Ruthenium-Catalyzed (<i>Z</i>)-Selective Hydroboration of Terminal Alkynes with Naphthalene-1,8-diaminatoborane

Abstract

The metal-catalyzed (<i>Z</i>)-selective hydroboration of terminal alkynes is synthetically challenging due to the usually (<i>E</i>)-selective nature of the hydroboration and the formation of the thermodynamically unstable (<i>Z</i>)-isomer. Herein, we report that N-heterocyclic-carbene-ligated ruthenium complexes catalyze the (<i>Z</i>)-selective hydroboration of terminal alkynes with H-B(dan) (dan = naphthalene-1,8-diaminato), which generates a diverse range of synthetically valuable (<i>Z</i>)-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 (<i>Z</i>)-selective hydroborations. The direct cross-coupling of the obtained (<i>Z</i>)-alkenyl-B(dan) enables the rapid synthesis of biologically active Combretastatin A-4 analogues.