Cations Lewis acids

Basic molecules such as pyridine and NH3 have been the popular choice as the basic probe molecules since they are stable and one can differentiate and quantify the Bronsted and Lewis sites. Their main drawback is that they are very strong bases and hence adsorb nonspecifically even on the weakest acid sites. Therefore, weaker bases such as CO, NO, and acetonitrile have been used as probe molecules for solid acid catalysts. Adsorption of CO at low temperatures (77 K) is commonly used because CO is a weak base, has a small molecular size, a very intense vc=0 band that is quite sensitive to perturbations, is unreactive at low temperature, and interacts specifically with hydroxyl groups and metal cationic Lewis acid sites.26... 

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 solvent, in addition to permitting the ionic charges to separate and the electrolyte solution to conduct an electrical current, also solvates the discrete ions, by ion-dipole or ion-induced dipole interactions and by more direct interactions, such as hydrogen bonding to anions or electron-pair donation to cations. Lewis acidity and basicity of the solvents affect the latter. The redox properties of the ions at an electrode depend on their being solvated, and the solvation effects electrode potentials or polarographic half-wave potentials. 

Historically, the concept of coordination chemistry was associated with complexation of a metal cation (Lewis acid) by a ligand behaving as a Lewis base. Such was traditionally the case for macrocyclic molecules as ligands. In the early 1970s, however, the concept of coordination chemistry was extended in the area of macrocyclic chemistry to include molecular cations, neutral molecules and anions as substrates. Complexes of all of these species are to be included in the scope of this chapter section. Examples of the types of substrates are discussed below. 

Common examples of cationic Lewis acids are metal ions, such as Al3+ and Cu2+. Hydration of the Al3+ ion, for example, is a Lewis acid-base reaction in which each of six H20 molecules donates a pair of electrons to Al3+ to form the hydrated cation A1(H20)63+ ... 

The nickel and palladium compounds described above are useful in processes for polymerising various olefins, and optionally also for copolymerising olefinic esters, carboxylic acids or other functional olefins with these olefins. When (I) is used as a catalyst, a neutral Lewis acid or a cationic Lewis or Bronsted acid whose counterion is a weakly coordinating anion is also present as part of the catalyst system. The neutral Lewis acid is originally uncharged (i.e. not ionic). Suitable neutral Lewis acids include SbFs, A B and BF3. By a cationic Lewis acid is meant a cation with a positive charge such as Ag+, H+ and Na+. 

Bridging by fluorides may also occur. For example SbFg may coordinate to 1 the very strong Lewis acid SbFs to foran [FjSb—F—SbF5] (see Probleir 3.42), but it is very difficult to abstract a fluoride ion from SbF. It has beenl suggested that [SbFJ should be the amion of choice in the synthesis off reactive yet potentially isolable cationic Lewis acids. ... 

Useful solvents must themselves resist oxidation or reduction, should dissolve suitable ionic solutes and nonelectrolytes, and in addition should be inexpensive and obtainable in high purity. Kratochvil indicated that the most potentially useful solvents are those that have a dielectric constant greater than about 25 and have Lewis-base properties. Some solvents meeting these criteria are acetonitrile, dimethyl-sulfoxide, dimethylformamide, dimethylacetamide, propylene carbonate, ethylene carbonate, formamide, sulfolane, and y-butyrolactone. Solvents of the Lewis-base type show specific solvation effects with many metal cations (Lewis acids). Thus acetonitrile functions as a Lewis base toward the silver ion. At the same time it reacts but little with the hydrogen ion. 

It has been noted that although the linear gemnetry is consistently predicted for cationic Lewis acid carbonyl ctxnplexes in ab initio calculations, extrapolation of these results to neutral Lewis acid c< n-plexes may not be justified. Semiempirical MNDO calculations predicted the bent conformation as the lowest energy structure for neutral Lewis acidic derivatives of beryllium, boron and aluminum cmn-plexed with rranr-2,3-dimethylcyclopropanone, whereas linear structures were predicted for the cationic complexes of beryllium and aluminum (Table 1). ... 

Chiral oxazaborolidine salts are useful catalysts not only for enantioselective reductions (i.e., CBS reduction), but also for promoting enantioselective D-A reactions. The cationic Lewis acids formed by reaction of chiral oxazaborolidines with triflic acid [TfOH] or trifluoromethanesulfonimide [(Tf)2NH] coordinate to dienophiles to direct subsequent cycloadditions in a highly controlled fashion. The D-A reactions using these chiral salts proceed in CH2CI2 under very mild conditions at temperatures as low as -95 °C (Table 9.4). ... 

I See the Saunders Interactive General Chemistry CD-ROM, Screen 1 7.11, Lewis Acids and Bases, and Screen 17.12, Cationic Lewis Acids. 

This cationic Lewis acid can form complexes with the Lewis bases undergoing oxidation (species that are oxidizable are frequently electron rich and hence are Lewis basic). Therefore, at low pH the oxidant (NO" ) is a different chemical species than at higher pH (N02 ) (see Section 15.13(b) Nitrogen(IV) and mtrogen(IIl) oxides and oxoanions). 

Table 1.4 Empirical Hardness Parameters for Cationic Lewis Acids ...

For efBdent cationic polymerization of vinyl monomers it is necessary that the carbon-carbon double bond be the strongest nucleophile (electron donor) in the molecule. If more than one nucleophilic site exists in a monomer and the rr electron system of the double bond does not represent the most nucleophilic site, the other site(s) may complex with the electrophile (proton, cation, Lewis acid). Thus, for methyl methacrylate ... 

In an important development of these generalizations by Pearson, cations (Lewis acids) and ligands (Lewis bases) were classed as being either hard or soft . The principle of hard and soft acids and bases (HSAB) is used to rationalize observed patterns in complex stability. In aqueous solution, complexes formed between class (a), or hard, metal ions and ligands containing particular donor atoms exhibit trends in stabilities as follows ... 

Proline-derived oxazaborolidines 45 have shown to be effective pre-catalysts with triflic acid as an activator to generate cationic Lewis acids.18,15 The optimal proportions of 45 and triflic acid was found to be 1.2 1. Protonation of 45 produced a 1.5 1 mixture of 46 and 47 as determined by low temperature H NMR. Their interconversion at low temperature (-80 °C) is slow on the NMR timescale. However, this interconversion increases as the temperature rises and at 0 °C this becomes rapid (Tc). Phenyl or o-tolyl were determined to be the best substituents for the R group in 45. For the Ar group of 45, phenyl and 3,5-dimethylphenyl were determined to be optimal. 

This in situ formed cationic Lewis acid catalyst coordinates enals in a highly organized fashion (48) that allows for the execution of asymmetric Diels-Alder reactions. Thus for the initially disclosed acrolein examples, the Diels-Alder adducts 51 produced from enals 49 and dienes 50 could be isolated in good to excellent yields with very high optical purities. 

---