Abstract
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.
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