Ketone reduction, chemical reaction mechanism

The nature of the chemical reaction mechanisms can be conveniently illustrated by considering the ones advanced for the pH-independent carbon-halogen fission process and for the pH-dependenl ketone carbonyl reduction. These two processes are examples of two fundamentally different types of electron addition processes (cf. subsequent discussion of Energetic Reaction Mechanism). 

Electrocatalysis is manifested when it is found that the electrochemical rate constant, for an electrode process, standardized with respect to some reference potential (often the thermodynamic reversible potential for the same process) depends on the chemical nature of the electrode metal, the physical state of the electrode surface, the crystal orientation of single-crystal surfaces, or, for example, alloying effects. Also, the reaction mechanism and selectivity 4) may be found to be dependent on the above factors in special cases, for a given reactant, even the reaction pathway [4), for instance, in electrochemical reduction of ketones or alkyl halides, or electrochemical oxidation of aliphatic acids (the Kolbe and Hofer-Moest reactions), may depend on those factors. 

Benzene was introduced in Chapter 5 (Section 5.10). Chapter 21 will discuss many benzene derivatives, along with the chemical reactions that are characteristic of these compounds. In the context of dissolving metal reductions of aldehydes, ketones, and alkynes, however, one reaction of benzene must be introduced. When benzene (65) is treated with sodium metal in a mixture of liquid ammonia and ethanol, the product is 1,4-cyclohexadiene 66. Note that the nonconjugated diene is formed. The reaction follows a similar mechanism to that presented for alkynes. Initial electron transfer from sodium metal to benzene leads to radical anion 67. Resonance delocalization as shown shordd favor the resonance contributor 67B due to charge separation. 

The mechanism of the chemical reduction of enones with metal (Li, Na, etc.) in liquid ammonia can be described by the following equation in which the substrate 212 receives two electrons from the metal to give the dianion intermediate 213. This intermediate is then successively transformed into the enolate salt 214 and the ketone 2T5 with an appropriate proton donor source. It can readily be seen that the stereochemical outcome of this reaction depends on the stereochemistry of the protonation step 213 - 214. An excellent review on this topic has been recently written by Caine (60). This subject will be only briefly discussed here. 

The cyclic mechanism, in which the concertedness of the reaction is so clearly depicted, has found applications in several other instances in organomagnesium chemistry. This is particularly true for the reduction reaction with the aid of Grignard compounds. It was reported in 1950, in a paper on the asymmetric reduction of ketones with the aid of chiral Grignard reagents [58], that Whitmore had already presented the idea of a cyclic mechanism (Scheme 23) at an American Chemical Society meeting in 1943 [59] ... 

Chemical Quenching. Many photochemical reactions of organic compounds are known to occur via intermolecular interactions of the excited species with reactant molecules in solution. Common examples which may involve this mechanism include the photo-reduction of ketones by hydrogen donor solvents and many photo-dimerizations (227). The scarcity of quenching reactions of metal complexes that have been attributed to chemical processes has already been discussed by Balzani et al. (204). With the increasing interest in the mechanistic aspects of OTM complex photochemistry, however, many more examples should be discovered. 

Photoreduction of the herbicide paraquat dichloride in aqueous propan-2-ol is more efficient in the presence of a sensitizer such as benzophenone than on direct irradiation.84 Hyde and Ledwith84 propose that the paraquat cation radical is formed by electron transfer from ketyl radicals, in turn produced during the conventional photoreduction of the sensitizer ketone. The suggested mechanism is given in reactions (23)—(25), where PQ2+ is the paraquat dication. The reduction process therefore involves chemical sensitization, rather than electronic energy transfer. 

The combustion of kinetically stable fuel releases lots of energy by a very complex mechanism. To understand how energy is involved in the progress of a reaction we will need to take a much more simple and familiar mechanism. The reduction of a ketone to an alcohol with sodium borohydride will do. You met this reaction in Chapters 5 and 6 and it should by now be a familiar part of your chemical vocabulary. An example is shown in the margin in this particular case, the ketone is rather hindered by the adjacent ferf-butyl group, and the reaction must be heated to form the product. Evidently, then, there is an activation barrier that must be overcome. 

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