Addition of halides

Addition of halide ions to aqueous copper(II) solutions can give a variety of halo-complexes for example [CuCl4] (yellow square-planar, but in crystals with large cations becomes a flattened tetrahedron) [CuClj] (red, units linked together in crystals to give tetrahedral or distorted octahedral coordination around each copper). 

If the X formed during the reaction can decrease the rate, at least in some cases, it should be possible to add X from the outside and further decrease the rate in that way. This retardation of rate by addition of X is called common-ion effect or the mass law effect. Once again, addition of halide ions decreases the rate for diphenylmethyl but not for tert-butyl halides. 

Scheme 2.17. Intramolecular domino Michael/aldol process initiated by the addition of halide to an enone.

Addition of halogens and pseudohalogens to the cyclopropylthiocarbene chromium complexes 122 affords the 1,4-dihalo-1-phenylthio-l-alkenes 123 stereoselectively [65]. Electrophilic halogen is likely to activate the carbene complexes, followed by the homo-Michael addition of halide anion. (Scheme 44)... 

The addition of halide ions to 0.1M sulfuric acid solutions of triprolidine hydrochloride has a quenching effect on the fluorescent intensity. The degree of quenching is I > Br >C1 > F. A concentration of 10 M Cl has little effect on the fluorescent intensity, while 0.1M Cl reduces the intensity by approximately 75% of the chloride-free sulfuric acid solution. 

The only new report of a group 7 complex with thallium is the reaction of [Re7C(CO)2i]3 ion with T1PF6 (Equation (92)).94 The thallium adds in a triply bridging fashion opposite to the capping Re(CO)3 unit 99.94 The complex is stable in dichloromethane but dissociates in coordinating solvents. In acetone, infrared data indicated that the complex would be 99% dissociated at concentrations of the cluster of about 10-4M. Addition of halide ions to dichloromethane solutions causes a precipitation of the thallium(i) halide. 

Crown ether-assisted addition of halide to chloro-substituted ethylene-1,1-dicarboxylates [137] was reported to result in dechlorocarbomethoxy-lation (18) (Ykman and Hall, 1975). The authors proposed a mechanism in... 

The cathodic shifts of the redox potentials of compounds [68], [69] and [71] on addition of halide anions are due to the stabilization of the cobaltocenium cation by the bound anion which causes the redox couple to shift to a more... 

Scheme 65 Heterocycles by Nickel catalyzed intramolecular cathodic addition of halides to double bonds.

Secondary bromides and tosylates react with inversion of stereochemistry, as in the classical SN2 substitution reaction.21 Alkyl iodides, however, lead to racemized product. Aryl and alkenyl halides are reactive, even though the direct displacement mechanism is not feasible. With there halides the mechanism probably consists of two steps. The addition of halides to transition-metal species with low oxidation states is a common reaction in transition-metal chemistry and is called oxidative addition. An oxidative addition to the copper occurs in the first step of the mechanism, and the formal oxidation... 

Addition of halides to alkenes preparation of alkyl dihalides... 

Addition of halides and water to alkenes preparation of halohydrins... 

A variety of conditions were tried for the coupling, and aqueous KOH in DME were found to be optimal. Although Pd(PPh3)4 and PdCl2(PPh3)2 worked equally well as catalysts, the latter was easier to handle, and this becomes an important issue when many reactions must be run to make a library. The resin-bound vinyl boronates 29, which would be produced if the order of addition of halides were different, were not stable with respect to deboration, and subsequent yields using this strategy were low. 

The analogous Z aryl triflate 19.1 reacts under the cationic manifold to give, ultimately, oxindole (/ )-17.3a in 72% yield and 43-48% ee (Scheme 8G.19) [38]. An important synthetic advance is the observation that Heck cyclization of this substrate could be diverted to the more selective neutral pathway by addition of halide salts. For example, Heck cyclization of triflate 19.1 in the presence of 1 equiv. of n-Bu4NI gave (/ )-17.3a in 62% yield and 90% ee, which is similar to the enantioselectivity obtained for cyclization of the corresponding iodide 18.1c under neutral conditions (see entry 6, Table 8G, 1). Conversely, cyclization of iodide 18.1c in the... 

