Phenyl carbene, laser flash photolysis

Insight into the nature of the intermediate in the cyclopropanation of electron-rich and electron-poor alkenes was given by laser flash photolysis (LFP) studies of phenyl-bromodiazirine in the presence of various amounts of tetrabutylammonium bromide.23 Electron-rich alkenes react exclusively with the carbene (11) leading to (12). Electron-poor alkenes yield cyclopropanes (13) only slowly with the carbene (11) and more rapidly with the carbanion (14) arising from the addition of the bromide to the carbene. 

In a laser flash-photolysis study, 2-phenyladamantene was generated in benzene at room temperature from 3-noradamantyl(phenyl)diazomethane. This strained cycloalkene decays with second-order kinetics to give a dimer, and reacts much faster with O2 and Bu3SnH than with methanol, thus revealing a substantial radical character. Diphenyldiazomethanes possessing stable /er -butylaminoxyl and Ullman s nitronyl nitroxide radicals, e.g. (25), have been prepared by photolysis of the parent diazomethanes. Analysis of ESR fine structures showed that the carbene and radical centres couple ferromagnetically in these molecules, as expected. 

We add to these investigations a recent study of Tomioka s group (Hirai et al., 1994), in which reaction of O2 was used for trapping a triplet carbene and where it was possible to observe the triplet carbene at ambient temperature by laser flash photolysis. (2,4,6-Tri( err-butyl)phenyl)phenyldiazomethane (8.51) afforded photo-lytically 4,6-di( er butyl)-l,l-dimethyl-3-phenylindan (8.53) almost quantitatively. It is an insertion of the intermediate carbene into a CH bond of the tert-hMiy group at C(2). No solvent adducts were found in benzene, cyclohexane or methanol. As... 

Fujiwara, Y., Tanimoto, Y., Itoh, M., Hirai, K., and Tomioka, H., Laser Flash Photolysis Studies of Methoxycarbonyl Phenyl Carbene and Its Derived Carbonyl Oxide at Room Temperature, /. Am. Chem. Soc., 109, 1942,1987. 

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