followed by a hydrogen shift,
leads to tropone. We seem to have intercepted the
intermediate in the phenylcarbene rearrangement by
providing an energetically attractive intramolecular rear-
rangement. Only once before has a simple seven-
membered intermediate been diverted in such a fashion;
Tomioka and Taketsuji found 8-(methoxycarbonyl)hep-
tafulvene (5) on generation of carbene 6 by flash vacuum
pyrolysis.7 There is an earlier report by Wentrup and
Becker of isolation of seven-membered-ring-containing
compounds in a more complicated multi-ring system,8
and Dunkin and his group have exploited a nitrene
version of this process in a matrix synthesis of azepin-
4-ones.9
One major product in each case is the cyanophenol
from which the starting tetrazole was synthesized. The
retro 1,3-dipolar addition is a common reaction of these
tetrazoles and merely serves to diminish the useful yield
of carbene products; it is of no mechanistic significance.
Phenol, formed here in about 11% relative yield, is
representative of a minor, if not well-explained, process
(1) Portions of this work are taken from the A.B. Thesis of Adam
H. Golden, Princeton University, 1986. We thank the National Science
Foundation for support through grants CHE 83-18345 and CHE9322579.
(2) Very similar results have been obtained in the laboratories of
Harold Shechter at The Ohio State University. We thank professor
Shechter for stimulating conversations, for sharing his data, and for
agreeing to simultaneous publication.
(3) Gaspar, P. P.; Hsu, J .-P.; Chari, S.; J ones, M., J r. Tetrahedron
1985, 41, 1479 and references therein.
(4) For two examples of the phenylcarbene rearrangement used as
a “divalency transport system”, see: Chambers, G. R.; J ones, M., J r.
Tetrahedron Lett. 1978, 5193. Boxberger, M.; Volbracht, L.; J ones, M.,
J r. Tetrahedron Lett. 1980, 3669.
(6) Wentrup, C. In Methoden der Organische Chemie (Houben-Weyl);
Regitz, M., Ed.; G. Thieme Verlag: Stuttgart, 1989; Vol. E19b, pp 824-
976. 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.
(5) Gleiter, R.; Rettig, W.; Wentrup, C. Helv. Chim. Acta 1974, 57,
2111. Decomposition of the o-tetrazole leads only to the corresponding
nitrile. Apparently the proximate OH catalyzed the reverse cycload-
dition. At any rate, no sign of carbene chemistry appears.
(7) Tomioka, H.; Taketsuji, K. J . Org. Chem. 1993, 58, 4196.
(8) Wentrup, C.; Becker, J . J . Am. Chem. Soc. 1984, 106, 3705.
(9) Dunkin, I. R.; El Ayeb, A.; Lynch, M. A. J . Chem. Soc., Chem.
Commun. 1994, 1695.
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