Hydroboration. 64. Effect of Structure on the Relative Reactivity of Representative Alkenes and Alkynes toward Hydroboration by Dibromoborane-Methyl Sulfide

Article abstract of DOI:10.1021/jo00153a004

The relative reactivities of 22 alkenes and 5 alkynes toward hydroboration by dibromoborane-methyl sulfide (Br2BH*SMe2) have been determined in CH2Cl2 at 25 deg C by the competitive method.The data are compared and contrasted with those available on 9-borabicyclo<3.3.1>nonane (9-BBN), disiamylborane (Sia2BH), and thexylchloroborane-methyl sulfide (ThxBHCl*SMe2).Br2BH*SMe2 exhibits a uniquely high selectivity toward an internal triple bond compared to a terminal triple or double bond, while the reverse is true with 9-BBN.Consequently, 1-octen-4-yne has been selectively converted to (4Z)-1,4-octadiene by hydroboration with Br2BH*SMe2, followed by protonolysis.The terminal double bond, on the other hand, is selectively hydroborated by 9-BBN.The far higher reactivity of Br2BH*SMe2 toward the C(CH3)=CH2 structure, as compared with the CH=CH2 structure makes possible the selective hydroboration of the 1,2-positions of 2-methyl-1,5-hexadiene; Sia2BH, on the other hand, is known to hydroborate the 5,6-positions selectively.Thus we have at hand several monofunctional hydroborating agents with widely different selectivities, making them highly useful for selective hydroborations for synthesis.