Nitrones and nitrile oxides as 1,3-dipoles

The addition of a conjugated nitrone to unactivated olefins in solution was reported (Eq. 4). Sonication was expected to accelerate this slow process, which normally requires 1-2 days in refluxing benzene. The expectation seems to have been fulfilled, and yields equivalent to those of the thermal process were produced after less than 1 h irradiation. 

14 Daniel Little, R. in Comprehensive Organic Synthesis (Trost, B.M. Paquette, L.A. Eds.), Pergamon Press, Oxford, 1991, Vol. 5, pp. 239-270. 

However, the meaning of these results is questionable, since the sonochemical experiments were carried out without any temperature control. 

The addition of nitrile oxides to various olefins was studied by several groups. Similarly to ketenes, the dipolar compounds must be generated in the presence of the other partner, and several methods were used for this purpose. The first one consists of the oxidation of oximes (Z) or ( ), in a biphasic system with aqueous sodium hypochlorite (Fig. 4). Sonication provided a large rate increase, and the reaction was completed in hours instead of days. This result is most probably explained by an efficient emulsification (Ch. 4, p. 108). In a few cases, minor changes in stereoselectivity occur in the irradiated reactions. 

This observation is not general, and the same reaction made with a dipolarophile bearing a polar substituent on the allylic position showed that sonication influences the rate, but not the yield and stereoselectivity (Eq. 

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