Pearson’s hard-soft acid-base theory

The utilization of Pearson s hard-soft acid-base theory to interpret the reactions of lignin has also been described in work from the Soviet Union by Zarubin and Kirysun (59). Beyond this specifically related paper, researchers in the Soviet Union have been quite active in the application of numerical methods to lignin-related problems (60,61). 

Woerpel and Nevarez demonstrated the synthetic potential of silaaziridines by selective insertion reactions (Scheme 7.52).123 Silver-catalyzed aldehyde insertion into the Si-N bond of 169b produced the N,0-cyclic acetal 180 as the cis isomer. In contrast to this process, insertion of tert-butyl isocyanide occurred into the weaker C-Si bond to afford imine 181. The authors rationalized the chemoselectivity for these two processes on the basis of Pearson s hard-soft acid-base theory 124-126 the more ionic Si-N bond reacted with harder benzaldehyde electrophile, whereas the more covalent Si-C bond reacted with the softer isocyanide. 

Formation constants for many metal complexes have been compiled by Ramunas Motekaitis and Art Martell, and these as well as techniques for measuring them in the laboratory will be covered in Chapters 3 and 8. One can, however, predict the relative stability of a desired complex based on simple bonding theories. Crystal field theory, as well as the Irving-Williams series and Pearson s hard-soft-acid-base theory (see the next section) enable us to predict what might happen in solution. 

Drago [Dr 73] considered how his concept could be correlated with Pearson s hard-soft acid-base theory [Pe 63b, Kl 68]. Setting out from the assumption that the terms E Eg and C Cg may serve as measures of the electrostatic and covalent interactions, respectively, and identifying the hard and soft interactions of the Pearson concept with the electrostatic and the covalent interactions, respectively, Drago wished to employ the C/E ratios for the acids or bases examined as a measure of the soft nature of the molecules. He considered that the higher the value of C/E, the softer is the given acid or base, whereas low value of C/E is indicative of a hard character. 

Further examination of the results indicated that by invocation of Pearson s Hard-Soft Acid-Base (HSAB) theory (57), the results are consistent with experimental observation. According to Pearson s theory, which has been generalized to include nucleophiles (bases) and electrophiles (acids), interactions between hard reactants are proposed to be dependent on coulombic attraction. The combination of soft reactants, however, is thought to be due to overlap of the lowest unoccupied molecular orbital (LUMO) of the electrophile and the highest occupied molecular orbital (HOMO) of the nucleophile, the so-called frontier molecular orbitals. It was found that, compared to all other positions in the quinone methide, the alpha carbon had the greatest LUMO electron density. It appears, therefore, that the frontier molecular orbital interactions are overriding the unfavorable coulombic conditions. This interpretation also supports the preferential reaction of the sulfhydryl ion over the hydroxide ion in kraft pulping. In comparison to the hydroxide ion, the sulfhydryl is relatively soft, and in Pearson s theory, soft reactants will bond preferentially to soft reactants, while hard acids will favorably combine with hard bases. Since the alpha position is the softest in the entire molecule, as evidenced by the LUMO density, the softer sulfhydryl ion would be more likely to attack this position than the hydroxide. 

The extent to which an atom or molecule s charge distribution is affected by an external electric field E (which could be due to an approaching reactant) is governed, to first order, by its polarizability a. It was really a to which Pearson was referring in his hard and soft acid-base theory, which rationalizes a large number of chemical reactions. The terms hard and soft refer, respectively, to low and high polarizability. 

According to Yatsimirskii, group (2) and (3) species are equivalent to Pearson s hard acids and bases, and group (4), (5) and (6) species correspond to Pearson s soft acids and bases. This classification is of more value than HSAB theory to our subject. It can be seen that all cementforming anions come from group (3) and cations from groups (3), (4) and (5). Thus, the bonding in cement matrices formed from cation-anion combinations is not purely a but contains some n character. 

