Ligand‐Controlled Regiodivergent Cyanoboration of Internal Allenes by Copper Catalysis

The first copper‐catalyzed regiodivergent cyanoboration of internal allenes with B2pin2 (bis(pinacolato)diboron) and NCTS (N‐cyano‐N‐phenyl‐p‐toluenesulfonamide) derivatives is reported. The β,γ‐ and α,β‐cyanoborylated products were synthesized with high regio‐ and stereo‐selectivity. Computational studies revealed that nucleophilic addition of allylcopper or related intermediates on cyanation reagent is the regio‐ and stereo‐determining step, while transmetalation with B2pin2 is the rate‐determining step. The nucleophilic addition step proceeds via inner‐sphere mechanism in the CuI/P(o‐tol)3 and CuI/Xantphos (P(o‐tol)3=tris(o‐methylphenyl)phosphine, Xantphos=4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene) catalytic systems and via outer‐sphere mechanism in the CuII/Xantphos catalytic system, respectively. The first copper‐catalyzed regiodivergent cyanoboration of internal allenes with B2pin2 and NCTS derivatives has been developed. MeOH was identified as the necessary additive for the regeneration of LCu‐Bpin species. Computational studies revealed that the copper‐mediated allylic cyanation is the regio‐ and stereo‐determining step, while the MeOH‐assisted transmetalation with B2pin2 is the rate‐determining step.