Arenes mechanisms

Scheme 6.1 Arene-mechanism of electrophilic aromatic substitution.

Steps (a) and (b) of Scheme 6 constitute the Arens mechanism. Therefore, equation (244) was simply a case of attack on Cl in which the ion-molecule (PhC=C CISC,-H,-p) was the key intermediate. Attack of acetylide on the sulphur of the sulphenyl chloride leads to the product given in equation (244). Support for this step is the well-known reaction of sulphenyl chloride with carbanions to yield sulphides . The disulphides which sometimes turn up in the haloalkyne-thiolale processes (see Table 6) are easily explained by the sulphenyl halide reacting with the thiolates. 

Neither the Viehe nor the Arens routes to the ethynyl ether are plausible for this system. The l-phenyl-l-methoxy-2-haloalkenes of equation (259), for example, may be recovered intact when treated in MeOH with 4m NaOMe at 155 °C . Though these conditions are presumably suitable for the generation of or C vinyl anions, no onium process (equation 253) seems to have occurred. Further, it seems improbable that phenylacetylide could be a precursor of the ethynylether (Arens mechanism), since this ion abstracts protons from protic solvents (/r lO M s at 25 °C in water) and halogen from hypohalite (OX") [k(Cl) = 2-3 x IQ- m" s" at 25 °C in water] . Thus, the possibility that there is an Arens ion-molecule intermediate, which can survive long enough in methanol to rearrange and form the alkynyl ether... 

Consider the Viehe mechanism, i.e. steps (c), (f), (i) and (j) of equation (264) the problem with this route is that it does not reach product 54. This is because triethyl-amine is a stronger base (nucleophile) than 55 and hence the direction shown in equation (264), namely 54 -> 55, is preferred rather than 55 -> 54. Attack on bromine (Arens mechanism) is not shown in the above scheme because no acetylene was produced. If the ion-pair [HC=C BrNEt ] had formed, it would almost certainly have yielded acetylene in the presence of the proton source, HC=CBr. 

DCHA is normally obtained in low yields as a coproduct of aniline hydrogenation. The proposed mechanism of secondary amine formation in either reductive amination of cyclohexanone or arene hydrogenation iHurninates specific steps (Fig. 1) on which catalyst, solvents, and additives moderating catalyst supports all have effects. 

Based on these results, it was concluded that the transfer of ions such as Ag, Hg, and Hg with polymeric calix[4]arene follows a different mechanism than that of calix[4]arene. 

Bis-arene iron dications [20] are easily accessible from arenes, A1C13 and FeCl3 (for C6Me6, FeCl2 must be used). It is advisable to use tris-sublimated A1C13 to avoid problems of isomerization [23], With toluene, this isomerization due to the re/ro-Friedel-Crafts mechanism [24] is too extensive to give any clean complex. 

However, the hydride reduction of FeCp(arene)+ salts [124, 125] gives [FeCp(r 5-cyclohexadienyl)] complexes [125, 126] (via an ET mechanism [127] for the directing effect of substituents see Refs. [126, 128-130]. The electrochemical reduction of the carboxylic substituents at an Hg cathode in water leads to the primary alcohol [131-133] Eq. (39) ... 

Notice what reagent we use to pull off the proton. We use H2O, rather than using a hydroxide ion (HO ). To understand why, remember that we are in acidic conditions there really aren t many hydroxide ions floating around. But there is plenty of water, and a mechanism must always be consistent with the conditions that are present. 

Considerable attention has been directed to the formation of nitroarenes that may be formed by several mechanisms (a) initial reaction with hydroxyl radicals followed by reactions with nitrate radicals or NO2 and (b) direct reaction with nitrate radicals. The first is important for arenes in the troposphere, whereas the second is a thermal reaction that occurs during combustion of arenes. The kinetics of formation of nitroarenes by gas-phase reaction with N2O5 has been examined for naphthalene (Pitts et al. 1985a) and methylnaphthalenes (Zielinska et al. 1989) biphenyl (Atkinson et al. 1987b,c) acephenanthrylene (Zielinska et al. 1988) and for adsorbed pyrene (Pitts et al. 1985b). Both... 

The oxidation by strains of Pseudomonas putida of the methyl group in arenes containing a hydroxyl group in the para position is, however, carried out by a different mechanism. The initial step is dehydrogenation to a quinone methide followed by hydration (hydroxylation) to the benzyl alcohol (Hopper 1976) (Figure 3.7). The reaction with 4-ethylphenol is partially stereospecific (Mclntire et al. 1984), and the enzymes that catalyze the first two steps are flavocytochromes (Mclntire et al. 1985). The role of formal hydroxylation in the degradation of azaarenes is discussed in the section on oxidoreductases (hydroxylases). 

Arene oxides can be intermediates in the bacterial transformation of aromatic compounds and initiate rearrangements (NIH shifts) (Dalton et al. 1981 Cerniglia et al. 1984 Adriaens 1994). The formation of arene oxides may plausibly provide one mechanism for the formation of nitro-substituted products during degradation of aromatic compounds when nitrate is present in the medium. This is discussed in Chapter 2. 

A rearrangement (NIH shift) occurred during the transformation of 2-chlorobiphenyl to 2-hydroxy-3-chlorobiphenyl by a methanotroph, and is consistent with the formation of an intermediate arene oxide (Adriaens 1994). The occurrence of such intermediates also offers plausible mechanisms for the formation of nitro-containing metabolites that have been observed in the degradation of 4-chlo-robiphenyl in the presence of nitrate (Sylvestre et al. 1982). 

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