A New Family of Chelating Diphosphines with Transition-Metal and Carbon Stereocenters in the Backbone: A Second-Generation Rhenium-Containing System

The less stable ReC isomer of the enantiomerically pure benzylidene complex (S)-[(η5-C5H5)Re(NO)(PPh3)(CHPh)]+BF4- ((S)-(sc)-6) and PPh2H react to give the phosphonium salt (SReSC)-[(η5-C5H5)Re(NO)(PPh3)(CH(Ph)PPh2H)]+BF4- ((SReSC)-7; 91%), as confirmed by a crystal structure of the racemate. Reaction with t-BuOK gives the phosphine (SReSC)-(η5-C5H5)Re(NO)(PPh3)(CH(Ph)PPh2) ((SReSC)-8; 73%). An analogous sequence with the more stable ReC isomer ((S)-(ac)-6) but at −60 °C gives (SReRC)-8 (58%). NMR experiments show that the intermediate (SReRC)-7 epimerizes to (SReSC)-7 above −20 °C, and the mechanism has been analyzed. Reactions of (SReSC)- and (SReRC)-8 with t-BuLi and then PPh2Cl give the title diphosphines (SReSC)- and (SReRC)-(η5-C5H4PPh2)Re(NO)(PPh3)(CH(Ph)PPh2) (84−89%). Additions of [Rh(NBD)2]+PF6- give the heterobimetallic chelate complexes (SReSC)- and (87−91%). In efforts to access similar compounds with ReCPh2PPh2 linkages, the chlorobenzene complex [(η5-C5H5)Re(NO)(PPh3)(ClPh)]+BF4- and NNCPh2...