Hydrogenolysis, ionic

Aromatic aldehydes and aromatic ketones also can be reduced to hydrocarbons in a completely different manner, namely via the so-called ionic hydrogenation followed by an ionic hydrogenolysis. This kind of reduction is possible only if it can proceed via resonance-stabilized cationic intermediates. This resonance stabilization is readily achieved in a benzylic position, and it is therefore advantageous to employ aromatic carbonyl compounds in this kind of reduction. The carboxonium ion A, formed... 

Alkylation of (a-tetralone)chromiumtricarbonyl 37 with MeLi and subsequent ionic hydrogenolysis with excess EtsSiH and CF3CO2H gave (1-endo-methyltetralin)-chromiumtricarbonyl (39) via stereoselective exo-hydride displacement on 38. On the other hand, the endo-acstate complex 40 was converted into (1-exo-methyltetralin)-chromiumtricarbonyl (41) via exo-methyl attack to the carbocation by treatment with MejAl (Sch. 23) [47]. 

Cii. The carbonyl group of 3 is converted to an endo-methyl group by addition of CHjLi followed by ionic hydrogenolysis to provide the complex 4. Reaction of 4 with 2-lithio-l,3-dithiane results in substitution meta to the methyoxy group to provide 5 after demetalation. This product has the desired rra/is-arrangement of the groups at Ci and C4 as well as the precursor for the carboxyl group at for the complete synthesis of the diterpene 6. The conversion of 5 into 6 was completed in four conventional steps. 

Hydrogenolysis can be decreased, if necessary, by the addition of any of a variety of salts that increase the ionic strength of the medium U62). Iron salts have been used specifically for this purpose 102J73). Yields may depend markedly on the conditions of the reaction 92). 

Even if it is assumed that the reaction is ionic, Occam s Razor would lead to the conclusion that the system is too complex and that the effort to keep it ionic is too great. It is difficult to undersand why step 8c is slow and why a simple uncharged complex would not be equally reasonable. We prefer a mechanism in which the carbon monoxide molecule is adsorbed parallel to the surface and in which the oxygen orbitals as well as the carbon orbitals of C=0 bond electrons interact with the metal. It seems reasonable that hydrogenolysis occurs exclusively only because the oxygen is held in some way while the two bonds are broken and it finally desorbs as water. The most attractive picture would be (a) adsorption of CO and H2 with both atoms on the surface... 

In the absence of radical traps, the radical R is converted immediately into the carbanion R by an ECE or a DISP mechanism, according to the distance from the electrode where it has been formed. B is a strong base (or nucleophile) that will react with any acid (or electrophile) present. Scheme 2.21 illustrates the case where a proton donor, BH, is present. The overall reduction process then amounts to a hydrogenolysis reaction with concomitant formation of a base. This is a typical example of how singleelectron-transfer electrochemistry may trigger an ionic chemistry rather than a radical chemistry. This is not always the case, and the conditions that drive the reaction in one direction or the other will be the object of a summarizing discussion at the end of this chapter (Section 2.7). 

Fig. 17.72. Ionic hydrogena-tion/hydrogenolysis of an aromatic ketone (meto-nitroace-tophenone). CF3C00H causes a reversible protonation of the ketone to the ion A. The reducing agent triethylsilane then transfers a hydride ion onto A to form a benzylic alcohol. This alcohol presumably is silylated, protonated, and converted into the benzyl cation B. A second hydride transfer yields the final product.

Ionic hydrogenation is the noncatalytic addition of hydrogen to carbon-carbon or carbon-heteroatom double bonds, and the hydrogenolysis of carbon-oxygen or carbon-halogen single bonds. Thus, the conversion of aldehydes and ketones to the corresponding hydrocarbon is one of its many manifesta-... 

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