Ketene Reactions with the Aminoxyl Radical TEMPO:  Preparative, Kinetic, and Theoretical Studies

Abstract

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).

100 Years of the Wolff Rearrangement

“…[27a] 2,2,6,6-Tetramethylpiperidinyloxy (TEMPO, TO) was found to react with ketenes 2 to give 1,2 adducts 403 (Scheme 50). [305] were produced with α,β-unsaturated ketenes. Various ketenes were generated by Wolff rearrangement for kinetic studies which showed that the reactivities with TEMPO and with H 2 O are correlated with unit slope.…”

“…Various ketenes were generated by Wolff rearrangement for kinetic studies which showed that the reactivities with TEMPO and with H 2 O are correlated with unit slope. [305] The rates of TEMPO addition to pentafulvenones served as a test for cyclopentadienyl radical destabilization. [305d] The adducts 403 are derivatives of α-hydroxycarboxylic acids 408.…”

Asymmetric Synthesis, Structure, and Reactivity of Unexpectedly Stable Spiroepoxy-β-Lactones Including Facile Conversion to Tetronic Acids: Application to (+)-Maculalactone A

“…Ketene was not detected when this reaction was run at either ambient or low temperature (-78 °C), however this does not exclude the possibility of a short-lived, ketene intermediate. An alternative method for trapping ketene intermediates was also studied involving performing the reactions in the presence of TEMPO, a reagent known to react rapidly with ketenes to provide esters containing two TEMPO molecules 26. To compete with the intramolecular cyclization leading to tetronic acid, a large excess of TEMPO was employed.…”

Cycloaddition and Electrocyclic Reactions of Vinylketenes, Allenylketenes, and Alkynylketenes

“…Kinetic studies of ketene reactivity show the order PhCH=CHCH=C=O > PhCH=C=O ≫ n-BuCH=C=O for addition both of water and of the stable free radical TEMPO. 31 Comparable kinetic data for cycloaddition reactions are not available, but as many of these processes involve a polarized transition structure, a similar order of reactivity is to be expected.…”