Stereochemistry of Metalated Aldimines. 2. A Theoretical Study of Dimeric Ion-Pair Aggregates of Isomeric Lithioacetaldimines and of their Kinetically Controlled Reaction with Formaldehyde
Our recent studies of isomeric monomeric metaloimine ion pairs showed a thermodynamic anti preference and the syn structures offered no explanation for a kinetic advantage. We thus proposed a mechanism involving dimeric ion-pair aggregates as the reactive species in order to account for the experimentally observed syn specificity in the reactions of metaloimines with electrophiles in solvents of low polarity. This hypothesis has now been tested at the ab initio level. Equilibrium geometries, vibrational frequencies, relative stabilities, and electronic structures of the Ci-symmetric dimers 3a and 4a formed by two syn- or anti-configured monomeric ion pairs of lithioacetaldimine, 1 and 2, respectively, have been determined. The thermodynamic anti preference carries over from the monomers to the dimers; an anti-preference energy of 6.1 kcal/mol is found. The near perpendicular arrangement between the Li2N2 plane and the best plane(s) containing the anions emerges as an important structural concept and electrostatic interactions are likely as its electronic origin. Topologically related C2h structures of 3 and 4 are disfavored. The analysis suggests and computations of the isomeric dimers formed by 1 or 2 with LiH, syn-5 and anti-6, respectively, confirm that this mode of dimerization is maintained so long as at least one of the anions contains a (pseudo)- π-system. Potential energy surface scans of 5 and 6 show that distortions of the metal coordination that retain this structural feature require little energy while deviations from it require substantial energy, The activation energies required of 5 and 6 to meet distance requirements are discussed as well as ligand repulsion effects within the Li coordination sphere in the model system 7, an H2CO coordinated dimer formed by LiH and LiNH2. Distance requirements alone cannot account for the kinetic advantage of the syn reaction, but the structures with optimal distance requirements suggest that precoordination and orientation of the reagent cause a kinetic advantage for the metal-assisted syn reaction as a result of ligand repulsion effects in the coordination sphere of lithium. As a concrete example, the transition-state structures 8 and 9 for the reactions of 5 and 6 with H2CO have been determined. The Curtin-Hammett principle applies to the competition between the syn and the anti reactions and a preference of 0.9 kcal/mol for the transition-state structure of the syn reaction is found. These results suggest that the experimentally observed stereochemistry can be accounted for with a mechanism that involves a kinetically controlled reaction featuring the dimer as the reactive species. In contrast to metalated oxyimines, cooperative effects appear necessary to account for the stereochemistry of the additions of electrophiles to metaloimines under conditions that favor ion pairing and aggregation and the dimeric ion-pair aggregates appear as the smallest conceivable reactive species.
R. Glaser et al., "Stereochemistry of Metalated Aldimines. 2. A Theoretical Study of Dimeric Ion-Pair Aggregates of Isomeric Lithioacetaldimines and of their Kinetically Controlled Reaction with Formaldehyde," Journal of Organic Chemistry, vol. 56, no. 23, pp. 6625-6637, American Chemical Society (ACS), Nov 1991.
The definitive version is available at https://doi.org/10.1021/jo00023a031
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