Lewis acids cationic

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. 

Liquid ammonia, because of its lower dielectric constant, is in general a much poorer solvent for salts than water but this is offset to some extent toward salts of electron-acceptor, Lewis acid cations by its greater basic, electron-donor character. 

If cyclopentadienyliron dicarbonyl halides are allowed to react with themselves in the presence of Lewis acids, cations are formed in which the new substituent is the cyclopentadienyliron dicarbonyl halide itself, for example, [CjH6Fe(CO)i-X-(CO)2-FeC8H8]+ (X = Cl, Br, I). All three cations can be prepared best by treatment of the corresponding halides with boron trifluoride diethyl etherate all are isolated as tetrafluoro-borates.18 The bromine complex can also be obtained by a more complicated procedure by the reaction between C8H8Fe-(CO)2Br and AlBr3 in liquid sulfur dioxide 16 the iodine cation can be isolated from a melt of cyclopentadienyliron dicarbonyl iodide and aluminum chloride.17 In the latter two cases the hexafluorophosphate salts can be obtained. These binuclear cations are of special interest, because they are cleaved by electron donors,16-17 e.g., aniline, pyridine, benzonitrile, acetonitrile, acrylonitrile, with the formation of the corresponding [C8H8Fe(CO)2L]+ cations and the parent halide. Equations for preparation of the tetrafluoroborate are ... 

In cationic polymerizations, initiation occurs by attachment of a proton or some other Lewis-acidic cation X" to the H2C=CR2 double bond of a vinyl monomer to form a new carbon-centred cation of the type XH2C-CR2, which then grows into a polymer chain by subsequent H2C=CR2 additions (Figure 2, bottom). This type of polymerization works well - and is used in practice - only for olefins such as isobutene, where 1,1-disubstitution stabilizes the formation of a cationic centre. Since side reactions, such as release of a proton from the cationic chain end, occur rather easily, cationic polymerization usually gives shorter chains than anionic polymerization. 

This area has been further advanced by Coates and coworkers. The synthesis of succinic anhydrides by )3-lactone carbonylation nsing a Lewis acidic cation in conjnnction with an anionic cobalt carbonyl complex has been reported (eqnation 33). 

Since a direct and crucial proof for the occurrence of an initiation reaction consisting of the addition of a Lewis acid cation to the monomer double bond is not yet available, we will expound some relevant observations which seem to us highly suggestive indications of such an interaction. 

Simple Lewis acid cationic catalysts, e.g. BF3, SnCl4, AIR3, PFj, SbCls etc., have been extensively used in the study of the polymerization of THF [118]. With these initiators the problem of estimating the concentration of prop ating species becomes acute, and is not made any easier by the use of so-called promoters such as propene oxide,... 

It is known that iodide salts react with methanol to give methyl iodide which can subsequently react with transition metal complexes [1]. The substitution of hydroxy group with iodide would be promoted by the Lewis acidic cations coordinating to the oxygen atom as illustrated in Scheme 1. 

SN2-type nucleophilic attack on the less highly substituted carbon better Lewis acids cations, e.g., Zn2+, Li+, and Mg2+, are more effective in directing the attack of the amine to the benzylic carbon. 

These (or similar) interactions find their antecedents in the literature [74, 92, 142, 153, 179], The stronger the interaction of the anion with the hydroxyl group of the alcohol, the more activated the alcoholate will be for nucleophilic attack on the carboxyl carbon, which itself can be activated by a Lewis-type complex formed with a Lewis acidic cation. A third beneficial interaction with a basic anion may arise from strong interactions (hydration) with the water formed, which would drive the equilibrium reaction towards the product side [96-112],... 

The adsorption behavior of diazines in X and Y zeolites has been studied by infrared spectroscopy (IR), temperature-programmed desorption (TPD), and simulation techniques. The studies showed that the interaction is determined by a donation of electron density from the nitrogen atoms of the probe molecules to the Lewis-acidic cations. The individual nature of the adsorption strongly depends on the Si/Al ratio of the zeolites, the kind of extraframework cation, and the positions of heteroatoms in the probe molecules. 

Adsorption of toluene on zeolites Li-X, Na-X, K-X, Rb-X, and Cs-X has been investigated with quantum chemical methods. Calculations of geometries, Mulliken partial charges, and C chemical shift parameters of clusters representing the catalytically active site are presented. The polarisation of the toluene carbons is the first step in alkylation reactions catalysed by zeolites and, at an early stage, will influence the outcome of the reaction. We show the simultaneous influence of the Lewis acidic cation and the basicity of the zeolite is responsible for altering the electron distribution within the toluene and thus affecting the outcome of an alkylation reaction. 

Chromatograp hy Aluminium oxide Binding, H-Bridges, Steric Interactions, Bronsted and Lewis Acidity, Cation- Exchange. O-H Wsi I i i I H-O Structure Products, Isomers... 

Reduction of 3-trimethy1s11yl-2-propyn-l-ol exemplifies the problem of stereoselectivity in hydride reduction of acetylenic alcohols to E-allyl alcohols, Early reports that lithium aluminum hydride stereoselectively reduced acetylenic alcohols gave way to closer scrutiny which revealed a striking solvent dependence of the stereochemistry, Specifically, the percentage of trans reduction is seen to increase with increasing Lewis basicity of solvent. Similarly, the addition of less Lewis acidic cations to the reducing mixture leads to improved trans/cis ratios, Sodium b1s(2-... 

Bhugun, I., D. Lexa, and J.M. Saveant (1996). Catalysis of the electrochemical reduction of carbon dioxide by iron(O) porphyrins. Synergistic effect of Lewis acid cations. / Phys. Chem. 100, 19981-19985. 

The potentials observed for the reduction of CO2 under these conditions were still relatively high (e.g., -1.7 V, Table 7.5). The efficiency of the electrocatalytic reduction of CO2 by Fe°TPP improves in the presence of Lewis acid cations such as Mg +, Ca " ", Ba " ", Li+, and or Bronsted acid co-catalyst such as 2-... 

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