From 2,3-dihydrofuran to 2,2-dialkyl-2,3-dihydrofurans: new substrates for the intermolecular asymmetric Heck reaction

Palladium complexes of heterobidentate ferrocene-containing ligands were tested as catalysts in asymmetric intermolecular Heck reactions employing 2,3-dihydrofuran as the substrate and afforded a mixture of products in various ratios depending on the choice of ligand and the conditions employed. The favoured kinetic isomer was obtained in enantioselectivities of up to 99%. Dihydrofurans disubstituted at the 2-position were postulated as new substrates as they form a single regioisomeric product, thus providing a true comparative test of enantioselectivity of a range of palladium complexes. The synthesis of 2,2-dimethyl-2,3-dihydrofuran and 2,2-diethyl-2,3-dihydrofuran and their application in the intermolecular asymmetric Heck reaction with both diphosphine and phosphinamine ligands is also described. For both phenylation and cyclohexenylation of 2,3-dihydrofuran and 2,2-dimethyl-2,3-dihydrofuran the t-Bu substituted diphenylphosphinoferrocenyloxazoline ligand gave best results. The use of 2,2-diethyl-2,3-dihydrofuran as substrate demonstrated that the increased bulk at the 2-position had a deleterious effect on both the chemical yields and ees in phenylations and cyclohexenylations although enantioselectivities of 94 and 93% were obtained, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.