Metals free cations

The less important positive CCAs are metal-free cationic molecules, such as the black dye nigrosine and the colorless quaternary compound cetyl pyridinium chloride.35,36... 

With transition-metal free cationic phosphenium, an amino substituent is known to stabilize the cation. Theoretical studies46-50 support the existence of conjugation involving the nitrogen lone pairs and the vacant phosphorus p orbital, and estimate that the N-P+-N lone pair conjugation in [P(NH2)2]+ contributes 68.1 kcal/mol.47... 

A possible strategy to improve control of the anionic polymerization of methacrylates relies on the substitution of metal-free cations for metal cations. Ammonium and phosphonium salts were investigated as discussed hereafter. 

Organocatalytic cationic polymerization has also been reported for the ROP of TMC and DTC [57-59]. These methods displayed relatively poor control over the polymerization, and were characterized by the incomplete conversion of monomer and broad polydispersities that resulted from extensive back-biting of the polymer chains and decarboxylation. It is worth noting here that the ROP of oxazolines is usually performed by the application of metal-free cations (for an extensive reviewed of this subject, see Chapter 6). 

One of the most common chemical reducing agents for metallurgy is coke, a form of carbon made by heating coal at high temperature until all of the volatile impurities have been removed. Metals whose cations have moderately negative reduction potentials—Co, Ni, Fe, and Zn—are reduced by coke. For example, direct reaction with coke in a furnace frees nickel from its oxide NiO(.j) + C( ) Ni(/) -F CO(g)... 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

The use of weakly coordinating and fluorinated anions such as B(C6H4F-4)4, B(C6F5)4, and MeB(C6F5)3 further enhanced the activities of Group 4 cationic complexes for the polymerization of olefins and thereby their activity reached a level comparable to those of MAO-activated metallocene catalysts. Base-free cationic metal alkyl complexes and catalytic studies on them had mainly been concerned with cationic methyl complexes, [Cp2M-Me] +. However, their thermal instability restricts the use of such systems at technically useful temperatures. The corresponding thermally more stable benzyl complexes,... 

Crown ethers have also been found to lower the rate of reduction by metal hydrides. Wiegers and Smith (1978) reported that the rate of reduction of camphor by LiAlH4 in tetrahydrofuran was depressed by a factor of 6 on addition of one equivalent of crown ether [201]. They also concluded that, although the free cation shows a catalytic effect in metal hydride reduction, it is not indispensable. Dibenzo-18-crown-6 [11] was also found to lower the rate of... 

Ciloslowski, J., Boche, G. (1997) Geometry-tunahle Lewis acidity of amidinium cations and its relevance to redox reactions of the Thauer metal-free hydrogenase - a theoretical study. Angeiv. Chem. Int. Ed. Engl. 36, 107-9. 

The fact that complex 38 does not react further - that is, it does not oxidatively add the N—H bond - is due to the comparatively low electron density present on the Ir center. However, in the presence of more electron-rich phosphines an adduct similar to 38 may be observed in situ by NMR (see Section 6.5.3 see also below), but then readily activates N—H or C—H bonds. Amine coordination to an electron-rich Ir(I) center further augments its electron density and thus its propensity to oxidative addition reactions. Not only accessible N—H bonds are therefore readily activated but also C—H bonds [32] (cf. cyclo-metallations in Equation 6.14 and Scheme 6.10 below). This latter activation is a possible side reaction and mode of catalyst deactivation in OHA reactions that follow the CMM mechanism. Phosphine-free cationic Ir(I)-amine complexes were also shown to be quite reactive towards C—H bonds [30aj. The stable Ir-ammonia complex 39, which was isolated and structurally characterized by Hartwig and coworkers (Figure 6.7) [33], is accessible either by thermally induced reductive elimination of the corresponding Ir(III)-amido-hydrido precursor or by an acid-base reaction between the 14-electron Ir(I) intermediate 53 and ammonia (see Scheme 6.9). 

