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The 2-phenyl-2-butyl cation, formed from optically active or racemic 2-phenyl-2-butanol (2) or 2-phenyl-I-butene, was reduced by optically active a-naphthylphenylmethylsilane (3) or triethylsilane to 2-phenylbutane (1) in trifluoroacetic acid-dichloromethane. When ( R ) -( + ) -3 was used as the reducing agent, 1 was obtained with the same small predominance of R enantiomer (-2% ee) regardless of whether the cation precursor was (S)-2, (R)-2, (-f)-2, or 2-phenyl-1-butene. Similarly, ( S ) -( -) -3 yielded the same small predominance of (S)-1 regardless of starting material. Triethylsilane gave completely racemic 1 from ( R ) -2 . The results are interpreted in terms of enantioface-differentiating capture of a prochiral carbenium ion which is common to all the precursors.In a previous communication we reported the formation of 2-phenylbutane (1) which was enriched with a small predominance (2-3%) of one enantiomer from the reduction of racemic 2-phenyl-2-butanol (2) by optically active cu-naphthylphenylmethylsilane (3) in a trifluoroacetic acid-dichloromethane medium.' Organosilane of ( R ) -( +) configuration gave an enantiomeric excess of hydrocarbon with ( R ) -( -) configuration, whereas (S)-( -) organosilane yielded an excess of (S)-(+)-2-phenylbutane. These results may be explained as enantioface-differentiating reactions* in which the (R)-( +) or ( S ) -( -) silane delivers hydride preferentially to the si or re face, respectively, of a prochiral 2-phenyl-2-butyl cation (4) formed by loss of water from either enantiomer of the protonated alcohol (Scheme I). Additional studies have since been performed using other alcohols and chiral organosilicon hydrides with similar results. In every case optically active hydrocarbon products have been ~b t a i n e d .~If the interpretation is correct, then these results are of special interest. They indicate that relatively unstable, highly reactive carbocation intermediates may be subject to observable discriminate partitioning among two or more accessible reaction pathways, even when these pathways differ only slightly in free energy and lead to energetically indistinguishable products (enantiomers). That carbocations of such CF,COOH. 1 I CIH-, 1 CH,CI, 0002-7863/78/1.500-7641$01.00/0 high reactivity may exhibit this subtle kind of stereoselectivity is ~n e x p e c t e d .~Such an interpretation is justified, however, only if the carbocation intermediate is symmetrically solvated or, perhaps equivalently, if a set of water-separated, enantiomerically related ion pairs (5 and 5') interconvert at a rate much faster than the rate of capture by organosilane. If the rate of interconversion of such asymmetric ion pairs were comparable to the rate of hydride transfer, then the reactions would be more properly regarded as enantiomer differentiating rather than as enantioface differentiating.2 Although the two cases are in principle distinguishable, our original experiments did not allow an unambiguous distinction to be made. Evidence favoring interpretation in the former terms...

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