Quaternary ammonium borohydrides

Hydroboration of alkenes in non-ethereal solvent has been reported using diborane generated in situ from a quaternary ammonium borohydride and bromoethane (see Section 11.5). Almost quantitative yields of the alcohols are reported [e.g. 1 ]. As an alternative to the haloalkane, trimethylsilyl chloride has also been used in conjunction with the ammonium borohydride [2]. Reduction of the alkene to the alkane also occurs as a side reaction (<20%) and diphenylethyne is converted into 1,2-diphenylethanol (70%), via the intermediate /ra 5-stilbene. 

Quaternary ammonium aluminium hydrides have also been prepared from the reaction of the quaternary ammonium borohydride and lithium aluminium hydride in THF at 20°C. LV Titov and VD Sasnovskaya, Zh. Neorg. Khim. 1974,19, 258 Chem. Abstr., 1974,80,81952. 

For many years, prior to the development of current phase-transfer catalytic techniques, tetraalkylammonium borohydrides have been used in non-hydroxylic solvents [see, e.g. I, 2], Originally, the quaternary ammonium borohydrides were obtained by metathesis in water or an alcohol [3, 4], However, with greater knowledge of the phase-transfer phenomenon, an improved procedure has been developed in which the ammonium salt is transferred into, and subsequently isolated from, dichloromethane [5, 6], In principle, it should be possible to transfer the quaternary ammonium borohydride for use in any non-miscible organic solvent. It should be noted, however, that quaternary ammonium cations are susceptible to hydrogeno-lysis by sodium borohydride in dipolar aprotic solvents to yield tertiary amines [4]. 

Early use of the low-molecular-weight quaternary ammonium borohydrides in hydrocarbon solvents showed little advantage over the use of sodium borohydride in aqueous or alcoholic media. Although the ammonium salts in benzene appealed to be capable of effecting all the normal reductions exhibited by the sodium salt in water, they appeared to be generally less reactive. This is well illustrated by the recrystallization of tetra-n-butylammonium borohydride from acetone, if the operation is performed rapidly [5,6],... 

Although there is evidence that quaternary ammonium salts are cleaved by sodium borohydride at high temperature [7], initial studies suggested that the quaternary ammonium borohydrides might have some synthetic value in their selectivity, e.g. aldehydes are reduced by an excess of the quaternary ammonium salts under homogeneous conditions in benzene at 25 °C, whereas ketones are recovered unchanged and are only partially reduced at 65 °C [2], The reduction of esters also requires the elevated temperature, whereas nitriles are not reduced even after prolonged reaction at 65 °C. Evidence that the two-phase (benzene water) reduction of octan-2-one by sodium borohydride was some 20-30 times faster in the presence of Aliquat, than in the absence of the catalyst [8], established the potential use of the mote lipophilic catalysts. 

Much emphasis has been placed on the selectivity of quaternary ammonium borohydrides in their reduction of aldehydes and ketones [18-20]. Predictably, steric factors are important, as are mesomeric electronic effects in the case of 4-substituted benzaldehydes. However, comparison of the relative merits of the use of tetraethyl-ammonium, or tetra-n-butylammonium borohydride in dichloromethane, and of sodium borohydride in isopropanol, has shown that, in the competitive reduction of benzaldehyde and acetophenone, each system preferentially reduces the aldehyde and that the ratio of benzyl alcohol to 1-phenylethanol is invariably ca. 4 1 [18-20], Thus, the only advantage in the use of the ammonium salts would appear to facilitate the use of non-hydroxylic solvents. In all reductions, the use of the more lipophilic tetra-n-butylammonium salt is to be preferred and the only advantage in using the tetraethylammonium salt is its ready removal from the reaction mixture by dissolution in water. 

Demercuration of organomercury compounds is a critical step in synthetic procedures, which involve mercuration-initiated cyclization reactions [e.g. 41], Many of the standard procedures for demercuration result in rearrangement or ring cleavage of the system, but reductive carbon-mercury cleavage (e.g. Scheme 11.4) with an excess of the quaternary ammonium borohydride is effective under phase-transfer conditions [e.g. 42,43]. 

Quaternary ammonium borohydrides react with diborane to produce the corresponding ammonium diboronoheptahydrides, which behave both as borohydrides and as diborane [3]. 

It was initially proposed that reduction of ketones by quaternary ammonium borohydrides in dichloromethane (see Section 11.3) might be accounted for by the initial slow formation of diborane. However, the generation of diborane under such conditions is too slow to have any influence on the reduction. 

The reactive anionic hydridometalcarbonyl complexes can be preformed from the neutral metal carbonyls using quaternary ammonium borohydrides either under homogeneous conditions or two-phase catalytic conditions [5] and are used in a range of reductive processes. The preparation of tetraethylammonium hydridotri-iron undecylcarbonyl is used as an illustrative example. 

Polymeric Quaternary Ammonium Borohydride Reducing Agents... 

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