Halide transfer

The above indicates the importance of the equilibrium in Eq. (18) for the termination of a polymeric chain. The halide transfer from the complex counterion to the cationic chain end was experimentally investigated using the model system Ph3C+MtX +1... 

Table 20. Reaction energies AElr (eV) for the halide transfer according to SbX5 + SbY5Z-- SbX5Z" + SbY5...

Thus, the conjugated anion represents an intermediate for the halide transfer from a complex anion to a Lewis acid. The quantum chemical reaction energies for the halide transfer AE(r can be calculated using the values of the interaction energies from Table 18 in the equation AEtr = AE(I) — AE(II). The results are presented in Table 20 and allow the following generalization ... 

Systems consisting of both Lewis acids SbX5 and complex anions SbYsZ possess the tendency to form the anion preferably with the largest number of F-, after which those of Cl-atoms by halide transfer. 

Therefore, the halide transfer should cause an increase in molecular weight. 

These reactions proceed with retention of double-bond configuration in both the boron derivative and the alkenyl halide. The oxidative addition by the alkenyl halide, transfer... 

Aryl Halides from Diazonium Ion Intermediates. Replacement of diazonium groups by halides is a valuable alternative to direct halogenation for the preparation of aryl halides. Aryl bromides and chlorides are usually prepared by a reaction using the appropriate Cu(I) salt, which is known as the Sandmeyer reaction. Under the classic conditions, the diazonium salt is added to a hot acidic solution of the cuprous halide.99 The Sandmeyer reaction occurs by an oxidative addition reaction of the diazonium ion with Cu(I) and halide transfer from a Cu(III) intermediate. 

When isobutene is polymerised in an inert solvent such as w-hexane by a metal halide with water as co-catalyst, the same end-groups are formed [9, 10] However, with other solvents, especially alkyl halides, transfer reactions may also introduce end-groups derived from the solvent [11, 12], for example ... 

Haloperoxidases act as halide-transfer reagents in the presence of halide ions and hydrogen peroxide. In the first step, the halide ion is oxidized to a halonium-ion carrier, from which the positive halogen species is then transferred to the double bond. In an aqueous medium, the intermediary carbocation is trapped and racemic halohydrins are formed (Eq. 7). Selective examples of CPO-cata-lyzed formation of halohydrins are given in Table 9. In CPO-catalyzed reaction. 

Concerning the first field of application, the kinetics and equilibrium constants for several halide transfer reactions (equation 1) were measured in a pulsed electron high pressure mass spectrometer (HPMS)4 or in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR)5. From measurements of equilibrium constants performed at different temperatures, experimental values were obtained for the thermochemical quantities AG°, AH° and AS° for the reaction of equation 1. The heat of formation (AH°) of any carbocation of interest, R+, was then calculated from the AH0 of reaction and the AH° values of the other species (RC1, R Cl and R +) involved. 

A couple of significant examples of the use of halide transfer reactions for the investigation of reaction mechanisms and of the structure of gaseous cations, the second field of application of these processes, are given below. More examples are reported in Section II.B. The first example concerns the reaction of the 2-fluoropropyl cation (1) with its neutral precursor /-butyl fluoride to yield the /-butyl cation, a reaction which could proceed either via F abstraction or via H+ transfer and HF elimination, both routes being exothermic (Scheme l)6. Collection of the neutral products by means of the EBFlow... 

Halide transfer reactions to metal cations are also very common processes. The reactions of halo compounds with metal cations are discussed in Section II.A.3.b. 

Halide transfer to other elements has also been observed. In particular, in view of the strength of the silicon-halogen bond, it is not surprising that silyl ions, H3Si+, readily undergo CT transfer from the chloromethanes CH Cl (x = 0-3) and F" transfer from CF4 and CHF38. 

The products most commonly observed in the reactions of bare metal cations (M+) with alkyl halides, summarized in Scheme 11, can be described as arising from halide transfer (a), halogen transfer (b) dehydrohalogenation (c) and alkene elimination (d). 