W. B. Jensen, The Lewis Acid-Base Concepts An Overview, Wiley-Interscience, New York, 1980, and H. L Finston and Allen C. Rychtman, A New View of Current Acid-Base Theories, John Wiley Sons, New York, 1982, provide good overviews of the history of acid-base theories and critical discussions of the different theories. R. G. Pearson s Hard and Soft Acids and Bases, Dowden, Hutchinson, Ross, Stroudsburg, PA, 1973, is a review by one of the leading exponents of HSAB. For other viewpoints, the references provided in this chapter should be consulted. 

According Pearson s Hard and Soft Acid and Base Theory, oxygen is a hard donor atom, sulfur is a soft donor atom and nitrogen is intermediate (14,15), To probe the influence of replacing one or two oxygen atoms in lipophilic crown ethers 5, 6 and 8 upon metal perchlorate transport, the lipophilic macrocyclic carriers 9-16 (Figure 3) were investigated. Macrocycles 9 and 10 are lipophilic diaza-15-crown-5 compounds and 11,14 and 15 are diaza-18-crown-6 derivatives. Macrocyle 16 is a lipophilic monothia-15-crown-5 compound and macrocycles 12 and 13 are dithia-15-crown-5 and -18-crown-6 compounds, respectively. 

Priebe and coworkers [107,178] have attempted to rationalize the product distribution in terms of Pearson s theory of hard and soft acids and bases (HSAB) [179], concluding as a broad generalization that soft bases (S-, N- and C-nucleophiles) form bonds at the softer C-3 electrophilic center, whereas hard bases (O-based nucleophiles) react preferentially at the harder C-l center to give glycosides. They acknowledge that other factors may overrule this interpretation, such as when C-nucleophiles give kinetic C-l-alkylated products whose formation is not reversible. 

Although several research groups have been interested in transition metal enolates to use the metal centre as a potential site of asymmetry in the design of chiral catalysts, examples of well defined redox reaction involving middle to late transition elements and lanthanides are scarce in the literatnre. Based on Pearson s theory of hard and soft acids and bases", it has been proposed that combining a hard ligand with a soft late transition metal centre may lead to weak metal-heteroatom links, resnlting in reactive late metal-heteroatom bonds. 

Particularly relevant to the present crmtext is the fact that the olefinic double bond is considered as a soft base in Pearson s theory, while many Lewis acids used in cationic polymerisation (BF3, BCI3, AICI3, etc.) are classed as hard acids. Obviously, n-acceptors like chloranil or tetracyanoethylene are considered as soft acids. Thus, the interactions between Lewis acids and olefins must be considered as very weak in the context of the HSAB theory. This prediction is well substantiated by the tenuous character of the complexes observed in experimental studies (see Chap. IV). On the other hand, carbenium ions are usually placed at the borderline between hard and soft acids and are definitely softer than the Lewis acids mentioned above. Consequently, their interactions with olefins must be rather strong, which suggests that that propagation in cationic polymerisations promoted by Lewis acids should be faster than initiation. 

It has been previously pointed out that the tribological conditions used in this work are milder than those typically reported in the literature [23]. Under very severe conditions the formation of a bi-layered tribofilm, consisting of polyphosphates overlying mixed short chain phosphates, has been predicted, according to a model of the mechanism of formation of ZnDTP trihofilms on steel proposed by Martin and based on the Pearson s theory of hard and soft acids and bases (HSAB) [21]. The same model predicts the formation of short-chain iron/zinc phosphates under less severe conditions, such as those used in this work. 

A plausible explanation of the preference for 1,2- or 1,4-addition or -reduction processes of a/S-unsaturated carbonyl compounds has been advanced, based on Pearson s theory of hard and soft acids and bases, the 4-carbon being considered softer than the 2-carbon rational variation of reagent then permits alteration of this preference. Whereas the catalysed addition of trialkylsilanes to ketones gives silyl ethers, a/5-unsaturated ketones react by a process of 1,4-addition to give silyl enol ethers (Scheme 103) only conjugated double bonds are affected the potential utility of... 

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