Titanocene complexes hydrogenation

The polymers were converted to supported catalysts corresponding to homogeneous complexes of cobalt, rhodium and titanium. The cobalt catalyst exhibited no reactivity in a Fischer-Tropsch reaction, but was effective in promoting hydroformylation, as was a rhodium analog. A polymer bound titanocene catalyst maintained as much as a 40-fold activity over homogeneous titanocene in hydrogenations. The enhanced activity indicated better site isolation even without crosslinking. 

In addition to phosphine ligands, a variety of other monodentate and chelating ligands have been introduced to functionalized polymers [1-5]. For example, cyclo-pentadiene was immobilized to Merrifield resins to obtain titanocene complexes (Fig. 42.13) [102]. The immobilization of anionic cyclopentadiene ligands represents a transition between chemisorption and the presently discussed coordinative attachment of ligands. The depicted immobilization method can also be adopted for other metallocenes. The titanocene derivatives are mostly known for their high hydrogenation and isomerization activity (see also Section 42.3.6.1) [103]. 

A chiral titanocene complex catalyzes enantioselective hydrogenation of imines in a moderate to high optical yield (Scheme 78) (117). 

New hydrogenated tetrahydroindenyl and substituted Ind titanocene complexes have been synthesized in an effort to modify the steric environment around the titanium atom in complexes containing chiral bridges connecting two binaphthyl substituted Ind ligands (Scheme 658). The molecular structure of some of these complexes has been determined by X-ray crystallography, -symmetry is observed in the solid state, although in solution at room temperature the NMR spectra indicate 6 -symmetry.1678... 

Only a few chiral catalysts based on metals other than rhodium and ruthenium have been reported. The titanocene complexes used by Buchwald et al. [109] for the highly enantioselective hydrogenation of enamines have aheady been mentioned in Section 3.4 (cf. Fig. 32). Cobalt semicorrin complexes have proven to be efficient catalysts for the enantioselective reduction of a,P-unsaturated carboxylic esters and amides using sodium borohydride as the reducing agent [ 156, 157]. Other chiral cobalt complexes have also been studied but with less success... 

The most widely studied approach for the enantioselective hydrogenation of non-functionalized alkenes has been the use of reduced chiral titanocene complexes. The initial promising demonstration utilized bis(menthylcyclopentadi-enyl)titanium dichloride (12) in the presence of Red-Al to catalyze the hydrogenation of 2-phenylbutene in 23% ee (determined by optical rotation) [13,14, 15]. Two mechanisms have been postulated for hydrogenations involving reduced titanocene catalysts a Ti(II)/Ti(IV) cycle and the more commonly invoked Ti(III) cycle shown in Scheme 2. 

Chiral Cyclopentadienyl Complexes. Since the discovery of the polymerization activity of cyclopentadienyl complexes, they also play a key role in asymmetric catalysis (Fig. 13). Titanocene complexes of chiral tricyclic monocy-clopentadienyl ligand catalyze the enantioselective hydrogenation of unfunctionalized oleflns (105). A similar reaction has been performed with related catalysts such as chiral Ziegler-Natta systems (106) and organolanthanide systems (107). 

The basic compound of Brintzinger s ansa-titanocene complexes is ethylenebis-(tetrahydroindenyl)titanium dichloride, (EBTHI)TiCl2. Further analogues ((EBTHI)TiH, (EBTHI)Ti(Me)2, and (EBTHI)Ti(CO)2) have been wddely used for asymmetric hydrogenation, hydrosilylation, and Pauson-Khand reaction (121). Novel optically active titanium complexes containing a linked amido-cyclopentadienyl ligand have been developed and used for asymmetric hydrogenation (122). 

The titanocene dichloride complexes derived from the camphor- and pinene-annulated ligands 126 and 127 were tested as enantioselective hydrogenation catalyst and using 2-phenylbutene as substrate 2-phenylbutane was obtained with ee up to 34% [148, 149]. 

Deactivation of a homogeneous catalyst by dimerization can usefully be prevented by supporting the monomeric unit [note also that most multicenter catalysts require dissociation to an active monomeric form (/, p. 405 also Section VI)]. This concept has been used to maintain the hydrogenating activity of titanocene intermediates, which normally dimerize to inactive fulvalene complexes (334-336). Similarly maintained... 

Asymmetric hydrogenation of 3,4-hydroisoquinolines with Ir-chiral phosphorus ligand complexes has been studied. Although the highest enantioselectivity to date is obtained with a chiral titanocene catalyst,308,308a 308c chiral BCPM-Ir or BINAP-Ir complexes with additive phthalimide or F4-phthalimide have shown some good selectivity. Some examples are listed in Table 24. 

Early transition-metal complexes have been some of the first well-defined catalyst precursors used in the homogeneous hydrogenation of alkenes. Of the various systems developed, the biscyclopentadienyl Group IV metal complexes are probably the most effective, especially those based on Ti. The most recent development in this field has shown that enantiomerically pure ansa zirconene and titanocene derivatives are highly effective enantioselective hydrogenation catalysts for alkenes, imines, and enamines (up to 99% ee in all cases), whilst in some cases TON of up to 1000 have been achieved. 

The most selective - and also most general - titanocene catalyst is complex 35 d, also studied by Buchwald and coworkers. This catalyst was used to hydrogenate a variety of functionalized and unfunctionalized cyclic and acyclic alkenes with excellent ee-values in most cases [46]. Enamines could also be hydrogenated with enantiomeric excesses of 80-90% [47]. However, high catalyst loadings (5-8 mol%) and long reaction times were required to drive the reactions to completion. 

Figure 1.31 illustrates a mechanism proposed for this hydrogenation. The titanocene hydride 31A is expected to be a catalytic species. The imine substrate is inserted into the Ti—H bond of 31A with a 1,2-fashion to form a titanocene amide complex 31B. Then the hydrogenolysis of 31B through a a-bond metathesis produces the amine product with regeneration of 31A. The enantioface selection... 

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