Radical dechlorination

The gas phase isomerisation of substituted halobenzenes occurs readily on zeolites. Similarities appear with the same reaction catalyzed by AlClj in the homogeneous phase, in particular the applicability of the Hammett equation. However, several differences stand in the mechanism and the reaction scheme. Part of the transformation of halobenzenes occurs by means of a radical dechlorination/chlorination mechanism. For the same reasons the formal reaction follows a triangular scheme which differs from the consecutive scheme occuring in the liquid phase. Moreover, selectivity can be strongly affected by the restriction to diffusion in the porous volume of the solid. 

SO far, to contain the previously unknown 176-nor-cobamide norpseudovitamin Bi2 (17) (54). The cofactor role of the corrinoid and its presumed participation in the reduction suggest a radical dechlorination mechanism (see Fig. 11) (55). 

AT-Acyl Derivatives. — O-Protected 2-deoxy-2-trichloroacetamido-D-glucopyranosyl trichloroacetimidates, e.g. 54, have been used in the synthesis of -linked disaccharides with some success (see Chapter 3). The products could be converted into the corresponding 2-acetamido-derivatives by radical dechlorination (BuaSnH, AIBN). The 2-trichIoromethyl-oxazoline derivative 55, formed from 3,4,6-tri-0-acetyl-2-deoxy-2-trichloroacetamido-D-glucosyI acetate (on reaction... 

Atom Transfer Atom Transfer (AT) takes place typically in the case of d7 complexes, which abstract the halogen atom from RX. The radical formed combines then with a second metal [193, 194]. A classical example of this mechanism is the hydrodehalogenation with cyanocobaltates(II) (see Section 18.2.1) [8, 9], but an analogous pathway was suggested recently for the Co(II) corrin-catalyzed dechlorination of CC14 in the presence of S2 /cysteine as reductant (Eqs. (11)—(12))... 

The metabolism of carbon tetrachloride proceeds via cytochrome P-450-dependent dehalogenation (Sipes et al. 1977). The first step involves cleavage of one carbon-chlorine bond to yield Cl- and a trichloromethyl free radical, which is then oxidized to the unstable intermediate trichloromethanol, the precursor of phosgene. Hydrolytic dechlorination of phosgene yields C02 and HC1 (Shah et al. 1979). Although there are similarities in the metabolism of chloroform and carbon tetrachloride, metabolic activation of chloroform produces primarily phosgene, whereas the level of phosgene production from... 

Thereby, either a localized or delocalized radical anion is formed in the first step that undergoes cleavage to a radical and X . The radical is further reduced to an anion that is finally protonated. Tetrachloropicolinate is regioselectively dechlorinated at the 4- and 5-position, which is attributed to the controlled orientation of the substrate at the electrode surface owing to the dipole moment of the molecule (Eq. 3) [117]. 

Pentachloropyridine undergoes a regio-selective dechlorination to give 2,3,5,6-tetrachloropyridine [118]. The reduction of 1- and 2-phenoxynaphthalene at the cathode or by an alkali metal afforded phenol and naphthalene. The intermediacy of the naphthalene radical has been made probable by the use of an internal radical trap [119]. 5-Aryloxy-l-phenyltetrazoles cleave reductively predominantly at the tetrazolyl-O bond. The relative rates of... 

Many reactions demonstrate a high activity of the phenylthiolate ion in trapping aryl radicals and inability of the phenolate ion to do so. Thus, the phenylthiolate ion acting on 5-chloro-2//, 3//-benzo[b]thiophenedione-2,3 produces the substitution product (in anion-radical form), whereas the phenolate ion initiates only the reductive dechlorination (Ciminale et al. 1978 Scheme 4.2). 

An useful alternative to the already known retropinacol reactions is presented by Liu and co-workers [7], This works demonstrates that pinacols bearing (dimethylamino)phenyl substiments can be subjected to fast oxidative fragmentation via photoinduced electron transfer with chloroform as the electron acceptor in yields up to 80%. The extremely fast dechlorination of the chloroform radical anion inhibits back-electron transfer and thus leads to effective fragmentation of the pinacol radical cation (Scheme 8). 

