Uncatalyzed hydroboration of alkenes and alkynes

Odorless borane-sulfide complexes 36a-e65-68 and borane-amine complexes 37a-g69-75 have been recently prepared because of the growing importance of diborane for the synthesis of pharmaceuticals or other compounds and certain inconveniences of well-established reagents, for example, the low concentration and instability of BH3-THF and high volatility, flammability, and unpleasant odor of dimethyl sulfide from BH3-SMe2. Trialkylsilanes, for example, Me3SiH, were found to be an efficient reducing reagent for the in situ preparation of 

36a S[CH2ch2ch(CH3)2]265 36b S(CH2ch2och3)265 36c (ch3och2ch2sch2ch2o)3b66 Sia2BH 36d c8f17ch2ch2sch367 38 

Hydroboration of divinyltin with 9-BBN unexpectedly resulted in a previously unknown cyclization (Equation (3)).109 This unusual behavior contrasts markedly with that of other metalloids (Si, Ge) which undergo normal dihydroboration. 

1-Alkenylboronic acids and esters have been prepared by thermal or catalyzed hydroboration of 1-alkynes with catecholborane (HBcat), pinacolborane (HBpin), or dihaloboranes 41-43, followed by hydrolysis to boronic acids or alcoholysis to boronic esters. A convenient alternative to improve chemo- and regioselectivity is the hydroboration of alkynes with dialkylboranes. For selective removal of dummy groups, the oxidation of two cyclohexyl groups was conduced by treatment of l-alkenyl(dicyclohexyl)borane intermediates with Me3N-0 (Equation (7)).116 The 

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