Iminium salts regiochemistry

The regiochemistry of ketene iminium salt cycloadditions can also differ from ketene cycloadditions. Whereas reaction of styrene with a series of ketene iminium salts gave 3-phenyI-cyclobutanones7 (60-70% yield) similar to the regiochemistry of ketene cycloadditions, reaction with a series of acrylates and a,/J-unsaturated ketones gave cyclobutanones 5 with regiochemistry opposite to what would be expected from electrostatic considerations of ketene cycloadditions.s... 

Unlike ketcnc cycloadditions, very few mechanistic studies have been carried out with ketene iminium salt cycloadditions. Differences in regiochemistry in the latter examples suggest that these reactions are not concerted and that a carbcne-type addition to the alkene leading to an intermediate such as 6 is responsible for these reactions.8... 

In a regiochemical investigation of the reaction of iminium salt (32) with a series of unsymmetrical methyl ketones, Jasor et al. have observed that the site of aminomethylation is solvent dependent (equation 7, Table S). It is found that in MeCN, reaction occurs exclusively at the least-substimted position, while in TFA it occurs mainly at the most-substituted position. Yields are good (70-90%) in TFA but are somewhat reduced in MeCN due to dialkylation, which can be overcome if the more bulky NA -diisopro-pyliminium salt is used. How the solvent influences the two possible rate-determining steps, enolization versus aminomethylation, and the extent of reversibility of the reaction need further investigation in order to determine the basis for the observed dichotomy in regiochemistry. 

---