(Z)-Selective Hydrosilylation and Hydroboration of Terminal Alkynes Enabled by Ruthenium Complexes with an N-Heterocyclic Carbene Ligand.

Metal-catalyzed trans-1,2-hydrosilylations and hydroborations of terminal alkynes that generate synthetically valuable (Z)-alkenylsilanes and (Z)-alkenylboranes remain challenging due to the (E)-selective nature of the reactions and the formation of the thermodynamically unfavorable (Z)-isomer. The development of new, efficient catalytic systems for the (Z)-selective hydrosilylation and hydroboration of terminal alkynes is thus highly desirable from a fundamental perspective as it would deepen our understanding of the metal-catalyzed (Z)-selective hydrosilylation and hydroboration of terminal alkynes. This personal account describes our research for developing a ruthenium complex that can efficiently catalyze the hydrosilylation and hydroboration of terminal alkynes, and for exploring the factors controlling (Z)-selectivity of the reactions. Our effort into the activation of B-protected boronic acids, R-B(dan) (dan=naphthalene-1,8-diaminato), that was believed not to participate in Suzuki-Miyaura cross-coupling, is also discussed.

Ruthenium-Catalyzed (Z)-Selective Hydroboration of Terminal Alkynes with Naphthalene-1,8-diaminatoborane.

The metal-catalyzed (Z)-selective hydroboration of terminal alkynes is synthetically challenging due to the usually (E)-selective nature of the hydroboration and the formation of the thermodynamically unstable (Z)-isomer. Herein, we report that N-heterocyclic-carbene-ligated ruthenium complexes catalyze the (Z)-selective hydroboration of terminal alkynes with H-B(dan) (dan = naphthalene-1,8-diaminato), which generates a diverse range of synthetically valuable (Z)-alkenylboranes. Mechanistic studies, particularly the isolation of a catalytically relevant borylruthenium complex, revealed a mechanism that involves the insertion of the alkyne into a Ru-B bond, which provides a catalytic cycle that is distinctly different from that of previously reported (Z)-selective hydroborations. The direct cross-coupling of the obtained (Z)-alkenyl-B(dan) enables the rapid synthesis of biologically active Combretastatin A-4 analogues.

(Z)-Selective Hydrosilylation of Terminal Alkynes with HSiMe(OSiMe3)2 Catalyzed by a Ruthenium Complex Containing an N-Heterocyclic Carbene.

The N-heterocyclic-carbene-ligated ruthenium complex [RuHCl(CO)(H2IMes)(PCy3)] exhibits high catalytic activity for the (Z)-selective hydrosilylation of various terminal alkynes with 1,1,1,3,5,5,5-heptamethyltrisiloxane (HSiMe(OSiMe3)2). The stereoretentive derivatization of the (Z)-alkenylsiloxanes allows the synthesis of biologically active compounds, e.g. potent antitumor agents and inhibitors for induced-NO synthase.