Dedicated to Professor Andreas Pfaltz on the occasion of his 60th birthday
Ene-1,1-diamines (ketene aminals, A) are a special class of olefins distinguished by a very electron rich and strongly polarized double bond. [1] This dipolar character is caused by the significant contribution from the mesomeric extreme A' to their ground-state structure (Scheme 1), and becomes particularly prominent if the nitrogen atoms are part of a heterocyclic ring able to accommodate positive charge. Likewise, coordination to a suitable (transition) metal template enhances the inherent ylide character of compounds such as 1 [2,3] and its relatives. Surprisingly, however, few such metal complexes of ene-1,1-diamines have been reported, [3, 4] and no attempts have been made to generalize the underlying concept of charge separation. Outlined below are the results of our initial foray into this promising territory, with emphasis on the coordination chemistry of gold as a particularly "carbophilic" and catalytically relevant Lewis acid. [5, 6] Although compound 1 has been investigated in some detail, [2,3,7] the parent 1,3-dimethyl-2-methyleneimidazoline (3) itself has not been isolated in pure form. Gratifyingly, we found that this particular compound is readily obtained as an air-sensitive solid by deprotonation of imidazolium iodide 2 with KH in Et 2 O (Scheme 2). However, first attempts at preparing the corresponding cationic gold complex through reaction of 3 with [AuCl(PPh 3 )] and AgSbF 6 in THF were thwarted by the instantaneous formation of a black, intractable precipitate. On the assumption that the electron-rich