Evidence for Electron Transfer in Reactions of Thianthrene Cation Radical with Dialkylmercurials

Article abstract of DOI:10.1021/jo00296a029

Reactions of dialkylmercurials (R2Hg) with thianthrene cation radical perchlorate (Th^.+ClO4^-) in acetonitrile solution have been studied in quantitative detail.Evidence was obtained from reactions of MeHgR (R = Et, i-Pr, tBu) that reaction begins with electron transfer rather than with electrophilic cleavage of an alkyl-mercury bond.That is, each reaction gave MeHg⁺ and R^., diagnostic of the formation and decomposition of MeHgR^.+, rather than 5-methylthianthreniumyl perchlorate (1a), which would have been diagnostic of electrophilic displacement of the least hindered group (Me).The radicals R^. either were trapped at the sulfur atom of Th^.+ to form a 5-alkylthianthreniumyl perchlorate (Et^., 1b) and at the ring positions of Th^.+ to form 1- and 2-alkylthianthrenes (Et^., i-Pr^.) or were oxidized to the cations R⁺ (Et^., i-Pr^., and t-Bu^.).Products of R⁺ were obtained, after workup with 4 M aqueous LiCl, as alkene, ROH, RNHCOCH3 and RCl.These reactions had the stoichiometric ratio of reactants 2Th^.+ClO4^-/MeHgR.Reactions of symmetrical R2Hg sometimes followed this stoichiometry (R = Me, Et, Bu) and led to RHg⁺ and 5-R-thianthreniumyl perchlorates (1a,b,e).Other R2Hg (R = t-Bu, benzyl, allyl) underwent oxidation by 4 equiv of Th^.+ClO4^-.Di-tert-butyl- and dibenzylmercury gave products derived entirely from the respective cation, R⁺.Diallylmercury gave some of the sulfonium product, 5-allylthianthreniumyl perchlorate (1g).None of the reactions with R = i-Pr, t-Bu, and benzyl led to the isolation of a thianthreniumyl perchlorate (i.e., 1c,d,f).Oxidations at the 4:1 molar ratio produced Hg(ClO4)2, which formed a partly insoluble complex (2) with thianthrene having the composition Th3Hg(ClO4)2.This product could be isolated if removed before workup treatment with aqueous 4 M LiCl, which, otherwise caused its decomposition into its components.The complex 2 was also prepared directly from reaction of Th with Hg(ClO4)2 in acetonitrile.Oxidation of benzylmercuric chloride by Th^.+ClO4^- in methylene chloride solution also occurred quantitatively, giving benzyl chloride, 1- and 2-benzylthianthrene, and a mixture of dibenzylthianthrenes.Oxidation of t-BuHgCl in acetonitrile solution led to a quantitative mixture of isobutene, t-BuOH, and t-BuNHAc.Th^.+ClO4^- also oxidized metallic Hg to either Hg⁺ or Hg²⁺, depending on the amount of oxidant used.