Synthesis and Characterization of Ruthenium (II) Complexes and their Applications to Atom Transfer Radical Addition Reactions
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Authors
Nair, Radhika P.
Issue Date
2009
Type
Dissertation
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Abstract
A series of mixed-phosphine ruthenium complexes of the type
Cp#Ru(PR3)(PPh3)Cl, ,here Cp# 0 Cp*, Dp, Ind, Cp, Tp; PR3 = PTA or PMe3 have been
synthesized by li<and e>chan<e reactions ,ith Cp#Ru(PPh3)2Cl, 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, CHCl3, p-TsCl, CCl3CO2Et, and CH2ClCO2Et) to
styrene in the presence of AIBN as a radical source. For comparison purposes, we also
investi<ated the activity of Cp#Ru(PPh3)2Cl and Cp#Ru(PTA)2Cl 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-donatin< ability of the Cp# ancillary li<ands and on the
nature of the phosphines. Among the ruthenium (II) complexes studied
Cp*Ru(PTA)(PPh3)Cl and Cp*Ru(PMe3)(PPh3)Cl were very active at 60° C with TOFs
of 1060 h-1
and 933 h-1
, respectively; Cp*Ru(PPh3)2Cl was the most active for the
addition of CCl4 to styrene with a TOF >960 h-1
at room temperature. The reactivity
decreased significantly upon substitution of the labile PPh3 with stronger binding
phosphines such as PTA or PMe3 (Cp#Ru(PPh3)2Cl F Cp#Ru(PR3)(PPh3)Cl >
Cp#Ru(PTA)2Cl)G Based on the Cp# ancillary li<and, the order of reactivity was found to
be: Cp*Ru >> DpRu > IndRu > CpRu > TpRu.
The activity of Cp*Ru(PTA)(PPh3)Cl, Cp*Ru(PMe3)(PPh3)Cl, and
Cp*Ru(PPh3)2Cl was further explored for the addition of CCl4 to more challenging
ii
olefins. Both terminal and internal olefins were utilized for the addition reactions. All the
three complexes exhibited high reactivity towards CCl4 addition to terminal olefins. Total
turnovers (TTO) in excess of 80,000 were obtained for the addition of CCl4 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 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, is also described. The rate of H/D exchange of the hydride complex
was found to be very low (t1/2 ~ 5.5 d). We explored the activity of IndRu(PTA)(PPh3)Cl
for the selective transfer hydrogenation of ,-unsaturated carbonyl compounds in
aqueous-biphasic mediaG Jlectrochemical studies on some of the Cp#Ru(PR3)(PPh3)Cl
complexes in dichloromethane are also reported.
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