Sulfur Containing Ligands

More recendy, molecular molybdenum-sulfur complexes and clusters have been used as soluble precursors for M0S2 in the formulation of lubricating oils for a variety of appHcations (70). Presumably, the oil-soluble molybdenum—sulfur-containing precursors decompose under shear, pressure, or temperature stress at the wear surface to give beneficial coatings. In several cases it has been shown that the soluble precursors are trifunctional in that they not only display antifriction properties, but have antiwear and antioxidant characteristics as weU. In most cases, the ligands for the Mo are of the 1,1-dithiolate type, including dithiocarbamates, dithiophosphates, and xanthates (55,71). 

No complexes can be isolated with hindered sulfur-containing ligands such as VII and VIII ... 

Sulfur-containing ligands form complexes with Hg(I) salts other than the nitrate and perchlorate. The ligand PhCH2SCH2Ph forms a 1 1 complex with Hg2Sip4... 

Interaction of Metal Centers Through Sulfur-Containing Ligands (O. Kahn, J.-J. Girerd)... 

Sulfur-Containing Ligands in Metalloproteins and Enz mies (W. E. Newton)... 

Scheme 1.12 Test reaction with (5)-proline-derived sulfur-containing phosphine ligands.

In 2006, these workers successfully expanded the previous study to several acyclic and cyclic allylic substrates (Scheme 1.17)." In all cases, the best enantioselectivity (up to 91% ee) was obtained by using the ligand that contained the more bulky sulfur substituent (t-Bu). The methodology was also applied to monosubstituted acyclic substrates but, however, this ligand proved to be inadequate in terms of regioselectivities, whereas a good enantioselectivity of up to 89% ee was obtained. 

In 1999, Ikeda et al. reported a new type of sulfur-oxazoline ligands with an axis-fixed or -unfixed biphenyl backbone prepared in good yields by coupling reactions of methoxybenzene derivatives substituted with a chiral oxazoline and a sulfur-containing Grignard reagent. These ligands were subsequently evaluated for the test palladium-catalysed asymmetric allylic alkylation... 

In contrast to the large number of chiral pyridine derivatives used as ligands of metal complexes in asymmetric catalysis, only a few examples of chiral sulfur-containing pyridine ligands have so far been reported, such as pyridine thioethers derived from ( + )-camphor depicted in Scheme 1.33, which were assessed in the test reaction providing enantioselectivities of up to 76% ee. The related 2,2 -bipyridine thioethers were also prepared but showed a lower stereodilferentiating capability in the test reaction. 

Scheme 1.66 Cu-catalysed allylic alkylations of allylic phosphates with vinylalumi-num reagents in the presence of sulfur-containing diaminocarbene ligands.

Scheme 1.67 Test reaction with sulfur-containing ferrocenyloxazoline ligands.

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