Density functional theory investigation into mechanisms of metal ion-promoted and base-promoted phosphorothioate cleavage reactions and Lanthanide ion catalysts for kinetic resolution of amino acid esters
Catalysis , Chemistry
Described here are chiral Ln3+ complexes of Schiff-base ligands and their use as catalysts for the kinetic resolution of the 4-nitrophenyl esters of N-Boc glutamine and phenylalanine through an enantioselective methanolysis/ethanolysis reaction. Catalysts were screened in various solvents and temperatures to optimize the selectivity value k2L/k2D or k2D/k2L. The greatest selectivity at 25°C determined was 7.1, however, reducing reaction temperature increases selectivity and after kinetic resolution at -40°C a product mixture with >99% e.e. with ~80% of the desired product remaining. A computational method is presented for the prediction of the Brønsted parameter βeq, for a phosphorothioyl group transfer in methanol by comparing relative free energy values of starting materials and phenolate products for a series of related substrates. Direct comparison of starting material and anionic leaving group free energies results in a poor approximation of βeq, however given that the βeq value for the equilibrium of phenol and phenoxide is -1, the comparison of computed free energies of neutral starting material with neutral phenol product provides βeq approximations that are satisfactorily close to the experimentally derived values. The detailed mechanism of base-promoted hydrolysis and methanolysis of O,Oʹ-dimethyl O-aryl phosphorothioates and phosphates was examined computationally. Generally, with increased leaving group acidity, the reactions showed a tendency toward being concerted (a single transition state) and while substrates with leaving groups with lower acidity tend to involve 5-coordinate phosphorane intermediates. Brønsted βlg values are also computed, and found to be similar to experimentally determined values. The mechanism of palladacycle-promoted phosphorothioate methanolysis is also modeled computationally, showing a change in rate determining step from substrate binding (sspKa of leaving group phenol < 13) to leaving group departure (sspKa of leaving group phenol > 13). The calculations also predict the existence of a stable metal-bound 5-coordinate thiophosphorane intermediate which has been tentatively verified experimentally by others. The preliminary results of a computational investigation into the mechanism for the methanolytic cleavage of a series of N-methyl N-aryl thiobenzamides catalyzed by a similar palladacycle complex is also presented, showing metal-assisted nucleophilic attack and leaving group departure.