Acridizinium ions cycloaddition

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... 

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. 

In the first cycloaddition reaction of the acridizinium ion, that with maleic anhydride, it had been observed that addition had occurred with great stereoselectivity, although it iVas not ascertained whether the product (4) was syn or anti with respect to the benzenoid ring. It was later demonstrated by use of NMR and IR evidence (derived from the... 

Not shown in Table I are several adducts obtained by cycloaddition of the acridizinium ion with 5,6-endo-substituted norbomene derivatives. These adducts each have two (imequally shielded) methylene hydrogen atoms which make simple the NMR analysis of mixtures of syn (8) and anti (9). When the 5,6-endo chain (R) was of the type... 

Although dimethyl acetylenedicarboxylate was reported earlier not to undergo cycloaddition with the acridizinium ion, it was later... 

Another important observation by Fields et al ° was that the acridizinium ion (25) will undergo cycloaddition with benzyne, affording... 

Fields cf oZ. have pointed out that the differences observed between the rate of cycloaddition of ketene acetal with the acridizinium ion and that of various acridizinium benzologs qualitatively parallels those which are encountered when the rate of cycloaddition of maleic anhydride with... 

The most recent extension of the 4 + 2 cycloaddition to aromatic quaternary salts has been carried out with isoquinolinium salts (42), in effect, dispensing with ring A of the acridizinium ion. Although there was an earlier claim that the addition of an ynamine to 2-methyl-isoquinolinium iodide led to a 2 1 adduct, the assigned structure... 

Change in the Rate of Cycloaddition Produced by Substitution at the Meso Positions of the Acridizinium Ion... 

In cycloaddition reactions involving the acridizinium and isoquino-linium ions, the presence of a positive charge leads with great stereoselectivity to products which could not be predicted by the rule of Alder and Stein. A charge-transfer complex arising from the attraction... 

It is now clear that acridizinium compounds and their benzo analogs are more susceptible to cycloaddition than are their quinolizinium counterparts. Fields et al. (68JOC390) showed that, even with the strong nucleophile 1,1-diethoxyethylene, 2,3-dimethylquinolizinium ion (35a) does not react. As yet the only quinolizinium derivative known to undergo cycloaddition is the anhydride (35b) of quinolizinium-2,3-dicarboxylic acid which, because of the increased electrophilicity provided by the electron-withdrawing substituents, undergoes cycloaddition even with the more moderately nucleophilic styrene to yield (36). 

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