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

Cheng, Li-Jie, Mankad, Neal P.
Journal of the American Chemical Society 2019 v.141 no.8 pp. 3710-3716
alkynes, catalysts, catalytic activity, chemical bonding, copper, iron, manganese, regioselectivity
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.