Dehalogenation. See

Tris(phenylthio)antimony, 337 Dechlorination (see Reduction reactions) Deconjugation (see Isomerization) Defluorination (see Reduction reactions) Dehalogenation (see Elimination reactions, Reduction reactions) Dehydration (see also Elimination reactions)... 

The main hydrotreating reaction is that of desulfurization, but many others take place simultaneously to various degrees, such as denitrogenation, various hydrogenations and hydrocracking (in some cases deoxidation and dehalogenation) (see Table 1). 

Apparatus 500-ml flask (see Fig. 1) for the dehalogenation of the trichlorovinyl-amine (note 1). 

Preparation. Thermal elimination of HCl from l-chloro-l,l-difluoroethane (HCFC-142b) [75-68-3] is the principal industrial route to VDF covered by numerous patents (8—19). Dehydrohalogenation of l-bromo-l,l-difluoroethane (20), or 1,1,1-trifluoroethane (HFC-143a) (21—25), or dehalogenation of l,2-dichloro-l,l-difluoroethane (26—28) are investigated alternative routes (see Fluorine compounds, organic-fluorinated aliphatic compounds). 

The dehalogenation of the a-haloalkyl radical is a fast step which can take place by several possible routes . Dibromides are reduced much faster than dichlorides and rra j-l,2-dibromocylohexane is reduced 100 times faster than the m-isomer. This accords with neighbouring group assistance which bromine seems particularly capable of offering (see subsection 6.4.10). 

An important synthetic application of this reaction is in dehalogenation of dichloro- and dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce deuterium at a specific site. The mechanism of the reaction involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

Dialkyldichlorostannanes undergo reduction to dialkylstannanes (100), as shown in reaction 26. Product 100 is a good dehalogenation reagent for more complex stannyl chlorides, such as compound 32 (see Table 5) yielding compound 19 (see Table 2)134. 

Another investigation180 181 can be taken as a lesson about the borderline in elucidating structures by comparison of experimental and calculated IR spectra. In the hope that dibromoformoxime may give carboxime (isofulminic acid) (135) (the only missing member in the series 132-135) upon irradiation in a similar manner to the dehalogenation of diiodomethane (see Section m.B.l), we carried out such an experiment. At first glance it looked as if the anticipated reaction had occurred.180 It needed a lot of effort to show that in reality the photochemistry of matrix-isolated dihaloformoximes is very complex.181... 

The product-forming steps of dehalogenations by free radical pathways were discussed earlier (see Section 18.3.1.1). In non-radical mechanisms, the dehalo-genated products (RH) will be formed mostly by reductive elimination [193, 194] however, concerted processes lead directly from RX to RH (see Sections 18.3.1.2 and 18.3.1.3). 

In a similar type of reaction, polymer-supported hydridoiron tetracarbonyl anion reacts with simple non-benzylic aliphatic bromides and iodides to produce aldehydes (Table 8.15), presumably through the intermediate formation of RCOFeH(CO)3, which undergoes reductive extrusion of the aldehydes [3], In contrast, benzylic halides and a-halocarbonyl compounds are reductively dehalogenated by the HFe(CO)4 anion (see Chapter 11). 

The reductive dehalogenation of haloalkanes has also been achieved in high yield using polymer supported hydridoiron tetracarbonyl anion (Table 11.15). In reactions where the structure of the alkyl group is such that anionic cleavage is not favoured, carbonylation of the intermediate alkyl(hydrido)iron complex produces an aldehyde (see Chapter 8) [3]. 

The relevance of this mechanism to mammalian enzymes is an important question, but we are not aware of any detailed study that affords a definitive answer. Proof that reactions of hydrolytic dehalogenation ofhaloalkyl groups occur in animals is presented in the next subsection, but much remains to be discovered regarding the enzymes involved or the reaction mechanisms. Furthermore, nonenzymatic reactions remain a distinct possibility when the C-atom bearing the halogen is sufficiently electrophilic, as seen, e.g., with (2-chloroethyl)amino derivatives (see Sect. 11.4.2). 

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