Competitive Reactivity of Nitrenium and Carbenium Ion Contributors of Purinium Cations with "Soft" Bases

Article abstract of DOI:10.1021/jo00343a010

Delocalized cations formed by ionization of "activated" esters of carcinogenic purine N-oxides react with many nucleophiles to yield C-substitution products but afford oxidation-reduction products with others.The present study provides experimental support for the proposals (1) that these two reactivities result from nucleophilic substitutions at different sites of the delocalized cations and (2) that HSAB "hard" bases react only at carbenium ion sites to form C-substitution products, while "soft" bases react preferentially at nitrenium ion contributors to afford adducts that ultimately yield redox products. "Soft" bases showed the following order of reactivity at a purine nitrenium ion: iodide ca. selenourea >> thio amides ca. thio acids ca. biselenide > bisulfide ca. thiols ca. disulfides > thiosulfate.This order differs significantly from that observed for the double S_N2 displacement reaction of nucleophiles with compounds of the type NH2-X.This appears to be the first report of differing orders of nucleophilicities of bases involved in S_N1 and S_N2 reactions at electron-deficient nitrogen centers.No evidence was found for radical intermediates formed by electron transfer.