Ketene reactions with the aminoxyl radical TEMPO: Preparative, kinetic, and theoretical studies

Tetramethylpiperidinyloxy (TEMPO, TO .) reacts with ketenes (RRC)-C-1=C=O g enerated by either Wolff rearrangement or by dehydrochlorination of acyl ch lorides to give products resulting from addition of one TEMPO radical to th e carbonyl carbon and a second to the resulting radical. Reactions of pheny lvinylketenes 4b and 4f, phenylalkynylketene 4c, and the dienylketene AcOCM e= CHCH=CHCMe=C=O (11) occur with allylic or propargylic rearrangement. Eve n quite reactive ketenes were generated as rather long-lived species by pho tochemical Wolff rearrangement in isooctane solution, characterized by IR a nd UV, and used for kinetic studies. The rate constants of TEMPO addition t o eight different ketenes have been measured and give a qualitative correla tion of log k(2)(TEMPO) = 1.10 log k(H2O) -3.79 with the rate constants for hydration of the same ketenes. Calculations at the B3LYP/6-311G**//B3LYP/6 -311G** level are used to elucidate the ring opening of substituted cyclobu tenones leading to vinylketenes and of a,4-cyclohexadienone (17) forming 1, 3,5-hexatrien-1-one (18).