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 giv es a cyclopropane (3) by addition of the generated carbene to the olef in and a vinyl chloride derivative (2) resulting from a 1,2-H shift re arrangement. 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 me chanism). Results obtained by time-resolved photoacoustic calorimetry as well as by thermolysis and photolysis of ClCH2C(N-2)Cl and CH3(CH2) (2)C(N-2)Cl in the presence of tetramethylethylene clearly indicate th at 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 d ue to the COC mechanism.