A study of hydride nucleophile reactions with q6-(2R, 5R)-dimethylpyrrolidinylbenzene-manganese tricarbonyl hexafluorophosphate 1 reveals an unexpected reversal of selectivity during addition of hydride to the diastereotopic meta carbons, which is explained on the basis of changes in transition state location as the reactivity of the nucleophile is varied.Addition of nucleophiles to arene-transition metal complexes leads to important methodology for the conversion of aromatic molecules to substituted cyclohexadienes or cyclohexenones,l but few successful approaches have been documented for obtaining asymmetric induction during the key nucleophile addition reactions2 We recently reported3 on nucleophile additions to arene-manganese complex 1, which gave products of structures 2 and 3, with the former in excess. We rationalized our observations on the basis of steric approach control, whereby nucleophilic attack is preferred at the sterically more accessible meta position (C6 in structure 1; this numbering is consistent with that used in our earlier X-ray structure).We now report an unexpected reversal of selectivity which occurs when less reactive nucleophiles are added to complex 1. This phenomenon was first observed when 1 was allowed to react with NaBH4 or LiBH4, whereupon a 1 : 4 mixture was obtained in favour of 3 (R = H).? The assignment was confirmed by an X-ray crystal structure determination on the major product.$ The results of a series of hydride additions are collected in Table 1, which includes the reactions of 1 with PhMgBr and MeLi, the latter also giving reversed selectivity compared with PhMgBr.5 Clearly, our earlier analysis of this stereoselectivity is inadequate.There are a number of possible explanations for such a reversal of selectivity. First, a change in mechanism was ruled out by deuterium labelling studies, wherein it was observed that
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