Substitution oxidative, mechanisms

Aromatic ethers and furans undergo alkoxylation by addition upon electrolysis in an alcohol containing a suitable electrolyte.Other compounds such as aromatic hydrocarbons, alkenes, A -alkyl amides, and ethers lead to alkoxylated products by substitution. Two mechanisms for these electrochemical alkoxylations are currently discussed. The first one consists of direct oxidation of the substrate to give the radical cation which reacts with the alcohol, followed by reoxidation of the intermediate radical and either alcoholysis or elimination of a proton to the final product. In the second mechanism the primary step is the oxidation of the alcoholate to give an alkoxyl radical which then reacts with the substrate, the consequent steps then being the same as above. The formation of quinone acetals in particular seems to proceed via the second mechanism. ... 

Oxidation of higher fatty acids was first studied in 1904 by Knoop who fed animals with phenyl-substituted fatty acids and analyzed the products in the urine. He showed that the fatty acid oxidation results in the successive cleavage of two carbon moieties from the carboxyl end. Knoop coined the fatty acid oxidation mechanism as n-oxidation. As has been established by Kennedy and Lehninger in 1948-1949, oxidation of fatty acids occurs in the mitochondria only. Lynen and coworkers... 

FIGURE 4.80 Oxidative mechanisms for the formation of hydroquinone from / -substituted phenols. 

The R—0—B bonds are hydrolysed in the alkaline aqueous solution, generating the alcohol. The oxidation mechanism involves a series of B-to-0 migrations of the alkyl groups. The stereochemical outcome is replacement of the C—B bond by a C—O bond with retention of configuration. In combination with the stereospecific syn hydroboration, this allows the structure and stereochemistry of the alcohols to be predicted with confidence. The preference for hydroboration at the least substituted carbon of a double bond results in the alcohol being formed with regiochemistry which is complementary to that observed in the case of direct hydration or oxymercuration, that is, anti-Markownikoff. 138... 

DPT calculations indicated that the mechanism most Hkely involves three steps electrophilic substitution, oxidation and reductive ehmination. The inactivity of the iodine complexes prompted us to investigate the counterion dependence. For the methyl-substituted complexes (Scheme 23, R = CH3) we synthesized the acetate (X = OCOCH3) 22 and the chloride complex (X = Cl) 23. The catalytic conversions are within experimental error identical to the results of the bromide complex 20. This indicates that the dissociation of a counterion is a necessary condition for the activity of the complex [59]. 

Since then, a number of studies of model systems have confirmed that dialkenes, cyclic alkenes, and aromatics form substituted monocarboxylic acids, dicar-boxylic acids, and organic nitrates in the condensed phase (e.g., see O Brien et al., 1975a Grosjean and Friedlander, 1979 Dumdei and O Brien, 1984 Izumi and Fukuyama, 1990 and Forstner et al., 1997a, 1997b). For example, Table 9.21 shows the products identified in particles formed in the 1-octene- and 1-decene-NO,-ambient air systems. In both bases, only 40% of the total particle mass could be identified, and the yields shown in Table 9.21 are those relative to the total identified compounds. That is, the absolute product yields are about factor of 2.5 larger. As expected from the known oxidation mechanisms (see Chapter 6.E), heptanal and heptanoic acid are the major condensed-phase oxidation products of 1-octene and nonanal and nonanoic acid from 1-decene (see Problem 4). The mechanism of formation of the fura-nones, which are formed in relatively high yields, is not... 

The Hammett correlation for nitrobenzenes is almost identical to the correlation at pH 9 therefore, the degradation of substituted benzenes at pH 3 can also be described by the same hole oxidation mechanism. Figure 9.21 demonstrates the oxidation of nitrobenzene by positive hole. At pH 3, substituted benzenes are oxidized by the formation of a positive hole. An electron transfer from nitrobenzene to Ti02 creates this positive hole. 

Figure 44 Depth profiles using stable isotopic substitution of reactants to reveal oxidation mechanism of water reaction with uranium and its inhibition in the presence of oxygen. N2+ primary beam used. See text. (From Refs. 142 and 143.)...

Photochemical reactions of substituted phenylethylenes, frans-stilbene and 1,1-diphenylethylene, on silica gel, has been reported by Sigman et al. [40]. Irradiation of fraus-slilbcnc adsorbed on silica gel produced two dimers, along with ds-stilbene, phenanthrene, and a small amount of ben-zaldehyde which arose from a type II oxidation mechanism (Scheme 9). The formation of photodimers was claimed to be promoted by inhomogeneous surface loading and slow diffusion of fraus-slilbcnc on silica [40]. 

The E2-like process depicted for the general oxidation mechanism in Table 4.1 is supported by the observation that deuterium substitution of the a-H in isopropanol slows the rate of chromic acid oxidation by sevenfold. Deuterium replacement at the methyl positions does not diminish the oxidation rate. Since C-D bonds are broken more slowly than C-H bonds, these results suggest that the a-H is removed in a slow step. 

Almost all of the reactions of metals can be classified into just a few typical reactions, and the reactions that metals promote in organic chemistry are simple combinations of these typical reactions. If you learn these typical reactions, you will have no trouble drawing metal-mediated mechanisms. The typical reactions of metal complexes are ligand addition/ligand dissociation/ligand substitution, oxidative addition/reductive elimination, insertion/j8-elimination, a-insertion/ a-elimination, cr-bond metathesis (including transmetallations and abstraction reactions), [2 + 2] cycloaddition, and electron transfer. 

In this review, attention is focused primarily on the oxidation mechanisms under the given conditions, which is the essential topic of interest for organic chemists. Reaction pathways will be outlined if they seem to be well established. However, even small differences in medium properties used by different researchers can lead to serious variations as will be shown in some examples. Anodic oxidation of unsubstituted aniline is discussed in Section II and electrode reactions of /V-substifilled and C-substituted anilines in Sections III and IV, respectively. In the last case, the oxidation of reactants with monosubstituted ring is presented first (para-substituents separately from the effects of ortho- and mefa-substituents), and next the oxidation of di- and trisubstituted anilines. In each part the processes in dipolar aprotic solvents, in particular in acetonitrile (ACN) and /V. /V-dimethylformamide (DMF), are compared with those proceeding in aqueous solutions, chiefly in commonly used acidic media. 

Little is known about the mode of action of hydra-zinecarboxamide-derived fungicides. Since diazene formation is involved in the fungitoxic action of phenyl-thiosemicarbazide (2 2) and is implicated in a glutathione-oxidation mechanism to account for fungi-toxicity of similarly structured compounds (2 1 ), it is conceivable that diazenes described in this study may well play a critical role in the action of fluorine-substituted hydrazinecarboxamide fungicides and perhaps larvicides as well. 

With Ti-substituted Keggin polyoxometalates, for example, Na5 H PTiWn04o, the oxidation of cyclohexene with HP in an acetonitrile solvent yields traws-1,2-cydohexandiol as the main reaction product, via a heterolytic oxygen-transfer mechanism, when n> 2 in the compound. If the polyoxometalate contains only one proton, the main products are those of allylic oxidation, namely 2-cyclohexene-l-ol and 2-cydohexene-l-ol, produced via a homolytic oxidation mechanism [36tj. 

---