147239-56-3Relevant academic research and scientific papers
Chiral benzylic carbocations: Low-temperature NMR studies and theoretical calculations
Stadler, Daniel,Goeppert, Alain,Rasul, Golam,Olah, George A.,Prakash, G. K. Surya,Bach, Thorsten
supporting information; experimental part, p. 312 - 318 (2009/04/10)
(Chemical Equation Presented) The α-chiral secondary and tertiary benzylic carbocations 19-30 were generated from the corresponding benzylic alcohols 1, 2, and 5-14 by treatment with FSO3H or FSO 3H/SbF5 in SO2ClF as the solvent at -70°C and characterized by one- and two-dimensional NMR spectroscopy. Coupling constants and NOESY measurements suggest a preferred conformation in which the α-hydrogen atom occupies the 1,3-allylic-strain position and the diastereotopic faces of the cations are differentiated by the alkyl substituent and a functional group (FG). The existence of this preferred conformation is further supported by calculations using a DFT method at the B3LYP/6- 311+G** level. Quenching experiments with an arene nucleophile showed a preferential attack from the less shielded diastereotopic face delivering high diastereomeric ratios, supporting the hypothesis that these carbocations are involved as intermediates in previously studied SN1 reactions. A strong shielding effect at the benzylic carbocationic center is observed for most of the secondary benzylic carbocations (derived from precursors 5-13) investigated, indicating a strong mesomeric distribution of the positive charge to the carbon atom in the para-position of the anisyl substituent. For α-halogen-substituted carbocations (5-7, 12), no neighboring halogen participation leading to halonium ion formation was observed.
