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Introduction to Hydroboration Reactions

Diborane acts readily with alkynes, but the creation of substituted alkene products leaves open the possibility of a second addition reaction. A witty method for avoiding this event takes benefit of the fact that alkynes do not usually suffer from steric hindrance near the triple-bond. Subsequently, large or bulky electrophilic reagents add easily to the triple-bond, but the resulting alkene is essentially more sterically or crowded hindered and resists additional additions. The bulky hydroboration reagent required for this strategy is prepared by reaction of diborane with 2-methyl-2-butene, a highly branched alkene. Due to the alkyl branching, just two alkenes add to a BH3 moiety (steric hindrance again), leaving one B-H covalent bond presented for reaction with an alkyne, as displayed below. The resulting dialkyl borane is known as disiamylborane, contraction of di-secondary-isoamylborane (amyl is an older name for pentyl).

2 (CH3)2C=CHCH3   +   BH3 in ether   -->  [ (CH3)2CH-CH(CH3) ]2B-H     disiamylborane

A significant application of disiamylborane is its addition reaction to terminal alkynes. The B-H reagent group adds in an apparently anti-Markovnikov manner, like with alkenes, because of the fact that the boron is the electrophile, not the hydrogen. Further addition to resulting boron-substituted alkene does not occur, and the common oxidative removal of boron by alkaline hydrogen peroxide gives an enol which quickly rearranges to the aldehyde tautomer. So, by the correct selection of reagents, terminal alkynes might be converted either to aldehydes (hydroboration followed by oxidation) or methyl ketones (mercuric ion catalyzed hydration).

RC≡CH   +   (C5H11)2B-H   -->   [ RCH=CH-B(C5H11)2 ]   +   H2O2 & NaOH   -->   [ RCH=CH-OH ] -->   RCH2-CH=O

Hydroboration of inner alkynes is not a specifically helpful procedure because a mixture of products will frequently be obtained, except the triple-bond is symmetrically substituted. Mercuric ion catalyzed hydration gives identical results.

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Oxidations

With oxidizing agents Reactions of alkynes like potassium permanganate and ozone generally result in cleavage of the triple-bond to give carboxylic acid products. A usual equation for this type of transformation follows. The symbol [O] is often employed in a general way to designate an oxidation.

RC≡CR'   + [O]   --> RCO2H   +   R'CO2H

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