Oxidative Bond-cleavage Processes

Oxidative Bond-cleavage Processes Alkylaromatic radical cations... 

Although a previous chapter in this volume provides a broader perspective on the reactivity of radical cations, in this section we will examine intramolecular electron-transfer reactions coupled with or followed by cleavage of a bond in odd electron species such as radical cations, radical zwitterions and radical anions. In particular, this paragraph will be divided in oxidative and reductive bond-cleavage processes. Because this field is however too large to be covered extensively here, the discussion will be limited to selected examples—for oxidative cleavages, side-chain fragmentation reactions of alkylaromatic radical cations and decarboxylation reactions of radical zwitterions derived from benzoic and arylalkanoic acids, and for reductive... 

C-C bond cleavage process have been proposed (Figure 6.48) . The first involves a Baeyer-Villiger-like oxidation of the keto tautomer of the phenol and the second a rearrangement of the epoxide intermediate in aromatic oxidation. Delineation of the mechanism will require experimentation with purified enzyme, mutants, and substrate analogues. 

The complete bond cleavage process shown in Eq. (1.1), termed oxidative addition (OA), and the reverse process, reductive elimination (RE), are fundamen-... 

Other cleavages of the C-C bonds of substrates that lack a dative ligand have occurred after addition of the C-H bond. Crabtree and co-workers reported the oxidative addition of an unstrained C-C bond (Equation 6.64), most likely after a C-H bond cleavage process and just prior to establishment of the cyclopentadienyl-type ligand. The reaction of... 

The C—P bond cleavage processes responsible for the observed interchange has been described in the literature. Novak and coworkers reported on the reversible formation of tetraarylphosphonium ion, via a reductive elimination pathway, as a possible key intermediate in the exchange of aryl groups on the central palladium with aryl groups on the phosphine moiety (Scheme 4.4) [5]. A subsequent oxidative addition of a different C-P bond in the phosphonium ion to the intermediary palladium(O) complex would then create the interchanged arylpaUadi-um(II) complex also observed by Chernard and coworkers [6]. A similar C-P bond... 

A Cul-catalyzed synthesis of acridones via intramolecular cyclization including C(sp )-H bond activation and C(0)-CHj bond cleavage process using air as the oxidant was reported in 2013 (Scheme 8.100). Many substituents on the aromatic rings are tolerable in the reaction, and the acridin-9(10//)-ones could be obtained in moderate to excellent yields. C-labeling experiments show that only about 86% of carbon atom of carbonyl originates from the substrate. They proposed that a copper-catalyzed intramolecular Friedel-Crafts-type reaction pathway is disfavored [172]. Shortly after the previous report, Fu and coworkers developed a relevant aerobic synthesis of acridone derivatives from l-[2-(arylamino)aryl]ethanones under Cu(02CCF3)2/pyridine/02 catalytic system [173]. 

The initially proposed mechanism [14], and one that continues to be considered as the likely pathway for most variants, involves the oxidative cyclization of a Ni(0) complex of an aldehyde and alkyne to a metallacycle (Scheme 18). Metallacycle formation could proceed independently of the reducing agent via metallacycle 19, or alternatively, metallacycle 20a or 20b could be formed via promotion of the oxidative cyclization transformation by the reducing agent. Cleavage of the nickel-oxygen bond in a o-bond metathesis process generates an alkenyl nickel intermediate 21. In the variants involv-... 

The complete process for synthesizing such species using this approach would entail the acquisition of an appropriate natural silicate or the preparation of an appropriate synthetic silicate and then the conversion of this silicate into the alkyl silicate or organosiloxane by suitable substitution reactions. In terms of bond cleavage, this process could entail no destruction and reformation of framework silicon-oxygen bonds, and, in terms of oxidation number, it would entail no reduction and reoxidation of the silicon. 

This process contrasts with the elemental-silicon processes sometimes used for alkyl silicates (8) and the elemental-silicon processes generally used for oligomeric and polymeric organosi-loxanes ( ,7) Since the silicon in these processes is obtained from quartz, these processes entail, in terms of bond cleavage, the destruction of four silicon-oxygen bonds per silicon and the subsequent reformation of the required number of such bonds. In terms of oxidation number, they entail the reduction of the silicon from four to zero and then its reoxidation back to four, Figures 2 and 3. 

Redox mediators, such as flavins or quinones, are usually involved in the azo bond reduction. Therefore, the azo bond cleavage is a chemical, unspecific reaction that can occur inside or outside the cell, relying on the redox potential of the redox mediators and of the azo compounds. Also the reduction of the redox mediators can be both a chemical and an enzymatic process. As a consequence, it is an evidence that environmental conditions can affect the azo dyes degradation process extent both directly, depending on the reductive or oxidative status of the environment, and indirectly, influencing the microbial metabolism. 

A SET process has been postulated between Rh(III) oxidative adducts and an NAD(P)H model compound (cf. Section 18.2.4) [91]. Oxidative adducts formed by Sn2, SNAr, or inner-sphere SET pathways may produce radicals by homolytic M-C bond cleavage [130, 155, 176, 199]. 

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