Steric balance within chiral dirhodium(II) carboxamidate catalysts enhances stereoselectivity

The substitution of larger groups onto the ester moiety of various dirhodium(II) carboxamidates have led to the development of three catalysts for use in various transformations. These new chiral imidazolidinone-ligated catalysts, Rh-2(4S-EPPIM)(4), Rh-2(4S-BACIM)(4), and Rh-2(4S-BPPIM)(4), have been tested in a variety of systems, including carbon-hydrogen insertion, intramolecular cyclopropanation, and Hetero-Diels-Alder reactions. While all display high levels of selectivity, only Rh-2(4S-BACIM)(4) demonstrates higher levels of selectivity than its base catalyst, Rh-2(4S-MACIM)(4). These results show that increased levels of selectivity cannot be achieved through simply increasing the steric bulk around the active site. Instead, a delicate balance in steric distribution around the active site of the catalyst must be achieved to maximize selectivity with reactivity. (C) 2002 Elsevier Science B.V. All rights reserved.