A New Catalytic Route to Boryl‐ and Borylsilyl‐Substituted Buta‐1,3‐dienes

Abstract

Vinyl-substituted boronates in the presence of complexes containing Ru-H bonds (preferably [Ru(CO)ClH(PCy(3))(2)], Cy: cyclohexyl) react regioselectively with terminal ethynes (involving silylethynes), albeit with the exception of phenylacetylene, to produce boryl- and borylsilyl-substituted buta-1,3-dienes with a preference for E,E-diene. The reaction opens a new catalytic route for the preparation of dienylboronates, and particularly dienylsilylboronates, that are functionalised building blocks in the synthesis of organic and natural products. The mechanism of this new reaction was proved to involve an insertion of alkyne into Ru-H bonds followed by an insertion of coordinated vinyl boronate into the Ru-C= bond and beta-hydrogen transfer to the metal to eliminate boryldiene or borylsilyldiene.

Stereo- and Regioselective Formation of Silyl-Dienyl Boronates

“…This effect is presumably due to competitive formation of boronate dimer (D) which consumes vinyl boronate (B) prior to coupling. 22 We also noted that product isolation was difficult most likely due to boronate hydrolysis upon exposure to silica gel. Measures were taken to deactivate the silica gel with triethylamine, trimethylchlorosilane, or boric acid 23 with no improvement in isolated yield.…”

Mechanism of co-dimerization of vinylboronates with terminal alkynes catalyzed by ruthenium-hydride complex

“…2.). The best results were obtained for the most active complex [Ru(CO)ClH(PCy 3 ) 2 ] [18]. Lower reactivity of vinylboronates in reactions with terminal alkynes in comparison to metalative coupling of alkynes with vinylsilanes and vinylgermanes is responsible for different pathway.…”

“…According to the proposed mechanism [18], the catalytic cycle reported by Lin and Marder [16] starts with the dissociation of tricyclohexylphosphine and the formation of a four-coordinative catalyst as suggested by calculations of the trans-borylation of an olefin with boronates. Such 14-electron complex can easily coordinate a terminal alkyne (A1).…”

“…We have shown that this type of complex is formed as an intermediate in the codimerization reaction of vinylboronates with terminal organic alkynes and ethynylsilanes [18].…”

“…The reaction of vinylboronates with terminal alkynes took, unexpectedly, a different course, giving borylsubstituted buta-1,3-dienes as products instead of alkynylboranes [18]. Co-dimerization of vinylboronates with terminal alkynes (organic and silylacetylenes) occurs in presence of complexes with a Ru H bond (Fig.…”