Quaternary ammonium borohydride reducing

Polymeric Quaternary Ammonium Borohydride Reducing Agents... 

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. 

One-pot conversions of 2-hydroxy(acylbenzenes) with anhydrides or acid chlorides to produce coumarins [52-54] and flavones [54-58] under mild liquiddiquid or solidtliquid two-phase conditions via a Baker-Venkataraman type reaction (Scheme 6.19) are catalysed by quaternary ammonium salts. 3-Substituted coumarins are produced from salicylaldehyde and malonodinitrile, or substituted acetonitriles, in high yield (>85%) in a one-pot catalysed sequential aldol-type reaction and cycliza-tion in the absence of an added organic solvent [59]. When 2 -hydroxychalcones are reduced under catalytic two-phase conditions with sodium borohydride, 2,4-cis-flavan-4-ols are produced [60] (see Section 11.3). 

Polymer-bound reagent. Reaction of a quaternary ammonium anion-exchange resin (Amberlites) wth NaBH4 results in an immobilized borohydride reducing agent (1), which is somewhat less reactive than NaBH4.4... 

Most alkynones are reduced to the corresponding / -alkynols with high (62— 82%) enantiomeric excess using a complex formed from lithium aluminium hydride and the amino-alcohol (10). The asymmetric reduction of ketones by borohydride in the presence of quaternary ammonium derivatives of (25,35)-l,4-diaminobutane-2,3-diol has been described. When lithium aluminium... 

Sodium borohydride adsorbed on alumina is a new reagent combination for the reduction of aldehydes and ketones to alcohols in aprotic solvents such as ether. Cyanoborohydride anion, [BHsCN] , supported on an anion-exchange resin containing quaternary ammonium groups has been found in preliminary results to be a successful and convenient reagent for, inter alia, the reduction of enones to allylic alcohols. Ease of work-up and retention of cyanide by the resin are obvious advantages. Carbonyl compounds can also be reduced to alcohols by tributyltin hydride in a cyclohexane-silica gel slurry aldehydes are selectively reduced in the presence of ketones and products are obtained by simple elution from the silica. Adsorption, and consequent polarization, of the carbonyl group by the silica seems to be important in the mechanism of this mild, non-basic, process. 

Rather surprisingly alcohols are poor at reducing imines, yet TEAF works well. During our studies we rationalized that the TEAF system was sufficiently acidic (pH approximately 4) to protonate the imine (pK l approximately 6) and that it was an iminium that was reduced to an ammonium salt [14]. When an iminium was used in the I PA system, it was reduced albeit with a low rate and moderate enan-tioselectivity. Quaternary iminium salts were also reduced to tertiary amines. Hydrogen will not reduce ketones or imines using the CATHy catalysts, but hydrides such as sodium borohydride have been shown to work. 

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