Owing to the high Lewis acidity the group 14 organometallic cations are polymerization catalysts par excellence. so Silanorbonyl cations and triethylsilyl arenium have been shown to be efficient catalysts for metal-free hydrosilylation reactions. Chiral silyl cation complexes with acetonitrile have been applied as cata -lysts in Diels Alder-type cyclization reactions °792 intramolecularly stabilized tetracoordinated silyl cations have been successfully used as efficient catalysts in Mukaiyama-type aldol reactions. 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

This review will concentrate on metal-free Lewis acids, which incorporate a Lewis acidic cation or a hypervalent center. Lewis acids are considered to be species with a vacant orbital [6,7]. Nevertheless, there are two successful classes of organocatalysts, which may be referred to as Lewis acids and are presented in other chapter. The first type is the proton of a Brpnsted acid catalyst, which is the simplest Lewis acid. The enantioselectivities obtained are due to the formation of a chiral ion pair. The other type are hydrogen bond activating organocatalysts, which can be considered to be Lewis acids or pseudo-Lewis acids. 

It has been shown that metal-free Lewis acids have been applied as catalysts in a broad variety of reactions. However, in several cases the asymmetric induction in the reactions has to be improved. While many of the highly active salts are moisture sensitive, ionic liquids with the right choice of cation and anion, are quite stable. Therefore their catalytic Lewis acidic activity is weak. The research field presented still has much room for improvanent and further investigations and results are continuously reported in the literature in an uiCTeasing number due to the large potential of metal-free Lewis acids. 

Substrate not only arrested spontaneous inactivation of the enzyme, but prevented the restoration of activity by Zn2+ in a preparation that had already undergone spontaneous decay. It was concluded that, regardless of whether it contains Zn2+ or a toxic cation, the substrate so combines with the protein-metal complex as to prevent dissociation, and, hence, inactivation or reactivation as the case might be. On the other hand, the addition of Zn2+ caused immediate reactivation of an EDTA-treated preparation, even in the presence of substrate, suggesting that the substrate cannot combine with the metal-free, enzyme protein. 

Although most other cations have little effect on the activity of a-D-mannosidase, certain bivalent cations, notably Cu2+, Cd2+, and Co2+, combine firmly with the enzyme, displacing Zn2+ and causing inactivation in every case.39,46,60 Unlike the metalloenzyme carboxy-peptidase86 (EC 3.4.2.1), a-D-mannosidase in the metal-free state cannot combine with substrate so as to prevent subsequent restoration of activity by the metal metal-free preparations are immediately activated by Zn,2+ even in the presence of substrate.39,48,60 On the other hand, substrate combines so firmly with metal complexes of a-D-mannosidase, regardless of whether the metal ion is Zn2+ or an inactive cation, that it lessens dissociation (and, consequently, metal exchange) to small proportions. 

Giernoth, R. and Bankmann, D., Transition-metal free ring deuteration of imidazolium ionic liquid cations. Tetrahedron Lett., 47,4293,2006. 

The nse of complexation to allow codeposition of alloys is well known in electroplating. The best-known example is that of brass (Cu/Zn) plating, where cyanide, which is a stronger complex for Cu than it is for Zn, brings the deposition potentials of the two metals, originally far apart, to almost the same value. There is a direct connection between this effect and the equivalent one for CD. This arises from the fact that, for both CD and electrodeposition of alloys (we in-clnde mixed metal compounds in the term alloy), the effect of the complexant is to lower the concentration of free cations. For CD this affects the deposition throngh the solnbility product, while for electrodeposition it affects the deposition potential through the Nemst equation ... 

