Lewis acid catalysis: catalytic hydroboration of alkynes initiated by Piers' borane

Abstract

Terminal and internal alkynes are efficiently hydroborated to (E)-alkenyl pinacol boronic esters with excellent yields and selectivities using a Lewis acid catalyst. In the case of Piers' borane (HB(C6F5)2) the borane acts as a pre-catalyst generating dissymmetrically gem-diborylated species of the form RCH2CR'(Bpin)(B(C6F5)2) which are the active catalysts.

Unsymmetrical 1,1-Bisboryl Species: Valuable Building Blocks in Synthesis

“…Interestingly, 1,1-bis(boryl)alkanes can behave as catalysts, too. Piers's borane V [HB(C 6 H 5 ) 2 ] precatalyzed the hydroboration of terminal and internal alkynes for the synthesis of E-alkenyl pinacol boronic ester 36, and excellent selectivities were reported by Stephan and coworkers in 2016 [34]. In this hydroboration, they found that unsymmetrical 1,1-bis(borane) 34 catalyzed the reaction.…”

“…The proposed mechanism for the unsymmetrical 1,1-bis(borane) 34 catalyzing the hydroboration of alkynes for the synthesis of E-alkenyl boronic ester 36 is shown in Scheme 10. In this mechanism, unsymmetrical 1,1-bisboranes 34 acts as a Lewis acid catalyst, which activates alkyne to form complex VI: then, HBpin reacts with complex VI in a concerted syn-1,2-hydroboration manner (complex VII) to afford the hydroborylated product 36 and regenerate the unsymmetrical 1,1-bis(borane) 34 [34].…”

“…The proposed mechanism for the unsymmetrical 1,1-bis(borane) 34 catalyzing the hydroboration of alkynes for the synthesis of E-alkenyl boronic ester 36 is shown in Scheme 10. In this mechanism, unsymmetrical 1,1-bisboranes 34 acts as a Lewis acid catalyst, which activates alkyne to form complex VI: then, HBpin reacts with complex VI in a concerted syn-1,2-hydroboration manner (complex VII) to afford the hydroborylated product 36 and regenerate the unsymmetrical 1,1-bis(borane) 34 [34]. In 2019, Sharma research group reported a novel and concise method for the synthesis of a wide range of MIDA (MIDA = N-methyliminodiacetic acid) acylboronates (41) via the chemoselective oxidation of 1,1-bisboranes (39) (Scheme 12) [35].…”

Alkenyl Boronates: Synthesis and Applications

“…Later,K nochel reported as imilar hydroboration of alkynes at room temperature with two equivalents of pinacolb orane,w hich is nowadays the most popularb oronate reagent owing to its great stability in air or during chromatography purifications (Scheme 1b). [8] Dicyclohexylborane [9] or HB(C 6 F 5 ) 2 [10] have been reported as catalysts for this reaction.…”

B(C6F5)3‐Enabled Synthesis of a Cyclic cis‐Arsaphosphene

“…[13,14] Similar reactivity has also been observed with substituted perfluoroaryl borane HB(C 6 F 5 ) 2 . [15] Herein, we discuss the synthesis of an arsanyl-phosphaketene which,u pon reactionw ith B(C 6 F 5 ) 3 ,w as found to yield an arsaphosphene in ar are cis-configuration. Moreover,w hen catalytic amounts of B(C 6 F 5 ) 3 were used, we found that dimeric species are generated through af ormal [2+ +2] cycloadditiono f the cyclic arsaphosphene.…”