Reactions of the "stable" nitroxyl radical TEMPO with ketenes: Formation of a unique peroxidic source of aminyl radicals

Calculations at the B3LYP level predict addition of the radical H2NO to the carbonyl carbon of CH2=C=O to be exothermic by 18.7 kcal/mol. Consistent with this prediction, the ketene Ph2C=C=O reacts at 25 degrees C with tetramethylpiperidinyloxy radical (TEMPO, TO) to yield an unstable species that reacts with oxygen to form the peroxide (OCPh2CO2T)(2) (6, T = 2,2,6, 6-tetramethylpiperidinyl), whose structure was confirmed by X-ray crystallography. Heating of 6 at 100 degrees C in toluene with TEMPO leads to Ph2C=O, tetramethylpiperidine, and PhCH2OT, indicating that 6 decomposes to form two 2,2,6,6-tetramethylpiperidinyl radicals 15. Kinetic studies of the thermal decomposition of 6 show a 100-fold rate acceleration relative to (PhMe2CO)(2). Thermal reactions of TEMPO with the bisketene (Me3SiC=C=O)(2) (23) at 90 degrees C and with the allenylketene 26 also lead to deoxygenation of TEMPO, forming radicals 15, together with 2,3-bis (trimethylsilyl)maleic anhydride (24) and the alkylidenelactone 27, respectively.