Bulky Diphosphine Acetate Ruthenium Complexes: Synthesis and Catalytic Activity in Ketone Transfer Hydrogenation and Alkyne Dimerization

Abstract

Several mononuclear ruthenium complexes containing bulky diphosphines have been easily prepared from acetate ruthenium precursors. The cyclohexyl complex [Ru(n 2-OAc)2(DCyPF)] (1-cy; DCyPF = 1,1′-bis(dicyclohexylphosphino)ferrocene) is synthesized from [Ru(n 2-OAc)2(PPh3)2] and DCyPF in toluene at reflux, whereas the ethylenediamine (en) derivatives trans-[Ru(n 1-OAc)2(PP)(en)] (PP = 1,1′-bis(diisopropylphosphino)ferrocene (DiPPF) (2), DCyPF (3)) have been obtained in a one-pot reaction from [Ru(n 2-OAc)2(PPh3)2], PP, and en in n-heptane. Treatment of the isopropylphosphine complex [Ru(n 2-OAc)2(DiPPF)] (1-ip) with 2-(aminomethyl)pyridine (ampy) in methanol affords the isolation of a mixture of the cationic [Ru(n 2-OAc)(DiPPF)(ampy)]OAc (5a) and cis-[Ru(n 1-OAc)2(DiPPF)(ampy)] (5b) (4/1 molar ratio), via trans-[Ru(n 1-OAc)2(DiPPF)(ampy)] (4) characterized at low temperature. The analogous cyclohexyl compound trans-[Ru(n 1-OAc)2(DCyPF)(ampy)] (6), synthesized from [Ru(n 2-OAc)2(PPh3)2], DCyPF and ampy in heptane, slowly converts into [Ru(n 2-OAc)(DCyPF)(ampy)]OAc (7) at RT. The monocarbonyl derivatives [Ru(n 1-OAc)(n 2-OAc)(PP)(CO)] (PP = 1,3-bis(cyclohexylphosphino)propane (DCyPP) (8), (R)-1-[(S)-2-(dicyclohexylphosphino)ferrocenyl]ethyldicyclohexylphosphine (Josiphoscy) (9)) are obtained from [Ru(n 1-OAc)(n 2-OAc)(PPh3)2(CO)] and the suitable diphosphine. Treatment of [Ru(n 1-OAc)(n 2-OAc)(DiPPF)(CO)] (10) with phenylacetylene and in the presence of pyridine leads to the alkynyl complex [Ru(n 2-OAc)(CCPh)(DiPPF)(CO)] (11), characterized by X-ray diffraction analysis. Protonation of 10 with 2 equiv of TFA at RT gives [Ru(n 1-OCOCF3)(n 2-OCOCF3)(DiPPF)(CO)] (12). The dicarbonyl trans,cis-[Ru(n 1-OAc)2(DiPPF)(CO)2] (13) is obtained by reaction of 1-ip with CO (1 atm) in CH2Cl2, whereas the isomer cis,cis-[Ru(n 1-OAc)2(DiPPF)(CO)2] (14) can be prepared from carbonylation of 10. When 13 is heated in the solid state, 10 is formed by decarbonylation. These complexes promote the transfer hydrogenation of acetophenone in 2-propanol (S/C = 1000-2000) with NaOiPr (2 mol %), affording a TOF value of up to 81000 h-1 for complex 9 at 30 °C. Complex 10 catalyzes the head-to-head dimerization of terminal alkynes to 1,4-enynes in toluene at reflux with high stereoselectivity for the kinetic Z isomer.