title: 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.
abstract: Cu-catalyzed methods for site-selective hydroboration of terminal alkynes, where the internal or α-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 °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 α-selectivity is achieved with substrates that bear an electron-withdrawing substituent. α-Selective Cu-catalyzed hydroborations are amenable to gram-scale procedures (1 mol % catalyst loading). Mechanistic studies are presented, indicating that α 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 β selectivity with the same class of substrates and under similar conditions.
authors.affiliation: Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, USA.
authors.forename: Hwanjong
authors.initials: H
authors.lastname: Jang
authors.forename: Adil R
authors.initials: AR
authors.lastname: Zhugralin
authors.forename: Yunmi
authors.initials: Y
authors.lastname: Lee
authors.forename: Amir H
authors.initials: AH
authors.lastname: Hoveyda