Metal cyanides displacement reactions

Complexation constants of crown ethers and cryptands for alkali metal salts depend on the cavity sizes of the macrocycles 152,153). ln phase transfer nucleophilic reactions catalyzed by polymer-supported crown ethers and cryptands, rates may vary with the alkali cation. When a catalyst 41 with an 18-membered ring was used for Br-I exchange reactions, rates decreased with a change in salt from KI to Nal, whereas catalyst 40 bearing a 15-membered ring gave the opposite effect (Table 10)l49). A similar rate difference was observed for cyanide displacement reactions with polymer-supported cryptands in which the size of the cavity was varied 141). Polymer-supported phosphonium salt 4, as expected, gave no cation dependence of rates (Table 10). 

Alkylthiocyanates have been prepared in higji yield by reaction of alkali metal thiocyanates with various primary and secondary alkyl halides under phase transfer conditions. Quaternary alkylammonium salts [12—14], crown ethers [15], cryptates [16], and tertiary amines [14] have all proved effective phase transfer catalysts for this reaction (Eq. 13.7 and Table 13.4). The mechanism of the thiocyanate displacement is probably similar to that of the cyanide displacement reaction (see Sect. 7.2). 

Ethylene chlorohydrin vapors form an explosive mixture with air the LEE and UEL values are 4.9% and 15.9% by volume of air, respectively. Among the hazardous reaction products are ethylene oxide, formed by internal displacement of the chlorine atom by the alkoxide ion, ethylene glycol formed by hydrolysis with sodium bicarbonate at 105°C (221°F), and ethylene cyanohydrin resulting from the reaction with alkali metal cyanides. 

Aromatic and heteroaromatic halides fail to react with alkali metal cyanides under the conditions normally used for aliphatic nucleophilic displacement reactions. Traditionally, the preparation of aryl [ CJnitriles has been accomplished either by the Sandmeyer reaction or the Rosenmund-von Braun reaction. In the former, cyano-dediazoniation of aryl diazonium salts is accomplished with Cu CN or a mixture of and CuCN (or CuCl)... 

The commonest reactions involve the displacement of halide by hydroxide or cyanide ion to yield co-ordinated phenols or nitriles. Once again, the metal may play a variety of different functions. The polarisation of the C-Cl bond is the most obvious, but stabilisation of the product may be of equal importance, as could the involvement of a metal coordinated nucleophile. The availability of a one-electron redox inter-conversion between copper(n) and copper(i) also opens up the possibilities of radical mechanisms involving homolytic cleavage of the C-Cl bond. All of these different processes are known to be operative in various reaction conditions. In other cases, organocopper intermediates are thought to be involved. 

Complexes of 82 have also been formed by the reaction of 2,6-diacetylpyridine and Af.Af-hwQ-aminopropyOamine in the presence of nickel(II) chloride and copper(II) chloride50). Other metals that have been used include copper(II) 63,64), nickel64165), cobalt(II) 66), manganese(II)73>, cobalt(I)69), eobalt(III)68,70-72), zinc(II)73>, and ruthenium(II)74). Kam and Busch 51 have reported the catalytic hydrogenation of the nickel(II) perchlorate complex of 82 to afford two nickel(II) complexes of 83 a yellow minor component and a red major component which preliminary studies indicate to be the meso form (84). The isomeric ligands can be displaced from the respective reduced complexes by cyanide ion. Ligand 84 has also been isolated and characterized as the cobalt(III) 67), iron(II)61,62), iron(III) 62>, and copper(II) complex 75,76). Dehydro — 82 has also been synthesized and complexed with nickel(II) 65,65a), and nickel(III)65 a. ... 

Formation and equilibration of allylic metal derivatives such as those from the allylic bromides 24 and 25 we have just made. Reaction of the Grignard reagent with C02 gives only 27 but displacement of bromide with cyanide leads to the isomeric acid -28 both in good yield.5... 

While the rate of dissociation of Fe(phen)i" is not sensibly affected by hydrogen ions, hydroxyl (200) as well as cyanide and azide ions 201) greatly accelerate the rate of displacement of the diimine ligand. For the reaction with OH, the activation energy for the catalized path is lowered by ca. 6 kcal/mole 202). The preferred mechanism 201) consists in the replacement of water molecules in one of the pockets between the hgands, by the nucleophilic ion. Interaction between the i r-orbitals of the latter and the antibonding metal i-orbitals is believed to destabilize the metal-diimine bonds. 

Displacement of multidentate ligands from square-planar nickel(ii) has been the subject of several studies. Interest in cyanide as the displacing ligand has continued, and the nature of the intermediate species and the rate-determining steps in these reactions have been discussed. " Interactions between the multidentate leaving ligands and axial co-ordination sites of the metal have been suggested for these intermediates. The mechanistic description of an earlier study has been revised. ... 

The PMR spectra of the o - and tt -allyl complexes correspond very well with the spectra of the corresponding manganese and cobalt carbonyl complexes (9). Although the exact location of the tt -allyl group with respect to the metal is not known, the reaction with cyanide ion indicates that the TT -allyl group may be considered to be bidentate, a conclusion in full accord with the displacement of carbon monoxide in the conversion of a -to-tt-allyl cobalt and manganese carbonyls (9), and with the coordination of dimethyl-sulfoxide in the conversion of 7r-to-o -allyl palladium chloride (10). Structure(I) is tentatively proposed for the tt -allyl cyanocobaltate complex. 

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