Cyclic Amidinium Salts

The selectivity of acylations or alkylations of polyamines can also be modulated by protonation [78] or by conversion to cyclic aminals [71, 79, 80]. The latter strategy has, for instance, been used successfully for the preparation of monofunctionalized 1,4,7-triazacyclononanes (Scheme 10.18). In this reaction the first alkylation leads to the formation of an amidinium salt, which is more difficult to alkylate than a tertiary amine. Thus, the use of only one equivalent of alkylating agent leads to a clean monoalkylation. 

Deprotonation of amidinium salts gives 1,1-enediamines112 and the method is well suited for the preparation of simple cyclic 1,1-enediamines18, 41. 4,5-Dihydroimidazolium salts 99, prepared easily by alkylation of imidazolines or cyclization of ethylenediamine with orthoesters, react with sodium hydride to afford 2-alkylideneimidazolidines 100 in good to excellent yields (equation 34)18. 

Cyclic amidinium salts (71) are obtained by [2 + 2] cycloaddition of ketenimmonium compounds (69) to Schiff bases (70 R = H, Ph, MeOaC) or thioimidates (70 R = SBn) their thiolysis provides an elegant access to 3-thiolactams (72 equation 32). ... 

Fig. 8. Structure of the parent diamino-substituted carbocation (amidinium salt) J and of its cyclic valence isomer 2wNa...

Before going further, note that quantum ehemieal calculations predicted the amidinium salt 39 (R = R = H) to be 541 kJ/mol more stable than its unknown cyclic valence isomer 40. 

Jazzar, R., Bourg, J.-B., Dewhurst, R.D.et a/. (2007) Intramolecular hydroiminiumation and-amidiniumation of alkenes a convenient, flexible, and scalable route to cyclic iminium and imidazolinium salts. The Journal of Organic Chemistry, 72, 3492-2499. 

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