Hydroboration: LVIII. Monohydroboration of alkynes with representative dialkylboranes of varying steric requirements. a general synthesis of dialkylvinylboranes

Abstract

The hydroboration of alkynes with representative dialkylboranes (R₂BH) covering a range of structural types has been examined. Convenient methods now available for the preparation of a variety of R₂BH, via hydridation of the corresponding dialkylchloroboranes (R₂BCl), permitted a systematic investigation of the hydroboration of terminal and internal alkynes with these reagents. The monohydroboration internal alkynes generally proceeds cleanly. However, in the monohydroboration of terminal alkynes, varying amounts of dihydroborated products are also formed, the amount of such side products varying with the steric requirements of R₂BH. Thus, relatively less hindered dialkylboranes, such as borinane, 9-borabicyclo [3.3.1]nonane (9-BBN), and di-n-hexylborane, produce considerable quantities of 1, 1-diboraalkanes, whereas, sterically more demanding reagents, such as bis(3-hexyl)borane, dicyclohexylborane, and disiamylborane, afford insignificant amounts of dibora derivatives, Dihydroboration is noticeably suppressed by carrying out the hydroboration at lower reaction temperatures.