Chiral benzylic carbocations: low-temperature NMR studies and theoretical calculations.
The alpha-chiral secondary and tertiary benzylic carbocations 19-30 were generated from the corresponding benzylic alcohols 1, 2, and 5-14 by treatment with FSO(3)H or FSO(3)H/SbF(5) in SO(2)ClF as the solvent at -70 degrees C and characterized by one- and two-dimensional NMR spectroscopy. Coupling constants and NOESY measurements suggest a preferred conformation in which the alpha-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 S(N)1 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 alpha-halogen-substituted carbocations (5-7, 12), no neighboring halogen participation leading to halonium ion formation was observed.
Stadler
Daniel
D
Lehrstuhl fur Organische Chemie I, Technische Universitat Munchen, 85747 Garching, Germany.
Goeppert
Alain
A
Rasul
Golam
G
Olah
George A
GA
Prakash
G K Surya
GK
Bach
Thorsten
T
D001593:Benzyl Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D002412:Cations / Q000138:chemical synthesis / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy / Q000592:standards; D008956:Models, Chemical; D008968:Molecular Conformation; D012015:Reference Standards; D013237:Stereoisomerism; D013696:Temperature
D001593:Benzyl Compounds; D002412:Cations