Hydroboration. 59. Thexylchloroborane-Methyl Sulfide. A New Stable Monohydroborating Agent with Exceptional Regioselectivity

Article abstract of DOI:10.1021/jo00344a022

Under selected conditions, the hydroboration of 2,3-dimethyl-2-butene with 1 equiv of BH2Cl*SMe2 proceeds cleanly in solution (CH2Cl2, ClCH2CH2Cl, Et2O) or under neat conditions to give exclusively the monohydroboration product, thexylchloroborane-methyl sulfide (ThxBHCl*SMe2).Stock solutions of ThxBHCl*SMe2 in CH2Cl2 or Et2O have unusual thermal stability at ambient temperatures or below.The hydroboration of reactive olefins, such as terminal or unhindered disubstituted alkenes, with ThxBHCl*SMe2 proceeds quantitatively with high regiospecificity in CH2Cl2,ClCH2CH2Cl, Et2O, and THF to produce isomerically pure thexylalkylchloroborane intermediates.Subsequent oxidation produces the desired alcohols in nearly quantitative yield with high regiospecificity.With less reactive olefins, such as 1-methylcyclopentene, cyclohexene, or α-pinene, 11B NMR showed that the desired thexylalkylchloroborane products were contaminated with alkyldichloroborane species, indicating that a significant amount of product redistribution had occured.This was reflected in a lower observed regioselectivity in the hydroboration of some less reactive alkenes with ThxBHCl*SMe2.