MECHANISTIC STUDY OF ANODIC INTRA-MOLECULAR COUPLING REACTIONS

Anodic cyclization reactions of several methoxybibenzyls were studied. The reactions were: 2-metiiyl-3’,4’,5-trimethoxy-4-ethoxybibenzyl (1) to 9,10-dihydro-10a-methyl-2-ethoxy-6,7-dimethoxy-3(10aH)phenanthrone; laudanosine (2) to O-metliylflavinatine (5) and 4,4’-dimethoxy[2,2]metacyclopliane (3) to 2,7-dimethoxy-4,5,9,10-tetrahydropyrene (6). The solvent was acetonitrile and the anode was platinum. When using cyclic voltammetry, each of these compounds shows a 2e- oxidation peak at + 0.5-1.1 V vs. Ag|0.1 M AgNO3 which is irreversible at all sweep rates due to the rapid chemical reactions following electron transfer. At slightly more positive potentials, there is a reversible couple due to the products. Preparative electrolysis gave n = 2 and high isolated yields of the products. The reaction kinetics were studied by using the shift in the first peak potential (Ep) in linear sweep voltammetry as a function of sweep rate, concentration of substrate, and the acidity of the medium. The rate law was also elucidated by using the convolution potential sweep voltammetry method. The two techniques led lo similar conclusions. For compound 1 the mechanism involves reversible electron transfer (e) and disproportionation (d) steps followed by a rate-limiting step which is most probably cyclization (c) of the dication. Compound 2 was studied in acidic media so that the amine group was initially protonated. The mechanism under these conditions involves reversible electron transfer from 2H+, cyclization, and deprotonation of the amine followed by a rate-limiting solution phase electron transfer from a cyclized cation radical to a dication radical 2H+. The importance of the amine/ammonium function in controlling the reaction pathway is discussed. Compound 3 reacts via an ec-type process in which the initial electron transfer and follow-up ehcmical reaction have a similar rate. © 1979, American Chemical Society. All rights reserved.