Lewis metals

Zacharion, M. Traverso, L Hearn, M.T.W. High-performance liquid chromatography of amino acids, peptides and proteins CXXXI. o-Phosphoserine as a new chelating ligand for use with hard Lewis metal ions in the im-mobilized-metal affinity chromatography of proteins. J. Chromatogr. 1993, 646, 107-120. 

Lewis metal. Alloy of one part tin and one part bismuth. 

In the present work, we have carried out the rearrangement of acetals of a-bromopropiophenone to 2-phenylpropanoates catalyzed by large pore zeolites containing Lewis metal ions. We have found that our results can be interpreted taking into account the Lewis nature of the active sites but also assuming that their softness-hardness is modified by the zeolite framework. 

Influence of the zeolite framework on the softness-hardness of the Lewis metal site. 

Aimed to explore the extent of these radical processes using our Lewis metal ion containing zeolites, we have treated the a-bromopropiophenone (2) in methanol/trimethyl orthoformate mixtures, which besides to be a better hydrogen donor medium than chlorobenzene and hence more propiophenone should be now expected, could shift the equilibrium to the acetal 1b formation. The results obtained... 

Table 1 presents the acid and base site densities measured for all the samples by TPD of preadsorbed NH3 and CO2, respectively. The base site density (nb) of the MgyAlOx oxides was always between the values measured for MgO and AI2O3. Basic sites of different chemical nature were observed by FTIR of CO2 and deconvolution of the CO2 TPD traces isolated low-coordination O ions, O in metal-oxygen pairs and OH groups. The acid site density (na), on the other hand, increases with increasing A1 content. By deconvolution of the NH3 TPD traces it was found that the MgyAlO oxides contain both Brdnsted (OH groups) and Lewis (metal cations) acid sites. 

From the slope of the plots of Fig. 6, the initial deactivation rate was calculated as do = [- da/dt ]h). In Fig. 7, the do values obtained for all the samples are represented in open squares as a fimction of the carbon content measured after the catalytic runs (Table 2) clearly, it does not exist any correlation between do and the amount of coke. Initial deactivation is lower on AI2O3 (do = 0.14 h" ) than on MgO (do = 0.53 h ) in spite that alumina forms more coke during reaction and that the coke is more difficult to oxidize as compared to MgO (Table 2 and Fig. 4). These results show that neither the coke amount nor its polymerization degree account for the catalyst deactivation order observed in Fig. 6. A better explanation is obtained by considering the nature of the surface sites that are responsible for the formation of coke precursors on pure AI2O3 or MgO. Alumina contains Brbnsted (OH groups) and Lewis (metal... 

Addition of a Lewis acid to the reaction mixture can significantly increase the rate of the hydrogen exchange process. HCl will not exchange with the aromatic hydrogens of toluene at temperatures up to 140° upon addition of SnCU the reaction proceeds at room temperature (30). Lewis metal halides have been used to... 

According to the general acid-base concepts of Bronsted and Lewis, metal cations are generally regarded as acids. Therefore, transition metal cations or coordina-tively imsaturated compoimds can undergo addition of neutral or anionic nucleophiles to give cationic (Eq. 2-17), anionic (Eq. 2-18), and ir-acceptor complexes (Eq. 2-19). 

Metzner et al. reported an acetal (Mukaiyama-type) MHB reaction with a combination of a sulfide catalyst and TBDMSOTf under basic conditions, i.e. without Lewis metal ion co-activation of the electrophilic enone. A substoichiometric amount of 189 (20 mol%) at -40°C gave 42% of 190 (Scheme 7.30) [157],... 

There are many compounds which do not conduct electricity when solid or fused indicating that the bonding is neither metallic nor ionic. Lewis, in 1916. suggested that in such cases bonding resulted from a sharing of electrons. In the formation of methane CH4 for example, carbon, electronic configuration l.s 2.s 2p. uses the tour electrons in the second quantum level to form four equivalent... 

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

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... 

Bcamples of metal-ion catalysed organic reactions in water where the catalyst acts exclusively as Lewis acid are the hromination of diketones" " and the decarboxylation of oxaloacetate. The latter reaction has been studied in detail. In 1941 it was demonstrated that magnesium(II) ions catalyse this reaction" Later also catalysis by other multivalent metal ions, such as Zn(II), Mn(II), Cu(II), Cd(ir), Fe(II), Pb(II), Fe(III)... 

First, the use of water limits the choice of Lewis-acid catalysts. The most active Lewis acids such as BFj, TiQ4 and AlClj react violently with water and cannot be used However, bivalent transition metal ions and trivalent lanthanide ions have proven to be active catalysts in aqueous solution for other organic reactions and are anticipated to be good candidates for the catalysis of aqueous Diels-Alder reactions. 

The effect of substituents on the rate of the reaction catalysed by different metal ions has also been studied Correlation with resulted in perfectly linear Hammett plots. Now the p-values for the four Lewis-acids are of comparable magnitude and do not follow the Irving-Williams order. Note tlrat the substituents have opposing effects on complexation, which is favoured by electron donating substituents, and reactivity, which is increased by electron withdrawirg substituents. The effect on the reactivity is clearly more pronounced than the effect on the complexation equilibrium. 

So far the four metal ions have been compared with respect to their effect on (1) the equilibrium constant for complexation to 2.4c, (2) the rate constant of the Diels-Alder reaction of the complexes with 2.5 and (3) the substituent effect on processes (1) and (2). We have tried to correlate these data with some physical parameters of the respective metal-ions. The second ionisation potential of the metal should, in principle, reflect its Lewis acidity. Furthermore the values for Iq i might be strongly influenced by the Lewis-acidity of the metal. A quantitative correlation between these two parameters... 

Unfortunately, addition of copper(II)nitrate to a solution of 4.42 in water did not result in the formation of a significant amount of complex, judging from the unchanged UV-vis absorption spectrum. Also after addition of Yb(OTf)3 or Eu(N03)3 no indications for coordination were observed. Apparently, formation of a six-membered chelate ring containing an amine and a ketone functionality is not feasible for these metal ions. Note that 4.13 features a similar arrangement and in aqueous solutions, likewise, does not coordinate significantly to all the Lewis acids that have been... 

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 a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

Silyl enol ethers are other ketone or aldehyde enolate equivalents and react with allyl carbonate to give allyl ketones or aldehydes 13,300. The transme-tallation of the 7r-allylpalladium methoxide, formed from allyl alkyl carbonate, with the silyl enol ether 464 forms the palladium enolate 465, which undergoes reductive elimination to afford the allyl ketone or aldehyde 466. For this reaction, neither fluoride anion nor a Lewis acid is necessary for the activation of silyl enol ethers. The reaction also proceed.s with metallic Pd supported on silica by a special method[301j. The ketene silyl acetal 467 derived from esters or lactones also reacts with allyl carbonates, affording allylated esters or lactones by using dppe as a ligand[302]... 

All the following compounds are charactenzed by ionic bonding between a group I metal cation and a tetrahedral anion Wnte an appropriate Lewis structure for each anion remembenng to specify formal charges where they exist... 

Their polar carbon-oxygen bonds and the presence of unshared electron pairs at oxygen contribute to the ability of ethers to form Lewis acid Lewis base complexes with metal ions... 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

Ethers form Lewis acid Lewis base complexes with metal ions Certain cyclic polyethers called crown ethers, are particularly effective m coor dinatmg with Na" and K" and salts of these cations can be dissolved m nonpolar solvents when crown ethers are present Under these conditions the rates of many reactions that involve anions are accelerated... 

A Lewis base that binds with a metal ion. 

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