Hydrogen-transfer catalyzed by half-sandwich Ru(II) aminophosphine complexes

The syntheses of and catalytic studies on some Ru() complexes bearing the a minophosphine ligands N,N-dimethyl-2- diphenylphosphinoethylamine (PN), opt ically pure (R-C,S-pl)-2-{1-(N,N-dimethylamino)ethyl}-1-diphenylphosphinofe rrocene (PPFA), and N,N dimethyl-2-diphenylphosphinoaniline (DBD) are descr ibed, [RuCp(CH3CN)(3)](+) reacts with these ligands to give the cationic co mplexes [RuCp(PN-kappaN,kappaP)(CH3CN)](+) (1a), [(S-Ru,R-C,S-pl)-RuCp(PPFA -kappaN,kappaP)( CH3CN)](+) (1b), and [RuCp(DBD-kappaN,kappaP)(CH3CN)](+)(1 c), respectively, in high yields. From these, in turn, the residual CH3CN l igand can be replaced by Br(-)upon addition of NEt4Br in CH2Cl2, resulting in the formation of the neutral complexes RuCp(PN)Br (2a), (S-Ru,R-C,S-pl)- RuCp(PPFA) Br (2b), and RuCp(DBD)Br (2c), again in good yields. Similarly, [Ru(eta (6)-p-cymene)Cl-2](2) reacts with 1 equiv. of PN or PPFA to give Ru (eta (6)-p-cymene)(PN-kappaP)Cl-2 (3a) and Ru(eta (6)-p-cymene)(PPFA-kappaP )Cl-2 (3b). Furthermore, treatment of 3 with TlCF3SO3 in THF at room temper ature affords the cationic complexes [Ru(eta (6)-p-cymene)(PN-kappaN,kappaP )Cl]CF3SO3 (4a) and [(R-Ru,R-C,S-pl)-Ru(eta (6)-p-cymene) (PPFA-kappaN,kapp aP) Cl] CF3SO3 (4b). The absolute configuration at the metal center of 2b a nd 4b' (BPh4- salt of 4b) was determined by X-ray crystallography. Catalyti c studies were performed with the racemic complexes 1a, 2a, 2c, and 4a and the diastereopure complexes 1b, 2b, and 4b. All of these proved to be excel lent precatalysts for the transfer hydrogenation of acetophenone and deriva tives thereof, and cyclohexanone. With the enantiomerically pure systems 2b and 4b, only racemic products were obtained. This testifies to the hemilab ile nature of the aforementioned aminophosphine ligands giving transient ka ppa -P-bonding coordination. Since diastereoface selection of incoming subs trates is based on the planar chirality of the ferrocene moiety, rather tha n the metal centered chirality, no enantioselective reaction takes place.