Cationic structures bimolecular reactions

Concerning their structure and reactions, organic radical cations have been the focus of much interest. Among bimolecular reactions, the addition to alkenes and their nucleophilic capture by alcohols, which lead to C—C and C—O bond formation, respectively have been investigated in detail. Unimolecular reactions like geometric isomerization and several other rearrangements have also attracted attention. 

This bimolecular coupling is a structure-sensitive reaction, and it illustrates a key characteristic of metal oxides multiple coordinative unsaturation of surface metal cations may facilitate coupling of ligands in a manner similar to that for unsaturated metal complexes in solution. Examples of other coupling reactions... 

The bimolecular reaction of the [Fe(terpy)J + cation with cyanide ion shows the expected increase in rate on going from water (kt = 0.0191 mol s at 35 °C) to 50% aqueous ethanol (k = 0.068 1 mol s" at 35 °C). This increase can be ascribed to the decreased solvation, and thus increased chemical potential, of the cyanide ion in the aqueous ethanol. It is interesting to contrast this situation with the dissociative-interchange reaction between iron(iii) and thiocyanate mentioned above, where the small decrease in rate in going from water to less-solvating DMSO was used as evidence against bimolecular attack by thiocyanate. The bimolecular substitution redox reaction of iron(ii) with the [Co(NH3)6Br] + cation shows a more complicated reactivity pattern in mixed aqueous solvents. The pattern is discussed in terms of the effects of the organic co-solvents on water structure and thence on reaction rates. ... 

Bimolecular ion/molecule reactions of dienes and polyenes have been extensively studied for several reasons. Some of them have been mentioned implicitly in the previous sections, that is, in order to structurally characterize the gaseous cations derived from these compounds. In this section, bimolecular reactivity of cationic dienes, in particular, with various neutral partners will be discussed, and some anion/molecule reactions will be mentioned also (cf Section IV). In addition, the reactions of neutral dienes with several ionic partners will also be discussed. Of this latter category, however, the vast chemistry of reactions of neutral dienes with metal cations and metal-centred cations will not be treated here. Several reviews on this topic have been published in the last decade178. 

Electron-spin resonance (e.s.r.) spectra with characteristic hyperfine structure have been recorded during the initial stages of the Maillard reaction between various sugar and amino compounds. The products responsible for the spectra appear to be IV, Af -disubstituted pyrazine radical cations. The pyrazine derivatives are assumed to be formed by the bimolecular condensation of two- and three-carbon enaminol compo-... 

For steric reasons the 2,6-disubstituted benzoic acids and esters are particularly susceptible to this type of cleavage reaction, and also particularly unreactive in the usual bimolecular solvolytic processes, and they have proved very convenient substrates for the study of the AacI mechanism. The kinetic work is discussed in a later section we are concerned at this point only with the qualitative behaviour of protonated esters amt acids, and of the structures of the cationic species. 

Formation of novel free radical products at an early stage of the Maillard reaction was demonstrated by use of ESR spectrometry. Analyses of the hyperfine structures for various sugar-amino compound systems led to the conclusion that the radical products are N,N -disubstituted pyrazine cation radicals. These new pyrazine derivatives are assumed to be formed by bimolecular condensation of a two-carbon enaminol compound involving the amino reactant residue. The presence of such a two-carbon product in an early stage reaction mixture of sugar with amine was demonstrated by isolation and identification of glyoxal dialkylimine by use of TLC, GLC, NMR, MS and IR. 

There are a number of claims4 40 51,69,150,247,268-274 for the isolation of bimolecular ethers from various other heterocyclic cations although the structures of these products have rarely been unambiguously established. The reaction mechanism outlined for the formation of 114 probably does occur in other heterocyclic systems, particularly in those cases in which alkoxide ion formation from the pseudobase readily occurs. Solubility considerations may dictate the precipitation of the bimolecular ether rather than the pseudobase from basic aqueous solutions containing relatively high concentrations of the heterocycle. However, such bimolecular ether formation will usually be in direct competition with the pseudobase disproportionation reaction (Section V,D) which shows the same pH dependence. 

Many attempts have been made to interpret the origin of the cracked products (propene and pentenes) formed during the reaction of n-butenes and to relate the formation of isobutylene, propene, and pentenes to the acidic and structural properties of the molecular sieve catalyst. In the introduction it was noted that the key reaction intermediates are (i) methylcyclo-propyl cations (formed in the monomolecular path) and (ii) di- (or tri-) branched methyl Ce (or C5) cations (formed in the bimolecular path) and that high selectivity to isobutylene can be achieved only when the monomolecular path predominates. 

The consumption of one amine group in reaction (93) increases the acidity of the medium. In order to establish the equilibria (90) and (91), new amine groups and acyllactam structures are formed. As soon as at least one lactam molecule or lactam cation is involved in the disproportionation reaction (90), the sequence of disproportionation (90) and bimolecular aminolysis (93) results in the incorporation of one or two monomer units into the polymer molecule. The participation of this type of chain growth in cationic lactam polymerization, suggested by Doubravszky and Geleji [182—184], has been confirmed both for polymerization [185—188] as well as for model reactions [189, 190]. The heating of an equimolar mixture of acetylcaprolactam with cyclo-... 

Such exploded transition states, in which the reaction centre carries a pronounced positive charge, are the commonest type of nucleophilic displacements at acetal centres. The reactions are unambiguously bimolecular, but in quantitative measures of transition state structure they differ only slightly from those of true 5 nI reactions. Thus, the value of for the hydrolysis of aryloxytetrahydropyrans, in which the solvent-equilibrated tetrahydropyranyl cation is a true intermediate, is —1.18, modestly but significantly more negative than the value of —0.82 obtained for hydrolysis of methoxymethyl esters of the type CH30CH20C0R. ... 

Nucleophilic substitution reactions in ionic liquids have recently been the subject of both synthetic [33] and kinetics and mechanistic studies [34]. Ionic liquids may be efficient promoting media for nucleophilic displacement reactions and important information about the ionic liquid properties has been obtained from the study of these reactions. The kinetic investigation of bimolecular substitution reactions of the halides on methyl p-nitrobenzenesulfonate, recently carried out by Wdton et al. [34] in several ionic media characterized by the same anion, [Tf2N] , and different cations ([BMIM]+, [BMMIMJ+ and [BMPY]+) or by the same cation, [BMIM] and different anions ([BF4] , [PFs] , [SbF ] and [Tf2N] ), has shovrathat the halides nucleophilicities depend on the ionic liquid structure (Scheme 5.1-7). 

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