Magnesium dimers synthesis

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. 

Telomerization of Isoprene.—Reviews have appeared on isoprene and chloro-prene, and on the complex reactions of isoprene to form terpenoids (in Japanese). Isoprene reacts with magnesium, especially in the presence of Lewis acids, and the resulting complex gives adducts with aldehydes. As usual in this type of reaction, a very complex mixture is obtained. The palladium-chloride-catalysed reaction of isoprene with acetic acid gives different products in different solvents. Monomers predominate in benzene [2-methylbut-2-enyl acetate (5) and 3-methylbut-2-enyl acetate (6)] while dimers [(7), (8), neryl (9), and geranyl (10) acetates] tend to be formed in tetrahydrofuran. Further details of the synthesis of Cio alcohols from isoprene and naphthyl-lithium are available, as well as of the in situ oxidation,but there is little of novelty (see Vol. 1, p. 17). 

The in situ synthesis of organoboranes via reaction of alkyl halides with magnesium in the presence of diborane can also be used to prepare coupled products (equations 20 and 21). Oxidation of the reaction mixture with alkaline silver nitrate leads to good yields of dimeric products. The reaction is successful for primary and secondary halides. A related reaction is the coupling of secondary alkyl halides in the presence of catalytic quantities of thallium salts. This procedure fails for primary alkyl halides and gives modest yields for secondary alkyl halides (equation 22). 

Reports that thermolysis of metal chalcogenolate complexes offers a low-temperature route to the synthesis of novel solid state materials (32) is also stimulating research in this area. As is the case with copper(I) thiolates, lithium thiolates are often aggregated species (Section III.C) and from the few magnesium thiolate structures known (Section III.D) one sees a preference for monomeric and dimeric formulations when bulky substituents are present. 

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. 

Synthesis of polyheteroarenes, heteroprotophanes, ketazines by Cu catalyzed dimerization of magnesium or Li derivatives. 

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). 

---