A study of N-nitrosoamide-mediated Friedel-Crafts type benzylation of benzene-toluene and benzene-anisole

Nitrogen-separated carbocation-carboxylate ion pairs were employed as sources of carbocations in the alkylation of aromatic compounds. The N-nitrosoamide approach to these nitrogenous-molecule-separated ion pairs is an excellent alternative to the standard acid-catalyzed Friedel-Crafts approach for studies of the alkylation because of the following variables: high reactivity of the electrophile, stability of the products, strict kinetic control, homogeneity, lack of overalkylation, straightforward chemistry, and good product balance. In deaminative benzylations of benzene-toluene and anisole-benzene mixtures, the values of k(X)/k(B) and % meta isomer are significantly different from those observed in the standard benzylations in a;manner that indicates the deaminative electrophiles are more reactive than those generated by the standard Friedel-Crafts approach. The reactions show a direct proportionality between inter- and intramolecular selectivities and thus follow the Brown selectivity relationship (BSR). The benzylation of 2,4,6-trideuteriotoluene provided;no evidence for deuterium rearrangements (or coupled benzyl rearrangements) in the arenium ion intermediate. Kinetic isotope effects were not detected. The methyl substituent.on toluene appears to affect intermolecular selectivity (k(T)/k(B)) and intramolecular selectivity (o, m, p distribution) to similar degrees. A mechanism is proposed in which both selectivities are determined by activation energy differences in the transition states leading to the sigma-complex intermediates. The observation that most standard Friedel-Crafts benzylations do not follow the BSR is discussed in terms of possible systematic errors in those cases. Silver ion-assisted Friedel-Crafts benzylations were performed under dry, basic conditions to investigate whether the standard approach could lead to data that obey the BSR.