TEMPO reactions with ketenes

Figure 4.20 Plot of log k for ketene reactions with TEMPO vs. log/c for reactions with H2O. Reproduced with permission from Ref. 131a.

The reaction of TEMPO with ketenes (in particular Ph2C=C=0) proceeds to yield unstable species that upon reaction with O2 form a stable peroxide (60). Heating (60) at 100°C in toluene with added TEMPO leads to benzophenone, PhCH20TEMP0, and tetramethylpiperidine, indicating that the peroxide is a source of aminyl radicals (61) (Scheme 42). ° Kinetic studies of the reaction of TEMPO with ketenes in hydrocarbon solvents have also been reported. ... 

Huang, W.-W, Henry-Riyad, H., and Tidwell, T.T., Reactions of the Stable Nitroxyl Radical TEMPO with Ketenes Formation of a Unique Peroxidic Source of Aminyl Radicals, /. Am. Chem. Soc., 121, 3939, 1999. 

AUen, A.D., Cheng, B., Fenwick, M.H., Givehchi, B., Henry-Riyad, H., Nikolaev, V.A., Shikhova, E.A., Tahmassebi, D., Tidwell, T.T., and Wang, S., Ketene Reactions with the Aminoxyl Radical TEMPO Preparative, Kinetic and Theoretical Studies,/. Org. Chem., 66, 2611, 2001. 

A novel use of nitroxides in synthesis involves the addition of TEMPO to ketenes [83], Tidwell carried out ab initio computational studies to predict that the addition of nitroxides to the carbonyl carbon of ketene would be exothermic. This was then verified experimentally by the reaction of TEMPO with various ketenes to give diaddition products. For example, ketene 54 was allowed to react with TEMPO to give product 56 (Scheme 28). The intermediacy of radical 55, suggesting initial ni-troxide addition to the carbonyl carbon, is supported by the isolation of alternative products derived from the trapping of molecular oxygen at the a position. 

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