Another, and on the face of it, rather different example, is the coprecipitation of solid solution compounds, such as CulnSi and CulnSei—semiconductors of particular interest due mainly to their applicability for photovoltaic cells. It was shown, by X-ray diffraction, that the precipitate resulting from reaction between H2S and an aqueous solution containing both Cu" and In " ions was, at least in part (depending on the concentrations of the cations), single-phase CulnSi [3]. Two factors were found to be necessary for this compound formation (1) the presence of sulphide on the surface of the initially precipitated colloidal solid metal sulphide and (2) one of the cations being acidic and the other basic. The monovalent Cu cation is relatively basic, while the trivalent In cation is relatively acidic. It is not clear what the physical reason is for this latter requirement. A difference in practice between acidic and basic cations is that, in an aqueous solution of both cations, the acidic cation is more likely to be in the form of some hydroxy species (not to be confused with hydrated cations), while the basic cation is more likely to exist as the free cation. 

The earliest of these studies was on PbS. PbS can have either p- or n-type conductivity, although CD PbS is usually p-type. Based on the belief that the p-type conductivity may be due to alkali metal cations from the deposition solution, an alkali metal—free deposition, using lead acetate, thiourea, and hydrazine hydrate was used [33]. While initially n-type, the film converted to p-type in air. Attempts to stabilize the p-type material by adding trivalent cations to the deposition solution were unsuccessful. However, deposition of the PbS on a trivalent metal, such as Al, did stabilize the n-PbS, at least for a time. In this way, p-n junctions were made (the PbS close to the trivalent metal was n-type, while the rest of the film was p-type). Photovoltages up to 100 mV were obtained from these junctions at room temperature and almost 300 mV at low temperatures (90 K). 

The effectiveness with which the solvent promotes migration decreases in the order methanol > ethanol water. The extremely low stability of complexes in water can be explained by the relatively great tendency of metal ions to associate with water molecules.70 The difference between the rate in ethanol and that in methanol can be attributed, at least partly, to the fact that salts are more highly dissociated into free ions in methanol71 a higher concentration of free cations would permit a higher concentration of positively charged carbohydrate species. 

Both associated and nonassociated electrolytes exist in sea water, the latter (typified by the alkali metal ions U+, Na-, K+, Rb+, and Cs-) predominantly as solvated free cations. The major anions. Cl and Br, exist as free anions, whereas as much as 20% of the F in sea water may be associated as the ion-pair MgF+. and 103 may be a more important species of I than I-. Based on dissociation constants and individual ion activity coefficients the distribution of the major cations in sea water as sulfate, bicarbonate, or carbonate ion-pairs has been evaluated at specified conditions by Garrels and Thompson (19621. 

The first two steps in the biosynthesis of tryptophan in Salmonella typhimurium involve the enzyme complex anthranilate synthase-phosphoribosyltransferase, which is a tetramer having two subunits of each enzyme. The anthranilate synthase catalyzes reaction (7) and the phos-phoribosyltransferase catalyzes two reactions the N-terminal portion cleaves glutamine to glutamate giving NH3 for the anthranilate synthase, while the C-terminal portion catalyzes reaction (8).3,1,312 All these reactions require M2+ cations. Orotate phosphoribosyltransferase binds four Mn2+ ions in a cooperative fashion kinetic data have been interpreted in a scheme where both metal-free and metal-containing enzyme catalyze the reaction.313... 

Suzuki et al. examined the effect of various divalent cations on purified recombinant human GCH expressed in Escherichia coli to clarify the molecular mechanism of action of divalent cations on the GCH enzymatic activity [150]. They demonstrated that GCH utilizes metal-free GTP as the substrate for the enzyme reaction. Inhibition of the GCH activity by divalent cations such as Mg(II) and Zn(II) was due to a reduction in the concentration of metal-free GTP substrate by complex formation. Many nucleotidehydrolyzing enzymes such as G proteins and kinases recognize Mg-GTP or Mg-ATP complex as their substrate. In contrast with these enzymes, Suzuki et al. demonstrated that GCH activity is dependent on the concentration of Mg-free GTP [150]. 

The metal-free hydrogenase enzyme system was also studied by theoretical calculations.52 This study involved the reactions of two model systems (115 and 116) with molecular hydrogen (Table 3). Cation 115... 

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