Titanium reductive coupling reactions

From cyclopentadienyl titanium complexes 49 and di-p-tolylcarbodiimide, a product 50, derived from a reductive coupling reaction, is obtained. ... 

Reductive coupling reactions, titanium complexes Carbon monoxide insertion, titanium complexes Acetylene insertion, titanium complexes Carbon dioxide insertion, titanium complexes Isocyanide insertion, titanium complexes Carbonylatlon, titanium alkyls... 

It is clear that McMurry reactions can be performed with low-valent titanium species in various oxidation states. Pinacol and alkene-forming reductive coupling reactions have been achieved with organometallic complexes of Ti(0), Ti(II), and Ti(III) (see Section 6.2.2.3). While no evidence was found for the presence of 11(0) on reduced Ti02 surfaces active in benzaldehyde coupling, X-ray photoelectron spectroscopy showed that the active site required for gas-solid reductive coupling is an ensemble of Ti cations in the +1, +2, and +3 oxidation states that collectively effect the four-electron reduction [258]. 

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). 

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

A modified procedure suitable for intramolecular reductive coupling is achieved using low-valence titanium prepared by reduction of titanium trichloride with a zinc-copper couple followed by the extremely slow addition of ketone to the refluxing reaction mixture (0.0003 mol over a 9-hour period by use of a motor-driven syringe pump) [S60. ... 

Asymmetric pinacol coupling reactions with a stoichiometric amount of titanium salts have been observed using a combination of TiGL and chiral amines such as A,A,A,A-tetramethyl-l,2-cyclohexanediamine and 2-methoxy-methylpyrrolidine. " " In these reports, the titanium salt is considered to be TiGl2- However, the titanium salt prepared by reduction of TiGU with McsSiSiMcs has proved to be TiGL. The enantioselectivities of the reactions using the chiral amines are modest (Table 2). 

Reductive coupling of dialdehydes may also be accomplished by use of samarium(II) iodide514. The reactions is stereoselective and has been used to prepare myo-inositol derivatives (equation 132)515. The equivalent reaction, using low-valent titanium species as catalysts, results in a mixture of products516. The production of cyclic /1-amino alcohols may be accomplished in good yields, and with a high degree of cis selectivity by the treatment of carbonyl hydrazones with samarium(II) iodide (equation 133)517. This reaction is effectively equivalent to an aza-Barbier reaction. 

Zizaene sesquiterpenes were the subject of considerable synthetic interest in the 1970s and several synthetic strategies to construct the tricyclo[6.2.1.0 ]undecane skeleton involved an intramolecular diazoalkane-carbonyl ring expansion, an intramolecular magnesium-ene reaction, and a titanium-promoted reductive coupling. - ... 

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