Preparation of 2-Alkylidene-Substituted 1,3,4,5-Tetramethylimidazolines and Their Reactivity Towards RhI Complexes and B(C6F5)3

In addition to the known 1,3,4,5-tetramethyl-2-methyleneimidazoline (1a), which exhibits a highly polarized exocyclic CC bond, a series of novel 2-alkylidene-substituted 1,3,4,5-tetramethylimidazolines 1be were synthesized and characterized. The molecular structures of 1b, 1d, and 1e were determined by X-ray diffraction analysis and revealed an increase in the polarization of the exocyclic CC bond with increasing steric demand of the 2-substituent. On the basis of their ylidic nature, 1ae show enhanced basicity and reactivity towards Lewis acidic centers. Treatment of 1a and 1b with [{RhCl(cod)}2] or B(C6F5)3 afforded complexes of the type [(L)RhCl(cod)] (4a,b), [(L)RhCl(CO)2] (7a,b) (L = 1a,b) or classical Lewis acid/base adducts [(1a)B(C6F5)3] (8a) and [(1b)B(C6F5)3] (8b). In contrast, complexes [(L)RhCl(cod)] with 1c and 1d as ligands are not stable, and imidazolium dichlororhodate salts 6a and 6b were isolated instead. Rhodiumalkyl complexes 5a and 5b are assumed to be intermediates in this decomposition process, and 5a was characterized by X-ray diffraction analysis. Furthermore, treatment of 1c and 1d with B(C6F5)3 did not afford classical Lewis adducts, and instead imidazolium hydridoborate salts 9a and 9b are formed by hydride abstraction. Surprisingly, we found that 1e does not react with [{RhCl(cod)}2] and forms an abnormal Lewis adduct 10 when treated with B(C6F5)3.