Lewis acids monovalent

The principal study in this field made in our laboratory during the past year has been on acid-base reactions between the Lewis acid, monovalent silver, and a number of bases, both ionic and molecular. All transition metals are, of course, Lewis acids and should undergo these reactions. The number of positions available for complex formation on a metallic ion will vary with the atomic number and can, in some instances, present a complex problem. 

Addition of Lewis acids under the form of monovalent or divalent metal ions produces similar effects,5 as shown in Figure 4.7. Analysis of the reactions metal ion and C02 reaction orders suggests the mechanism depicted in Scheme 4.7 for monovalent cations. The carbene-like adduct is stabilized, in this case by addition of one metal ion and one C02 molecule. 

In contrast to the previously discussed carbon-carbon bond rearrangements, these results clearly show that Lewis acid sites can also act as catalytically active sites for nucleophilic substitutions. Note that if catalysts without Bronsted acid sites are used (i.e., with zeolites exchanged with monovalent cations) the competitive side reaction leading to (i)-hydroxybutryonitrile via protonation of the acid amide can be completely suppressed (Scheme 6). 

Stepwise addition of two pyridine molecules to Ph-Si , whose reversibility was established in collision-induced dissociation (CID) experiments, seems to be due to the formation of one bond at a time, the monovalent silicon cation reacting as a Lewis acid. That two, but no more than two, pyridine molecules are accepted by Ph-Si points to the silicon atom as the site of addition. In this scenario, addition of the first pyridine forms a distonic silylene. That this is a plausible process is indicated by the reaction of the parent silanetriyl cation H-Si with diethylamine HNEt2 CID of the product ion established its structure as a four-membered ring whose most likely source is a two-step process formation of a silylene intermediate by a Lewis acid-Lewis base reaction followed by intramolecular insertion of the silylene into a methyl C-H bond. Three bonds are formed in a single reactive encounter, but the stepwise process is much more likely than the more interesting concerted reaction. 

The reaction of //-methyl-2-butenylsilanes 36 and stannanes with chiral a-al-koxyaldehydes has also been reported [33]. Surprisingly, the anti homoallylic alcohols were predominantly observed (94/6, ant i/syn) when a bivalent Lewis acid such as SnCl4 was used (Scheme 10-13). A synclinal transition structure is proposed to account for the observed selectivity. In the chelation-controlled reactions the synclinal transition structure is favored over the corresponding antiperiplanar transition structure because there exists an open space wherein the complexed Lewis acid can reside. The monovalent Lewis acid BF3-OEt2 provides the expected syn homoallylic alcohol, presumably through the antiperiplanar transition structure shown (66% of the product was the syn alcohol 37). 

We would like to know the bond strengths of A B with respect to separation into the free acid and the free base. This only takes us back to the same problem, so we avoid it, since only comparisons will be made, by using the better documented gas-phase bond dissociations D° for separation into the pair of radicals A and B. For monovalent Lewis acids, we can then take a pair of reference monovalent bases, such as fluoride ion and iodide ion, one hard and one soft, and for which there are plenty of data. We use the reaction in Equation 3.8 to define the scale of local hardness at the atom in the bond we are using by the difference APi using Equation 3.9. 

Similarly, IR investigation of CO adsorption on molecular sieves was used to characterize Lewis acidity of cations (C-sites) and true Lewis acidity (L-sites) [ 740]. The interaction of CO with cations (acid C-sites) was dealt with already in Sect. 5.5.2.2. In particular, Angell and Schaffer [595] have carried out a detailed study of CO adsorption on a series of X- and Y-type zeoHtes containing monovalent and divalent cations of alkali, alkaline earth and transition metals. A linear relationship was found between the position of the IR stretching band of adsorbed CO and the Coulomb field, q/r, of the respective cationic adsorption center. This is similar to the observation made by Ward in the case of pyridine attached to cations (vide supra). It should be noted, however, that CO, like pyridine, is not capable of entering the sodalite cages and the hexagonal prisms of the faujasite structure, so that the cations located there are not detected by these probes. 

Considerably less information is available concerning the D structure of water molecules hydrating vermiculites that form surface complexes with monovalent cations. It appears that both Li- and Na-vermiculite can form mono- and bilayer hydrates, whereas K-, Rb-, and Cs-vermiculite cannot. For the latter, inner-sphere surface complexes are stable against solvation of the cations because of Lewis acid character and stereochemistry. These factors are especially notable when comparing with which have almost identical ionic radii. Barium-vermiculitc contains a monolayer of... 

The introduction of monovalent and bivalent transition metal cations into zeolites is also possible and introduces in zeolites sites with redox activity. Several of these systems have wide application in catalysis. In particular, Co-zeolites, such as Co-MFI and Co-FER, have been deeply investigated for their activity in the CH4-SCR reaction [246]. In this case the adsorption of bases such as nitriles and ammonia, followed by IR and by TPD technique, show that they act as medium-strong Lewis acid sites. The current opinion is that these sites are catalytically active for the DeNO c reaction just when they are isolated in the zeolite cavities. A recent investigation provided evidence for the deposition of part of Co ions also at the external surface of the zeolite upon cation exchanging [85] and to their likely nonnegligible catalytic activity [247]. The deposition of Co species at the external cavities can be a reason for only apparent over-exchanging (i.e., production of zeolites with Co +/AP+ atomic ratios >0.5). 

The AE value obtained from the deviation of the g z value from the free spin value increases in order monovalent cations (M ) < divalent cations (M ) < trivalent cations (M )." The AE value also increases with decreasing ion radius when the oxidation state of the metal ion is the same. The same trend has been reported for 02 adsorbed on the surface of various metal oxides, which act as Lewis acids as well. " The scandium ion, which has the smallest ion radius among the trivalent metal cations, gives the largest AE value." ... 

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