The title carbene has been generated by photolysis of the corresponding diazo precursors and studied by spectroscopic means, i. e., electron paramagnetic resonance (EPR) and UV/vis spectroscopy in matrixes at low temperature and laser-flash photolysis in solution at room temperature, with the product analysis. The results are compared with triplet di(2,3,5,6-tetramethylphenyl)carbene, an open-chain counterpart, which revealed that bicycloalkyl groups are acting as a fairly good kinetic protector for the triplet carbene as opposed to the open-chain counterpart. The formation of all-hydrocarbon triplet carbenes having a half-life over a second under normal conditions was realized for the first time. Effects of para-substituents on the structure and reactivities of the carbene are also investigated and discussed in terms of polar and spin electronic effects.

While the history of persistent radicals is long and impressive, that of persistent triplet carbenes is short, and stable triplet carbenes are not attainable yet.2 Two basic strategies, i.e., thermodynamic and kinetic stabilization, are possible for the stabilization of reactive species. However, each strategy encounters serious problems when applied to triplet carbenes. Thus, thermodynamic stabilization not only results in the stabilization of the singlet state but also poses the issue of electronic configuration as a pure carbene (one centered diradical) as a result of possible conj~gation.~-~ While steric protection (kinetic stabilization) is therefore a much better method of stabilizing the triplet$-6 a voracious appetite of carbenes for electrons makes it difficult to explore the usual protecting groups for this extremely reactive center. Carbenes react even with very poor sources of electrons, such as C-H bonds. In this light, it is crucial to develop a protecting group which is sterically much more congesting and unreactive toward triplet carbenes. We report here that the triptycyl group is exceptionally effective in protecting triplet carbenes, increasing the lifetimes of some arylcarbenes by a factor of ca. lo5.Irradiations (A =-300 nm) of 9-triptycyl-a-naphthyldiazomethane (la)7 in a 2-methyltetrahydrofuran (MTHF) glass at (1) (a) Forrester, A. R.; Hay, J. M.; Thomson, R. H. Organic Chemistry ofstable Radicals; Academic Press: London, 1968. (b) Griller, D.; Ingold, K. U. Acc. Chem. Res. 1976, 9, 13. (c) Ballester, M. Ibid. 1985, 18, 380. See also: Ballester, M.; Pascual, I.; Riera, J.; CadtaAer, J. J. Org. Chem. 1991, 56, 217. (2) Phosphinocarbene and imidazol-2-ylidene were prepared as "bottleable" carbenes in 1988 and 1991, respectively. See: Igau, A,; Griitzmacher, H.; Baceiredo, A.; Bertrand, M. Okada, H.; Watanabe, T.; Hirai, K. Angew. Chem., Inr. Ed. Engl. 1994, 33, 873. (f) Tomioka, H.; Watanabe, T.; Hirai, K.; Furukawa, K.; Takui, T.; Itoh, K. J. Am. Chem. SOC. 1995, 117, 6376. (6) It has been reported that 9,9'-dianthrylmethylene is stable up to 160 "C in anthracene (Wasserman, E.; Kuck, V. J.; Yager, W. A,; Hutton, R. S.; Greene, F. D.; Abegg, V. P.; Weinshenker, N. M. J. Am. Chem. SOC. 1971, 93, 6335). However, the stability is due to the rigidity of the environment and not to an intrinsic lack of reactivity. For instance, flash photolysis studies show that it undergoes self-reaction at the diffusioncontrolled limit in benzene at room temperature (Astles, D. J.; Girard, M.; Griller, D.; Kolt, R. J.; Wayner, D. D. M. J. Org. Chem. 1988, 53, 6053). (7) Characteristic spectroscopic data for la: mp 183 'C; IH NMR (270 MHz, CDC13) 6 8.50 (d, J = 8.58 Hz, lH), 8.02 (d, J = 7.92 Hz, lH), 7.87-7.61 (m, 3H), 7.47-7.30 (m, 8H), 7.05-6.91 (m, 6H), 5.47 (s, IH); IR (KBr) 2047 cm-I. For lb: mp 178 OC; 'H NMR (270 MHz, CDCI3) 6 7.80-7.75 (m, 2H), 7.70 (d, J = 8.58 Hz, lH), 7.66-7.30 (m, IOH), 7.15 (d, J = 8.58 Hz, IH), 7.06-6.85 (m, 6H), 5.47 (s, 1H); IR (KBr) 2045 cm-'. For IC: red liquid; 'H NMR (270 MHz...

A series of triplet 9-triptycyl(aryl)carbenes, where aryl groups are phenyl, 1- and 2-naphthyl, and 9-anthryl, is generated by photolysis of the corresponding diazomethanes and observed directly by spectroscopic means. Their structures are characterized by electron spin resonance (ESR) spectroscopy in a 2-methyltetrahydrofuran matrix at 77 K, and the reactivities are investigated by laser flash photolysis in degassed benzene solution at room temperature. Comparison of the data with other arylcarbenes bearing a series of substituents, i.e., hydrogen, phenyl, naphthyl, and anthryl groups, revealed an interesting relationship between structures and reactivities of triplet arylcarbenes.Key words: steric protection, stability of triplet carbenes, electron spin resonance, laser flash photolysis, structure-reactivity relationship.

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