Rearrangement of Alkylchlorocarbenes: 1,2-H Shift in Free Carbene, Carbene−Olefin Complex, and Excited States of Carbene Precursors

Photolysis of alkylchlorodiazirines (1) in the presence
of olefins gives a cyclopropane (3) by addition
of
the generated carbene to the olefin and a vinyl chloride derivative
(2) resulting from a 1,2-H shift rearrangement.
This rearrangement may occur either in the carbene or in some
excited state, precursor of the carbene (RIES
mechanism), or in a “carbene + olefin complex” on the way to the
formation of 3 (COC mechanism). Results
obtained by time-resolved photoacoustic calorimetry as well as by
thermolysis and photolysis of
ClCH2C(N2)Cl and
CH3(CH2)2C(N2)Cl
in the presence of tetramethylethylene clearly indicate that both the
RIES and COC mechanisms
play a role but with efficiencies which greatly depend on the nature of
the diazirine. Reexamination of the results
previously obtained with benzylchlorodiazirines indicates that, for
this class of diazirines, the RIES mechanism is
temperature dependent and has a very low efficiency at room temperature
and below, whereas the nonlinearity of
the plots [3]/[2] vs [olefin] is
mainly due to the COC mechanism.