Oxophilicity of titanium

The titanium methylidene interacts with the carbonyl carbon-oxygen double bond in a sequence that is believed to resemble the metathesis process. With the metallacycle (4), a titanium oxametallacycle is believed to be formed, which then decomposes to generate the new carlxm-carbon double bond and a titanium-oxygen product. The driving force for the reaction has been attributed to the oxophilicity of titanium (equation 18). ... 

The infrared data suggest that the sulfinato group is oxygen-bonded, in agreement with the oxophilicity of titanium(IV). 

The oxophilicity of titanium has been exploited in the well-known reductive carbonyl coupling reactions pioneered by McMurry [37-41] and others [42,43] using low-valent titanium. This reaction has been shown to occur with many other early transition metals [44, 45]. The oxophilicity of the early transition metals, lanthanides, and actinides is well known. The standard enthalpies of... 

In the case of the epoxide openings, Me3SiCl is not an appropriate reagent for the cleavage of a titanium alkoxide because of the high oxophilicity of silicon. This results in the... 

Titanium alkoxides are also effective and sought-after initiators for the ROP of lactides due to a low toxicity, which minimizes the problems linked to the presence of catalyst residues in commercial PLA products [18, 19]. Despite impressive advancements in the use of Lewis acidic metal initiators in the preparation of PLAs, surprisingly little attention has been paid to the group 4 metal (Ti, Zr, Hf) initiators, probably due to the highly oxophilic nature of M(1V) which has a natural tendency to form aUcoxy-bridged multinuclear complexes. Verkade and coworkers previously demonstrated a series of titanium aUcoxide complexes 118-122 (Fig. 17) that function as moderately efficient initiators in bulk homopolymeization of L-lactide and rac-lactide, some of these initiators displaying a well-controlled polymerization behavior [119]. 

Titanium. The high reducing ability and the pronounced oxophilicity of early transition metals in low oxidation states act jointly as a formidable driving force in many transformations. However, such processes are usually hampered by the fact that the metal oxides or alkoxides formed as the inorganic by-products usually resist attempted re-reductions to the active species and thus render catalysis a difficult task. 

The early stages of the hydrolysis-condensation of [M(OR)n]m can be characterized by x-ray diffraction for oxophilic metals (Table 4). These include centrosymmetric polynuclear oxoalkoxides of titanium, zirconium, and niobium. Their stmcture is often related to that of polyoxoanions TiyOalOEOao is isostructural with Mo7024, and Nb80io(OEt)20 has a cagelike structure that compares well with that of the paratungstate H2Wi2042 b-. These oxoalkoxides are soluble as a result of their closo structure, and NMR has been developed as a technique to follow the hydrolysis of titanium(IV) alkoxides with... 

The high oxophilicity of early transition metal catalysts (titanium, zirconium, or chromium) causes them to be poisoned by most functionalized olefins, particularly the commercially available polar comonomers. However, there are examples of copolymerizations with special substrates or with very high levels of a Lewis acid incorporated into the polymerization system to protect the polar functionality through complexation. " Alternative routes to polar copolymers involving metathesis of cyclic olefins and functionalization of the resulting unsaturated polymer or metathesis of polar cycloolefins followed by hydrogenation to remove the resulting unsaturation have been published.The cost of these multistep... 

McMuriy Reaction A coupling reaction of great interest is McMurry s titanium-mediated synthesis of alkenes from two ketones (Eq. 14.69). This involves a reduced form of titanium, perhaps Ti(0), which may give the sequence of reactions shown in Eq. 14.70. These ideas are supported by the fact that 1,2-diols are also reduced to the alkene. Whatever the mechanism, the reaction shows the strongly oxophilic character of this early metal. 

The convenient preparation of thiolates of titanium " and uranium from their respective dialkylamides, but not from alkoxide analogues, demonstrates the oxophilic nature of these metals in their higher oxidation states. 

The results presented in Section 6.4.3 revealed that the McMurry reactions of ster-ically hindered ketones can in fact be viewed as Wittig-like olefination reactions. While these two reactions were for a long time thought to be mechanistically different, formation of carbenoid species by reduction of ketones with low-valent titanium complexes is not really surprising in view of the highly oxophilic and reducing character of titanium. Carbonyl olefinations by means of titanium carbe-... 

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... 

On the other hand, catalytic effects of this surface titanium suboxide are not so clear. There is evidence in the literature (2) and in this Symposium (1JD that geometric effects are dominant in Pt. But in the case of Ni, the bonding of CO and H2 is altered (6, 11). Sachtler (12) suggested that the oxophilic nature of reduced titanium may weaken the C-0 bond upon CO chemisorption near such sites. If this is true, other oxophilic ions should show similar characteristics. In this paper, we report Auger and EELS studies of Ni/MnOx model catalysts prepared by vacuum evaporation of Ni onto an oxidized manganese foil and subsequent reduction at 500 K. 

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