Activation energies for the 1,2-carbon migration of ring-fused cyclopropylchlorocarbenes

Fused-ring cyclopropylchlorocarbenes 2 and 3 ring expand to fused-ring chlorocyclobutenes 10 and 11, respectively, with activation energies of 3–4 kcal/mol and activation entropies of ∼−20 e.u. Carbon tunneling appears to be unimportant at low temperatures.