Titanium-based coupling

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 soluble titanium-based modified Ziegler-Natta catalyst [Ti(OR)4-Et3Al, R = n-Bu, isoPr] is employed in the reaction.42 Since similar catalysts may be used for the oligomerization and polymerization of ethylene, the nature and oxidation state of the metal and reaction conditions determine selectivity. Ti4+ was found to be responsible for high dimerization selectivity, whereas polymerization was shown to be catalyzed by Ti3+. According to a proposed mechanism,42,43 this catalyst effects the concerted coupling of two molecules of ethylene to form a metal-lacyclopentane intermediate that decomposes via an intramolecular p-hydrogen transfer ... 

Of more general application are the titanium-based reagents first introduced by Mukaiyama and by Trylik (equation 8). Mukaiyama demonstrated that TiCl4/Zn selectively coupled both aromatic or aliphatic aldehydes and ketones to either pinacols or alkenes by judicious choice of the appropriate reaction... 

The IFP-SABIC Alphabutol process utilizes a proprietary homogeneous titanium based catalyst which demonstrates high dimerization activity coupled with excellent selectivity to butene-1 at moderate pressures and temperatures. This performance is influenced by the catalyst composition and reaction parameters. 

Figure 2. Anionic synthesis of four-arm star poly(dimethylsiloxanes) via coupling to titanium based cores...

Galvanic corrosion currents and potentials of several titanium-based implant materials were studied by Arslan et al. [34]. The corrosion tests were performed on couples between titanium and three alloys a gold alloy, a Co-Cr alloy, and a Cr-Ni alloy. Each couple was tested in Ringer s solution (0.31 g sodium lactate, 0.02 g calcium chloride, 0.6 g sodium chloride, and 0.03 g potassium chloride in 100 mL of water) at 37 °C. The titanium-gold couple was found to be the best for use in dental environments. 

The direct reductive coupling of benzyl and allyl alcohols with titanium-based complexes offers a convenient method for the synthesis of bibenzyl and 1,5-dienes. For example, when a mixture of titanium tetrachloride and sodium benzoxide is treated with potassium metal at 100°-140 , bibenzyl is produced in 51 % yield. Allyl alcohol is converted to biallyl in 38% yield. Squalene... 

Two related procedures for reductive coupling of aromatic ketones with titanium-based reagents have been reported (Tyrlik and Wolochowicz, 1973 Mukaiyama et al., 1973). The deoxygenative dimerization of aromatic carbonyl compounds can be achieved with low-valent tungsten complexes formed from tungsten hexachloride and alkyllithium as well (Sharpless et al., 1972). 

No hydrogen uptake and embrittlement problems occur when titanium is coupled to fuUy passive materials in a given environment. These compatible materials may include other titanium alloys, passive stainless steels, copper alloys, and nickel-base alloys, depending on conditions. 

Metal-Based Coupling Agents Example of Zirconium and Titanium... 

Among the appHcations of lower valent titanium, the McMurry reaction, which involves the reductive coupling of carbonyl compounds to produce alkenes, is the most weU known. An excellent review of lower valent titanium reactions is available (195). Titanium(II)-based technology is less well known. A titanium(II)-based complex has been used to mediate a stetio- and regio-specific reduction of isolated conjugated triple bonds to the corresponding polyenes (196). 

In primer formulations for adhesive bonding of metals, the coupling agents that are most frequently used are those based on epoxy and amine functionalities. Aqueous solutions of aminosilanes have been successfully used for obtaining stable adhesive bonds between epoxy and steel [10] and epoxy and titanium [11,12], while epoxy functional silanes are preferable for applications involving aluminum substrates [13,14], A simple solution of % epoxy functional silane in water is currently used for field repairs of military aircraft [15] where phosphoric acid anodization would be extremely difficult to carry out, and performance is deemed quite acceptable. 

A new process developed by Institut Francais du Petrole produces butene-1 (1-butene) by dimerizing ethylene.A homogeneous catalyst system based on a titanium complex is used. The reaction is a concerted coupling of two molecules on a titanium atom, affording a titanium (IV) cyclic compound, which then decomposes to butene-1 by an intramolecular (3-hydrogen transfer reaction. ... 

The successful clinical use of titanium and cobalt-chromium alloy combinations has been reported Lucas etal. also investigated this combination using electrochemical studies based on mixed potential and protection potential theories. Verification of these studies was made by direct coupling experiments. The electrochemical studies predicted coupled corrosion potentials of -0.22 V and low coupled corrosion rates of 0.02 ft A/cm. Direct coupling experiments verified these results. The cobalt-titanium interfaces on the implants were macroscopically examined and no instances of extensive corrosion were found. Overall, the in-vitro corrosion studies and the examination of retrieved prostheses predicted no exaggerated in-vivo corrosion due to the coupling of these cobalt and titanium alloys. 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

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