Additives/cosolvents Lewis bases

Medium—The use of various Lewis bases, either as solvent or as component of a predominantly hydrocarbon medium, offers advantages. When trying to improve solubility, even hydrocarbon cosolvent selection is important, keeping in mind that aromatic solvents, especially toluene, can be beneficial. The option of using limited solubility of the desired ArLi to advantage should be considered however, the trade-off may be mixing issues and longer addition times (Scheme 12.2, eq. 3). 

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. 

Since the start of investigations into asymmetric reactions with enolates it has been known that the reactivity and selectivity observed in enolate chemistry is influenced not only by the base employed, but also by the use of cosolvents such as HMPA, and the addition of metal salts or Lewis acids. [2-4, 11] Lithium enolates, in particular, tend to form aggregates by self-assembly. [3, 4] Decisive contributions to the explanation of this phenomenon and its consequences have been made by Seebach et al. by crystal structure analyses of crystalline lithium enolates [12] up to suggestions regarding the structure of the complexes in solution (Scheme 5). [3, 4, 13]... 

Shioiri s group also employed a 2-lithiooxazole to construct the bis-oxazolyl methanol core of 904. Here, lithiation of a silyl-protected 4-oxazolemethanol 905a/ b followed by condensation with 902 afforded a racemic bis-oxazolyl methanol 906a/b in modest yield (Scheme 1.242). Attempts to improve the yield of 906 using a Lewis acid to coordinate the oxazole nitrogen, alternate bases, or addition of cosolvents were not successful. Barton-McCombie radical deoxygenation was used... 

---