Radical additions of TEMPO to ketenes: correlation of free radical and nucleophilic reactivity

[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.