Formation Inert Cations

Complex Formation Inert Cations. Several reports deal with the formation of chromium(m) complexes in dipolar aprotic solvents, especially DMF and DMSO. Reactions of [Cr(DMSO)e] + with Ns or NCS are /a, the reaction with Cl perhaps more complicated. lon-triples as well as ion-pairs may be kinetically significant in DMSO. Kinetics of reactions of complexes of the type fm 5 -[Cr(DMSO)2-(en)al + and rraKj-[Cr(DMF)a(en)2] with halide ions have also been described. ... 

This topic can be conveniently, if rather arbitrarily, divided into two parts, viz. formation of complexes of inert cations and formation of complexes of labile cations. The latter aspect ranges from the simplest examples through to biochemical systems this whole range is covered in a unified discussion in Part III of this Report. Formation reactions at kinetically inert centres will be discussed in this present section, in which references will be grouped according to the nature of the cation. Two general and comprehensive reviews of the formation of complexes have appeared the formation of complexes of sp elements has also been reviewed. ... 

Pseudo-first-order rate constants (k bs) for the reaction of anionic fV-hydrox-yphthaUmide (NHP) with HO increased by > 3-fold and 15-fold in the presence of inert monocations (LP, Na+, K+, and Cs+) and dication (Ba ), respectively, in aqueous solvent containing 2% v/v acetonitrile." Catalytic effects of these cations increased with the increase in the contents of acetonitrile in mixed aqueous solvents." The presence of anions such as CE and C03 did not show a kinetically detectable effect on for the alkaline hydrolysis of NHP-. The catalytic effects have been explained quantitatively in terms of an ion-pair mechanism in which cations produced a predominantly stabilizing effect on TS rather than on GS. The overall catalytic effect of inert cations is apparently the combined effect of ion-pair formation between cation and anionic reactants, which causes the increase in electrophilicity of carbonyl carbon of NHP for nucleophilic attack and decrease in the nucleophilicity of nucleophile (HO ). 

Dependence of Product Formation on Solvent Composition in the Irradiation of the Trityl Cation (Inert Atmosphere)... 

Although this mechanism could explain the inertness of di-t-butyl sulphide towards oxidation due to the absence of a-hydrogen atoms, it was later ruled out by Tezuka and coworkers They found that diphenyl sulphoxide was also formed when diphenyl sulphide was photolyzed in the presence of oxygen in methylene chloride or in benzene as a solvent. This implies that a-hydrogen is not necessary for the formation of the sulphoxide. It was proposed that a possible reactive intermediate arising from the excited complex 64 would be either a singlet oxygen, a pair of superoxide anion radical and the cation radical of sulphide 68 or zwitterionic and/or biradical species such as 69 or 70 (equation 35). 

The third primary intermediate in the oxidation chemistry of a-tocopherol, and the central species in this chapter, is the orr/zo-quinone methide 3. In contrast to the other two primary intermediates 2 and 4, it can be formed by quite different ways (Fig. 6.4), which already might be taken as an indication of the importance of this intermediate in vitamin E chemistry. o-QM 3 is formed, as mentioned above, from chromanoxylium cation 4 by proton loss at C-5a, or by a further single-electron oxidation step from radical 2 with concomitant proton loss from C-5a. Its most prominent and most frequently employed formation way is the direct generation from a-tocopherol by two-electron oxidation in inert media. Although in aqueous or protic media, initial... 

An essential requirement for such stabilisation is that the carbocation should be planar, for it is only in this configuration that effective delocalisation can occur. Quantum mechanical calculations for simple alkyl cations do indeed suggest that the planar (sp2) configuration is more stable than the pyramidal (sp3) by = 84 kJ (20 kcal) mol-1. As planarity is departed from, or its attainment inhibited, instability of the cation and consequent difficulty in its formation increases very rapidly. This has already been seen in the extreme inertness of 1-bromotriptycene (p. 87) to SN1 attack, due to inability to assume the planar configuration preventing formation of the carbocation. The expected planar structure of even simple cations has been confirmed by analysis of the n.m.r. and i.r. spectra of species such as Me3C SbF6e they thus parallel the trialkyl borons, R3B, with which they are isoelectronic. 

