Theoretical investigation of dichloromethane ligand substitution in [(η5-C5H5)Re(NO)(PPh3)(ClCH2Cl)]+, a functional equivalent of a chiral Lewis acid

The chiral-at-metal rhenium complex [(η⁵-C₅H₅)Re(NO)(PPh₃)(ClCH₂Cl)]⁺ X^– (1⁺ X^–) undergoes dichloromethane ligand substitution by various neutral donor ligands (:L), i.e., ketones and sulfides, to form the corresponding adducts [(η⁵-C₅H₅)Re(NO)(PPh₃)(L)]⁺ X^– with retention of configuration of rhenium. Reactions are first order in both 1⁺ X^– and :L concentrations. However, the experimental data could not identify the most favorable mechanism. Thus, DFT calculations were applied with :L = cyclohexanone, dimethyl sulfide, ethyl chloride, and dichloromethane. Two pathways were found: interchange (I) and neighboring group participation (NGP), anchimerically assisted by a PC=CH moiety of the PPh₃ ligand. The stronger nucleophiles, i.e., cyclohexanone and dimethyl sulfide, undergo the I mechanism with frontside displacements of the dichloromethane ligand, while the weaker nucleophiles, i.e., ethyl chloride and dichloromethane, can react by either the same I or NGP mechanisms. Both lead to retention of configuration of rhenium.

Summary of dichloromethane ligand substitution in (S_Re)-1⁺ by :L via I and NGP mechanisms