Coupling carbonylative, reviews

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). 

Stoichiometric, nucleophilic addition of alkylcobalt compounds to carbonyl compounds were reviewed in 1996.374 This chapter focuses on recent progress in the reactions of cobalt enolates with carbonyls and cobalt-catalyzed coupling reactions. 

For a review encompassing the intermolecular nickel-catalyzed reductive coupling of dienes to carbonyl compounds, see Tamaru, Y. /. Organomet. Chem. 1999, 576, 215-231. 

For a review encompassing the transition metal-catalyzed reductive coupling of carbonyl compounds, see Avalos, M. Babiano, R. Gintas, P. ... 

The alcohol-unsaturate couplings developed in our laboratory provide products of carbonyl addition. In contrast, related hydrogen auto-transfer processes provide products of alcohol substitution via pathways involving oxidation-condensation-reduction and the use of preactivated nucleophiles. For recent reviews, see [22-25]. 

For selected reviews encompassing intra- and intermolecular direct reductive coupling of alkynes to carbonyl partners, see [151-158]... 

Allyl- and vinylsilane chemistry was one of the first areas of reagent synthesis impacted by CM methodology. Allylsilanes are commonly employed in nucleophilic additions to carbonyl compounds, epoxides, and Michael acceptors (the Sakurai reaction) vinylsilanes are useful reagents for palladium-coupling reactions. As the ubiquitous application of CM to this substrate class has recently been described in several excellent reviews, this topic will not be discussed in detail, with the exception of the use of silane moieties to direct CM stereoselectivity (previously discussed in Section 11.06.3.2). 

The reduction of carbonyl compounds to form pinacol dimers can be accomplished photochemically, electrochemically or with chemical reducing agents. When conducted under acidic conditions or in protic solvents, pinacols are likely produced by coupling of two neutral ketyl radicals (vs radical anions). The electrochemical reduction is especially complicated in terms of the role of the electrode surface, counterion and solvent, and an excellent review has appeared on the subject32. 

This review attempts to provide an overview of microwave-promoted metal-catalyzed transformations of aryl and vinyl halides (or pseudo halides), providing a personal selection of both pioneering and very recently published work. Covered areas include carbonylative transformations, Heck and Sono-gashira reactions, nucleophilic substitutions and cross-couplings. Because of the diversity of the microwave systems used, the reader should consult the original references for detailed descriptions of settings and instrumentation. 

Since the introduction of the titanocene chloride dimer 67a to radical chemistry, much attention has been paid to render these reactions catalytic. This field was reviewed especially thoroughly for epoxides as substrates [123, 124, 142-145] so only catalyzed reactions using non-epoxide precursors and a few very recent examples of titanium-catalyzed epoxide-based cyclization reactions, which illustrate the principle, will be discussed here. A very useful feature of these reactions is that their rate constants were determined very recently [146], The reductive catalytic radical generation using 67a is not limited to epoxides. Oxetanes can also act as suitable precursors as demonstrated by pinacol couplings and reductive dimerizations [147]. Moreover, 5 mol% of 67a can serve as a catalyst for the 1,4-reduction of a, p-un saturated carbonyl compounds to ketones using zinc in the presence of triethylamine hydrochloride to regenerate the catalyst [148]. 

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