Progress towards asymmetric intermolecular and intramolecular cyclopropanations using α-nitro-α-diazo carbonyl substrates

Abstract

Chiral Rh(II) catalysts were tested in the cyclopropanation of styrene with α-nitro-α-diazo carbonyls giving high yields and modest enantioselectivities up to 41% e.e. Copper-based catalysts with bis(oxazoline) ligands gave e.e. up to 72% and yields up to 55%. The first intramolecular cyclopropanation with these substrates is also reported with enantioselectivities up to 61% e.e. and yields up to 66%. A variety of Rh(II) catalysts were screened for enantioselectivity in the cyclopropanation of styrene with α-nitro-α-diazo carbonyl compounds and found to give modest to high yields in a wide range of solvents but modest enantioselectivities (up to 41% e.e. for substrate 9c ). Copper catalysts with bis(oxazoline) ligands gave higher enantioselection, with e.e. up to 72% for the major diastereoisomer and yields up to 55%. The use of ethyl diazoacetate as an additive was necessary to obtain improved yields. The first example of intramolecular cyclopropanation involving α-nitro-α-diazo carbonyls is also reported. Rh(II) carboxylate 4b catalysed the formation of 9-membered nitro cyclopropyl lactones giving up to 66% yield and 61% e.e.