Chiral α-branched benzylic carbocations:: Diastereoselective intermolecular reactions with arene nucleophiles and NMR spectroscopic studies

The chiral benzylic alcohols 1-6 were prepared and subjected to S(N)1-type displacement reactions with various arene nucleophiles in acidic medium. Under optimized conditions (HBF4 center dot OEt2, CH2Cl2, -78 degrees C -> r.t.) the corresponding 1,1-diarylalkanes 11-18 and 20 were obtained in good chemical yields (48-99%). The facial diastereoselectivity of the reaction is high (d.r.) 91/9-97/3) when the substrate bears a stereogenic carbon center -CHtBuMe in the alpha-position to the electrophilic carbon atom. If the starting material was enantiomerically pure, no significant racemization was observed (94% ee -> 92% ee). The reactions proceed stereoconvergently as demonstrated by the conversion of the separated diastereoisomers syn-1a and anti-1a in separate reactions to the same product syn-11 (d.r.) 97/3). Further evidence for long-lived chiral benzylic carbocations as reaction intermediates was obtained from NMR studies in superacidic medium. The chiral cation 24 was generated in SO2CIF as the solvent at -70 degrees C employing SbF5 as the Lewis acid and characterized by its H-1 and C-13 NMR spectra. NOE measurements suggest a preferred conformation in which the diastereotopic faces of the cation are differentiated by the two carbon substituents R and Me at the stereogenic carbon center in the alpha-position. The hypothesis is further supported by the observation that the diastereoselectivity of the substitution reaction decreases if the bulky tert-butyl (R) = tBu) substituent in the substrate 1a is replaced by a smaller ethyl group (2a, R = Et).