Lewis acid metal

Perhaps the most extensively studied catalytic reaction in acpreous solutions is the metal-ion catalysed hydrolysis of carboxylate esters, phosphate esters , phosphate diesters, amides and nittiles". Inspired by hydrolytic metalloenzymes, a multitude of different metal-ion complexes have been prepared and analysed with respect to their hydrolytic activity. Unfortunately, the exact mechanism by which these complexes operate is not completely clarified. The most important role of the catalyst is coordination of a hydroxide ion that is acting as a nucleophile. The extent of activation of tire substrate througji coordination to the Lewis-acidic metal centre is still unclear and probably varies from one substrate to another. For monodentate substrates this interaction is not very efficient. Only a few quantitative studies have been published. Chan et al. reported an equilibrium constant for coordination of the amide carbonyl group of... 

Perchloric acid (HCIO4 Ho —13.0), fluorosulfuric acid (HSO3F Ho — 15.1), and trifluoromethanesulfonic acid (CF3SO3H Ho —14.1) are considered to be superacids, as is truly anhydrous hydrogen fluoride. Complexing with Lewis acidic metal fluorides of higher valence, such as antimony, tantalum, or niobium pentafluoride, greatly enhances the acidity of all these acids. 

In accordance with the electropositive nature of the bridgehead atoms, all di(pyridyl) substituted anions behave like amides with the electron density accumulated at the ring nitrogen atoms rather than carbanions, phosphides or arsenides. The divalent bridging atoms (N, P, As) in the related complexes should in principle be able to coordinate either one or even two further Lewis acidic metals to form heterobimetallic derivatives. According to the mesomeric structures, (Scheme 7), it can act as a 2e- or even a 4e-donor. However, theoretical calculations, supported by experiments, have shown that while in the amides (E = N) the amido nitrogen does function as... 

Removing electrons from a metal atom always generates vacant valence orbitals. As described in Chapter 20, many transition metal cations form complexes with ligands in aqueous solution, hi these complexes, the ligands act as Lewis bases, donating pairs of electrons to form metal-ligand bonds. The metal cation accepts these electrons, so it acts as a Lewis acid. Metal cations from the p block also act as Lewis acids. For example, Pb ((2 g) forms a Lewis acid-base adduct with four CN anions, each of which donates a pair of electrons Pb ((2 ( ) + 4 CN ((2 q) -> [Pb (CN)4] (a g)... 

Guilard R, Brandes S, Tardieux C, Tabard A, L Her M, Miry C, Gouerec P, Knop Y, Collman JP. 1995. Synthesis and characterization of cofacial metaUodiporphyrins involving cobalt and lewis acid metals New dinuclear multielectron redox catalysts of dioxygen reduction. J Am Chem Soc 117 11721. 

Fe3+, La3+ and Ce3+ ion-exchanged MAPO-36 was prepared by wet ion-exchange method. The materials were characterized by XRD, TGA and TPD (ammonia). Lewis acid metal ions are suggested to remain as charge compensating MO+ species after calcination in ion-exchanged MAPO-36. Beckmann rearrangement of cyclohexanone oxime was studied over these catalysts in the vapour phase. 

Beckmann rearrangement of oxime is an acid catalysed reaction. The environmental problems associated with the use of sulphuric acid instigated interest to use number of solid acid catalysts [1], There are only scanty references about Lewis acid ion-exchanged MeAlPOs. Beyer et al. [2], Mihalyi et al. [3] and Mavrodinova et al. [4] already suggested the presence of Lewis acid metal ions as MO+ species in zeolites. The present study focussed the synthesis and characterisation of Fe3+, La3+ and Ce3+ ion-exchanged MAPO-36. The catalytic results of Beckmann rearrangement of cyclohexanone oxime over ion-exchanged catalysts are delineated in this article. 

Since both a acceptors and n donors at the alkyne termini are known to facilitate the Bergman cyclization, Zaleski and coworkers established a model46 in which the coordination of a Lewis acid (metal ion) would change the electronic environment in favor of diradical formation (Scheme 11). 

Perhaps the most attractive method of introducing enantioselectivity into the Diels-Alder reaction is to use a chiral catalyst in the form of a Lewis acidic metal complex. In recent years, this area has shown the greatest progress, with the introduction of many excellent catalytic processes. Quite a number of ligand-metal combinations have been evaluated for their potential as chiral catalysts in Diels-Alder reactions. The most commonly used metals are boron, titanium, and aluminum. Copper, magnesium, and lanthanides have also been used in asymmetric catalytic Diels-Alder reactions. 

