Highly Selective Methods for Synthesis of Internal (α-) Vinylboronates through Efficient NHC-Cu-Catalyzed Hydroboration of Terminal Alkynes. Utility in Chemical Synthesis and Mechanistic Basis for Selectivity

Cu-catalyzed methods for site-selective hydroboration of terminal alkynes, where the internal or alpha-vinylboronate is generated predominantly (up to >98%) are presented. Reactions are catalyzed by 1-5 mol % of N-heterocyclic carbene (NHC) complexes of copper, easily prepared from N-aryl-substituted commercially available imidazolinium salts, and proceed in the presence of commercially available bis-(pinacolato)diboron[B-2(pin)2] and 1.1 equiv of MeOH at -50 to -15 degrees C in 3-24 h. Propargyl alcohol and amine and the derived benzyl, tert-butyl, or silyl ethers as well as various amides are particularly effective substrates; also suitable are a wide range of aryl-substituted terminal alkynes, where higher alpha-selectivity is achieved with substrates that bear an electron-withdrawing substituent. alpha-Selective Cu-catalyzed hydroborations are amenable to gram-scale procedures (1 mol % catalyst loading). Mechanistic studies are presented, indicating that alpha selectivity arises from the structural and electronic attributes of the NHC ligands and the alkyne substrates. Consistent with suggested hypotheses, catalytic reactions with a Cu complex, derived from an N-adamantyl-substituted imidazolinium salt, afford high beta selectivity with the same class of substrates and under similar conditions.