Catalytic conjugate addition of a boron nucleophile (M-B) to unsaturated carbonyl substrates [1] has emerged as a novel synthetic tool to afford b-boryl carbonyl compounds, [2] an important class of organoboron compounds. While several important and efficient methodologies for conjugate boration have been developed, [3] the asymmetric conjugate boration of b,b-disubstituted a,b-unsaturated esters is especially challenging because of the low reactivity and poor enantioselectivity exhibited by these substrates. The lower reactivity of b,b-disubstituted a,b-unsaturated esters, compared to enones [1b, j] and b-monosubstituted substrates, is due to the inherently lower electrophilicity and greater steric bulkiness at the reaction site. As is often practiced with catalysts to address this type of challenge, the development of a more efficient catalytic system to enhance reactivity and enantioselectivity is required. In this communication, we describe a highly efficient catalytic system for the asymmetric conjugate boration of b,b-disubstituted a,b-unsaturated esters that produces tertiary organoboron compounds with a quaternary boron-substituted carbon center [4] and ester functional group. We also report the successful application of these organoboranes as carbon nucleophiles in a rhodiumcatalyzed addition reaction.
Recently, Shibasaki and co-workers reported an efficient copper-catalyzed asymmetric conjugate boration of b-substituted cyclic enones in DMSO solvent. [5] However, in light of a major difficulty, the ketoQenol isomerization of b-borylated C-bound Cu ester enolate intermediates in copper-catalyzed borations, compared with enones, [6] we predicted and observed that b,b-disubstituted a,b-unsaturated esters would not be efficiently converted to products under the same conditions (CuPF 6 A C H T U N G T R E N N U N G (CH 3 CN) 4 , LiOtBu, (R,R)-QuinoxP, DMSO, [a] X.