J. Am. Chem. Soc. 136, 10605–10608 (2014)

Alkenyl boron derivatives are potentially useful synthetic intermediates that can, through subsequent cross-coupling reactions, be used to prepare highly substituted alkenes. Nickel- and palladium-catalysed additions of alkynyl or cyanoboron derivatives to alkynes have been described, but these methods have not been extended to more commonly available organoboranes. Now, Masaya Sawamura, Hirohisa Ohmiya and Kazunori Nagao from Hokkaido University, Japan, have reported a phosphine-catalysed addition of alkyl-, alkenyl- and arylboranes across the triple bond of alkynoates to produce β-borylalkenoates.

While investigating metal-catalysed reactions of organoboron compounds, they observed that, in the presence of a catalytic quantity of phosphine, an alkylborane reacted with an alkynoate. The addition occurred with the less electronegative boron adding to the more positively charged alkynoate carbon and with the overall carboboration occurring with anti-stereoselectivity — a mode of addition not previously observed using metal catalysis. The proposed mechanism of the reaction begins with conjugate addition of the phosphine catalyst assisted by the Lewis-acidic borane to form an allenolate intermediate. An alkyl group from the borane then migrates to the central carbon of the allenolate to produce a phosphonium ylide. Several bond rotations allow the formation of a cyclic borate that can then form the product — transferring the boryl group to the β-carbon of the formed alkene and eliminating the phosphine catalyst.

The scope of the reaction was investigated using a series of organoboranes and alkynoates. Sterically hindered aryl-, as well as alkenyl- and alkyl-alkynoates were successfully reacted, albeit with lower yields. A series of alkylboranes — prepared in situ by hydroboration of terminal alkenes and used without purification — were also found to react smoothly, as did aryl boranes. So far, simple trialkylphosphines are the only suitable catalysts with triarylphosphines, amines and N-heterocyclic carbenes all failing to provide any product. Attempts to cross-couple the β-borylalkenoates using palladium catalysis were presumably hampered by the sensitivity of the ester to aqueous base, though conversion to an amide first alleviated this issue, providing access to a stereodefined tetrasubstituted alkene.