Helicene-based phosphite ligands in asymmetric transition-metal catalysis: Exploring Rh-catalyzed hydroformylation and Ir-catalyzed allylic amination

Abstract

Starting from the optically pure [6]helicene-like alcohol(P,3S)-3-methyl-4- (4-methylphenyl)-1,3,6,7-tetrahydrobenzo[c]benzo[5,6]phenanthro[4,3-e] oxepin-14-ol, four helical phosphites were prepared from the corresponding chlorophosphites. These ligands containing parent or substituted 1,3,2-dioxaphospholan-2-yl or dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl moieties were applied to the asymmetric hydroformylation of terminal alkenes catalyzed by Rh(acac)(CO)2 and the asymmetric allylic amination of cinnamyl-type carbonates catalyzed by [Ir(cod)Cl]2. The helical phosphite containing the dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl group was most successful in the asymmetric hydroformylation of styrene, leading to moderate enantiomeric excess values (up to 32 % ee), high regioselectivity in favor of the branched product, and mostly high conversion, whereas the helical ligand containing the 4,4,5,5-tetramethyl-1,3,2-dioxaphospholan-2-yl fragment was most effective in asymmetric allylic aminations, exhibiting high enantioselectivity (up to 94 % ee), excellent regioselectivity in favor of the branched products, and good reactivity. This study represents the first use of helicene-like ligands in asymmetric reactions, including hydroformylation and allylic amination, and the promising results indicate the potential of the helicene moieties as chiral inductors. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.