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 °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 °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 °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.

在B3LYP水平下进行的计算预测，自由基H₂NO与乙烯酮（CH₂=C=O）的羰基碳发生加成反应的放热量为18.7 kcal/mol。
与该理论预测相符，二苯基烯酮（Ph₂C=C=O）在25℃下与四甲基哌啶氮氧自由基（TEMPO, TO）反应，生成一种不稳定物种；该物种可与氧气反应生成过氧化物(OCPh₂CO₂T)₂（化合物6，其中T=2,2,6,6-四甲基哌啶基），其结构经X射线晶体学确认。
将化合物6于100℃下在甲苯中与TEMPO共热，可得到二苯甲酮（Ph₂C=O）、四甲基哌啶以及PhCH₂OT，这表明化合物6分解生成两个2,2,6,6-四甲基哌啶基自由基（物种15）。
对化合物6的热分解进行动力学研究发现，其反应速率相较于(PhMe₂CO)₂提升了100倍。
TEMPO与双烯酮(Me₃SiC=C=O)₂（化合物23）在90℃下的热反应，以及与联烯基烯酮26的热反应，均可使TEMPO发生脱氧反应，生成自由基15，同时分别生成2,3-双(三甲基硅基)马来酸酐（化合物24）和亚烷基内酯27。