Boranes conversion into alcohols

Another secondary borane useful for the conversion of alkenes into alcohols and of alkynes to aldehydes or ketones is catecholborane (1,3,2-benzodioxaborole), which is prepared from catechol and borane in tetrahydrofuran [1201] (equations 600 and 601). 

The hydrogenolysis of chloroform to hydro n chloride has been used in the production of labile amines catalytic reduction of azides, nitriles, oximes, and nitro-compounds in a solvent containing a small amount of chloroform results in the trapping of the produced amine as its hydrochloride. Sodium sulphide smoothly reduces nitroxides to amines. The utility of chiral a-phenylethylamine-borane complexes for the conversion of ketones into alcohols or amines has been studied whereas chemical yields are good, optical yields are poor. ... 

The reduction of epoxides withborane is noteworthy as it gives rise to the less substituted alcohol as the major product (7.96), in contrast to reduction with complex hydrides (compare with Scheme 7.71). The reaction is catalysed by small amounts of sodium or lithium borohydride and high yields of the alcohol are obtained. With 1-alkylcycloalkene epoxides, the 2-alkylcycloalkanols produced are entirely cis, and this reaction thus complements the hydroboration-oxidation of cycloalkenes described in Section 5.1, which leads to trans products. Reaction with borane in the presence of boron trifluoride has also been used for the reduction of epoxides and for the conversion of lactones and some esters into ethers. 

Initially looking at methods for the synthesis of the desired enantioenriched alcohol, a number of chemical reduction approaches were evaluated and failed. These focused on borane-type reductions and in all cases, both poor conversions and enantioselectivities were observed. For instance, DIP-Cl lead to 10% conversion. Brown and coworkers have also reported that acetophenone substrates with two ortho-substituents perform poorly in DIP-Cl reductions as these substituents serve to force the acetophenone out of the plane, and into an almost perpendicular orientation with respect to the ring. ... 

Arynes combine with all kinds of polar compounds such as lithium halide, alcohols, primary, secondary or tertiary amines, phosphines, and boranes. Of practical, even technical, importance are their reactions with metal hydroxides and alkoxides. The conversion of chlorobenzene by sodium hydroxide at 350 °C into sodium phenolate, an industrial process, obeys the elimination/addition mechanism. Again arynes can be postulated as plausible intermediates. Regardless of which isomer of bromophenol or of bromophenyl benzenesulfonate is heated with sodium hydroxide, resorcinol is always isolated as the main or sole product after neutralization. 

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