5-Benzyl-3-methylimidazolidin-4-one-Derived Reactive Intermediates of Organocatalysis - A Comforting Resemblance of X-Ray, NMR, and DFT Solid-Phase, Liquid-Phase, and Gas-Phase Structures

The X-ray crystal structures of three (E)-1-cinnamoylidene iminium PF6 salts of 5-benzyl-3-methylimidazolidin-4-ones (2,2-dimethyl-, cis-2-(tert-butyl)-, and cis-2-styryl-substituted; 2-4, resp.) are reported (Figs. 3,5). In the 2,2-dimethyl and in the cis-2-styryl derivative, 2 and 4, respectively, a C-H bond of the cis-substituent in 2-position points to the center of the benzene ring of the benzyl group above the five-membered ring (Fig. 6,a and b). NMR Measurements (Fig. 8) provide evidence that the same Structure is present in solution, and that a fourth derivative of this type, 5 (Scheme), has (Z)-instead of (E)-configuration around the C-N bond. In the cis-2-(tert-butyl) derivative 3, the benzyl group is located over the iminium pi-system (Figs. 4 and 6,c). Overlays with DFT-calculated crotonylidene analogs, A and B, show that the theoretical and experimental structures are almost superimposable (Fig. 9 and Table). The structures are discussed in view of their role as reactive intermediates in organocatalysis and in view of the help synthetic organic chemists may experience from theory,