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BH3·THF and BH3·Me2S – Versatile Diborane Alternatives
Borane Complexes serve as the preferred borane sources in various organic transformations
Borane-tetrahydrofuran (BTHF) and borane-dimethyl sulfide (DMSB) complexes have become a valuable part of the organic chemistry toolbox and have found their way into several commercial applications on an industrial scale. They are valuable reagents for the reduction of functional groups and for hydroboration reactions with carbon-carbon double and triple bonds. Functional groups that are readily reduced by BTHF and DMSB include aldehyde, ketone, carboxylic acid, amide, oxime, imine, and nitrile. The carboxylic acid group is reduced at a faster rate than most groups including non-conjugated alkenes. [1, Review].
Table. 1. Borane-tetrahydrofuran (BTHF) and borane-dimethyl sulfide (DMSB) complexes
Borane (BH3) is an electrophilic reducing agent. This electrophilicity of borane can be viewed as possessing Lewis acid character since the boron is electron deficient. Due to the electrophilic nature of borane and complexation with the electron-rich center of the functional group, reductions with borane are very selective and specific.
The catalytic effect of ethers in the hydroboration reaction led to the development of more stable borane ligand complexes (BH3·L) as preferred borane sources at commercial scales. Borane adducts of Lewis bases such as tetrahydrofuran (THF) and dimethyl sulfide (DMS) have been used very effectively for hydroboration of double and triple bonds as well as in reductions of other functional groups. The strength of the Lewis base determines the reactivity of the borane complex. Borane tetrahydrofuran complex (BTHF) is the most reactive.
From a practical point of view, diborane (B2H6), is a useful reagent with many applications, but it is a pyrophoric gas, that is not convenient to handle. Therefore, THF and Me2S ligands used for the stabilization of BH3 makes it convenient to use instead of the extremely toxic and flammable diborane. But it is not without its own risks. For example, BH3·THF complex reacts violently with water generating hydrogen. Therefore, BH3·THF is normally sold as a 1 M solution in THF. BH3·Me2S is more stable than BTHF, available in higher concentrations or even neat, however its unpleasant odor is regarded as a major disadvantage.
BH3·THF and BH3·Me2S have found utility in several organic transformations of highly complex functionalized molecules and in the synthesis of critical drug intermediates [1]. Recent applications are reviewed in the corresponding individual technical notes. Also, please check out our blog on Corey-Bakshi-Shibata (CBS) catalysts.
References:
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05-0175 CALLERY™ Borane tetrahydrofuran, 1M in tetrahydrofuran (14044-65-6)
05-0110 CALLERY™ Dimethylsulfide borane, min. 93% (13292-87-0)
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