Pinacol coupling metal reagents

Pinacol Coupling with Other Low-valent Early Transition Metals 11.02.4.3.1 Vanadium-based reagent systems... 

They are much less basic than RLi or Grignard reagents, and the reactions can be carried out on molecules that tend to undergo enolization, metal-halogen exchange, reduction, or pinacol coupling when RLi is used. 

Pinacol formation. The reductive coupling of aldehydes by the metal reagent in the presence of MejSiCl is syn-selective. Aliphatic aldehydes give 2,4,5-trisubstituted dioxolanes. ... 

As indicated in Section 3.2 Background , a diverse set of metals and metal complexes can effect the pinacol coupling of carbonyl groups. In the present section, newly discovered families of reagents for pinacol couplings will be described (modifications of existing families of reagents will not be discussed). 

Low-valent compounds of other early transition metals were shown to be elfective reagents for the pinacol coupling. Zirconium,... 

The two reactions discussed here are new to you—they are reductive processes where an alkali or alkaline earth metal supplies electrons, to convert carbonyl compounds into radical anions, which then couple together on the metal surface. The first is the pinacol coupling reaction of ketones, which, classically, uses magnesium metal as an electron source. The reaction mechanism is shown in Figure 20.8. In recent years, many other metals have been used similarly titanium and samarium reagents have been particularly popular, and some applications are shown in Figure 20.9. 

Another well-known transformation of carbonyl derivatives is their conversion to pinacols (1,2-diols) via an initial one-electron reduction with highly active metals (such as sodium, magnesium, aluminum, samarium iodide, cerium(III)/ I2, yttrium, low-valent titanium reagents (McMurry coupling), etc.), amines, and electron-rich olefins and aromatics as one-electron donors (D).43 Ketyl formation is rapidly followed by dimerization44 (equation 22). 

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