Transition Metal-Catalyzed, Asymmetric Reactions of Diazo Compounds

Abstract

The past ten years have seen impressive advances in asymmetric synthesis. This review summarizes the recent advances in a particular set of asymmetric reactions, the reactions of diazo compounds catalyzed by transition metal complexes. Additionally, the emphasis of this summary is on reactions wherein the induction arises from a catalyst or an auxiliary, rather than some inherent asymmetry of the substrate. The covered reactions fall into two reaction types; cyclopropanations and insertions. The cyclopropanation section of this review describes how high stereoselectivities are possible using either chiral auxiliaries or various metal complexes. Both these strategies are effective for producing optically-enriched intermediates; however, the use of catalysts to control the stereochemistry of the cyclopropanation reaction is much more common than the corresponding use of auxiliaries Workers in the asymmetric cyclopropanation field have primarily used Cu(l) and Rh(ll) complexes as catalysts for these reactions, although several complexes of other metals do afford high asymmetric induction. Both inter- and intramolecular cyclopropanations afford synthetically useful selectivities. The insertion section of this review summarizes recent advances in the use of auxiliaries and catalysts for controlling the stereoselectivity of the insertion into various bonds. Insertion into C-H bonds are by far the most intensively studied, although there has been some success with asymmetric insertions into 0-H, S-H, Si-H and C-0 bonds. Complexes of Rh(ll) are almost universally employed for asymmetric insertions. As in the case of cyclopropanations, both inter- and intramolecular insertions can proceed with useful selectivities. Again, catalyst control has proven a more versatile way to control absolute stereochemistry than auxiliary control.