Polar Cycloadditions in Which Cationic Aromatic Systems Act as Electrophiles

Polar Cycloadditions in Which Cationic Aromatic Systems Act as Electrophiles 

Although it is possible to have a polar cycloaddition with a cation containing only carbon and hydrogen, the majority of those which have been found to undergo 1,4-cycloaddition are aromatic quaternary salts. The acridizinium cation (1) used in the first polar cycloaddition reaction of a quaternary salt has been used in the largest number of polar cycloaddition studies to date. The acridizinium ion (1) is particularly suitable for such a study since it is easily prepared, is stable (permitting sealed 

4 Note Added in Proof For a more adequate presentation of Dr. R. R. Schmidt s present views on the subject of polar cycloaddition, see Angew. Ghem., Int. Ed. Engl. 12, 212 (1973). 

The yields of product isolated when para-substituted styrenes were allowed to react with the acridizinium ion (Table I) are not indicative of the rates of reaetion. In an experiment patterned after that used by Sauer and Wiest in the first demonstration of the existence of cycloaddition with inverse electron demand, it was shown that the relative rates of addition of para-substituted styrenes to the acridizinium nucleus were as follows CHgO, 4.3 CHg, 1.7 H, 1.0 NOg, 0.34 or in the order expected from the nucleophilicity of the styrenes. 

Fields et al. showed that a variety of unsymmetrical alkenes added regiospecifically to the acridizinium nucleus and pointed out that the great majority of cases could be rationalized by the assumption that the more negatively polarized end of the alkene was preferentially attracted toward position 6, the previously demonstrated center for nucleophilic attack on the acridizinium ring. At the same time they reported that the addition of acrylonitrile to yield a 12- rather than a 13-cyano adduct is the reverse of what would be expected from the polarization of the acrylonitrile molecule. Possible explanations for this exception are offered in Section V. 

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II. Polar Cycloadditions in Which Cationic Aromatic Systems Act as Electrophiles. ... 

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