Selectivity in reactions of allyl diazoacetates as a function of catalyst and ring size from gamma-lactones to macrocyclic lactones.

Abstract

Catalytic reactions of diazoacetates tethered through zero, one, two, and three ethylene glycol units to an allyl group have been investigated for chemoselectivity, diastereoselectivity, and enantioselectivity. Results from cyclopropanation, carbon-hydrogen insertion, and oxonium ylide generation are compared from reactions of achiral and chiral catalysts of copper(I) and dirhodium(II) carboxylates and carboxamidates. Relative to results from intermolecular reactions of ethyl diazoacetate with allyl ethyl ether, intermolecular reactions show a diversity of selectivities including preference for the opposite configurational arrangement from the one preferred in corresponding intermolecular cyclopropanation reactions. Enantioselectivities for cyclopropanation are dependent on the catalyst ligands in a manner that reflects divergent trajectories of the carbon-carbon double bond to the reacting carbene center. Enantioselectivity increases as a function of ring size with chiral copper catalysts, but the reverse occurs with chiral dirhodium(II) carboxamidates. Mechanistic implications, including those related to the conformation of the reacting metal carbene, offer a new dimension to understanding of enantioselectivity in catalytic asymmetric cyclopropanation reactions.