Significant Cooperativity Between Ruthenium and Silicon in Catalytic Transformations of an Isocyanide

Abstract

Complexes [PhBP_3]RuH(η^3-H_2SiRR′) (RR′ = Me,Ph, 1a; RR′ = Ph_2, 1b; RR′ = Et_2, 1c) react with XylNC to form carbene complexes [PhBP_3]Ru(H)═[C(H)(N(Xyl)(η^2-H–SiRR′))] (2a–c; previously reported for 2a,b). Reactions of 1a–c with XylNC were further investigated to assess how metal complexes with multiple M–H–Si bonds can mediate transformations of unsaturated substrates. Complex 2a eliminates an N-methylsilacycloindoline product (3a) that results from hydrosilylation, hydrogenation, and benzylic C–H activation of XylNC. Turnover was achieved in a pseudocatalytic manner by careful control of the reaction conditions. Complex 1c mediates a catalytic isocyanide reductive coupling to furnish an alkene product (4) in a transformation that has precedent only in stoichiometric processes. The formations of 3a and 4 were investigated with deuterium labeling experiments, KIE and other kinetic studies, and by examining the reactivity of XylNC with an η^3-H_2SiMeMes complex (1d) to form a C–H activated complex (6). Complex 6 serves as a model for an intermediate in the formation of 3a, and NMR investigations at −30 °C reveal that 6 forms via a carbene complex (1d) that isomerizes to aminomethyl complex 7d. These investigations reveal that the formations of 3a and 4 involve multiple 4-, 5-, and 6-coordinate silicon species with 0, 1, 2, or 3 Ru–H–Si bonds. These mechanisms demonstrate exceptionally intricate roles for silicon in transition-metal-catalyzed reactions with a silane reagent.

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