Chengxue Zhao, Y. Gong
Sep 1, 1991
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Journal
Journal of Fluorine Chemistry
Abstract
Abstract In the reactions of 2,5-di- tert -butyl-1,4-dimethoxybenzene (1) and 2-alkyl-5- tert -butyl-1,4-dimethoxybenzenes (2) with different oxidizing systems, S 2 O 2 8 − Cu 2+ CH 3 CO 2 H, Ce 4+ CH 3 CO 2 H and (φ F CO 2 ) 2 CF 2 CL CFCL 2 , 1 + · and 2 + · are always detectable by EPR. However, the observed reactivity of 1 + · and 2 + · depends greatly on the oxidation systems. In the former two, 1 + · and 2 + · have long lifetimes and do not undergo fragmentation spontaneously; the nucleophile CH 3 CO 2 − in the bulk does not attack 1 + · and 2 + · effectively. In contrast, in (φ F CO 2 ) 2 CF 2 CL CFCL 2 solution, both 1 + · and 2 + · become short-lifed; and large amounts of de- tert -butylation products in the reactions of 1, and de- tert -butylation as well as deprotonation-perfluoro-benzoloxylation products in the reactions of 2, have been isolated. Experimental results imply that the CC bond cleavage involved in de- tert -butylation and CH bond cleavage in deprotonation could be a consequence of an attack by φ F COO · on 1 + · and 2 + · in their original solvent cage. The fact that addition of MeOH to the reaction mixture leads to the formation of t-BuOCH 3 suggests that the tert -butyl group leaves as a carbocation. In the reactions of 2, the deprotonation of 2 + · is further assured by the EPR observation of the corresponding benzyl radicals. The relative importance of deprotonation over de- tert -butylation is greatly dependent on the structure of −CHR 1 R 2 substituents and the solvents. For deprotonation, the reactivity order is found to be cy -PrCH 2 >Me>PhCH 2 >Et⪢ i -Pr. We conclude that the reactions of 1 and 2 with (φ F CO 2 ) 2 are initiated bv electron transfer and followed by fast coupling of 1 + · with φ F CO 2 · in the solvent cage, to form σ-complexes which collapse or react with nucleophile to yield the de- tert - butylation product in the case of 1; and in the cases of 2, fast deprotonation generating benzylic radisals and their fast coupling with φ F COO . in the cage, to form side-chain acyloxylation products. Perfluorodiacyl peroxides are proved to be effective oxidants for generating aromatic radical cations. By taking advantages of the simultaneous generation of the most reactive radical φ F COO · and the most common nucleophile φ F CO − 2 from the extremely fast collapse of their radical anion (φ F CO 2 ) − · 2 in original solvent cage rightafter the electron transfer, we have been able to probe such of the important and complicated mechanistic details of the reactions involving aromatic radical-ions.