Olefins titanium catalysts

Certain half-sandwich phenoxides have been shown to be highly active olefin polymerization catalysts. For example, the zirconium complex (60) polymerizes ethylene with an activity of 1,220 gmmol-1 h-1 bar-1.181 A similar titanium complex (61) displays an activity of 560gmmol ll bar 1 at 60°C.182-189 Comparable activities were also recorded for the copolymerization of ethylene with 1-butene and 1-hexene. 

A conveniently prepared amorphous silica-supported titanium catalyst exhibits activity similar to that of Ti-substituted zeolites in the epoxidation of terminal linear and bulky alkenes such as cyclohexene (22) <00CC855>. An unusual example of copper-catalyzed epoxidation has also been reported, in which olefins are treated with substoichiometric amounts of soluble Cu(II) compounds in methylene chloride, using MCPBA as a terminal oxidant. Yields are variable, but can be quite high. For example, cis-stilbene 24 was epoxidized in 90% yield. In this case, a mixture of cis- and /rans-epoxides was obtained, suggesting a step-wise radical mechanism <00TL1013>. 

In contrast to the limited success with vinyl sulfides as components of [2 + 2] cycloadditions, allenyl sulfides show wide applicability. As illustrated in Scheme 8.91, Lewis acid-catalyzed [2 + 2] cycloadditions of l-trimethylsilyl-l-methylthio-1,2-propadiene (333) with a variety of electron-deficient olefins 336 provide cycloadducts 337 with excellent regioselectivity but with moderate stereoselectivity [175c], Nara-saka and co-workers reported the first Lewis acid-catalyzed asymmetric [2 + 2] cycloaddition of C-l-substituted allenyl sulfides 319 with a,/3-unsaturated compounds 338 using a chiral TADDOL-titanium catalyst. The corresponding cycloadducts 339 were obtained with 88-98% ee, but a low level of trans/cis selectivity (Scheme 8.92) [169,175d[. 

Phenanthroline in the presence of heavy metals acts as an activator of the polymerization of vinyl compounds558,559 and other olefins.560-564 It also assists the dimerization of olefins in the presence of titanium catalysts.565,566 It enhances the metal catalyzed oxidation of ascorbic acid567 and dimethyl sulfoxide.568 On the other hand, on its own it can inhibit several polymerization processes.545,569 It also stabilizes butadiene and isoprene and prevents their dimerization.570 It prevents peroxide formation in ether,571 inhibits the vinylation of alcohol572 and stabilizes cumyl chloride.573 It accelerates the vulcanization of diene rubbers574 and copolymers.575 1,10-Phenanthroline catalyzes the autooxidation of linoleic and ascorbic acids in the absence of metals.567... 

The bis(rj-cyclopentadienyl)titanium(III) aryl complexes form interesting dinitrogen derivatives (Section III,B) the 1-methyl, 17-allyl complex (Tj-C5H5)2Ti(i73-C4H7) is the precursor to a particularly effective homogeneous olefin hydrogenation catalyst (Section VI). Attempts to prepare bis(i7-cyclopentadienyl)titanium(III) monoalkyl complexes were unsuccessful (40). 

Utility Co-components in Zirconium or Titanium Catalysts in a-Olefin... 

The catalytic oxidation of hydrocarbons with peroxides, especially the epoxidation of olefins, in liquid phase by titanium catalysts is one of the most actively investigated reactions (60). The active species for this epoxidation reaction is usually assumed to be titanium peroxo moieties, derived from four-coordinate titanium and peroxides. However, the isolation of the active intermediate remains a challenge owing to the inherent instability of such species. We have been able to synthesize and stabilize the related cubic p-oxo-silicon-titanium complex (35) by reacting a bulky... 

However, once again, the main obstacle to further development was one of limited substrate scope resulting from the oxophilic titanium, molybdenum, and tungsten metal centers. The problem is illustrated in Figure 6.2, which summarizes the reactivity of early and late transition metal olefin metathesis catalysts with common... 

Other applications of titanium catalysts are hydromagnesation and carbomagnesation of unsaturated compounds, such as olefins and acetylenes with Grignard reagents. These are described in Chapter 3. 

Metallocene derivatives are the most extensively studied class of homogeneous catalysts for the polymerization of olefins. Less saturated and less hindered mono-Gp group 4 metal species of the type [Cp MR2]+ (Cp denotes Cp or a substituted cyclopentadienyl ring) may also behave as useful catalysts or initiators for olefin polymerization. Catalysts for syndiospecific polymerization of styrene based on mono-Cp titanium derivatives with different substituents on the Gp ligand and with various types of tetraphenylborates have been examined. A good relationship between the... 

Mono-Cp titanium catalyst systems are also suitable for the polymerization of polar and non-polar olefinic monomers. The reduction of a mixture of Cp TiMe3 and Ph3C[B(C6F5)4] with zinc produces a catalyst for the syndiotactic homopolymerization of styrene. The same catalyst mediates the polymerization of methyl methacrylate to poly(methyl methacrylate) (PMMA) with >65% of syndiotacticity. This system is also effective for the co-polymerization of styrene/methyl methacrylate upon optimal conditions. A new polymerization mechanism to explain the characteristics of the polymers is proposed based on sequential conjugate addition steps.541... 

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