A New Family of Chelating Diphosphines with Transition-Metal and Carbon Stereocenters in the Backbone: A Second-Generation Rhenium-Containing System
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The less stable Re=C 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(N O)(PPh3)(CH(Ph)PPh2) ((SReSC)-8; 73%). An analogous sequence with the more stable Re=C 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-C5H4PPh 2Re(NO)(PPh2)(CH(Ph)PPh2) (84-89%). Additions of [Rh(NBD)2]-PF6- give the heterobimetallic chelate complexes (SReSC)- and (SReRC)-[(η5-C5H4PPh 2)Re(NO)(PPh3)(μ-CH(Ph)PPh2)Rh(NBD)]+ PF6- (87-91%). In efforts to access similar compounds with ReCPh2PPh2 linkages, the chlorobenzene complex [(η5-C5H5)Re(NO)(PPh3)(ClPh)] +BF4- and NNCPh2 have been reacted. Instead of the target Re=CPh2 adduct, the diphenyldiazomethane complex [(η5-C5H5)Re(NO)(PPh3)NNCPh 2)]+BF4- (73%) has been isolated and crystallographically characterized.
author list (cited authors)
Kromm, K., Hampel, F., & Gladysz, J. A.