Photoinduced metal-free diboration of alkynes in the presence of organophosphine catalysts

In sharp contrast to the transition metal-catalyzed diboration of alkynes with diboron species such as B(2)pin(2), which affords vicinal cis-diborated alkenes, the organophosphine-catalyzed addition of B(2)pin(2) to alkynes proceeds under metal-free conditions upon photoirradiation to give the corresponding vicinal trans-diborated alkenes with good stereoselectivity. For example, the photoinduced diboration of 1-octyne with B(2)pin(2) in the presence of tris(o-methylphenyl)phosphine afforded 1,2-diborated octene with excellent trans selectivity (trans/cis=95/5). The influence of various organophosphines on the diboration is described in detail. The diboration does not occur in the dark or in the absence of an organophosphine catalyst. Moreover, ESR (electron spin resonance) measurements of the reaction mixture revealed a radical species around g=2.003, supporting a radical pathway for the organophosphinecatalyzed diboration. In addition, the photoinduced cis to trans isomerization of vicinal cis-diborated alkenes in the presence/absence of organophosphines is discussed. (C) 2016 Elsevier Ltd. All rights reserved.