Experimental and Theoretical Conformational Analysis of 5‐Benzylimidazolidin‐4‐one Derivatives – a ‘<i>Playground</i>’ for Studying Dispersion Interactions and a ‘<i>Windshield‐Wiper</i>’ Effect in Organocatalysis

<jats:title>Abstract</jats:title><jats:p>The PF<jats:sub>6</jats:sub> salts of 5‐benzyl‐1‐isopropylidene‐ and 5‐benzyl‐1‐cinnamylidene‐3‐methylimidazolidin‐4‐ones <jats:bold>1</jats:bold> (<jats:italic>Scheme</jats:italic>) with various substituents in the 2‐position have been prepared, and single crystals suitable for X‐ray structure determination have been obtained of 14 such compounds, <jats:italic>i.e.</jats:italic>, <jats:bold>2</jats:bold>–<jats:bold>10</jats:bold> and <jats:bold>12</jats:bold>–<jats:bold>16</jats:bold> (<jats:italic>Figs. 2–5</jats:italic>). In nine of the structures, the Ph ring of the benzyl group resides above the heterocycle, in contact with the <jats:italic>cis</jats:italic>‐substituent at C(2) (staggered conformation <jats:bold>A</jats:bold>; <jats:italic>Figs. 1–3</jats:italic>); in three structures, the Ph ring lies above the iminium <jats:italic>π</jats:italic>‐plane (staggered conformation <jats:bold>B</jats:bold>; <jats:italic>Figs. 1</jats:italic> and <jats:italic>4</jats:italic>); in two structures, the benzylic CC bond has an eclipsing conformation (<jats:bold>C</jats:bold>; <jats:italic>Figs. 1</jats:italic> and <jats:italic>5</jats:italic>) which places the Ph ring simultaneously at a maximum distance with its neighbors, the CO group, the NC‐<jats:italic>π</jats:italic>‐system, and the <jats:italic>cis</jats:italic>‐substituent at C(2) of the heterocycle. It is suggested by a qualitative conformational analysis (<jats:italic>Fig. 6</jats:italic>) that the three staggered conformations of the benzylic CC bond are all subject to unfavorable steric interactions, so that the eclipsing conformation may be a kind of ‘<jats:italic>escape</jats:italic>’. State‐of‐the‐art quantum‐chemical methods, with large AO basic sets (near the limit) for the single‐point calculations, were used to compute the structures of seven of the 14 iminium ions, <jats:italic>i.e.</jats:italic>, <jats:bold>3, 4</jats:bold>/<jats:bold>12, 5</jats:bold>–<jats:bold>7, 13</jats:bold>, and <jats:bold>16</jats:bold> (<jats:italic>Table</jats:italic>) in the two staggered conformations, <jats:bold>A</jats:bold> and <jats:bold>B</jats:bold>, with the benzylic Ph group above the ring and above the iminium <jats:italic>π</jats:italic>‐system, respectively. In all cases, the more stable computed conformer (‘isolated‐molecule’ structure) corresponds to the one present in the crystal (overlay in <jats:italic>Fig. 7</jats:italic>). The energy differences are small (≤2 kcal/mol) which, together with the result of a potential‐curve calculation for the rotation around the benzylic CC bond of one of the structures, <jats:bold>16</jats:bold> (<jats:italic>Fig. 8</jats:italic>), suggests that the benzyl group is more or less freely rotating at ambident temperatures. The importance of intramolecular <jats:italic>London</jats:italic> dispersion (benzene ring in ‘contact’ with the <jats:italic>cis</jats:italic>‐substituent in conformation <jats:bold>A</jats:bold>) for DFT and other quantum‐chemical computations is demonstrated; the benzyl‐imidazolidinones <jats:bold>1</jats:bold> appear to be ideal systems for detecting dispersion contributions between a benzene ring and alkyl or aryl CH groups. Enylidene ions of the type studied herein are the reactive intermediates of enantioselective organocatalytic conjugate additions, <jats:italic>Diels–Alder</jats:italic> reactions, and many other transformations involving <jats:italic>α</jats:italic>,<jats:italic>β</jats:italic>‐unsaturated carbonyl compounds. Our experimental and theoretical results are discussed in view of the performance of 5‐benzyl‐imidazolidinones as enantioselective catalysts.</jats:p>