Brown hydroboration reaction

Boron has only three electrons in the valence shell, and therefore its compounds are electron deficient and there is a vacant p-orbital on the boron atom. Borane (BH3) exists as a mixture of BaHe/BHs, as dimerization partially alleviates the electron deficiency of the boron. This equilibrium is fast, and most reactions occur with BH3. The addition of borane to a double bond is a concerted process going through a four-centered transition state. The formation of the C-B bond precedes the formation of the C-H bond so that the boron and the carbon atoms are partially charged in the four-centered transition state. 

In the Cp2TiMe2-catalyzed hydroboration of alkenes, a titanocene bis(borane) complex is responsible for the catalysis. This bis(borane) complex initially dissociates to give a monoborane intermediate. Coordination of the alkene gives rise to the alkene-borane complex, which is likely to be a resonance hybrid between an alkene borane complex and a 3-boroalkyl hydride. An intramolecular reaction extrudes the trialkylborane product, and coordination of a new HBR2 regenerates the monoborane intermediate. 

In the enantiospecific total synthesis of the indole alkaloid trinervine, J.M. Cook and co-workers used the hydroboration/oxidation sequence to functionalize the C19-C20 exo double bond with excellent regioselectivity.  

During the enantioselective synthesis of (3aR,4R,7aS)-4-hydroxy-7a-methylperhydro-1-indenone, a suitable CD-ring fragment for vitamin D-anaiogs, M. Vandewalle et al. realized that the hydroboration/oxidation of (1,1)-ethylenedioxy-8a-methyl-1,2,3,4,6,7,8,8a-octahydronaphtalene led to a c/s-decalin structure instead of the literature reported trans-fusion.  

Knochel and co-workers used diphosphines as ligands in the rhodium-catalyzed asymmetric hydroboration of styrene derivatives. The best results were obtained with the very electron rich diphosphane, and (S)-1-phenylethanol was obtained in 92% ee at -35 °C, with a regioselectivity greater than 99 1 (Markovnikoff product). A lower reaction temperature resulted in no reaction, while a higher temperature resulted in lower enantioselectivity and regioselectivity. The regioselectivity was excellent in all cases. Irrespective of the electronic nature of the substituents, their position and size had a profound effect on the enantioselectivity. 

Addition of boranes to olefins, followed by basic oxidation of the organoborane adducts, resulting in alcohols. 

Pelter, A. Smith, K. Brown, H. C. Borane Reagents Academic Press New York, 1972. (Review). 

Hayashi, T. Comprehensive Asymmetric Catalysis I-III19995, 7, 351-364. (Review). 

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These last results combine the Wittig ylide chemistry with Brownes hydroboration reaction. We hope that a preparatively interesting ylide-borane chemistry will arise from this new "alloy". 

The Brown hydroboration reaction is the addition of B-H across a n-system (1) in an anti-Markovnikov fashion. Most commonly, this reaction utilizes BH3 THF as the hydroborating reagent and is followed by an oxidation of the newly formed C-B bond to afford an alcohol product (2). ... 

Clay, J. M. Brown hydroboration reaction. In Name Reactions for Functional Group Transformations, Li, J. J., Corey, E. J., Eds. Wiley Sons Hohoken, NJ, 2007 pp 183-188. (Review). 

Clay JM (2007) Brown hydroboration reaction. Name Reactions for Functional Group Transformation, 183-188... 

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