Reactions of Tetrahydroborates with Organic Compounds

Tetrahydroborates are widely used in organic chemistry and to cover all their uses here would be impossible hence, only selected examples are included herein. 

Reactions of tetrahydroborates with acidic species were reinvestigated and it was found that Li[BH4] and Na[BH4] react in certain instances to form different products. For example, Li[BH4] reacts with phenol at 25 C in tetrahydrofuran to give equimolar mixtures of Li-[HB(0C6H5)3] and Li[B(OC6H5)4] Na[BH4] reacts similarly. On the other hand, Na[BH4] was found not to produce any Na[H3B-0C(0)CH3] when allowed to react with acetic acid, since Na[H3B-0C(0)CH3] disproportionates in solution. This result is in conflict with previous reports which indicated that Na[H3B-0C(0)CH3] is formed as a stable product and is used for hydroboration and reduction. Li[BH4] reacts with acetic acid to produce more than one equivalent of H2 and BH3, which is trapped as the adduct. Thus, in (C2H5)20/S(CH3)2 (1 1) the reaction proceeds at 25 C (15 minutes) as follows [1]  

A kinetic study of the reduction of 4-substituted camphors with Na[BH4] indicated that the major product is the exo-alcohol. The reaction rates are greater for electron-withdrawing substituents and the results are ascribed to increasing the ground state double bond character and stabilization of the transition state [10]. 

The use of polymer supports in tetrahydroborate reductions was investigated. The effects of polymer texture and whether phase-transfer catalysts are used was found to be very important [11, 12]. 

The utility of Na[BH4] as a reducing agent is enhanced by the presence of catalysts. For example, CgHs-C C-CeHs is not reduced by Na[BH4], but in the presence of Rh(CO)Cl3 or HRh[P(C6H5)3]3 in C6H6/C2H5OH (1 1 25 C), a 5 1 mixture of trans- to c/s-stllbene is formed with the former, and pure frans-stilbene is formed with the latter [13]. Similarly, porphyrin metal complexes catalyze the reduction of ketones with Na[BH4] in tetrahydrofuran [14]. 

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