Solution Chemistry of N,N'-Disubstituted Amidines: Identification of Isomers and Evidence for Linear Dimer Formation

Amidines have found widespread use, but their solution chemistry remains poorly understood. In this work, X-ray crystallographic and detailed 1D and 2D NMR spectroscopic studies have been performed to elucidate the preferred isomers and their interconversion mechanisms. Amidines are shown to exist as a mixture of E-syn and Z-anti isomers in solution and to form dimeric H-bonded aggregates that are also observed in the solid state. Rapid proton exchange/tautomerization reactions occur within the dimers, allowing fast interconversion of E-syn and Z-anti isomers even at very low temperatures. Three different exchange processes were identified in solution, and on this basis the unusual concentration and temperature dependence of the NMR spectra of these amidines could be explained. This work thus resolves some of the puzzles of the complex solution chemistry of this prominent class of compounds.