The Benzylidenecarbene–Phenylacetylene Rearrangement: An Experimental and Computational Study

Benzylidenecarbene was generated from a new photochemical source, 1-benzylidene-1a,9b-dihydro-1<i>H</i>-cyclopropa­[<i>l</i>]­phenanthrene, in deuterated benzene at ambient temperature. The carbene undergoes a facile rearrangement to phenylacetylene and could not be trapped by olefins. Generation of the carbene bearing a ¹³C label at the β-carbon produced phenylacetylene in which the label was found exclusively at the carbon adjacent to the phenyl ring. This overwhelming preference for H shift is consistent with B3LYP and CCSD­(T) calculations. The label distribution observed in this work, however, contrasts previously reported high-temperature flash vacuum pyrolysis results where the interconversion of carbene and alkyne leads to the scrambling of labels over both alkynyl (sp) carbons