A more practical BLA has been designed to improve the scope of dienophiles in reactions with less reactive dienes. 3,5-Bis(trifluromethyl)-phenylboronic acid (80) was chosen as the Lewis acidic metal component in the new BLAs 77-79. These new BLAs show higher catalytic activity in enantioselective cycloaddition of both a-substituted and a-unsubstituted a,/ -enal with various dienes. 

An application of industrial importance of Lewis acidic metal salts is the condensation of carboxylic diacids and diols to give polyesters. This is an acid catalysed reaction that in the laboratory is usually catalysed by protic acids. For this industrial application salts of manganese, nickel, or cobalt and the like are used. From a chemical point of view this chemistry may not be very exciting or complicated, the large scale on which it is being carried out makes it to an important industrial reaction [29],... 

Because of the fractional R-Li bond, clusters and polymers can reversibly form an open cluster, which traps the unsaturated substrate through multiple-point bonding (cf Schemes 10.5 and 10.7). Lithium cations assist the electron flow from the cuprate to the electrophile and, to achieve such cooperative action, a cluster of a particular size may be necessary. Lewis acid metals other than lithium (Zn , for example) will also play similar roles. 

Lee, K. Y. Kawthekar, R. B. Kim, G. J. (2007) Synthesis of chiral intermediates eatalyzed by new ehiral polymeric (salen) cobalt complexes bearing Lewis acidic metal halides., Korean Chem. Soc., 28 1553-1561. 

In 1996, Carreira reported a catalytic enantioselective aUylation of aldehydes using BlNOL-modified Tip4. More recently, Kobayashi reported a catalytic enantioselective aUylation of hydrazono esters in aqueous media using ZnF2-chiral diamine complex, [Eq. (13.36)]. In both reactions, reaction mechanism via dual activation of the substrate by Lewis acid metals and aUylsUanes by fluoride is proposed ... 

Finally in Chapters 11-13, some of the more recent discoveries that have led to a renaissance in the field of organocatalysis are described. Included in this section are the development of chiral Brdnsted acids and Lewis acidic metals bearing the conjugate base of the Bronsted acids as the ligands and the chiral bifunctional acid-base catalysts. 

A variety of strategies for effecting dipolar cycloadditions from stable, neutral precursors, including carbon-carbon bond heterolysis induced by Lewis acidic metal complexes and chiral organic catalysts ... 

Having generated suitable (partially) cationic, Lewis acidic metal centers, several factors need to be considered to understand the progress of the alkene polymerisation reaction the coordination of the monomer, and the role (if any) of the counteranion on catalyst activity and, possibly, on the stereoselectivity of monomer enchainment. Since in d° metal systems there is no back-bonding, the formation of alkene complexes relies entirely on the rather weak donor properties of these ligands. In catalytic systems complexes of the type [L2M(R) (alkene)] cannot be detected and constitute structures more closely related to the transition state rather than intermediates or resting states. Information about metal-alkene interactions, bond distances and energetics comes from model studies and a combination of spectroscopic and kinetic techniques. 

A major limitation of such Group IVB metallocene catalysts is that they are air- and moisture-sensitive and not tolerant to heteroatom-containing monomers. In the case of heteroatom-containing monomers the unbonded electron pairs on the heteroatom, such as oxygen, preferentially coordinate to the Lewis acid metal center in place of the carbon-carbon double bond. Some so-called middle and late transition metal organometallics are more tolerant to the presence of such heteroatoms and can be used as effective cocatalysts. These include some palladium, iron, cobalt, and nickel initiators. 

Application of metal salts and well-defined metal complexes in ROP has enabled the exploitation of a three-step coordination-insertion mechanism, first formulated in 1971 by Dittrich and Schulz [17]. This proceeds through coordination of lactide by the carbonyl oxygen to the Lewis acidic metal center, leading to the initiation and subsequent propagation by a metal alkoxide species. This species can be either isolated or generated in situ by addition of an alcohol to a suitable metal precursor to result in the formation of a new chain-extended metal alkoxide, as shown in Scheme 3 [16]. 

---