Rhodium complexes chelating ligands

Figure 4.7. Approximate crystal structures of chelating biphosphine rhodium complexes (other ligands omitted for clarity).

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. 

Chelucci et al. [41] synthesized further chiral terpyridines derived from (-)-yd-pinene, (-i-)-camphor, and (-l-)-2-carene and tested their ability to chelate copper or rhodium for the asymmetric cyclopropanation of styrene. The copper catalysts were poorly efficient and selective in this reaction. The corresponding rhodium complexes led to the best result (64% ee) with the ligand derived from (-l-)-2-carene (ligand 33 in Scheme 17). 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

Rhodium complexes based on the chiral ligand (120) have been used in the asymmetric hydrogenation of functionalized chelating olefins in methanol and water. The results are compared to those obtained using the corresponding non-sulfonated catalysts in water all sulfonated... 

In the iron, ruthenium, osmium, cobalt, and rhodium complexes the xanthato ligands are isobidentate chelating. Selected examples are zra s-Ru(S2COEt)2(P-Me2Ph)2,265 cis- and zra s-Os(S2COMe)2(PPh3)2,266 Co(S2 COMe)3.267... 

Another structurally well characterized rhodium complex containing the phosphino(stibino)methane ligand as a chelate is [H3Rh2(r 2-C)2CCF3)2 r 2-(z -Pr)2P-CH2-Sb(z-Bu)2 2]PF6.101... 

Pannetier et al. (8) observed that the presence of chelating ligands on the rhodium gave a much poorer methanol conversion over a limited reaction period than did reactions using rhodium complexes with mono-dentate ligands. This may reflect the slower removal and subsequent quatemization of the chelating ligands. 

Scheme 8 Rhodium complexes of trans-chelating diphosphine ligands...

The alkynylphosphine (56) reacts with Wilkinson s catalyst to give an intermediate rhodium complex, which, when treated with diphenylacetylene followed by cyanide ion, yields the diphosphine (57), of interest as a rigid chelating ligand of fixed geometry.47... 

This system involves a mononuclear rhodium(II) a complex derived from the bis(dimethylglyoximato) chelating ligand, the... 

In principle, the mechanism of homogeneous hydrogenation, in the chiral as well as in the achiral case, can follow two pathways (Figure 9.5). These involve either dihydrogen addition, followed by olefin association ( hydride route , as described in detail for Wilkinson s catalyst, vide supra) or initial association of the olefin to the rhodium center, which is then followed by dihydrogen addition ( unsaturate route ). As a rule of thumb, the hydride route is typical for neutral, Wilkinson-type catalysts whereas the catalytic mechanism for cationic complexes containing diphosphine chelate ligands seems to be dominated by the unsaturate route [1]. 

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