Chemoselective reduction of functional groups in the presence of esters is observed with tetrabutylammonium borohydride in chloro solvents.Chemoselective reduction of esters in the presence of alkenes by metal borohydrides has always resulted in the tandem reduction hydroboration reaction. Significant differences in the reactivity has been observed with different metal ions. Thus lithium borohydride in ether reacts faster and also hydroborates nearly three equivalents of alkenes per ester 1 . Calcium borohydride is less reactive and hydroborates only one equivalent of alkene, and needs excess alkene to proceed to an uptake of two equivalents of the unsaturated compound 2 . On the other hand zinc borohydride exhibited borane like electrophilicity 3 . Evidently polarisability of the anion and the coordination of the cation are responsible for the observed results.In an attempt to find a suitable borohydride which would reduce the ester chemoselectively, we chose tetrabutylammonium as the cation, since its coordination ability is very poor. Interestingly, this reagent did not reduce even the highly reactive methyl-10-undecenoate in THF. However, in chlorinated solvents this resulted in the reversed selectivity i.e., hydroboration of the double bond occurred in preference to the reduction of the ester group. This prompted us to undertake a study on the selective reduction /hydroboration using tetrabutyl ammonium borohydride in chlorinated solvents.Tetrabutylammonium borohydride was prepared as reported earlier by the ion displacement reaction of sodium borohydride with tetrabutylammonium hydrogen sulphate 4 . The reagent undergoes slow decomposition in chloro solvents at room temperature and reflux temperature presumably due to the reduction of C-Cl bond by the reagent in the order CH 2 Cl 2

The newly developed chiral ligands 1 and 4 show opposite enantioselectivity in the asymmetric reduction of prochiral ketones resulting in the production of either enantiomer depending on the metal complex with high enantiomeric excess.

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reduction, hydrogenationreduction, hydrogenation O 0220 -053Reduction of Carboxylic Acids by Tetraalkylammonium Borohydride.-Tetrabutylammonium borohydride (as well as benzyltriethylammonium borohydride) reduces carboxylic acids to the corresponding alcohols in good yields in the absence of any Lewis acids. Functional groups such as ester, nitro and chloro are well tolerated whilst acetyl groups and olefinic double bonds are not compatible with these borohydrides. - (NARASIMHAN, S.; SWARNALAKSHMI, S.; BALAKUMAR, R.; Synth. Commun. 30 (2000) 5, 941-946; Cent. Agrochem. Res., SPIC Sci. Found., Guindy, Madras 600 032, India; EN)

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