2- -dehalogenation

Hydrolytic dehalogenation catalyzed by dehalogenases (or halido-hydrolases ) proceeds by formal nucleophihc substitution of the halogen atom with a hydroxyl ion [1810, 1811]. Neither cofactors nor metal ions are required for the enzymatic activity. Depending on the enzyme source, the reaction may either proceed with retention or inversion of configuration. It is this stereospecificity which makes 

Two drawbacks of a-haloacid dehalogenation - i.e., the hydrolytic instability of a-bromoacids in aqueous solvent systems and the limited substrate tolerance of a-haloacid dehalogenases — can be overcome by using anhydrous organic solvents [1828]. Thus, long-chain a-haloacids (which were not accepted as substrates in water) were successfully transformed with good specificity in toluene, acetone or even in dimethyl sulfoxide. 

A breakthrough was achieved by cloning and overexpression of halohydrin dehalogenases from Agrobacterium radiobacter, which allowed the preparative-scale application of these enzymes under well-defined ccmditions [1843]. 

Subsequent studies revealed that the natural nucleophile halide (Q, Br, I) could be replaced by nonnatural analogs, such as azide [1847], nitrite [1848], cyanide [1849], 

The /M ) )-nitrite (or formate) esters of v/c-diols obtained via enzymatic ring-opening of epoxides in presence of nitrite (or formate) are unstable and undergo spontaneous (nonenzymatic) hydrolysis to furnish the corresponding diols. This protocol offers a useful complement to the asymmetric hydrolysis of epoxides. Depending on the type of substrate and the enzymes used, enantio-complementary epoxide hydrolysis can be achieved [1851]. 

Mechanism of the radical chain dehalogenation of alkyl halides by tributyltin hydride. The various termination steps are not shown. 

CyP450 and other reduced Fe-porphyrins are reported to mediate reductive dehalogenation ofhaloalkanes and -alkenes [5], and of the aliphatic portion of DDT [6]. Products similar to anaerobic bacteria or CyP450 treatments were obtained by depositing stable ordered film of myoglobin (Mb) and a surfactant on electrodes  

RUO4 was shown to oxidize PCBs in water [21]. Water-soluble Ru complexes, such as [Ru(H20)2(dmso)4], were effective catalysts for the KHSO5 oxidation of a number of polychlorobenzenes and polychlorophenols, mainly converted into HCl and CO2 [22]. Replacement of the dmso- solvated ruthenium by RuPcS resulted 

WicKRAMANAYAKE, R. E. Hinchee (Eds.), Remediation of Chlorinated and Recalcitrant Compounds, Battelle Press, Columbus, USA 1998. 

Janssen, B. Witholt, in Metal Ions in Biolt cal Systems (Eds. H. Sigei, A. Sigel), Marcel Dekker, New York 1992, Vol. 28, p. 158 R. B. Winter, H. Zimmeemann, ibid., 

Allpress, A. Maule, Apjd. Environ. Microbiol. 1989, 55,1258. 

A novel entry to enantiomerically pure alcohols from racemic compounds is the use of dehalogenases. For example, the L-2-halo acid dehalogenase Pseudomonas putida was used for the synthesis of D-3-chlorolactic add from racemic 2,3-dichloropropionic acid (Fig. 10.37(a)).The enzyme catalyzes hydrolytic release of halogen from 2-halocarboxylic acids and produces 2-hydroxy acids with inversion of configuration. L-2-Halo acid dehalogenase acts on the L-isomer of 2-halo adds and produces n-2-hydroxy acid in excellent ee. 

In the reaction using halohydrin dehalogenase from Agrobacterium radiobacter (Fig. 10.37(c)), the R-enantiomer was converted to the corresponding epoxide, which was further converted to (S)-diol (ee 91%) by epoxide hydrolase from the same organism to prevent attack of chloride at the /3-position. 8 The unreacted (S)-dichloropropanol was obtained in enantiomerically pure form (ee 99%). 

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Homologues in principle can also be prepared from RCH(Br)-C(8r)=CH2 but for the synthesis of these starting compounds three steps have to be carried out. The dehalogenation procedure has also been used in the preparation of di- and tri-fluoroal1enes. ... 

