Reductions using Organoboron Compounds

Applications of Organoborates. Details have appeared of reactions of tri-alkylethenylborates with aldehydes, epoxides, and iodine, respectively useful for syntheses of 1,3-diols, 1,4-diols, and alk-l-enes. Trialkylalkynylborates cannot normally be acylated successfully because of cyclization and further rearrangement of. the initial intermediates. However, the trialkylalkynylborates 

The so-called copper trialkylmethylborates , formed by addition of Cu salts to lithium trialkylmethylborates, transfer an alkyl group from boron to carbon with direct displacement of halide in their reactions with allyl, propargyl, acyl, or 2-ethoxycarbonylvinyl halides. In the last case the stereochemistry about the double bond is retained. 

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The Petasis method involves the use of organoboron compounds to produce a side chain on a Mannich product derived from the imine condensation product between an amine and an a-ketocarboxylic acid. Thus, as shown in Scheme 12.64, as an example of this three-component reaction, when the styrylboronic acid derived from the reaction of phenylacetylene with catecholborane was allowed to react with glyoxylic acid (CHOCO2H) and (5)-2-phenylglycinol, a single diastereomer was isolated, which gave an enantiomerically pure (7 )-homophenylalanine hydrochloride on reduction. This procedure is limited only by the availability of the components. 

Hydroboration and asymmetric reductions proceed with high stereoselectivity because of the small atom radius and the large electronegativity of boron [22]. Hence, organoboron compounds are used for asymmetric reactions, especially for asymmetric reduction [9,22,36]. Both the yield and the optical yield of asymmetric reactions with organoboron compounds are high, and the isolation of chiral products is easy. Therefore, these reactions are exploited for syntheses of terpenes [22]. 

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