Enantiopure Phosphine Oxides with Electron-Rich Chiral Rhenium Stereocenters: Syntheses, Structures, and Brønsted Basicities

Abstract

The chiral racemic methyl complex (η⁵-C₅R₅)Re(NO)(PPh₃)(CH₃) (2) is converted to the rhenium-containing phosphorus donor ligands (η⁵-C₅R₅)Re(NO)(PPh₃)((CH₂)_nPR'₂) (n/R/R’ = a, 0/H/Ph; 4b, 0/H/t-Bu; 7a, 0/Me/Ph; 10a, 1/H/Ph; 10b, 1/H/t-Bu) via standard sequences involving intermediate phosphonium salts (4a,b and 7a; TfOH/CH₂Cl₂ or HBF₄/chlorobenzene, then PR₂H, then t-BuOK; 10a,b, Ph₃C⁺ X⁻, then PR₂H, then t-BuOK). These are converted to the title phosphine oxides (η⁵-C₅R₅)Re(NO)(PPh₃)((CH₂)_nP(O)R'₂) (n/R/R’ = 1a, 0/H/Ph; 1b, 0/H/t-Bu; 8a, 0/Me/Ph; 11a, 1/H/Ph; 11b, 1/H/t-Bu) using PhIO, Me₃SiOOSiMe₃ or (for 8a) air. The enantiopure methyl complex (S)-2 is similarly converted to (R)-4a,b, (R)-8a, and (S)-11a,b. The crystal structures of 1a, (S)-1a·(H₂O)_0.5, (S)-8a, and 11a·CHCl₃ are determined. The basicities of PO-containing species play key roles in certain enantioselective catalytic reactions. Those of the title and related compounds are computed by DFT (i.a. 8b > 8a > 1b > 1a > 11b > 11a > t-Bu₃PO > Ph₃PO). The enhancements in the rhenium-containing species are rationalized.