And pinacol coupling

The use of reducing metals nowadays is mainly restricted to acyloin and pinacol coupling reactions (see p. 53f.) and Birch reductions of arenes (A.A. Akhrcm, 1972 see p. 103f.) and activated C—C multiple bonds (see p. 103f.). 

Mn. Manganese is also effective for mediating aqueous carbonyl ally-lations and pinacol-coupling reactions. Manganese offers a higher reactivity and complete chemoselectivity toward allylation of aromatic aldehydes.178... 

Catalytic turn-over [59,60] in McMurry couplings [61], Nozaki-Hiyama reactions [62,63], and pinacol couplings [64,65] has been reported by Fiirst-ner and by Hirao by in situ silylation of titanium, chromium and vanadium oxo species with McaSiCl. In the epoxide-opening reactions, protonation can be employed for mediating catalytic turn-over instead of silylation because the intermediate radicals are stable toward protic conditions. The amount of Cp2TiCl needed for achieving isolated yields similar to the stoichiometric process can be reduced to 1-10 mol% by using 2,4,6-collidine hydrochloride or 2,6-lutidine hydrochloride as the acid and Zn or Mn dust as the reduc-tant (Scheme 9) [66,67]. 

Solutions to similar problems of achieving catalytic turnover [22] in McMurry couplings [23], Nozaki—Hiyama reactions [24], and pinacol couplings [25] have been reported by Fiirstner and by Hirao. The key step in these reactions is the in situ silylation of titanium and vanadium oxo species with Me3SiCl and reduction of the metal halides by suitable metal powders, e. g. zinc and manganese dust, as shown in Scheme 12.13. 

Dissolving metal reduction of camphor produces a mixture of bor-neol, isobomeol, and pinacol coupling products (Scheme 39). The ratios of the stereoisomers are affected profoundly by whether the starting ketone is enantiomerically pure or racemic, implying the chirality recognition at the stage of ketyl radical (65). 

One of the main issues with the use of magnesium and zinc as inexpensive co-reductants is their reactivity with alkyl halides. Also, additives such as TMSOTf are relatively expensive. In an attempt to address these limitations, Namy utilised mischmetal (La 33%, Ce 50%, Nd 12%, Pr 4%, Sm and other lanthanides 1%) as the stoichiometric reductant for the regeneration of the Sm(II).31,32 This reagent system provides an important alternative since it does not require the use of additives and mischmetal is relatively inexpensive. This system has been utilised successfully in Barbier and Reformatsky reactions, halide reductions and pinacol couplings (Scheme 7.7).31,32... 

The addition of Grignard reagents to C—ir-bonds is undoubtedly one of the most fimdamental and versatile reactions in synthetic organic chemistry. Nevertheless, it is also well recognized that these reactions are often accmnpanied by undesirable side reactions such as enoUzation, reduction, condensation, conjugate addition and pinacol coupling. In some cases, such abnormal reactions prevail over the normal addition reaction , resulting in poor yields of the desired products. 

Coupling reactions. In the presence of indium, Wurtz coupling of halides and pinacol coupling of aromatic aldehydes," the latter with assistance of ultrasound, have been reported. 

Pinacol formation from carbonyl compounds promoted by Smij continues to interest chemists. In the presence of methyl chloroformate the products are cyclic carbonates. In a cyclitol synthesis the diastereoselectivity for cyclization is found to be dependent on the a-substituents of the aldehyde groups. Lithium halides (chloride, bromide) have a profound effect on the reactivity of Sml2 and the pinacol coupling is favored. The reduction and pinacol coupling of ketones in THF is accelerated by MejSiCl (reaction time from hours reduced to a few minutes)." ... 

It was recommended that alkali-metal-NH3 reduction of ketones should be conducted in the presence of NH4+ this follows from the finding that reduction of [3,3-2H2]camphor and quenching in the absence of the ion gave epimeric alcohols with one or two atoms of tracer and complex products formed by disproportionation, abstraction of H from the medium, and pinacolic coupling. In the presence of NH4C1, exclusively [3,3-2H2]-alcohols were formed with Li, Na, or K. It was concluded that a mechanism proposed by House (1972) predominated when NH4+ was the proton source.120... 

Dehydrogenation and pinacol coupling. Allylamines give 1-aza-l,3-dienes, whereas aldehydes furnish a-hydroxycarbanion equivalents, which condense with other carbonyl compounds. ... 

The Stetter Reaction, Benzoin Condensation, and Pinacol Coupling... 

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