Lewis acid-bases molecular addition complexes

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

Although surface organometallic chemistry is still in its infancy, there are already several examples of surface reactions leading to well-defined surface complexes (Table l-I). It appears that these reactions obey the same principles as those encountered in molecular chemistry nucleophilic attack at the ligands, electrophilic attack of the metal-carbon bond, oxidative addition, Lewis acid-base adduct formation, redox reactions, disproportionation, and the cooperative effect of dual acid-base sites in an insertion reaction. 

The mechanisms for metal-catalyzed and organocatalyzed direct aldol addition reactions differ one from another, and resemble the mode of action of the type 11 and type I aldolases, respectively. Some metal-ligand complexes, for example, 1-4 and 9 are considered to have a bifunctional character [22], embodying within the same molecular frame a Lewis acidic site and a Bronsted basic site. Whereas base would be required to form the transient enolate species as an active form of the carbonyl donor, the Lewis acid site would coordinate the acceptor aldehyde carbonyl, increasing its electrophilicity. By this means, both transition state stabilization and substrates preorganization would be provided (see Scheme 5 for a proposal). 

MuUiken (140) has given a simple quantum-mechanical model of molecular complexes involving acids and bases (acceptors and donors) and points out that a loose complex of A and B should attract an additional A or B molecule additively. He gives a mechanism by which a weak Lewis acid like HCl may be transformed into a functioning proton... 

When a prochiral ( )-enolate is selectively (Si)-facially protonated, the result is the (H)-enantiomer. (Jle)-Facial protonation leads to the (S)-enantiomer. From the (Z)-enolate, the direct opposite is obtained. If it is not possible to control the ( )/(Z)-configuration of the enolate, in order to obtain good selectivity, one needs then an enantiomericaUy pure acid, whose protonation preference is dependent on the enolate configuration, i.e. for example, it transfers a proton (Si)-facially to the ( )-enolate, but (Re)-fadally to the (Z)-enolate. In many successful cases the enantiomericaUy pure acid is bonded to the metal of the enolate therefore, at the same time it acts also as a Lewis base. In addition, at least from a theoretical point ofview, enantioselective inter- and intra-molecular protonations with achiral acids are conceivable, in which another ligand of the enolate complex is enantionmericaUypure. 

A completely different method applied to a similar process is illustrated in Figure 4.73. The bis-Lewis acid l,8-bis(diethylgallanylethynyl)anthracene forms a dimer in solution, but it is decomplexed by pyrimidine, a bis-Lewis base. The DOSY method (Section 4.13.4) can measure the diffusion coefficients of NMR signals and thus we ean determine the hydrodynamic radius or size of the species in solution by carefiil comparison with reference substances. Upon addition of small amounts of pyrimidine the diffusion coefficient is decreased, i.e. the molecular size increased, then by addition of more pyrimidine the diffusion coefficient becomes drastically higher, indicating the formation of a smaller species, the 1 1 complex. With an excess of pyrimidine two molecules of base can be added to the bis-Lewis acid, which increases molecular size and reduces the diffusion coefficient. 

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