Catalytic B–H Bond Insertion Reactions Using Alkynes as Carbene Precursors

Abstract

Herein, we report transition-metal-catalyzed B-H bond insertion reactions between borane adducts and alkynes to afford organoboron compounds in excellent yields under mild reaction conditions. This successful use of alkynes as carbene precursors in these reactions constitutes a new route to organoboron compounds. The starting materials are safe and readily available, and the reaction exhibits 100% atom-economy. Moreover, an asymmetric version catalyzed by chiral dirhodium complexes produced chiral boranes with excellent enantioselectivity (up to 96% ee). This is the first report of highly enantioselective heteroatom-hydrogen bond insertion reactions of metal carbenes generated in situ from alkynes. The chiral products of the reaction could be easily transformed to widely used borates and diaryl methanol compounds without loss of optical purity, which demonstrates its potential utility in organic synthesis. A kinetics study indicated that the Cu-catalyzed B-H bond insertion reaction is first order with respect to the catalyst and the alkyne and zero order with respect to the borane adduct, and no kinetic isotopic effect was observed in the reaction of the adduct. These results, along with density functional theory calculations, suggest that the formation of the Cu carbene is the rate-limiting step and that the B-H bond insertion is a fast, concerted process.

Genetically programmed chiral organoborane synthesis

“…11 e , Rh 2 ( R -BTPCP) 4 -catalysed borylation using alkynes as carbene precursors. 42 f , I 2 -catalysed borylation of diazo esters with NHC-boranes. 43 …”

Copper-Catalyzed Ring-Opening/Borylation of Cyclopropenes

“…Since transition-metalcatalyzed insertion of carbenes into B-H bonds was first disclosed in 2013, it has become a good method for the construction of C-B bonds. [7][8][9][10] When diazo compounds, [11][12][13][14][15][16][17] alkynes, [18][19] or sulfoxonium ylides [20][21] are used as carbene precursors, this method enables the construction of many novel organoboranes in high yields with high regioselectivity and, in some cases, high enantioselectivity (Scheme 1a). Quite recently, insertion of unsaturated carbenes into B-H bonds has also been established as a method for constructing C(sp 2 )-B bonds.…”

Bottom-up Modular Synthesis of Well-Defined Oligo(arylfuran)s

“…Different transition metals were tested for this transformation. The coinage metal salts, which were typically good catalysts for enynones, 29,41,42 were almost ineffective for this transformation (entries 1-3). ZnCl2, another important catalyst for the cycloisomerization of enynones, 43 could not catalyze this reaction either (entry 4).…”