explored so far. Such nondiazo sources of the R-carbonyl-
carbenes could help to define the role of carbene intermedi-
ates in the photochemical Wolff rearrangement of R-diazo-
carbonyl compounds. These precursors also can become a
valuable alternative to thermally unstable diazo-based photo-
resist in short-wavelength microlithography and other ap-
plications.2 However, only few attempts at the photochemical
generation of R-carbonyl carbenes from non-nitrogenous
precursors have been reported. The close relative of 7-car-
bonyl-substituted dibenzonorcaradienes, 7-carbethoxybenzo-
norcaradiene, produces ethoxycarbonyl-carbene upon UV
irradiation but in rather low yield (8-11%).3 Platz and
coauthors recently observed generation of carbomethoxy-
chlorocarbene in the photodecomposition of the chlorocarbo-
methoxy-substituted analogue of norcaradiene, methyl 8-chlo-
ro-3a,7a-methanoindan-8-carboxylate.4
HPLC, as well as GC-MS chromatograms. X-ray analysis
of 1b showed trans (or exo) configuration.5
The irradiation of methanolic solution of 7-methoxycar-
bonyl-7-phenyldibenzonorcaradiene (1a) with 300 nm light
results in formation of methyl 2-(9-phenanthryl)-phenyl-
acetate (2a, Scheme 3).5 Small amounts of similar ring-
Scheme 3
This report describes investigation of the photochemistry
of three carbonyl-substituted dibenzonorcaradienes: an ester,
7-methoxycarbonyl-7-phenyldibenzonorcaradiene (1a), and
two stereoisomeric ketones, trans- (1b) and cis- (1c) 7-benz-
oyldibenzonorcaradiene.
opening products were also found in complex reaction
mixtures of 7-carbethoxybenzonorcaradiene photolysis.3b In
this case, however, 2a is the only product detected by HPLC.
The absence of detectable amounts of phenanthrene and
methyl 2-methoxy-2-phenylacetate, the expected product of
carbene reaction with the solvent, indicates that photolysis
of 1a does not produce carbomethoxyphenyl-carbene.
Photolysis of cis-7-benzoyldibenzonorcaradiene (1b) in
methanol produces two products, ω-(9-phenanthryl) aceto-
phenone 2b and cis-7-benzoyldibenzonorcaradiene 1c, the
cis(endo)-isomer of the starting material (Scheme 4), while
no benzoylcarbene-derived products were detected.
The dibenzonorcaradienes 1a and 1b were prepared by
rhodium-catalyzed cyclopropanation of phenanthrene with
corresponding R-diazocarbonyl compounds (Scheme 2).5
Scheme 4
Scheme 2
Stereoisomeric dibenzonorcaradienes 1b and 1c are easily
distinguished by the signals of cyclopropane protons in the
1H spectra. Resonance of the proton at the 7-position of
norcaradiene system in the trans isomer (1b) lies in an
unusually high field (δ 2.12 ppm) as a result of the shielding
effect of two aromatic rings. The same proton is directed
away from the aromatic system in the cis isomer (1c) and
gives a signal more than 1 ppm downfield (δ 3.21 ppm). In
addition, spin coupling of this proton with two other protons
at the cyclopropane ring is much stronger in the cis system
(1c, J ) 9.1 Hz) than in the trans (1b, J ) 3.6 Hz). The
latter data agrees well with known values for cis and trans
spin-spin coupling in 2,3-diphenyl-1-acetyl cyclopropanes.6
Exhaustive irradiation of the methanolic solution of 1b
resulted in formation of 2b as the only significant product.
It is interesting to note that this reaction is stereospecific,
producing only one isomer of 7-acyldibenzonorcaradienes,
1
as evidenced by only one set of signals in the H and 13C
NMR spectra of compounds 1a and 1b and one peak on
(2) (a) For example: Moreau, W. M. Semiconductor Lithography; Plenum
Press: New York, 1988. Reiser, A. PhotoreactiVe Polymers: The Science
and Technology of Resists; Wiley: New York, 1989. Reichmanis, E.;
Thompson, L. F. Chem. ReV. 1989, 89, 1273. (b) Yang, Y.; Huang, S.; He,
H.; Mau, A. W. H.; Dai, L. J. Am. Chem. Soc. 1999, 121, 10832.
(3) (a) Ciganek, E. J. Am. Chem. Soc. 1967, 89, 1458. (b) Swenton, J.
S.; Krubsack, A. J. J. Am. Chem. Soc. 1969, 91, 786.
(4) Likhotvorik, I.; Zhu, Z.; Tae, E. L.; Tippmann, E.; Platz, M. S. J.
Am. Chem. Soc. 2001, 123, in press.
(5) The preparation procedures for the compounds of 1a, 1b, and 1c,
the spectral data for these compounds, and details of photolyses of 1a, 1b,
and 1c are provided in the Supporting Information.
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Org. Lett., Vol. 3, No. 12, 2001