Chloride anion displacement

Figure 3.10. The cubic rock-salt, or sodium chloride, unit cell consists of two interlocking FCC sublattices, one of sodium cations and one of chloride anions, displaced by (1 /2 a) along (1 00).

Figure 1 illustrates the complexity of the Cr(III) ion in aqueous solutions. The relative strength of anion displacement of H2O for a select group of species follows the order perchlorate < nitrate < chloride < sulfate < formate < acetate < glycolate < tartrate < citrate < oxalate (12). It is also possible for any anion of this series to displace the anion before it, ie, citrate can displace a coordinated tartrate or sulfate anion. These displacement reactions are kineticaHy slow, however, and several intermediate and combination species are possible before equiUbrium is obtained. 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

A similar H2 activation mechanism was proposed for the [Pd(NN S)Cl] complexes (5 in Scheme 4.5) in the semi-hydrogenation of phenylacetylene [45] after formation of the hydride 14 (Scheme 4.9), coordination of the alkyne occurs by displacement of the chloride ligand from Pd (15). The observed chemos-electivity (up to 96% to styrene) was indeed ascribed to the chloride anion, which can be removed from the coordination sphere by phenylacetylene, but not by the poorer coordinating styrene. This was substantiated by the lower che-moselectivities observed in the presence of halogen scavengers, or in the hydrogenations catalyzed by acetate complexes of formula [Pd(NN S)(OAc)]. Here, the acetate anion can be easily removed by either phenylacetylene or styrene. 

The choice of the catalyst is an important factor in PTC. Very hydrophilic onium salts such as tetramethylammonium chloride are not particularly active phase transfer agents for nonpolar solvents, as they do not effectively partition themselves into the organic phase. Table 5.2 shows relative reaction rates for anion displacement reactions for a number of common phase transfer agents. From the table it is clear that the activities of phase transfer catalysts are reaction dependent. It is important to pick the best catalyst for the job in hand. The use of onium salts containing both long and very short alkyl chains, such as hexade-cyltrimethylammonium bromide, will promote stable emulsions in some reaction systems, and thus these are poor catalysts. 

The haloalkane dehalogenase DhlA mechanism takes place in two consecutive Sn2 steps. In the first, the carboxylate moiety of the aspartate Aspl24, acting as a nucleophile on the carbon atom of DCE, displaces chloride anion which leads to formation of the enzyme-substrate intermediate (Equation 11.86). That intermediate is hydrolyzed by water in the subsequent step. The experimentally determined chlorine kinetic isotope effect for 1-chlorobutane, the slow substrate, is k(35Cl)/k(37Cl) = 1.0066 0.0004 and should correspond to the intrinsic isotope effect for the dehalogenation step. While the reported experimental value for DCE hydrolysis is smaller, it becomes practically the same when corrected for the intramolecular chlorine kinetic isotope effect (a consequence of the two identical chlorine labels in DCE). 

Briefly, the Coordination Model attempts to account for the various species that form when solutes dissolve in various solvents. At low concentrations, iron(III) chloride, for example, forms [FeClaS] [FeCl2S4]+, [FeCU]- [FeS5Cl]2+ 2 Cl and [FeSe] + 3 Cl depending on the solvent employed. Basically, we wish to understand what solvent properties govern the extent of anion displacement. The overall process can be represented by a series of steps, each of which is exemplified by the general reaction ... 

The reaction proceeds to form the alkyl p-toluenesulfonate as expected, but the chloride anion formed in this step subsequently acts as a nucleophile and displaces p-toluenesulfonate from RCH2OTs. 

The nucleophilic chloride anion bonds to the carbon, displacing the pi electrons onto the oxygen. 

This is an example of the second step of an E2 (bimolecular elimination) reaction mechanism. Note the displacement of the chloride anion is the result of an anion present on an adjacent carbon atom. Arrow pushing is illustrated below ... 

Recognizing that nucleophiles can react at two different sites, an initial thought might be direct displacement of the chloride anion in an Sn2 manner. However, as alluded to in Problem 3(d), nucleophiles can add to carbonyl groups as shown below. 

This is a direct SN2 displacement of a chloride anion by a hydroxide anion. 

The second stage of the Swem oxidation, illustrated below, involves a nucleophilic displacement of the oxallyl group from the sulfur. In this step, the nucleophile is a chloride anion, and the reaction is facilitated by the decomposition of the leaving group into carbon dioxide gas, carbon monoxide gas, and a chloride anion. 

Nitro groups in aromatic rings activate substituents in ortho and para positions for nucleophilic displacements. The nucleophile here is a chloride anion that replaces fluorine. Of the live nitro groups, those in both ortho and para positions to fluorine activate fluorine for nucleophilic displacement very strongly, so that even a relatively weak nucleophile like chloride replaces fluorine under very mild conditions. Compound R is chloropentanitrobenzene [37],... 

In the case of 1-phenylpropyne a vinyl cation is suggested as intermediate. The kinetic law is rate = k3 [—C=C—] [HC1]2. The second order dependence on HC1 is explained by assuming that proton transfer to the triple bond results in the anion hydrogen dichloride, HClJ. The product distribution and the stereochemistry, under kinetic control, have been explained by assuming that a cw-oriented intimate ion-pair, 14 is initially formed which, following Scheme 2, may either collapse to cis chloride, undergo anion displacement by acetic acid to form tram acetate or a randomly oriented (solvent separated) ion-pair 15 which gives racemic material. 

At lower temperatures, a tosylate is formed from the reaction of p-toluenesulfonyl chloride and an alcohol. The new bond is formed between the toluenesulfonyl group and the oxygen of the alcohol. At higher temperatures, the chloride anion can displace the -OTos group, which is an excellent leaving group, to form an organochloride. 

The mechanism of action of these metal hydrides is somewhat uncertain. In the original report on [HFe(CO)4]", it was suggested that, by analogy with [Fe(CO)4], the anion displaces a chloride ion from the acyl chloride to give an intermediate (17) which then collapses to give the aldehyde (Scheme 7). However, there are alternative possibilities, particularly that the reagents act as H-atom donors (like... 

The mechanism of this last reaction may be postulated as shown in Scheme 52. Here, the initial nucleophilic attack by the sulfonamide anion (129) is followed by protonation of the intermediate and attack by the chloride anion to yield two moles of the sulfamoyl chloride (125). A wide range of sulfamoyl derivatives can be prepared by nucleophilic displacement of the chlorine atom in sulfamoyl chlorides (125). Examples include condensations with ureas, alcohols, compounds containing acidic hydrogens and nitrogen heterocycles to give the corresponding sulfamoyl derivatives (130)-(133) (Scheme 53). 

Alkyl chlorides can be prepared by the reaction of alcohols with thionyl chloride (SOCl2) in the presence of a nitrogen base (e.g. triethylamine or pyridine). An intermediate alkyl chlorosulfite (ROSOC1) is formed by nucleophilic attack of ROH on the sulfur atom of thionyl chloride. The OH group is converted to an OSOCl-leaving group, which is displaced on reaction with the chloride anion (e.g. in an SN2 mechanism when R is a primary alkyl group). 

Nucleophilic attack by N2 of pyrazol-3-ones 589a-e on chloramines displaced chloride anion to give 2-aminopyrazol-3-ones 590a-e in good yields (81JHC957) (Scheme 175). 

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