2.3- Dihydroperimidinium salts

There are two common approaches to the synthesis of these compounds, fruitfully complementing each other. The first is based on the alkylation of 1,8-diaminonaphthalene or its partially alkylated derivatives. In the second, l,l,3-trialkyl-2,3-dihydroperimidinium salts are used as the starting material. 

In the presence of transition metal complexes, such as those of Ir3+ (Scheme 24), Rh3+ and Ru3+, the proton sponge 1 behaves as a hydride donor from one of its methyl groups51,186. The methyleneimmonium cation 157 thus formed undergoes immediate cyclization to a l,l,3-trimethyl-2,3-dihydroperimidinium salt 158. 

Occasional formation of such salts in a small amount on nitration of compound 1, e.g. with N2O4 in 1,2-dichloroethane, indicates that the process could be initiated with common oxidants47. However, a question arises here as to why a similar cyclization is not observed for acenaphthene sponges (cf. Scheme 22) The answer may be that the N N distance in these sponges is remarkably longer (Table 4) and thus the cyclization becomes unfavourable. That the double proton sponge 35 with an N N distance yet shorter than 1 (Table 4) is very prone to cyclize to dihydroperimidinium salts44,64,137, confirms this explanation (Section IV.B.8). 

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