Hydroboration. 55. Hydroboration of Alkynes with Dibromoborane-Dimethyl Sulfide. Convenient Preparation of Alkenyldibromoboranes

Article abstract of DOI:10.1021/jo01291a004

Dibromoborane-dimethyl sulfide undergoes direct hydroboration of both terminal and internal alkynes with remarkable facility to give alkenyldibromoboranes.These reactive alkenylboranes, which may be isolated, undergo many synthetically useful transformations.Oxidation provides the carbonyl compounds, while protonolysis with acetic acid occurs stereospecifically to yield the corresponding alkenes. 1-Alkenyldibromoboranes can be converted easily to 1-iodo-1-alkenes by basic hydrolysis and iodination.Both internal and 1-alkenyldibromoboranes serve as convenient precursors to symmetrical conjugated dienes by reaction with 3 equiv of methylcopper.Hydroboration of alkynes with HBBr2*SMe2 is critically examined in terms of relative reactivities of both alkyne and alkene substrates.A very broad reactivity spectrum is evident, with internal acetylenes reacting with remarkable facility.The regiospecifity of the hydroboration of unsymmetrically substituted alkynes indicates HBBr2*SMe2 to be a highly selective reagent, sensitive to both steric and electronic effects.The regioselectivity is compared with that of other hindered hydroborating reagents, such as 9-BBN and disiamylborane.