Titanocene-based catalysts

The nature of the substituent directly attached to the N-atom influences the properties (basicity, reduction potential, etc.) of the C = N function more than the substituents at the carbon atom. For example, it was found that Ir-dipho-sphine catalysts that are very active for N-aryl imines are deactivated rapidly when applied for aliphatic imines [7], or that titanocene-based catalysts are active only for N-alkyl imines but not for N-aryl imines [8, 20, 21]. Oximes and other C = N-X compounds show even more pronounced differences in reactivity. 

As regards a comparison between the electrophilic properties of methylalu-minoxane-activated titanium-based homogeneous catalysts for syndiospecific polymerisation of styrene, half-sandwich titanocene-based catalysts are stronger electrophiles than non-cyclopentadienyltitanium-based catalysts, the former catalysts thus being more active in the polymerisation [55]. 

Zirconocene-based and None given titanocene-based catalysts... 

Venditto, V., De Tullio, G., Izzo, L. and Oliva, L. (1998) Ethylene-styrene copolymers by ansa-ziroonocene- and half- titanocene-based catalysts Composition, stereoregularity, and crystallinity. Macromolecules 31,4027-4029. 

Among ansa-metallocene catalysts with different group IV transition metals, ansa-titanocene-based catalysts (with rare exceptions [6]) lose most of their activity above 0°C, whereas ansa-hafnocene catalysts usually give lower activities than the analogous ansa-zirconocene catalysts [7, 8], which have thus received most research interest. 

Replacement of one of the Cp groups in the titanocene or zirconocene-based catalysts by an alkyl group destroys the... 

Titanocene catalysts do not catalyze the hydrosilation of most internal olefins, although they can attach active olefins such as styrene, or norbornene to the growing polymer chain ends. The zirconocene-based catalysts, on the other hand, can be powerful hydrosilation catalysts and the remarkable copolymer synthesis shown in Equation 3 can be easily achieved under mild conditions (V7). 

More recently, titanocene-based SiC>2-supported catalysts, analogous to that formed according to scheme (24), have seen prepared. Their precursors appear as follows ... 

Finally, carbohydrate ligands of enantioselective catalysts have been described for a limited number of reactions. Bis-phosphites of carbohydrates have been reported as ligands of efficient catalysts in enantioselective hydrogenations [182] and hydrocyanations [183], and a bifunctional dihydroglucal-based catalyst was recently found to effect asymmetric cyanosilylations of ketones [184]. Carbohydrate-derived titanocenes have been used in the enantioselective catalysis of reactions of diethyl zinc with carbonyl compounds [113]. Oxazolinones of amino sugars have been shown to be efficient catalysts in enantioselective palladium(0)-catalyzed allylation reactions of C-nucleophiles [185]. 

A variety of Group 4 metal complexes, in combination with common olefin polymerization activators, have been evaluated as potential catalysts for syn-diospecific polymerization of styrene (for reviews, see Refs. 114, 115, 123, and 426). Monocyclopentadienyl and monoindenyl titanocenes generally exhibit the highest activities (eq. 5) (112-127). Curiously, half-sandwich titanium-trifluoride-based catalysts are more active than their trichloride analogues (124,427,428). The polymerization mechanism for sPS formation is under debate. Kinetic studies and spectroscopic investigations of the catalytic systems suggest a cationic Ti(III) complex as the active species (123). 

Although the titanium-based methods are typically stoichiometric, catalytic turnover was achieved in one isolated example with trialkoxysilane reducing agents with titanocene catalysts (Scheme 28) [74], This example (as part of a broader study of enal cyclizations [74,75]) was indeed the first process to demonstrate catalysis in a silane-based aldehyde/alkyne reductive coupling and provided important guidance in the development of the nickel-catalyzed processes that are generally more tolerant of functionality and broader in scope. 

The very first example of the catalytic reductive cyclization of an acetylenic aldehyde involves the use of a late transition metal catalyst. Exposure of alkynal 78a to a catalytic amount of Rh2Co2(CO)12 in the presence of Et3SiH induces highly stereoselective hydrosilylation-cyclization to provide the allylic alcohol 78b.1 8 This rhodium-based catalytic system is applicable to the cyclization of terminal alkynes to form five-membered rings, thus complementing the scope of the titanocene-catalyzed reaction (Scheme 54). 

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. 

Whilst hydrogenation catalysts based on early transition metals are as active and selective as those based on late transition metals, they are usually not as compatible with functional groups, and this represents the major difficulty for their use in organic synthesis. Nonetheless, titanocene derivatives have been used in industry to hydrogenate unsaturated polymers. 

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

Various transition metal catalysts, including those based on Rh, Pt, Pd, Co, and Ti, have been bound to polymer supports—mainly through the phosphenation reaction described by Eq. 9-65 for polystyrene but also including other polymers, such as silica and cellulose, and also through other reactions (e.g., alkylation of titanocene by chloromethylated polystyrene). Transition-metal polymer catalysts have been studied in hydrogenation, hydroformylation, and hydrosilation reactions [Chauvin et al., 1977 Mathur et al., 1980]. 

Several cyclic imines were reduced with phenylsilane as a reducing agent in the presence of the chiral titanocene catalyst 11 followed by a workup process to give the corresponding cyclic amines in excellent ee [26]. The hydrosilylation of 2-propyl-3,4,5,6-tetrahydropyridine with (R)-ll (substrate Ti=100 l) in THF at room temperature was completed in about 6 h (Scheme 14) [29]. The reaction mixture was treated with an acid and then with an aqueous base to afford (S)-coniine, the poisonous hemlock alkaloid, in 99% ee. 

Haselwander, T., Beck, S. and Brintzinger H. H., Binuclear Titanocene and Zirconocene Cations with //-Cl and fi — CH3 Bridges in Metallocene-based Zieg-ler-Natta Catalyst Systems - solution-NMR studies , in Ziegler Catalysts, Springer-Verlag, Berlin, 1995, pp. 181-197. 

Among catalysts based on cyclopentadienyl-substituted half-sandwich titanocenes, the IndTiCb—[Al(Me)0]x catalyst in particular has extremely high activity and stereospecificity. It is relatively more sensitive to polymerisation conditions. A minimum concentration of about 50 mmol of methylaluminoxane was required to obtain the desired polymerisation activity. The activity also increases as the [Al(Me)0]x concentration increases, reaching a maximum at an Al/Ti molar ratio of 4000. For instance, the productivity of the IndTiCU—[Al(Me)0]x catalyst under optimum polymerisation conditions... 

Since the introduction of the titanocene chloride dimer 67a to radical chemistry, much attention has been paid to render these reactions catalytic. This field was reviewed especially thoroughly for epoxides as substrates [123, 124, 142-145] so only catalyzed reactions using non-epoxide precursors and a few very recent examples of titanium-catalyzed epoxide-based cyclization reactions, which illustrate the principle, will be discussed here. A very useful feature of these reactions is that their rate constants were determined very recently [146], The reductive catalytic radical generation using 67a is not limited to epoxides. Oxetanes can also act as suitable precursors as demonstrated by pinacol couplings and reductive dimerizations [147]. Moreover, 5 mol% of 67a can serve as a catalyst for the 1,4-reduction of a, p-un saturated carbonyl compounds to ketones using zinc in the presence of triethylamine hydrochloride to regenerate the catalyst [148]. 

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