Heterobimetallic Control of Regioselectivity in Alkyne Hydrostannylation: Divergent Syntheses of α- and ( E) -β -Vinylstannanes via Cooperative Sn-H Bond Activation.

Abstract

Cooperative Sn-H bond activation of hydrostannanes (Bu3SnH) by tunable heterobimetallic (NHC)Cu-[MCO] catalysts ([MCO] = FeCp(CO)2 or Mn(CO)5) enables the catalytic hydrostannylation of terminal alkynes under mild conditions, with Markovnikov/anti-Markovnikov selectivity controlled by the Cu/M pairing. By using the MeIMesCu-FeCp(CO)2 catalyst, a variety of α-vinylstannanes were produced from simple alkyl-substituted alkynes and Bu3SnH in high yield and good regioselectivity; these products are challenging to access under mononuclear metal-catalyzed hydrostannylation conditions. In addition, reversed regioselectivity was observed for aryl-substituted alkynes under the Cu/Fe-catalyzed conditions, affording the ( E)-β -vinylstannanes as major products. On the other hand, by using the IMesCu-Mn(CO)5 catalyst, a variety of ( E)- β-vinylstannanes were produced from primary, secondary, and tertiary alkyl-substituted alkynes, thus demonstrating divergent regioselectivity for alkyne hydrostannylation controlled by Cu/Fe vs Cu/Mn pairing. Both methods are amenable to gram-scale vinylstannane synthesis as well as late-stage hydrostannylation in a natural-product setting. Mechanistic experiments indicate the syn addition of Bu3SnH to the alkynes and imply the involvement of Sn-H bond activation in the rate-determining step. Two distinct catalytic cycles were proposed for the Cu/Fe and Cu/Mn catalysis based on stoichiometric reactivity experiments.