Pseudo-Lewis acids

The first step of the catalytic process is the hydrogen bond directed assembly and orientation of the reactants. In this example, the azlactone and methanol form a ternary starting complex with the organocatalyst (Fig. 1) [39]. The pseudo-Lewis acidic thiourea forms two bifurcated, nearly symmetric hydrogen bonds (2.147 A, < (0,H,N) = 155.5° and 2.146A, <(0,H,N) = 155.8°) to the carbonyl oxygen atom of the azlactone. 

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. 

Hydrogen bonding to substrates such as carbonyl compounds, imines, etc., results in electrophilic activation toward nucleophilic attack (Scheme 3.1). Thus, hydrogen bonding represents a third mode of electrophihc activation, besides substrate coordination to, for example, a metal-based Lewis acid or iminium ion formation (Scheme 3.1). Typical hydrogen bond donors such as (thio)ureas are therefore often referred to as pseudo-Lewis-acids. ... 

In addition, boron, aluminum, and gallium tris(triduoromethanesulfonates) (tridates), M(OTf)2 and related perduoroalkanesulfonates were found effective for Friedel-Crafts alkylations under mild conditions (200). These Lewis acids behave as pseudo haUdes. Boron tris(tridate) shows the highest catalytic activity among these catalysts. A systematic study of these catalysts in the alkylation of aromatics such as benzene and toluene has been reported (201). 

We wish to emphasise that the formation of esters (E) from alkenes (M) and acids (HA), the catalysis of the reactions of E by HA or MtXn, and the activation of E, such as organic chlorides, by the co-ordination of a Lewis acid, such as A1C13, are all very familiar chapters in conventional organic chemistry. It follows that the pseudo-cationic theory is nothing more than a generalisation of conventional organic-chemical ideas and a revival of some pre-Whitmore interpretations which had become occulted by the usefulness and novelty of the carbenium ion concept. 

In many respects the chlorine oxyfluorides resemble the chlorine fluorides. For example, they exhibit little or no self-ionization, but are amphoteric. With strong Lewis acids or bases they can form stable adducts. The tendency to form adducts was found (64) not to be so much a function of the relative acidity of the parent chlorine oxyfluoride but rather to depend on the structure of the amphoteric molecule and of that of the anion or the cation formed. The preferred structures are the energetically favored tetrahedron and octahedron. Consequently, a trigonal bipyramidal molecule, such as CIF3O (64), exhibits a pronounced tendency to form either a stable pseudotetrahedral cation or a pseudo-octahedral anion ... 

Mechanistically, the reaction initially proceeds through the formation of the zinc aUcox-ide, which then complexes a second equivalent of the reagent, and then undergoes a pseudo-intramolecular cyclopropanation (equation 48). It is therefore implicit that 2 equivalents of zinc are needed in these reactions, since the ethylzinc alkoxide does not form the corresponding iodomethylzinc alkoxide in the presence of iodomethane, and the latter does not cyclopropanate alkenes in the absence of a Lewis acid. 

Bronsted coefficient (Plg) is around 0.7. Thus, Lewis acid activation, metal-hydroxide activation and leaving-group activation could add up to over 1017-fold rate acceleration for DNA hydrolysis. The pseudo-first order rate constant for hydrolysis of a phosphate diester bond of DNA has been estimated to be about 1(T19 s"1 at neutral pH and 25 °C [13]. A 1017-fold rate acceleration for hydrolyzing DNA would reduce the half-life of the phosphate diester bond from billions of years to about a minute or two. 

Trivalent gadolinium with f7 configuration has isotropic distribution of electrons and hence cannot produce pseudo contact shift. However, when the Lewis acid-base interaction is partly covalent, the unpaired electron spin density influences the molecular framework of the base and causes an LIS known as contact shift. Gd(III) is used to ascertain the contributions of contact shift to the measured LIS. 

---