The Benzylidenecarbene–Phenylacetylene Rearrangement: An Experimental and Computational Study

Benzylidenecarbene was generated from a new photochemical
source,
1-benzylidene-1a,9b-dihydro-1H-cyclopropa­[l]­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.