Photostimulated tert-butylation of quinolines, quinolinium salts, quinoline N-oxides, N-benzylideneanilines, and azobenzenes
Photolysis of tert-butylmercury halides with quinolines, N-methyl or N-methoxyquinolinium salts, or quinoline N-oxides leads to alkylation via the intermediacy of adduct radicals or radical cations. The system t-BuHgCl/KI is a unique one because of the presence of both a mild oxidizing agent (t-BuHgI) and reducing agent (I[superscript]- or t-BuHgI[subscript]2[superscript]-). 4-Substituted quinolines, such as 4-chloro- or 4-methylquinoline, undergo only substitutive alkylation (at C-2). With 2-substituted quinolines, reductive alkylation (at C-4) occurs in the presence of KI. With quinoline itself, attack of t-Bu· at C-2 leads to the substituted product, but attack at C-4 leads mainly to a reduced quinoline derivative, 4-tert-butyl-1,4-dihydroquinoline which reacts rapidly in the presence of PTSA and t-BuHgI to form 2,4-di-tert-butyl-1,2,3,4-tetrahydroquinoline. Photolysis of t-BuHgCl/KI with N-methylquinolinium salts gives mainly reductive alkylation products. With N-methoxyquinolinium perchlorates, reductive tert-butylation occurs in the presence of KI, followed by, in most cases, demethoxylation with aromatization of the quinoline ring. Deoxygenation of quinoline N-oxides may occur before tert-butylation with t-BuHgCl/KI in Me[subscript]2SO, while photolysis of tert-butylmercury halides with 2-chloroquinoline N-oxide forms the dimer of the adduct radical. With quinoline N-oxide and its 4-substituted derivatives, the addition of DABCO yields the oxidative alkylation products, 2-tert-butylquinoline N-oxides.;Reaction of t-BuHgI with N-benzylideneanilines gives exclusively the reductive alkylation products. Quantitative yields of the products are obtained in the presence of t-BuHgI/KI/PTSA. Photostimulated tert-butylation of azobenzenes by t-BuHgI forms reductive alkylation products. Addition of K[subscript]2S[subscript]2O[subscript]8 speeds up the alkylation. The effects of I[superscript]- on the reaction are discussed.