Alkyl chlorides and bromides (RX) react with Mg+ 54, Al+, Ga+ and In+ 54,55 by halide transfer whenever this process is exothermic, i.e. when the halide affinity of the metal cation... 

Kinetics of the reactions of Me3Si+ with water (57) and alcohol (31,32) were investigated by ICR mass spectrometry. Proposed mechanisms involve formation of an oxonium ion as the primary product, which in the case of the methanol adduct is eventually decomposed to methoxy-silylenium ion according to Eq. (9). Reactions of halide transfer to silylenium ion [Eqs. (10) and (11)] have been studied by FT mass spectrometry (59) and by tandem mass spectrometry (44). Hydride transfer... 

Two reactions were explored in continuation of these attempts hydride transfer from silanes to a carbenium center and halide transfer from silanes to a Lewis acid. Media of low nucleophilicity and high ionization power were employed. Stable silylenium ions A and B were postulated to be formed by H- transfer from corresponding silyl hydride to triphenyl-methylium perchlorate in CH2C12 (71,72). However, soon after, and in view of results of additional experiments, this evidence was shown to be insufficient (19,73). 

Relative stability of the respective parent carbocations estimated from proton-transfer or halide-transfer equilibria, in kcal mol". po values in log unit of log( f/ATo) for the gas-phase ionization are obtained by multiplying the p values of the gas-phase stabilities AAG(cc) by a factor of 1000/2.303R7 . 

The stabilities of carbocations for which the appropriate alkene precursors are not available were determined from the gas-phase halide-transfer equilibria (29) of the corresponding benzylic halides (Mishima et al., 1995). 

The data thus obtained have been supplemented by relative heats of formation obtained by the study of proton, hydride, and halide transfer equilibria [Eqs. (3, 4)] in a high-pressure mass spectrometer, flow tube, or ion cyclotron resonance spectrometer [20]. 

Nevertheless, bimolecular halide transfer [Eq. (106)] reportedly occurs in polymerization of cyclohexyl vinyl ether [113]. 

Following the pioneer work of Kharasch [60], methods involving radical transfer of halides have been developed. The atom transfer method has emerged in the 1980s as one of the best method for conducting intra- and intermolecular radical additions to olefins [61]. This approach is particularly appealing from an atom economy point of view since all atoms remains in the final product. The non-reductive nature of these reactions is also particularly important for the preparation of functionalized molecules. Halides transfers and more particularly iodine atom transfers have found nice applications for cyclizations, annula-tions and cascade reactions [62]. These reactions are based on exothermic radical steps, such as the addition of an alkyl radical to an olefin, followed by an... 

A characteristically different mechanism appears to operate in alkyne trimerization systems based on PdCb." Cationic metal complexes are involved in which initial halide transfer to an alkyne carbon is followed by sequential linear insertion of two more alkyne moieties. Metallacyclopentadiene intermediates are not involved in this sequence. Unique to this mechanism is the subsequent ring-closure to a cy-clopentadienylmethyl metal derivative, which, via halide transfer back to the metal, eventually leads to benzene via a bicyclo[3.1.0] system (Scheme 27). Support for this mechanism comes in part from the isolation of methylcyclopentadienyl-derived structures in several cases, including pentalene derivatives from further alkyne insertion followed by a second ring-closure." ... 

Recent advances in the study of ion-molecule reactions have opened another approach to the determination of relative carbocation stabilities. The halide transfer... 

Oxetane undergoes ring-opening polymerization under the action of MAD in conjunction with onium salts, including quaternary ammonium and phosphonium halides, giving a narrow MWD polyether (Scheme 6.173) [221]. Use of MesAl in place of MAD resulted in no polymerization. The aluminum ate complex seemed to be an initiator, which underwent a trigger reaction involving halide transfer to the aluminum-oxetane complex. 

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