Several other methods have been utilized to produce the aminoalcohol 81. A ring expansion of prolinol 84 (derived from pyroglutamic acid (83) in eight steps) gave the trisubstituted chloropipendine 85. Dechlorination of 85 was accomplished under reductive radical conditions to give the aminoester 86, which was reduced to the aminoalcohol 81, thus completing the formal synthesis of (-)-paroxetine (6) (Scheme 15)." ... 

POLVINYLIDENE FLUORIDE. This product is made by the free-radical chain polymerization of vinylidene fluoride (H2C=CF2). This odorless gas which has a boiling point of —82°C is produced by the thermal dehydrochlorination of 1,1,1-chlorodifluoroethane or by the dechlorination of 1,2-dichloro-l.l-difluoro-ethane. As shown by the following equations, 1,1,1-chlorodifluoroethane may be obtained by the bydroflnorination and... 

The major part of isomers (> 90%) are formed by an acid mechanism, according to the consecutive scheme odCB mdCB pdCB, as in homogeneous phase (ref.6). However, the small odCB pdCB direct interconversion, and the appearance of CB and TCBs, evidenced a dechlorination/chlorination process initiated on Fe + impurities by a radical mechanism (ref.6). 

It is important to note that both H202 and Fe2+ have to be overdosed to maintain a steady-state concentration of hydroxyl radical and to obtain a satisfactory approximation of the mathematical model with the experimental data. When H202 and Fe2+ concentrations are 5 x 10-3 M and 2 x 1th4 M, respectively, the relative rate constants of 2-chlorophenol (2-CP) and 2,4,6-TCP with respect to 2,4-DCP can be calculated. The oxidation and dechlorination constants of 2,4-DCP were found to be 0.995 1/min (fc4) and 0.092 1/min (k2), as reported in a previous study (Tang and Fluang, 1996). For comparison, Table 6.1 summarizes all the kinetic constants as determined in this study and in the related literature. 

To evaluate the effect of the number of chlorines on the degradation rate constants of different chlorophenols, Table 6.2 shows the rate constants of elementary, oxidation, and dechlorination for the ratios of k2 CP/k2/l DcP and 2,4,6-tcp/ 2,4-dcp The relative rate constants are plotted against the number of sites unoccupied by chlorine atoms on the chlorinated phenols in Figure 6.3.A linear correlation between the rate constants and the number of sites available is found with a standard deviation of 0.132. Clearly, the more chlorine atoms the aromatic rings contain, the fewer sites are available for hydroxyl radical attack however, the correlation should not be used for... 

Using the developed model, the k values for 2-CP, 3-CP, and 4-CP are 1.12 x 107, 1.004 x 109, and 1.005 x 108 (1/s), respectively therefore, the dechlorination constants for monochlorophenols follow a decreasing order 3-CP > 4-CP > 2-CP. Because chloride ion can be released only after the rupture of the aromatic ring, the faster the hydroxylation of the parent compounds, the faster the dechlorination process should be. Therefore, the above order can be understood in terms of the effect of the substituents on the reactivity of their parent compounds. It is known that both OH and Cl are ortho and para directors. Under the influence of these directors, the following preference of hydroxyl radical attack is expected ... 

As discussed earlier, the effects of the meta, para, and ortho positions of chlorine on the dechlorination kinetics of monochlorophenols, dichlorophenols, and trichlorophenols during Fenton oxidation were evaluated by comparing the rate constants of the kinetic model (Tang and Huang, 1995). This study proposed a pseudo first-order steady state with respect to organic concentration. The proposed reaction pathways considered that the hydroxyl radicals would attack unoccupied sites of the aromatic ring. 

Electrophilic addition is of particular interest for a mechanistic interpretation of the rapid dechlorination of chlorinated phenols and leads to generation of chloride ions. One possible pathway could be electrophilic addition of the radical to the n bond followed by a subsequent fragmentation of the intermediate chlorohydrol as shown in Equation (7.26) ... 

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