Bis-chelates

By contrast to the metaphosphimates, the metaphosphate analogues in which two chalcogens are replaced by imido groups form dimeric, dianionic ligands (24). There is only one example of a metal complex of the oxo system (24, E = O). An N,0-chelated bis(dimethylalumimun) complex is prepared by the reaction of two equivalents of trimethylaliuninum with ds-[ BuNH(0)P(//-N Bu)2P-(0)NH Bu] [20]. The structure of this complex is comparable to that of (15). 

R. Colton Coord. Chem. Rev. 6, Steric effects in substituted halo- 16 Complexes with chelating bis-... 

Rhodium complexes with chelating bis(oxazoline) ligands have been described to a lesser extent for the cyclopropanation of olefins. For example, Bergman, Tilley et al. [32] have prepared a family of bis(oxazoline) complexes of coordinatively unsaturated monomeric rhodium(II) (see 20 in Scheme 13). Interestingly, the use of complex 20 in the cyclopropanation reaction of styrene afforded mainly the cis cyclopropane cis/trans = 63137), with 74% ee and not the thermodynamically favored trans isomer. No mechanistic suggestions are proposed by the authors to explain this unusual selectivity. 

Of the various results, these appear to the reviewer to be the most meticulously obtained and they do not fit the other two rate laws suggested. At 25 °C n =. 0M perchlorate), = 5.10x10 l mole. sec" and = 4.78x10 l .mole" . sec . The corresponding reaction intermediates are a neutral chelate mono-oxalato complex and a (non-chelate) bis-oxalato complex of Cr(VI). 

Other species tested in situ are chelating bis-imidazolium structures with phe-nylene spacers [48], and bidentate C-P ligands [49] that achieved, in the case of triarylphosphine-imidazolium salts, very good yields for a wide variety of substituted aryl bromides (Scheme 6.11) [50]. 

Early work by Strassner and co-workers showed that the chelating bis-NHC Pd complexes 32a and 32c were capable of promoting the oxidation of methane, whilst the iodo-analogues 32b and 32d were inactive under the same reaction condition [45], Indeed, in a mixture of TEA and TFAA, in the presence of potassium peroxodisulfate under 20-30 bar of methane, trifluoroactic acid methyl ester is produced, using 32a or 32c as catalyst (Scheme 10.14). hi a more recent work, the authors disclosed the use of pyrimidine-NHC Pd complexes for the same reaction. A shghtly better catalytic activity was obtained with the unexpected cationic complex 34 [46],... 

Figure 2.13 Selected examples of gold(lll) complexes containing chelating bis(amidate) ligands.

Kilpin, K.J., Henderson, W. and Nicholson, B.K. (2007) Organogold(III) complexes containing chelating bis (amidate) ligands Synthesis, characterisation and biological activity. Polyhedron, 26, 434. 

Elimination of the chlorine atom from [Au(C6E5)2Cl(dpma)] [29] with AgC104 gives the corresponding chelate bis(diphenylarsine)methanegold derivative, which evolves to the methanide [Au(C6F5)2(Ph2AsCHAsPh2)] under deprotonation reaction with NaH. 

In 2003, Livinghouse et al. also reported that chelating bis(thiophosphonic amidates) complexes of lanthanide metals, such as yttrium or neodymium, were able to catalyse intramolecular alkene hydroaminations. These complexes were prepared by attachment of the appropriate ligands to the metals by direct metalation with Ln[N(TMS)2]3- When applied to the cyclisation of 2-amino-5-hexene, these catalysts led to the formation of the corresponding pyrrolidine as a mixture of two diastereomers in almost quantitative yields and diastereos-electivities of up to 88% de (Scheme 10.81). 

Fast, selective conversion of internal alkenes to linear esters is catalyzed by Pd11 complexes of chelating bis(phospha-adamantyl)diphosphines, e.g., the mesa (168) and racemic (169) isomers, and the catalysis is acutely sensitive to the ligand backbone and even to the diastereomer used the results are compared with those for the P Bu2 analogous.686... 

Chelating bis(imine) ligands such asA-alkyl(2-pyridyl)methanimine (AlkPMI) have also been successfully utilized in copper-mediated ATRP [74,75,76,77,78]. In... 

FIGURE 33. Preparation of (0->-Ge)-chelate bis-(lactamo-W-methyl)-cA-dichlorogermanes. Reproduced from Reference 129 by permission of Elsevier sequoia S.A. 

Although the latter product is a solvated mononuclear [Rh(MeOH)2(diphos)]+ cation, in the solid state it is isolated as a binuclear complex of formula [Rh2 (diphos)2](BF4)2, in which each rhodium center is bonded to two phosphorus atoms of a chelating bis(diphenylphosphino)ethane ligand, and to a phenyl ring of the bis(diphenylphosphino)ethane ligand of the other rhodium atom. This dimer reverts to a mononuclear species on redissolving. The mechanism of hydrogenation of the prochiral alkene methyl(Z)-a-acetamidocinnamate, studied in detail by Halpern [31], is depicted in Scheme 1.7. 

The use of six equivalents of dihydrogen peroxide leads to a clean conversion of the dithiolate complex to the disulfonate compound. Earlier studies on oxidation of nickel thiolates showed that oxidations with dioxygen stop at monosulfinates. Our observation and the characterization of the first chelating bis-sulfonato nickel complex formed from the direct oxidation of a mononuclear nickel dithiolate, may also provide new insight into the chemistry of sulfur-rich nickel-containing enzymes in the presence of oxygen. 

A -Heterocyclic carbene complexes of Ir(I) and Ir(III) have also demonstrated high reactivity in transfer hydrogenation reactions of ketones (Scheme 2) [4]. Complex 4 catalyzed the reduction of a range of ketones into the corresponding alcohols, including the reduction of pinacolone 7 into alcohol 8 with a low catalyst loading and short reaction time [5]. The chelating bis(Af-heterocyclic carbene) complex 5 was shown to catalyze the reduction of ketones, and in the case of the reduction of benzophenone 9 to alcohol 10, the reaction was complete within 4 min [6]. 

A major group of rhenium(II) complexes is characterized by two chelating bis-phosphines forming the equatorial plane of a distorted octahedron (263). The axial ligands can be varied and representatives with halides, pseudohalides, thiolates, and nitriles have been studied extensively. 

Thus, the methanolysis of the lactone 35 by MeONa or (MeO)2Mg produces initially the triketoester 36, which is subsequently deacylated in situ to give after hydrolysis the diketone ester 38. The yield of 38 is considerably better with (MeO)2Mg, as the result of the formation in the reaction medium of a chelated bis-enolate 37 which protects 38 from degradation. Extensive degradation is observed with the sodium enolate analogue of 37 which is unstable in these conditions (equation 56). ... 

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