One-carbon ring expansion of azetidines via ammonium ylide [1,2]-shifts: a simple route to substituted pyrrolidines.

Simple N-substituted azetidines were heated with diazocarbonyl compounds in the presence of catalytic Cu(acac)(2) to furnish substituted pyrrolidines via Stevens [1,2]-shift. In all but two examples, complete selectivity was seen for ring expansion rather than migration of the other exocyclic group on the azetidinium nitrogen. The two exceptions, observed with ylides substituted with two carbonyl groups and lacking a stabilizing group at the 2-position of the azetidine, underwent exocyclic benzyl migration in preference to ring expansion.