Complexes of Ru, Rh and Ir

Amino acidate N,0-coordinated chiral complexes of Ru, Rh and Ir have found applications in the catalytic hydrogenations of ketones and unsaturated aldehydes... 

Reactions of half-sandwich complexes of Ru (Rh and Ir) with amino add esters have been used for sequence-specific peptide synthesis. N,N -Coordinated peptides undergo chain-lengthening at the N terminus by reaction with a-amino add esters (e.g. in MeOH in the presence of NEtj). Chain lengths of up to 9 amino add residues can be synthesized using a (ri -p-cymene)RuCl]" fragment [102]. 

Cyclometallated complexes of Ru, Rh and Ir have been used as ATH catalysts some examples of this catalyst class were prepared and reported previously however, their appUcation has now been extended to a broader range of target imines, both cyclic and acycUc (Fig. 35) [119]. 

The catalytic properties of chlorophosphine complexes of technetium such as lTcCl4(Ph3P)2]° for the hydrogenation of cyclohcxene and octcnc-1, dissolved in benzene, were studied in the presence of sodium amalgam and/or 0.1 MPa H2. The catalytic activity of the technetium complexes was shown to be evident but low as compared to the most active complexes of Ru, Rh or Ir [16],... 

While, transition metal complexes (mostly of Ru, Rh and Ir) have been widely studied in homogenous TH of ketones, much less attention has been devoted to the hydrogen transfer reduction of carbonyl compounds under heterogeneous conditions. Generally, homogeneous catalysts are far more active and selective than... 

In this reaction, a rhodium atom complexed to a chiral diphosphine ligand ( P—P ) catalyzes the hydrogenation of a prochiral enamide, with essentially complete enan-tioselectivity and limiting kinetic rates exceeding hundreds of catalyst turnovers per second. While precious metals such as Ru, Rh, and Ir are notably effective for catalysis of hydrogenation reactions, many other transition-metal and lanthanide complexes exhibit similar potency. 

The insertion of SnClj into M—Cl bonds to give M-SnC complexes is the standard method of preparing these catalytically active derivatives of Pt, Ru, Rh and Ir. Reviews are available ... 

Many other metals have been shown to be active in HDS catalysis, and a number of papers have been published on the study of periodic trends in activities for transition metal sulfides [15, 37-43]. Both pure metal sulfides and supported metal sulfides have been considered and experimental studies indicate that the HDS activities for the desulfurization of dibenzothiophene [37] or of thiophene [38, 39] are related to the position of the metal in the periodic table, as exemplified in Fig. 1.2 (a), 1.2 (b), and 1.2 (c). Although minor differences can be observed from one study to another, all of them agree in that second and third row metals display a characteristic volcano-type dependence of the activity on the periodic position, and they are considerably more active than their first row counterparts. Maximum activities were invariably found around Ru, Os, Rh, Ir, and this will be important when considering organometallic chemistry related to HDS, since a good proportion of that work has been concerned with Ru, Rh, and Ir complexes, which are therefore reasonable models in this sense however, Pt and Ni complexes have also been recently shown to promote the very mild stoichiometric activation and desulfurization of substituted dibenzothiophenes (See Chapter 4). 

One of the most successful preparative routes for r -bonded metal complexes of thiophenes has been the chemical or electrochemical reduction of the corresponding 18e 1)5 Ru, Rh, and Ir precursors [58-64] upon addition of two extra electrons to complexes like Cp Ir( ri -Th), the thiophenic ligand necessarily transforms from a 6e-donor into a 4e-donor situation in order to avoid oversaturation at the metal center, as exemplified for Cp Ir(2,5-Me2T) in Eq. 2.16. 

Metal complexes containing q -bonded thiophenes, where the ring formally donates 6 electrons and occupies three coordination sites (Lj-type ligand) are the most numerous and stable of the transition metal-thiophene derivatives, examples being available for Cr, Mn, Re, Fe, Ru, Rh, and Ir. Curiously, the synthesis of the first n-thiophene metal complex, viz. Cr(CO)3(q -T) reported by Fischer as early as 1958 [67] represents still today the only example available for a Group 6 metal it-bonded to thiophene its X-ray structure was solved by Dahl in 1965 albeit with a strong rotational disorder for the... 

It was originally thought that the high basicity of the amido complex would make the reprotonation a dillusion controlled process so that fc.., was always much greater than fc2- This is indeed true for the majority of Co" systems examined and for all studied cases of Cr", Ru", Rh" and Ir complexes. Under these conditions, the expression for the rate constant reduced to feg = S n ki k2 /k-i or 2 K k2 where K is the equilibrium constant for the proton transfer process (1). With proton transfer as a rapid preequilibrium, such systems exhibit specific base catalysis. 

Because of this increased nucleophihcity of the enolate, catalytic reactions with high atom economy such as chemoselechve aldol, Kndevenagel and Michael reactions catalyzed by Re, Fe, Ru, Rh and Ir complexes are achieved under neutral and mild conditions [199-202]. 

TTN has been used as an NO transfer agent in the formation of nitrosyl complexes of Ru, Rh, Ir, Co, Mo, and W (eq 7). No yields have been reported for these reactions and the by-products have not been defined. 

Fig. 4.2 Ligands tested in Ru, Rh and Ir complexes for enantioselective hydrogenation of enamino-ester 15, and e.e.s obtained...

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