Nitroxyl radical reactions with 4-pentenyl- and cyclopropylketenes: New routes to 5-hexenyl- and cyclopropylmethyl radicals

4-Pentenylketenes 4a and 9 and cyclopropylketenes 3a, 13, 14 (RCH=C=O) are generated by photochemical Wolff rearrangements and observed by IR as relat ively long-lived species at room temperature in hydrocarbon solvents. The r eactions of these ketenes with the nitroxyl radicals tetramethylpiperidinyl oxyl (TEMPO, TO.) and tetramethylisoindoline-2-oxyl (TMIO, IO.) form carbox y substituted 5-hexenyl and cyclopropylmethyl radicals which are either tra pped by a second nitroxyl radical or undergo rearrangements followed by tra pping. The rate constant of the reaction of 4a with TEMPO was similar to th at of n-BuCH=C=O (1b), while 3a was 4.3 times more reactive, indicating cyc lopropyl stabilization of the incipient radical.