The initiators were salts with reactive cations and stable and inert anions, so that the initiation should be fast and as unambiguous as possible, and there would be no scope for any BIE, involving the growing ends. Some exploratory experiments with HC104 and HSO3CF3 showed that these are unsuitable for kinetic work, because of the formation of conjugate anions of the type A2H". 

Secondly, the polymer may act as a protective shell, since it not only delays the crystallization of by-products but also induces the formation of unusual solids after thermal treatment under an inert atmosphere. The nature of these solids clearly depends on the cations initially present in the hydroxide layers... 

Formation of coordination complexes is typical of transition metals, but other metals also form complexes. The tendency to form complexes is a function of the metal s electron configuration and the nature of its outer electron orbitals. Metal cations can be classified into types A and B based on their coordination characteristics, as shown in Table 3.5. A-type cations, which tend to be from the left side of the Periodic Table, have the inert-gas type electron configuration with largely empty d-orbitals. They can be imagined as having electron sheaths not easily deformed under the influence of the electric fields around neighbouring ions. B-type cations have a more readily deformable electron sheath. 

Anodic oxidation in inert solvents is the most widespread method of cation-radical preparation, with the aim of investigating their stability and electron structure. However, saturated hydrocarbons cannot be oxidized in an accessible potential region. There is one exception for molecules with the weakened C—H bond, but this does not pertain to the cation-radical problem. Anodic oxidation of unsaturated hydrocarbons proceeds more easily. As usual, this oxidation is assumed to be a process including one-electron detachment from the n system with the cation-radical formation. This is the very first step of this oxidation. Certainly, the cation-radical formed is not inevitably stable. Under anodic reaction conditions, it can expel the second electron and give rise to a dication or lose a proton and form a neutral (free) radical. The latter can be either stable or complete its life at the expense of dimerization, fragmentation, etc. Nevertheless, electrochemical oxidation of aromatic hydrocarbons leads to cation-radicals, the nature of which is reliably established (Mann and Barnes 1970 Chapter 3). 

The proposed mechanism of the oxidative cleavage of S-protecting groups by the chlorosilane/sulfoxide procedure is outlined in Scheme 8. 95 The first reaction is considered to be formation of the sulfonium cation 9 from diphenyl sulfoxide (7) and the oxygenophilic silyl compound 8. The formation of a sulfonium ion of this type is known and has been utilized for the reduction of sulfoxides. 97 Subsequent electrophilic attack of 9 on the sulfur atom of the S-protected cysteine residue leads to the formation of intermediate 10, whereby the nature of the silyl chloride employed should be the main factor that influences the electrophilicity of 9. The postulated intermediate 10 may then act as the electrophile and react with another S-protected cysteine residue to generate the disulfide 11 and the inert byproduct diphenyl sulfide (12). This final step is analogous to the reaction of a sulfenyl iodide as discussed in Section 6.1.1.2.1. 

Inert markers have been used to obtain additional information regarding the mechanism of spinel formation. A thin platinum wire is placed at the boundary between the two reactants before the reaction starts. The location of the marker after the reaction has proceeded to a considerable extent is supposed to throw light on the mechanism of diffusion. While the interpretation of marker experiments is straightforward in metallic systems, giving the desired information, in ionic systems the interpretation is more complicated because the diffusion is restricted mainly to the cation sublattice and it is not clear to which sublattice the markers are attached. The use of natural markers such as pores in the reactants has supported the counterdiffusion of cations in oxide spinel formation reactions. A treatment of the kinetics of solid-solid reactions becomes more complicated when the product is partly soluble in the reactants and also when there is more than one product. 

Using this method, the step-wise complex formation constants of cations with basic solvents and of anions with protic solvents have been determined in relatively inert solvents like AN [25], PC [26] and acetylacetone (Acac) [27]. The original objective of this study is to determine the step-wise formation constants... 

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