New Mechanism for the Reaction of Thianthrene Cation Radical Perchlorate with tert-Butyl Peroxide

A new reaction mechanism is proposed for the reaction of thianthrene cation radical perchlorate (Th +·ClO4-) and tert-butyl peroxide in acetonitrile at room temperature on the basis of experimental and theoretical results. Rapid C-O bond rupture instead of O-O bond cleavage was observed by a good peroxy radical trapping agent, thianthrene cation radical. Products were N- tert-butyl acetamide, thianthrene 5-oxide (ThO), thianthrene 5,5- dioxide (SSO2), and thianthrene (Th). Thianthrene 5,10-dioxide (SOSO) was not obtained. A comparative computational study of the cation radical of tert-butyl peroxide is made by using B3LYP and CBS-4. The computational results are helpful to explain the reaction mechanism. In the course of studies in the C-O bond cleavages by one- electron oxidant, thianthrene cation radical perchlorate (Th+·ClO4-, 1), we have discovered that thianthrene cation radical is a good peroxy radical trapping agent. In this study, a new reaction mechanism is proposed on the basis of our experimental and theoretical results for the particular case of 1 and tert-butyl peroxide (2). Dialkyl peroxides usually give in the initial stage two alkoxy radicals via O-O homolysis on photochemical and thermal decomposition, which are apt to react further with other substrates or decompose to more stable compounds.1 Likewise, 2 undergoes conventional thermolysis into tert-butoxy radicals in solution at reason- able rates only if heated at above 100 oC.2 In contrast, reaction of this very stable 2 with 1 at room temperature leads to rapid C-O bond rupture instead of O-O cleavage. The subjects of this report are the issue of electron transfer versus nucleophilic addition, and of C-O cleavage versus O-O cleavage. In the present study, we have carried out the reaction of 2:1 Th +·/2 stoichiometry in MeCN at room temperature and products were N-tert-butyl acetamide (3, 80%, 0.80 mmol), thianthrene 5-oxide (ThO, 96%, 0.48 mmol), thianthrene 5,5-dioxide (SSO2, 52%, 0.13 mmol) and thianthrene (Th, 104%, 0.26 mmol) as determined by quantitative GC and GC/MS analyses (Scheme 1). Work somewhat analogous to ours has been reported by Kim and Shin 9, which describes the nucleophilic addition of 2 to 1 in a molar ratio of 1.23:1.00 and the characteristics of oxidation of thianthrene, reduced oxidant. Kim had shown that the major products were not characteristic of tert-butyl cations but acetone was obtained from tert-butyl peroxy radical. For example, products of the reaction in MeCN at room temperature were Th (66%), ThO (27%), thianthrene 5,10-dioxide (SOSO, 7.7%), 5-acetonylthianthrenium perchlorate (1.1%), and 3 (12%). Unusual is that, in the report of Kim nor SSO 2 was