Titanium complexes sulfur ligands

Polysulfides of several metals can be prepared by reaction of the metals with excess sulfur in liquid NH3 (group IA metals) or by heating sulfur with the molten metal sulfide. The polysulfide ion binds to metals to form coordination compounds in which it is attached to the metal by both sulfur atoms (as a so-called bidentate ligand). One example is an unusual titanium complex containing the S52-ion that is produced by the following reaction (the use of h to denote the bonding mode of the cyclo-pentadienyl ion is explained in Chapter 16) ... 

Titanium ester enolates are not only versatile reagents for asymmetric aldol additions but also function as starters of methacrylate polymerization. A representative titanium complex 24 was characterized by crystal structure and NMR spectroscopy and reveals the monomeric O-bound enolate character. The six-coordinated titanium atom in 24 is bound to two phenolic traws-oriented oxygen atoms and two sulfur donors the remaining ligands, methyl group, and enolate moiety are c/s-configured. Upon exposure to acetone, a spontaneous aldol occurs, and the aldolate 25 thus formed was also characterized by a crystal structure. Due to its coordinative saturation, the titanium obviously does not form a chelate with the carbonyl oxygen (Scheme 3.9) [57]. 

Doye s group [81] showed that a dinuclear titanium-sulfonamidate complex (Scheme 21), with a tetrahedral sulfur in the ligand backbone, can be used for intermo-lecular hydroaminoalkylation as well. This system gives mixtures of branched and linear products, although to date there has been no mechanistic rationale provided for the reduced regioselectivity of group 4 metal complexes in this transformation. There has been one report by Zi s group [44] that describes axially chiral bis(sulfonamidate) tantalum and niobium complexes for application as precatalysts for hydroamination and hydroaminoalkylation. Unfortunately, these complexes did not show any reactivity for either of these reactions. 

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