Enantioselective Synthesis of Allylboronates by NHC Copper Catalyzed Substitution

Significance

Chiral allylboronates represent a valuable class of synthetic intermediates. The authors developed a catalytic asymmetric protocol, which allows the preparation of allylboronates bearing B-substituted tertiary and (for the first time) quaternary carbon stereogenic centers. Subsequent oxidation furnished enantiomerically enriched carbinols. Transformations proceeded with high yields (up to >98%) and enantioselectivities (er > 99:1); high site selectivity (>98% S_N2′) is also notable. Aryl-containing carbonates containing a trisubstituted alkene favor benzylic C-H bond formation in contrast to the disubstituted counterparts, which react with the opposite sense of regioselectivity (Y. Lee, A. H. Hoveyda J. Am. Chem. Soc. 2009, 131, 3160).