Reductive elimination of chlorine

Roberts AL, LA Totten, WA Arnold, DR Burris, TJ Campbell (1996) Reductive elimination of chlorinated ethylene by zero-valent metals. Environ Sci Technol 30 2654-2659. 

Several trends emerge in these data (1) The reductive elimination of bromine is 6-13kJmol more facile than reductive elimination of chlorine in similar structures, which is consistent with weaker chalcogen-bromine bonds relative to chalcogen-chlorine bonds.(2) The reductive elimination of chlorine is accelerated by the presence of a chloride counterion as opposed to a less nucleophilic counterion such as hexafluorophosphate. (3) The rate of reductive elimination is accelerated by the presence of a more polar solvent (acetonitrile) relative to tetrachloroethane, which is consistent with development of charge in the rate-determining step. These observations suggest mechanisms for oxidative... 

Roberts, A.L., Totten, L.A., Arnold, W.A., Burris, D.R., and Campbell, T.J., Reductive elimination of chlorinated ethylenes by zero-valent metals, Environ. Sci. Technol., 30, 2654-2659, 1996. 

Fig. 15.33. Prototypical in situ generation of dichloroketene through reductive -elimination of chlorine from trichloroacetyl chloride. Stereospecific [2+2]-cycloaddi-tions of this dichloroketene with the stereoisomeric 2-butenes.

Figure 12.32 shows the second commonly employed method for the generation of dichloroketene, which involves the reductive / -elimination of chlorine from trichloroacetyl chloride by zinc (cf. Sections 4.7.1 and 14.4.1 for mechanistic considerations). The addition of the dichloroketene to the trisubstituted alkene A (Figure 12.32) exhibits ori-... 

The a-chloro-)S-oxosulfenyl chloride (321) on treatment with dipivaloylmethane and sodium ethoxide affords the intermediate a-ketothione (322) by reductive elimination of chlorine. Dimerization to (323) is spontaneous (Scheme 48) <77ACS(B)890>. 

Treatment of allyl sulfide 111 with dichloroketene, generated in situ by reductive elimination of chlorine from trichloroacetyl chloride, results in an intramolecular ketene Claisen rearrangement giving 779 with high, 2-syn selectivity (94% de). Reductive dechlorination and subsequent lactonization affords chiral butyrolactone 780, with an optical purity that exceeds 95%. 

It is often desirable to perform some kind of chemical modification to polymers once they have formed. Acetylation of cellulose, for example, represents the chemical modification of a naturally occurring polymer (see chapter 3) to give the technologically useful cellulose diacetate and triacetate polymers. Poly(vinyl alcohol) (PVA) is a well-known example of a polymer that can only be formed by chemical modification since vinyl alcohol monomer does not exist (except as its keto form, acetaldehyde). Instead, PVA is formed by hydrolysis of poly(vinyl acetate) (PVAc). Partly hydrolysed PVAc is, of course, simply a copolymer of VA and VAc. As another example, ethylene/vinyl chloride copolymers can be prepared by reductive elimination of chlorine from poly (vinyl chloride) (PVC). The driving force here is that, although ethylene and vinyl chloride can be copolymerised directly, the normal routes to these polymers give insufficient control over composition and sequence distribution. 

Highly reactive metal complexes serve as electrocatalysts for a number of detoxification reactions. An example is the use of Co(II) N, N -bis(salicylidene)ethylene (CoSalen) to facilitate the electrochemical reductive elimination of chlorine from hexachlorobenzene (HCB) [15]. Even though more environmentally friendly conditions have yet to be developed to scale up this process, the CV voltammogram in Fig. 6 shows separate dehalogenation electrochemical steps from the sequence below. 

The water insoluble, highly chemically and thermally stable PCBs used as insulating fluids for transformers and capacitors, in paints, copy paper, etc., are extremely toxic, persistent in the environment and bioaccumulating. PCBs are currently destroyed by incineration of concentrates at high temperatures or chemically with sodium metals or organosodium. Both processes are costly. The cathodic reduction/elimination of the chlorine from polychlorinated biphenyl... 

The following two procedures are typical of the selective reductive elimination of only one chlorine atom from the oc,a-dichlorocyelobutanones. 

Figure 15.33 is based on the first and most common method for the preparation of dichloroketene, i.e., the reductive /3-elimination of chlorine from trichloroacetyl chloride with zinc (for the mechanism see Sections 4.7.1 and 17.4.1). Upon addition of the dichloroketene to the isomerically pure 2-butenes perfect stereoselectivity (and hence overall stereospecificity) occurs /ra .v-2-butene reacts to give frans-dichlorodimethylcyclobutanone and cis-2-butene to furnish its cw-isomer. 

Fig. 12.32. Orientation-selective and diastereoselective [2+2]-cycloaddition with in situ generated dichloroketene II the dichloroketene is generated by way of a reductive /3-elimination of chlorine from trichloroacetyl chloride.

The addition of hydrogen chloride to atropic acid, followed by the replacement of the chlorine atom (Na2C03) by a hydroxyl group (213), or the addition of hypochlorous acid to the unsaturated acid followed by the reductive elimination of the chlorine (Zn-Fe + NaOH) (82) completed the synthesis of tropic acid but gave no clear insight into the position of the hydroxyl in the tropic acid molecule (V or VI). Although both V and VI... 

Fig. 14 Reductive elimination of halo-imidazolium salt via reaction of an abnormal Pd" bis(NHC) with molecular chlorine. ...

On the basis of the multinuclei NMR study and the fact that the reactions of Pt(IV) and Pt(II) chlorides with (CH2=CHSiMe2)20 yield polysiloxanes, vinyl chloride, 1,3-butadiene and ethene, Lappert and coworkers have proposed a plausible mechanism illustrated in Scheme 4, which includes a rather unique vinyl-chlorine exchange (26 -> 27) and reductive elimination of vinyl chloride (28 29). The homolytic fission of Pt—CH=CH2 bond is also suggested. If a divinyl-Pt complex is formed by double vinyl-chlorine exchange, the observed formation of 1,3-butadiene can be explained as well. This study concludes that 16-electron species such as 24 and 25 are considered to be highly active catalytic species due to the availability of a vacant site for oxidative addition by a hydrosilane. ... 

Reduction of RuCls XH2O and elimination of chlorine ions... 

H2 reduction method. The elimination of chlorines in catalyst is an important role and step during the preparation of ruthenium catalyst. In order to eliminate the Cl in activated carbon supported ruthenium catalyst prepared using RuClg as the precursors, it is necessary that RuCla is reduced to elementary ruthenium and... 

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