Pinacol coupling reactions mixed

Pinacol coupling reactions of aromatic aldehydes with commercially available TiCU /> a mixed solvent of THE and dichloromethane show high 47-selectivities." Also, high 47-selectivities are observed with TiCU-BuUTe in DME and TiCU-Bu"Li in Et20 at -50 °C. (see Table 1). 

Low-valent niobium and tantalum are also effective for homocoupling of aldehydes leading to 1,2-diols. " Not only commercially available NbClglDME) but also a combination of NbCls and zinc can be used for the pinacol coupling. Reactions in a mixed solvent of 1,4-dioxane and toluene (1 4) give better diastereoselectivity than those in DME or THE (Equation (52)). ... 

Photoinitiators are generally aryl alkyl ketones or diaryl ketones (Table 2.19). For aryl alkyl ketones two free radicals are produced by homolytic scission of a C-C bond (Eq. (2.96)). Diaryl ketones are usually mixed with a tertiary amine the mechanism of production of free radicals involves H abstraction from the tertiary amine by the excited state PI, via a charge-transfer stabilized exciplex (Eq. (2.97)). The a-amino alkyl radical formed is very reactive and is in fact the true initiator because the cetyl radical disappears rapidly through a coupling reaction (formation of pinacol). 

Iron-catalyzed Suzuki-Miyaura coupling reactions were also reported by Nakamura and colleagues (entry 27) [67]. Alkyl halides 1 and mixed pinacol aryl(butyl)borates, generated in situ from arylboronates and butyllithium, were used as the reagents and 10 mol% of the iron complexes 16a or 16b as the catalysts. The addition of 20 mol% of MgBr2 was essential for the success of the reaction. Products 3 were isolated in 65-99% yield. The methodology tolerates ester and nitrile functions. The reaction starts probably by initial boron-iron transmetalation to generate a diaryliron(II) complex. 

Reichwein, J. F., Pagenkopf, B. L. New Mixed Phosphonate Esters by Transesterification of Pinacol Phosphonates and Their Use in Aldehyde and Ketone Coupling Reactions with Nonstabilized Phosphonates. J. Org. Chem. 2003, 68,1459-1463. 

The CHDs formed by intramolecular addition often eliminate water to give another of,)6-unsaturated ketone. For mesityl oxide (4-methyl-3-pentene-2-one), 22b, the influence of the reaction conditions on the product distribution was examined in detail [96]. Mixtures of four dimeric species were observed, resulting from initial 4,4 -coupling (24b-d), or from mixed 2,4 -coupling, 24a. Pinacol formation (2,2 -coupling) was not observed [Eq. (6)]. 

The reductive coupling of a,p-unsaturated carbonyl compounds can lead to three types of dimerization products pinacols (1,2-coupling), 1,6-diones (1,4-coupling), and y-hydroxycarbonyl compounds (mixed coupling). These couplings are further complicated by the ability of the initially formed dimerization products to undergo further reactions and complex product mixtures can sometimes result. ... 

The photoreduction of benzophenone in methanol and ethanol gives benzopinacol and a cross-coupled product.Moreover, Weiner reported that photoreduction of benzophenone in 2-propanol yields a pinacol-type compound and the mixed pinacol (QH5)2C(OH)C(OH)(CH3)2-5 These reactions occur by the free ketyl radicals. 

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