Rhodium(II)-catalyzed inter- and intramolecular enantioselective cyclopropanations with alkyl-diazo(triorganylsilyl)acetates

Abstract

The intermolecular cyclopropanation of styrene with ethyl diazo(triethylsilyl)acetate (1a) proceeds at room temperature in the presence of chiral RhII carboxylate catalysts derived from imide-protected amino acids and affords mixtures of trans- and cis-cyclopropane derivatives 2a in up to 72% yield but with modest enantioselectivities (< 54%) (Scheme 1 and Table 1). Protiodesilylation of a diastereoisomer mixture 2a with Bu4NF is accompanied by epimerization at C(1) (→ 3). The intramolecular cyclopropanation of allyl diazo(triethylsilyl)acetate (8a), in turn, affords optically active 3-oxabicyclo[3.1.0]hexan-2-one (9a) with yields of up to 85% and 56% ee (Scheme 3 and Table 2). Similarly, the (2Z)-pent-2-enyl derivative 8d reacts to 9d in up to 77% yield and 38% ee (Scheme 3 and Table 3). In contrast, the diazo decomposition of (2E)-3-phenylprop-2-enyl and 2-methylprop-2-en-1-yl diazo(triethyl-silyl)acetates (8b and 8c, resp.) is unsatisfactory and gives very poor yields of substituted 3-oxabicyclo[3.1.0]hexan-2-ones 9b and 9c, respectively (Table 3).