Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT

ABSTRACT - Internet Electron. J. Mol. Des. January 2002, Volume 1, Number 1, 23-36

De Novo Structural Prediction: Computational Design of Organometallic Complexes for the Stereoselective Binding of Prochiral Olefins Aaron M. Gillespie and David P. White Internet Electron. J. Mol. Des. 2002, 1, 23-36

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Abstract:
Catalytic processes involving prochiral unsaturated species frequently involve the stereoselective binding of the prochiral moiety by a chiral organometallic catalyst. The precise factors that govern the transfer of chirality from the organometallic catalyst to the substrate are largely unknown. In this study, the chiral recognition abilities of the coordinatively unsaturated [(η⁵-C₅R₅)Re(NO)(L)]⁺ (R = H, Me; L = PMe₃, PPh₃) fragments towards prochiral α-olefins, CH2=CHR' (R' = Me, n-Pr, CH2Ph (Bn), Ph, i-Pr, t-Bu, SiMe₃), are investigated using a combined molecular mechanics/semiempirical quantum mechanics approach. Semiempirical quantum mechanics (genetics-algorithm optimized PM3(tm) Hamiltonian) is used to obtain an accurate geometry of the [(η⁵-C₅R₅)Re(η²-prochiral olefin)(NO)(L)]⁺ complexes and molecular mechanics (ligand repulsive energy methodology with modified MMP2 force field) is used to analyze the steric interaction between prochiral α-olefin and [(η⁵-C₅R₅)Re(NO)(L)]⁺ fragment. A computationally derived diastereoselective excess, de_SEQM, is developed. Computed diastereoselective excess is compared to experiment for the [(η⁵-C₅H₅)Re(NO)(PPh₃)]⁺ fragment. Computed diastereoselective excesses are compared across all [(η⁵-C₅R₅)Re(NO)(L)]⁺ fragments order to derive ligand design criteria pertaining to the effect of the steric nature of the cyclopentadienyl and phosphine ligands on diastereoselectivity. Geometries predicted by semiempirical quantum mechanical methods agree favorable with structural elements from the Cambridge database. Ligand repulsive energies are found to be sensitive measures of the steric demand of the α-olefins in the [(η⁵-C₅R₅)Re(NO)(L)]⁺ environments. Diastereoselective discrimination increases linearly with increasing steric demand of the [(η⁵-C₅R₅)Re(NO)(L)]⁺ fragments.

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