tert-Butylation of Pyridines, Quinolines, and Isoquinolines by tert-Butylmercury Halides

Photolysis of tert-butylmercury halides with pyridinium or quinolinium salts leads to alkylation via the intermediacy of adduct radical cations. With simple pyridines or the 2-adducts from quinolines, the radical cations readily lose a proton to form a substituted pyridinyl radical which is easily oxidized by the alkylmercury halide. Addition of t-Bu· at the 4-position of the quinolinium ions, the 1-position of the isoquinolinium ions, or the 9-position of the acridinium ions, yields in the presence of KI the dihydro derivatives formed via electron transfer to the adduct radical cation from I⁻ or its ate-complex with the tert-butylmercury halide. A similar reductive alkylation is observed for the radical cations formed by the addition of t-Bu· to the β-position of the 4-vinylpyridinium ion or to the N-methylated cations derived from pyridine-3,4-dicarboximide, acridine, quinaldine, or isoquinoline. Competition between substitutive (oxidative) and additive (reductive) alkylation reflects the ease of proton loss from the intermediate adduct radical cation. Because of reversibility in adduct formation and variable rates of deprotonation of the adducts, yields of substitutive alkylation products are often not a true measure of the selectivity in the initial radical addition step. 4-tert-Butyl-l,4-dihydro-2-methylquinoline can be isolated from the photolysis of quinaldine with t-BuHgCl in the presence of KI/PTSA, methylated at C-3 by methyl iodide during the tert-butylation reaction, reduced by NaBH₄ upon workup, or oxidized to the quinoline at long reaction times. 4-tert-Butyl-1,4-dihydroquinoline reacts rapidly in the presence of PTSA and t-BuHgCl to form 2,4-di-tert-butyl-1,2,3,4-tetrahydroquinoline while 4-tert-butyl-2-chloro-1,4-dihydroquinoline is readily hydrolyzed to form the amide. Although 1-tert·;-butyl-1,2-dihydro-3-methylisoquinoline is isolable, 1-tert-butyl-1,2-dihydroisoquinoline reacts via the iminium ion to form 1,3-di-tert-butyltetrahydroisoquinoline and the de-tert-butylated product 3-;tert-butyl-3,4-dihydroisoquinoline. De-tert-butylation with aromatization is also observed upon photolysis of 4-tert-butyl-3,4-dihydro-2,3-dimethylquinoline with t-BuHgCl.