ALKYLIDENECARBENES FROM 1-DIAZO-1-ALKENES - THEIR ELECTROPHILICITY AND STEREOCHEMISTRY OF [2+2]-CYCLOADDITION

Determination of the rho-value for the [2 + 2]-cycloaddition of the alkylidenecarbene derived from 1-diazo-2-methyl-1-propene shows that the carbene has an electrophilicity comparable to that of the species derived from alpha-elimination of the corresponding vinyl triflate. Measurements of rho as a function of solvent indicate that the carbene is free of encumbrance with solvent molecules, at least in the sense that the latter are functioning as Lewis bases. The observed stereochemistry of the [2 + 2]-cycloaddition is consistent with the alkylidenecarbene being produced in a singlet electronic state.