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Synthesis and Characterization of Ruthenium (II) Complexes and their Applications to Atom Transfer Radical Addition Reactions
AuthorNair, Radhika Parameswaran
AdvisorFrost, Brian J
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Abstract<bold>Abstract</bold>A series of mixed-phosphine ruthenium complexes of the type Cp'Ru(PR3)(PPh3)Cl<sub>3</sub>, where Cp' = Cp*, Dp, Ind, Cp, Tp; PR3 = PTA or PMe3<sub>3</sub> have been synthesized by ligand exchange reactions with Cp'Ru(PPh3)2Cl<sub>3,2</sub>, and characterized by multinuclear NMR spectroscopy and X-ray crystallography. We have explored the efficiency of these complexes as catalysts for the atom transfer radical addition (ATRA) of various chloro-substrates (CCl4<sub>4</sub>, CHCl3<sub>3</sub>, p-TsCl<italic>p</italic>, CCl3CO2Et<sub>3,2</sub>, and CH2ClCO2Et<sub>2</sub>) to styrene in the presence of AIBN as a radical source. For comparison purposes, we also investigated the activity of Cp'Ru(PPh3)2Cl<sub>3,2</sub> and Cp'Ru(PTA)2Cl<sub>2</sub> complexes towards ATRA. In general, these complexes efficiently catalyzed the radical addition reactions affording the 1:1 adduct in almost quantitative yields. Catalyst performance was found to depend mainly on the electron-donating ability of the Cp' ancillary ligands and on the nature of the phosphines. Among the ruthenium (II) complexes studied Cp*Ru(PTA)(PPh3)Cl<sub>3</sub> and Cp*Ru(PMe3)(PPh3)Cl<sub>3</sub> were very active at 60° C with TOFs of 1060 h-1<super>-1</super> and 933 h-1<super>-1</super>, respectively; Cp*Ru(PPh3)2Cl<sub>3,2</sub> was the most active for the addition of CCl4<sub>4</sub> to styrene with a TOF >960 h-1<super>-1</super> at room temperature. The reactivity decreased significantly upon substitution of the labile PPh3<sub>3</sub> with stronger binding phosphines such as PTA or PMe3 (Cp'Ru(PPh3)2Cl<sub>3,2</sub> > Cp'Ru(PR3)(PPh3)Cl<sub>3</sub> > Cp'Ru(PTA)2Cl)<sub>2</sub>. Based on the Cp' ancillary ligand, the order of reactivity was found to be: Cp*Ru >> DpRu > IndRu > CpRu > TpRu. The activity of Cp*Ru(PTA)(PPh3)Cl<sub>3</sub>, Cp*Ru(PMe3)(PPh3)Cl<sub>3</sub>, and Cp*Ru(PPh3)2Cl<sub>3,2</sub> was further explored for the addition of CCl4<sub>4</sub> to more challenging olefins. Both terminal and internal olefins were utilized for the addition reactions. All the three complexes exhibited high reactivity towards CCl4<sub>4</sub> addition to terminal olefins. Total turnovers (TTO) in excess of 80,000 were obtained for the addition of CCl4<sub>4</sub> to hexene, making the Cp* complexes the most active and robust catalysts for ATRA reported to date. Contrary to this, the internal olefins were less prone to CCl4<sub>4</sub> addition whatever the catalytic system may be. These results indicate that the rate of the reaction depends not only on the type of catalyst, but also on the nature of substrate employed.The synthesis and characterization of the air-sensitive hydride, IndRu(PTA)(PPh3)H<sub>3</sub>, is also described. The rate of H/D exchange of the hydride complex was found to be very low (t1/2<sub>1/2</sub> ~ 5.5 d). We explored the activity of IndRu(PTA)(PPh3)Cl<sub>3</sub> for the selective transfer hydrogenation of α,β-unsaturated carbonyl compounds in aqueous-biphasic media. Electrochemical studies on some of the Cp'Ru(PR3)(PPh3)Cl<sub>3</sub> complexes in dichloromethane are also reported.