Carbon—hydrogen bonds singlet carbene insertion reactions

A second process that has a central position in the analysis of the chemical properties of carbenes is their reaction with hydrocarbons. As is the case for alcohols, singlet and triplet carbenes react with hydrocarbons in distinctive ways. It has long been held that very electrophilic singlet carbenes can insert directly into carbon-hydrogen bonds (11) (Kirmse, 1971). On the other hand, triplet carbenes are believed to abstract hydrogen atoms to generate radicals that go on to combine and disproportionate in subsequent steps (12)... 

Particular interest has been shown in the reactions of photochemically generated arylcarbenes. A study of the photodecomposition of phenyldiazomethane in 2-chloropropane has revealed that carbon-chlorine bond insertion of singlet phenylcarbene predominates at low temperature in solution, whereas carbon-hydrogen bond insertion is preferred in a rigid matrix. The principal products of irradiation of diazoalkane (63) are phenylacetylene (64) and the cyclobutene (65), even in the presence of alkenes. At lower temperatures, however, carbene addition predominates as shown, for example, with isobutene as shown in Scheme 7. 

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