Tautomerism in the Solid State and in Solution of a Series of 6-Aminofulvene-1-aldimines

To study systems able to sustain intramolecular proton-transfer, we have prepared a series of six aminofulvene aldimines including several labeled with 15N and 2H. These compounds show coupling constants through the hydrogen bond, 1h J(15N−1H) and 2h J(15N−15N). The position of the tautomeric equilibria, i.e., on what nitrogen atom is the proton, was determined in the solid state and in solution. The crystal structure of N{[5-[(phenylamino)methylene]-1,3-cyclopentadien-1-yl]methylene}pyrrole-1-amine (3) has been determined by X-ray analysis. In solution, both N−H and C−H tautomers were observed and their structures assigned by NMR spectroscopy. Particularly useful is the value of the 1 J(15N−1H) coupling constant.