Oxidation-reduction reactions mechanistic principles

The plethora of entries in Tables 17.3 and 17.4 emphasizes that organic chemical redox reactions are not limited to one mechanism and are not even based on a small number of mechanistic principles. Hence, one should not expect any mechanistic homogeneity among the reactions to be discussed in Chapter 17. Sections 17.3 (oxidations) and 17.4 (reductions) are thus not organized on the basis of mechanistic considerations. Instead, the ordering principle reflects preparative aspects Which classes of compounds can be oxidized or reduced into which other classes of compounds, and how can these transformations be accomplished ... 

As already mentioned, the reverse reactions of Fig. 2.6 are reductive elimination reactions. By the principle of microscopic reversibility, the existence of an oxidative addition reaction means that reductive elimination, if it were to take place, would follow the reverse pathway. The reductive elimination of an alkane from a metal-bonded alkyl and hydride ligand in most cases poses a mechanistic problem. This is because clean oxidative addition of an alkane onto a metal center to give a hydrido metal alkyl, such as a reaction like Reaction 2.5, is rare. 

As mentioned above, treatment of the aldol adducts 150 a/b with NMO produced the phenol 152. The interesting oxidation properties of NMO had previously been investigated by Sulikowski et al. on the model compound 157 [85] (Scheme 40). They observed the formation of the hemiacetal 159 in 60% yield and assumed attack of the nucleophilic N-oxide on the quinonemethide tautomer 158 (or on the anion of 158). A related reaction was observed in our group in which the diol 94 was methoxylated at C-6 to 95 by treatment with methoxide ions [82] (Scheme 27). An internal redox step is postulated to account for the reductive 0-N-bond cleavage with concomitant oxidation of the hydroquinone back to the quinone. Without the presence of perruthenate, aromatization with formation of a C-5 phenolic hydroxy group was observed, a reaction later exploited in the synthesis of angucycline 104-2 [87] (see Scheme 49). Thus, based on similar mechanistic principles, the chemical results of the NMO oxidations were quite different compound 147 gave the C-6 phenol 152 [86] whereas 157/158 were converted to the C-5 phenol 160 [85]. 

Reduction of carbonyl to hydroxyl is part of the equilibrium process described in the section on oxidation of alcohols to ketones. The same mechanistic principles apply. Evidence for the correctness of the stoichiometry of the illustrated representation of this reaction has been provided, using crystalline enzyme, by Hubener, Schmidt-Thome, and co-workers (H-410, N-657, S-803). 

The generally accepted mechanistic alternatives (other than direct reductive elimination) are illustrated in Figure 4.12. In principle, the dimer could lose the acetate group and the corresponding cationic Pd(III) dimer would then lead to ArCl via reductive elimination. Alternatively, the dimer could disproportionate to Pd(IV) and Pd(II). Canty, Ritter, Sanford et al. demonstrated previously for oxidative trifluoromethylation reactions that dinuclear Pd and mononuclear Pd pathways are connected [48]. The Pd(IV) complex could then either directly eliminate ArCl or dissociate the acetate ion and reductively eliminate ArCl from the cation (see Figure 4.12). Using computational means, it is not trivial to answer whether the pathways via the cationic Pd complexes (Pd(III) dimer or Pd(IV) monomer) are favored over alternative pathways. It is possible to reproduce the... 

The chemistry of electrochemical reaction mechanisms is the most hampered and therefore most in need of catalytic acceleration. Therefore, we understand that electrochemical catalysis does not, in principle, differ much fundamentally and mechanistically from chemical catalysis. In addition, apart from the fact that charge-transfer rates and electrosorption equilibria do depend exponentially on electrode potential—a fact that has no comparable counterpart in chemical heterogeneous catalysis—in many cases electrocatalysis and catalysis of electrochemical and chemical oxidation or reduction processes follow very similar if not the same pathways. For instance as electrochemical hydrogen oxidation and generation is coupled to the chemical splitting of the H2 molecule or its formation from adsorbed hydrogen atoms, respectively, electrocatalysts for cathodic hydrogen evolution—... 

The book comprises 8 chapters. The first provides background, introduces the topic of asymmetric synthesis, outlines principles of transition state theory as applied to stereoselective reactions, and includes the glossary. The second chapter details methods for analysis of mixtures of stereoisomers, including an important section on sample preparation. Then follow four chapters on carbon-carbon bond forming reactions, organized by reaction type and presented in order of increasing mechanistic complexity Chapter 3 is about enolate alkylations. Chapter 4 nucleophilic additions to carbonyls. Chapter 5 is on aldol and Michael additions (2 new stereocenters), while Chapter 6 covers rearrangements and cycloadditions. The last two chapters cover reductions and oxidations. 

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