Magnesium reductive dimerization

Benzotelluropyrylium salts react with a variety of nucleophiles via distinct mechanistic pathways. 2-Benzotelluropyrylium salts 30 react with common Grignard reagents to afford products of reductive dimerization 31 (Equation 6) <1998J(P1)2123>. Under similar conditions, the reactions of the 2-benzotelluropyrylium salts with benzyl magnesium bromide take a completely different course leading to addition of the benzyl group to the... 

The hydrogen overvoltage of cadmium is rather large and cadmium may be used, plated on steel, as an alternative to lead. Cadmium has been used as cathode material in the hydrodimerization of acrylonitrile (Chapter 31). Magnesium may be used in reductive dimerization of aliphatic esters [459]. 

When using sacrificial magnesium anodes in DMF, an overconsumption of metal is observed it has been shown that electrochemically scouring of magnesium induces a chemical reduction of DMF, possibly primarily involving a reductive dimerization [176]. 

Electrochemical processes are often touted as being green chemistry because electricity is considered inexpensive, and toxic metal reagents are usually avoided. Electrochemical processes have produced tons of bulk chemicals [37], the best-known of which may be adiponitrile from reductive dimerization of acrylonitrile (Figure 13.17) [38]. An electrochemical synthesis to manufacture fenoprofen is shown in Figure 13.18, with the magnesium provided as a sacrificial electrode [39], Flow cell technology has been used for these operations on a commercial basis. 

Reductions. Sm(II) has been developed as a versatile one-electron reductant of broad utility in organic synthesis. Sm(II) can be prepared and regenerated in situ by reduction of Sm(III) in DMF with a consumable magnesium anode (52-54), Under these conditions aromatic esters are reductively dimerized to 1,2-diJketones with only 10% Sm(III) (52). Similarly, allylic chlorides can be added to ketones to give homoallylic alcohols (55). SmCl3-catalyzed electrosynthesis of y-butyrolactones from 3-chloro esters and ketones or aldehydes proceeds in 25-76% yield (54). 

The only other ambient stable low oxidation state magnesium compound to be reported outside our group is the dianionic magnesium(I) dimer, 14, which was prepared in one pot by the potassium reduction of the a-diimine,... 

Scheme 3 Two-electron reduction of organic substrates by magnesium(I) dimers...

Scheme 7 Further reductions of group 14 element halide complexes with a magnesium(I) dimer...

Diols (pinacols) can be synthesized by reduction of aldehydes and ketones with active metals such as sodium, magnesium, or aluminum. Aromatic ketones give better yields than aliphatic ones. The use of a Mg—Mgl2 mixture has been called the Gomberg-Bachmann pinacol synthesis. As with a number of other reactions involving sodium, there is a direct electron transfer here, converting the ketone or aldehyde to a ketyl, which dimerizes. 

The electrochemical reduction of azulene with carbon, platinum, lead or zinc cathode does not give any product, whereas that with magnesium electrode yields a dimeric compound as the only reduction product, though the dimeric compound is easily transformed to the corresponding monomeric compound by a mild oxidation as shown in equation 2825. 

The trimethylsilyl radical produced either rapidly dimerizes or reacts with solvent so that very clean ESR spectra of the radical anion, with minimum interactions with the counterion, can be obtained (116). Further reduction to dianions is very slow, and exhaustive reduction to anion radicals minimizes problems associated with exchange between anion radicals and unreduced substrate (115). It now appears that the solvent HMPA greatly facilitates the one-electron reduction, not only for trimethylsilylsodium, but also for organolithium and magnesium reagents (110). It was found that 0.1F solutions of methyl-, n-butyl-, or f-butyllithium or benzylmagnesium chloride will quantitatively reduce biphenyl to its radical anion in less than 10 minutes (110). 

Butadiene d4 complexes were obtained (i) from [NbCU(dmpe)2] and magnesium butadiene (equation 87) j706 (ii) by dimerization of ethylene using alkylidenes (equation 88) 707 or (iii) by metal vapor techniques (equation 89), which yielded sublimable methylallyl derivatives.70 Compound (62) could not be prepared by Na/Hg reduction of (22) in the presence of butadiene. Compound (63) is also accessible from [TaH2ClL4] (Scheme 9). 

These dimerizations are analogous to those of the radical anions R2C - 0 which are intermediates in the reduction of ketones to pinacols. Indeed, in the presence of magnesium amalgam, pyridine... 

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