Catalysis titanocene

The method was later extended to the synthesis of a number of meroter-penoids from epoxygeranyl carbonates or acetates in a two-step approach combining titanocene catalysis with Stifle reactions (carbonates) [108,109] or copper-catalyzed allylic substitutions (acetates) [110-112], The cyclizations... 

Scheme 27 Synthesis of 32 based on Stille couplings and titanocene catalysis [122]...

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. 

A titanium complex derived from chiral /V-arencsulfonyl-2-amino-1 -indanol [20], a cationic chiral iron complex [21], and a chiral oxo(salen)manganese(V) complex [22] have been developed for the asymmetric Diels-Alder reaction of oc,P-unsaturated aldehydes with high asymmetric induction (Eq. 8A.11). In addition, a stable, chiral diaquo titanocene complex is utilized for the enantioselective Diels-Alder reaction of cyclopentadiene and a series of a.P Unsaturated aldehydes at low temperature, where catalysis occurs at the metal center rather than through activation of the dienophile by protonation. The high endo/exo selectivity is observed for a-substituted aldehydes, but the asymmetric induction is only moderate [23] (Eq. 8A. 12). 

Atactic polypropylenes are produced in catalysis by C2v-symmetric metallocenes that are achiral, such as Cp2MCl2 or (Me2Si(FLu)2)ZrCl2. The only stereocontrol observed is both of the chain-end type and low because the chiral center of the terminal monomer unit of the growing chain is in the P position as a consequence of the 1,2 insertion of the monomers. A significant influence on the tacticity is observed only at low temperatures, being much more pronounced for titanocenes and hafnocenes than zirconocenes as a consequence of their shorter M-Ca bonds, bringing the chiral p-carbon closer to the active center (147,148). 

Radical A-exo cyclizations can also be catalyzed without recourse to the Thorpe-Ingold effect by applying the concept of template catalysis (Fig. 24) [157]. Here, 20 mol% of a cationic titanocene(IV) precatalyst 82 is applied, which contains a coordinating neutral tether. After reductive opening of epoxide 81, a cationic titanocene(III) complexed radical 83 is formed, in which both the epoxide and the xfi-un saturated carboxamide radical acceptor are coordinated. This provides the template to accelerate the slow A-exo radical cyclization step considerably. Cyclobutanes 84 were isolated in 46-84% yield with mostly good / / [Pg.145]

Polymerization catalysis with soluble complexes of group IV transition metals, in particular with hydrocarbon-soluble titanocene complexes, was discovered in the 1950 s, shortly after the appearance of Ziegler s and Natta s reports on solid-state catalysts, and rather thoroughly studied from then on. Alkylalu-minium compounds, such as AlEt2Cl, are required to activate also these soluble catalysts. In distinction to their solid-state counterparts, however, early soluble catalysts were able to polymerize only ethylene, and not any of its higher homologues. After their activation by methylalumoxanes had been discovered (Section 7.4.1), soluble catalysts became as efficient as solid-state catalysts - in... 

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]. 

In the Cp2TiMe2-catalyzed hydroboration of alkenes, a titanocene bis(borane) complex is responsible for the catalysis. This bis(borane) complex initially dissociates to give a monoborane intermediate. Coordination of the alkene gives rise to the alkene-borane complex, which is likely to be a resonance hybrid between an alkene borane complex and a 3-boroalkyl hydride. An intramolecular reaction extrudes the trialkylborane product, and coordination of a new HBR2 regenerates the monoborane intermediate. 

Enantiomerically pure bis-Gp derivatives with chiral Gp ligands have been used with success in the catalytic enantioselective opening of meso-epoxides via electron transfer (see Section 4.05.8). The structural features are of relevance for the understanding of activity and selectivity of these complexes in diastereoselective reactions and for the design of novel catalysts. A comparison of the structure of three of these bis-Gp Ti derivatives (Scheme 481) in the solid state and in solution determined by X-ray crystallography and NMR methods indicated that the structures in the crystal and in solution are the same, and that applications of these complexes in catalysis can de discussed on the basis of crystallographic data.1114 In a similar study, the 1-methylcyclohexyl-Cp, 1-butyl-1-methylbutyl-Cp, and cyclohexyl-Cp titanocene dichlorides (Scheme 481) have been prepared and their molecular structures compared. The use of these three compounds in radical addition reactions has been studied.1115... 

By using Brintzinger s ansa-titanocene C2H4(1-Ind)2TiCl2, Ewen first proved the correlation between metallocene chirality and isotacticity, a textbook example of shape selective catalysis. The C2-symmetric, racemic form yields isotactic polypropene while the achiral, meso form produces low molecular weight atactic polypropene. However, this titanocene is unstable at normal temperatures and has a quite low activity and a low stereoselectivity, producing... 

Minor amounts of thermally stable neoalkoxy titanate and zirconate additives may provide a means for postreactor, in situ metallocene-like repolymerization catalysis of a filled or unfilled polymer during the plasticization phase. This may result in the creation of metallocene-like (titanocene or zirconocene) behavior associated with effects such as increased composite strain to failure resulting in increased impact toughness or enhanced polymer foamabUity. Other effects to be discussed below with specific examples are related to enhanced processability, reduced polymer chain scission, shortened polymer recrystallization time, and compatibilization of dissimilar polymers. 

The ansa metallocenes of Ti and Zr, with structures of the fragments shown in Figure 3.18 have become extremely important in the stereoselective catalysis of olefin pol)m erization and other reactions. While the first ansa titanocene was prepared by Katz in 1970, and the first enantiopuie ansa titanocene was isolated in 1982 when Brintzinger resolved (EBTHI)TiClj (Figure 3.18 with M = Ti) using a binaphtholate derivative,the stereoselective synthesis of the rac form of the ansa metallocenes has been challenging. More recently, Jordan has foirnd... 

Early transition metal and lanthanide complexes also catalyze the hydroboration of alkenes. As shown in Equation 16.44, titanocene complexes catalyze the hydroboration of vinylarenes, and as shown in Equation 16.45, lanthanocene complexes catalyze the hydroboration of alkenes. Ligand-less lanthanides and zirconocene complexes have also been reported to catalyze these reactions, but the apparent catalysis of the hydroboration in these cases more likely occurs by catalytic generation of BH3 from catecholborane and uncatalyzed addition of BHj to the olefin. 

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). 

Rausch, M. D. Lin, Y.-G Winter, H. H. Two-state propagation mechanism for propylene polymerization catalyzed by rac-[anti-ethylidene(l-r -tetramethylcyclopentadienyl)(l-r -indenyl)] dimethylti-tanium. J. Am. Chem. Soc. 1991,113, 8569-8570. (e) Lhnas, G H. Day, R. O. Rausch, M. D. Chien, J. C. W. Ethylidene(l-r -tetramethylcyclopentadienyl)(l-r -mdenyl)dichlorozirconium Synthesis, molecular structure, and polymerization catalysis. Organometallics 1993, 12, 1283-1288. (f) Babu, G. N. Newmark, R. A. Cheng, H. N. Llinas, G H. Chien, J. C. W. Microstructure of elastomeric polypropylenes obtained with nonsymmetric afua-titanocene catalysts. Macromolecules 1992, 25, 7400-7402. 

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