Extensive studies by Amatore, Jutand, and co-workers have shed light on the structure and oxidative addition chemistry of a number of synthetically important palladium complexes [42], In particular, these workers have shown that the major species in a solution of Pd(dba)2 and BINAP is Pd(dba)BINAP and that oxidative addition of Phi to this complex generates (Bl-NAP)Pd(Ph)I [42d,43], In addition, it has been demonstrated that palladium halide complexes such as (PhjP jaryljPdCl do not dissociate the halide ligand in DMF solution [44], whereas the corresponding triflate complex is completely dissociated [44,45], As noted earlier, the nature of the oxidative addition intermediates defines two mechanistic pathways for the Heck reaction the neutral pathway for unsaturated halide substrates and the cationic pathway for unsaturated triflate substrates [2c-g,3,7-9]. Further, it is possible for halide substrates to be diverted to the cationic pathway by addition of Ag(I) orTh(I) salts [3], and it is possible to divert some triflate substrates to the neutral pathway by addition of halide additives [38]. Individual steps of these two pathways have recently received some scrutiny. 

Innovations in the chemistry of aromatic compounds have occurred by recent development of many novel reactions of aryl halides or pseudohalides catalysed or promoted by transition metal complexes. Pd-catalysed reactions are the most important [2,29], The first reaction step is generation of the arylpalladium halide by oxidative addition of halide to Pd(0). Formation of phenylpalladium complex 1 as an intermediate from various benzene derivatives is summarized in Scheme 3.1. 

Three transmetallation reactions are known. The reaction starts by the oxidative addition of halides to transition metal complexes to form 206. (In this scheme, all ligands are omitted.) (i) The C—C bonds 208 are formed by transmetallation of 206 with 207 and reductive elimination. Mainly Pd and Ni complexes are used as efficient catalysts. Aryl aryl, aryl alkenyl, alkenyl-alkenyl bonds, and some alkenyl alkyl and aryl-alkyl bonds, are formed by the cross-coupling, (ii) Metal hydrides 209 are another partner of the transmetallation, and hydrogenolysis of halides occurs to give 210. This reaction is discussed in Section 3.8. (iii) C—M bonds 212 are formed by the reaction of dimetallic compounds 211 with 206. These reactions are summarized in Schemes 3.3-3.6. 

Aromatic carboxylic acids, a,/f-unsaturated carboxylic acids, their esters, amides, aldehydes and ketones, are prepared by the carbonylation of aryl halides and alkenyl halides. Pd, Rh, Fe, Ni and Co catalysts are used under different conditions. Among them, the Pd-catalysed carbonylations proceed conveniently under mild conditions in the presence of bases such as K2CO3 and Et3N. The extremely high toxicity of Ni(CO)4 almost prohibits the use of Ni catalysts in laboratories. The Pd-catalysed carbonylations are summarized in Scheme 3.9 [215], The reaction is explained by the oxidative addition of halides, and insertion of CO to form acylpalladium halides 440. Acids, esters, and amides are formed by the nucleophilic attack of water, alcohols and amines to 440. Transmetallation with hydrides and reductive elimination afford aldehydes 441. Ketones 442 are produced by transmetallation with alkylmetal reagents and reductive elimination. 

Concurrent cyclopropanation by carbenes and carbanions has been investigated.163 It has been demonstrated that the deliberate addition of halide ions afforded concurrent cyclopropanation of electron-poor alkenes by an equilibrating mixture of phenyl-halocarbenes and phenyldihalomethide carbanions, permitting smooth modulation of selectivity between electron-rich and electron-poor alkenes. 

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