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

Formate is an excellent hydride source for the hydrogenolysis of aryl halides[682]. Ammonium or triethylammonium formate[683] and sodium formate are mostly used[684,685]. Dechlorination of the chloroarene 806 is carried out with ammonium formate using Pd charcoal as a catalyst[686]. By the treatment of 2,4,6-trichloroamline with formate, the chlorine atom at the /iiara-position is preferentially removed[687]. The dehalogenation of 2,4-diha-loestrogene is achieved with formic acid, KI, and ascorbic acid[688]. 

The reaction of MeO /MeOH with 2-Cl-5(4)-X-thiazoles (122) follows a second-order kinetic law, first order with respect to each reactant (Scheme 62) (297, 301). A remark can be made about the reactivity of the dichloro derivatives it has been pointed out that for reactions with sodium methoxide, the sequence 5>2>4 was observed for monochlorothiazole compounds (302), For 2.5-dichlorothiazole, on the contrary, the experimental data show that the 2-methoxy dehalogenation is always favored. This fact has been related to the different activation due to a substituent effect, less important from position 2 to 5 than from... 

The replacement of 2-amino group by a hydrogen can be achieved by diazotization, followed by reduction with hypophosphorous acid (1-8, 13). Another method starting from 2-aminothiazole is to prepare the 2-halo-thiazole by the Sandmeyer reaction (prepared also from the 2-hydroxy-thiazole), which is then dehalogenated chemically or catalytically (1, 9, 10). 

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). 

Catalysts. In industrial practice the composition of catalysts are usuaUy very complex. Tellurium is used in catalysts as a promoter or stmctural component (84). The catalysts are used to promote such diverse reactions as oxidation, ammoxidation, hydrogenation, dehydrogenation, halogenation, dehalogenation, and phenol condensation (85—87). Tellurium is added as a passivation promoter to nickel, iron, and vanadium catalysts. A cerium teUurium molybdate catalyst has successfliUy been used in a commercial operation for the ammoxidation of propylene to acrylonitrile (88). 

Nevertheless, an anaerobic system may be the method of choice under certain conditions (/) contamination with compounds that degrade only or better under anaerobic conditions, (2) low yield aquifers that make pump and treat methods or oxygen and nutrient distribution impractical, (J) mixed waste contamination where oxidizable compounds drive reductive dehalogenation of chlorinated compounds, or (4) deep aquifers that make oxygen and nutrient distribution mote difficult and cosdy. 

Although distibenes, the antimony analogues of azo compounds, have never been isolated as free, monomeric molecules (130), a tungsten complex, tritungsten pentadecacarbonyl[p.2-Tj -diphenyldistibene] [82579-41-7] C2yH2Q025Sb2W2, has been prepared by the reductive dehalogenation of phenyldichlorostibine (131) ... 

More recently, it has been prepared by the dehalogenation of / fZ-butyldichlorostibine [67877-43-4] with magnesium (133). The corresponding... 

Semicommercial production of 3,3/4,4 -biphenyltetracarboxyhc dianhydride [2420-87-3] in the United States has been announced by Occidental Chemical Corp. (74). This polyimide resin intermediate is prepared by dehalogenative dimerization of 4-chlorophthalate salts (75) or by oxidative coupling of phthalate esters (76). 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

Nitropyridazines are reduced catalytically either over platinum, Raney nickel or palladium-charcoal catalyst. When an N-oxide function is present, palladium-charcoal in neutral solution is used in order to obtain the corresponding amino N-oxide. On the other hand, when hydrogenation is carried out in aqueous or alcoholic hydrochloric acid and palladium-charcoal or Raney nickel are used for the reduction of the nitro group, deoxygenation of the N- oxide takes place simultaneously. Halonitropyridazines and their N- oxides are reduced, dehalogenated and deoxygenated to aminopyridazines or to aminopyridazine N- oxides under analogous conditions. 

Hydroxyphthalazin-l(2//)-one is obtained in a smooth reaction between phthalic anhydride and hydrazine hydrate and this is again the starting compound for many 1-substituted and/or 1,4-disubstituted phthalazines. The transformations of 1,4-dichloro-phthalazine, which is prepared in the usual manner, follow a similar pattern as shown for pyridazines in Scheme 110. On the other hand, phthalonitrile is the preferential starting compound for amino- and hydrazino-phthalazines. The most satisfactory synthesis of phthalazine is the reaction between a,a,a, a -tetrachloro-o-xylene and hydrazine sulfate in sulfuric acid (67FRP1438827), alt iough catalytic dehalogenation of 1-chloro- or 1,4-dichloro-phthalazine or oxidation of 1-hydrazinophthalazine also provides the parent compound in moderate yield. 

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