Efficient (Z)-selective semihydrogenation of alkynes catalyzed by air-stable imidazolyl amino molybdenum cluster sulfides

Abstract

Imidazolyl amino cuboidal Mo₃(μ₃-S)(μ-S)₃ clusters have been investigated as catalysts for the semihydrogenation of alkynes. For that purpose, three new air-stable cluster salts [Mo₃S₄Cl₃(ImNH₂)₃]BF₄ ([1]BF₄), [Mo₃S₄Cl₃(ImNH(CH₃))₃]BF₄ ([2]BF₄) and [Mo₃S₄Cl₃(ImN(CH₃)₂)₃]BF₄ ([3]BF₄) have been isolated in moderate to high yields and fully characterized. Crystal structures of complexes [1]PF₆ and [2]Cl confirm the formation of a single isomer in which the nitrogen atoms of the three imidazolyl groups of the ligands are located trans to the capping sulfur atom which leaves the three bridging sulfur centers on one side of the trimetallic plane while the amino groups lie on the opposite side. Kinetic studies show that the cluster bridging sulfurs react with diphenylacetylene (dpa) in a reversible equilibrium to form the corresponding dithiolene adduct. Formation of this adduct is postulated as the first step in the catalytic semihydrogenation of alkynes mediated by molybdenum sulfides. These complexes catalyze the (Z)-selective semihydrogenation of diphenylacetylene (dpa) under hydrogen in the absence of any additives. The catalytic activity lowers sequentially upon replacement of the hydrogen atoms of the N–H₂ moiety in 1⁺ without reaching inhibition. Mechanistic experiments support a sulfur centered mechanism without participation of the amino groups. Different diphenylacetylene derivatives are selectively hydrogenated using complex 1⁺ to their corresponding Z-alkenes in excellent yields. Extension of this protocol to 3,7,11,15-tetramethylhexadec-1-yn-3-ol, an essential intermediate for the production of vitamin E, affords the semihydrogenation product in very good yield.