Organometallic halides, reduction

Key Mechanism 10-1 Grignard Reactions 443 Summary Grignard Reactions 450 10-10 Side Reactions of Organometallic Reagents Reduction of Alkyl Halides 451... 

Side Reactions of Organometallic Reagents Reduction of Alkyl Halides... 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Formation of ketones. Ketones can be prepared by the carbonylation of halides and pseudo-halides in the presence of various organometallic compounds of Zn, B, Al, Sn, Si, and Hg, and other carbon nucleophiles, which attack acylpalladium intermediates (transmetallation and reductive elimination). 

Nitriles are similar in some respects to carboxylic acids and are prepared either by SN2 reaction of an alkyl halide with cyanide ion or by dehydration of an amide. Nitriles undergo nucleophilic addition to the polar C=N bond in the same way that carbonyl compounds do. The most important reactions of nitriles are their hydrolysis to carboxylic acids, reduction to primary amines, and reaction with organometallic reagents to yield ketones. 

The general mechanism of coupling reactions of aryl-alkenyl halides with organometallic reagents and nucleophiles is shown in Fig. 9.4. It contains (a) oxidative addition of aryl-alkenyl halides to zero-valent transition metal catalysts such as Pd(0), (b) transmetallation of organometallic reagents to transition metal complexes, and (c) reductive elimination of coupled product with the regeneration of the zero-valent transition metal catalyst. 

Stabilization of intermediates by strong adsorption will frequently be a necessary precondition for synthesis. Thus, in the case of the Kolbe reaction, further oxidation of the radicals is prevented the formation of metal-carbon bonds in the reduction of alkyl halides (Fleischmann et al., 1971a Galli and Olivani, 1970) or oxidation of Grignard reagents (Fleischmann et al., 1972c) is shown by the isolation of organometallic... 

Considerable progress has been made on C02 fixation in photochemical reduction. The use of Re complexes as photosensitizers gave the best results the reduction product was CO or HCOOH. The catalysts developed in this field are applicable to both the electrochemical and photoelectrochemical reduction of C02. Basic concepts developed in the gas phase reduction of C02 with H2 can also be used. Furthermore, electrochemical carboxyla-tion of organic molecules such as olefins, aromatic hydrocarbons, and alkyl halides in the presence of C02 is also an attractive research subject. Photoinduced and thermal insertion of C02 using organometallic complexes has also been extensively examined in recent years. 

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