Allylic alkylation catalysed by platinum complexes; structure and reactivity of intermediates, and the overall stereoselectivity

Abstract

η³-Butenyl[(R,R)-4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-l,3-dioxolane-P,P′]platinum tetrafluoroborate (4) has been synthesised and shown to exist as four diastereoisomers in solution. Their interconversion was shown to be promoted by added PPh₃ and to be rapid on the n.m.r. time-scale under appropriate conditions, with interconversion of E-isomers faster than that of Z-isomers and much faster than E⇌Z interchange. The palladium analogue was unaffected by added PPh₃. The platinum complex catalyses the C-alkylation of but-2-enyl acetate or trifluoroacetate by dimethyl sodiomalonate, giving dimethyl (1 -methylprop-2-enyl)malonate and (E)-dimethyl but-2-enylmalonate in a 5 : 1 ratio, the former in 11% optical yield. For a number of related complexes of chiral biphosphines, the optical yield in allylic alkylation varied between 0 and 23%. (Z)-But-2-enyl trifluoroacetate carrying ¹³C enrichment at C-l was synthesised and converted in part into the corresponding diop platinum ally1 complex. A series of reactions involving dimethyl sodiomalonate together with labelled (4) and unlabelled substrate, or unlabelled (4) and labelled substrate, was carried out. The results clearly demonstrate that the platinum allyl is a true intermediate in catalytic allylic alkylation.