Hydroboration. 54. New General Synthesis of Alkyldihaloboranes via Hydroboration of Alkenes with Dihaloborane-Dimethyl Sulfide Complexes. Unusual Trends in the Reactivities and Directive Effects

Article abstract of DOI:10.1021/jo01291a003

The reactions of alkenes with the dimethyl sulfide complexes of the dihaloboranes (HBX2*SMe2; X = Cl, Br, I) have been studied in detail.Dichloroborane-dimethyl sulfide (HBCl2*SMe2) hydroborates representative olefins relatively slowly and requires the presence of a strong Lewis acid, such as boron trichloride, to complete the hydroboration reaction rapidly.Unexpectedly, dibromoborane-dimethyl sulfide (HBBr2*SMe2) and diiodoborane-dimethyl sulfide (HBI2*SMe2) react readily with olefins, even in the absence of such Lewis acids.This is contrary to the trend expected on the basis of the strenghts of these methyl sulfide adducts and a hydroboration mechanism involving a prior dissociation of the addition compound.The hydroboration of olefins with these reagents, followed by distillation under reduced pressure, affords alkyldihaloborane-dimethyl sulfide complexes in good yields.These are readily converted by hydrolysis into the boronic acids or by methanolysis to the corresponding esters.Oxidation with alkaline hydrogen peroxide utilizing sufficient sodium hydroxide to neutralize the hydrogen halide readily provides the corresponding alcohols.HBBr2*SMe2 and HBI2*SMe2 exhibit an unusual directive effect in the hydroboration of trisubstituted olefins, giving unexpected enhanced amounts of the Markovnikov (tertiary) derivatives.