The\nElusive Ternary Intermediates of Chiral Phosphoric\nAcids in Ion Pair CatalysisStructures, Conformations, and\nAggregation

In ion-pair catalysis,\nthe last intermediate structures prior to\nthe stereoselective transition states are of special importance for\npredictive models due to the high isomerization barrier between <i>E</i>- and <i>Z</i>-substrate double bonds connecting\nground and transition state energies. However, in prior experimental\ninvestigations of chiral phosphoric acids (CPA) solely the early intermediates\ncould be investigated while the key intermediate remained elusive.\nIn this study, the first experimental structural and conformational\ninsights into ternary complexes with CPAs are presented using a special\ncombination of low temperature and relaxation optimized <sup>15</sup>N HSQC-NOESY NMR spectroscopy to enhance sensitivity. Combined NMR\ninvestigations and theoretical calculations revealed three conformers\nof the ternary complex, of which one also closely resembles the previously\ncalculated transition states. In addition, a 2:1:1 ternary complex\nas well as an unprecedent [3:3] dimeric species consisting of two\nternary complexes was revealed. Given the importance of the ground\nstate energies for the transition state interpretation in ion pair\ncatalysis we believe that the presented experimental insight into\nthe structural and conformational variety of the ternary complexes\nis a key to the future development of predictive models in ion pair\ncatalysis.