Tetramethylpiperidinyloxy (TEMPO, TO*) reacts with ketenes RR(1)C=C=O generated by either Wolff rearrangement or by dehydrochlorination of acyl chlorides to give products resulting from addition of one TEMPO radical to the carbonyl carbon and a second to the resulting radical. Reactions of phenylvinylketenes 4b and 4f, phenylalkynylketene 4c, and the dienylketene AcOCMe=CHCH=CHCMe=C=O (11) occur with allylic or propargylic rearrangement. Even quite reactive ketenes were generated as rather long-lived species by photochemical Wolff rearrangement in isooctane solution, characterized by IR and UV, and used for kinetic studies. The rate constants of TEMPO addition to eight different ketenes have been measured and give a qualitative correlation of log k(2)(TEMPO) = 1.10 log k(H(2)O) -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 cyclobutenones leading to vinylketenes and of 2,4-cyclohexadienone (17) forming 1,3,5-hexatrien-1-one (18).

[reaction: see text] Tetramethylpiperidinyloxy (TEMPO, TO*) reacts with a variety of ketenes R1R2C=C=O by rate-limiting attack on carbonyl carbon to give the 1,2-bis(adducts) R1R2C(OT)CO2T. The alpha,beta-unsaturated ketenes (E)-PhCH=CHCH=C=O (8b) and PhC=CCH=C=O (8c) give the 1,4-bis(adducts) PhCH(OT)CH=CHCO2T and PhC(OT)=C=CHCO2T. The ketenes may be generated in situ for these reactions in the presence of TEMPO by either dehydrochlorination of R1R2CHCOCl with Et3N or Wolff rearrangement. Ketenes PhCH=C=O (8a), 8b, and 8c had not previously been observed as long-lived species at room temperature, but when formed by photochemical Wolff rearrangement, these could be characterized in solution by conventional IR spectroscopy and used for kinetic studies for reaction with TEMPO using UV detection. The reactions of six ketenes with TEMPO in hydrocarbon solvents follow second-order kinetics, with a range of 2.5 x 10(5) in the rate constants, which are correlated with unit slope with the corresponding rate constants for hydration.

[reaction: see text] Bisketenes O=C=CH(CH(2))(n)()CH=C=O (1b,c,d, n = 4, 3, 6) and (E)-O=C=CHCH=CHCH=C=O (E-13) were generated in solution by dehydrochlorination of bis(acyl chlorides) and by photochemical Wolff rearrangements and identified by their characteristic IR signals. The bisketenes react with aminoxyl radicals to give tetraaddition products for 1b and conjugate 1,6-diaddition for E-13.

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