Bases and Complexing Agents

The 1 1 complex 5-BuLi/TMEDA is probably the most potent metalating organobthium reagent. With (-)-sparteine (Fig. 26.2), metalation leads to asymmetric complexes allowing the formation of enantiomerically enriched functionalized compounds [21]. 

Nonnucleophibc (sterically hindered) lithium amides can be prepared by the simple reaction of the corresponding amines with -BuLi in nonpolar organic solvents (Fig. 26.3). Lithium amides are more soluble in hydrocarbons than their heavier element congeners (Na, K). LiN(i-Pr)j (LDA) is also cheaper than KN(/-Pr)2 (KDA) and is more widely used. Lithium amides, which have a much lower Lewis acid character than alkyllithiums, also form aggregates in solution [26-28]. They usually react under thermodynamic control according to a classic acid-base mechanism. The p a of diisopropylamine is 36 [8]. Lithium 2,2,6,6-tetramethylpiperidide (LTMP) is slightly more basic 

Reactions of organolithium compounds are generally carried out under argon or dry nitrogen at low to very low temperatures (-20 -90°C). 

The choice of the solvent imposes its straightforward purification from water and peroxides, low-enough freezing point, and its stability against organolithium compounds [7]. Strong bases are not infinitely stable in ethereal solvents. The stability order is hydrocarbons EtjO THF. 

In the laboratory, THF is the solvent of choice for generating metalated aromatic species. Diethyl ether is highly flammable, more toxic, and prone to the formation of peroxides [7]. Aromatic hydrocarbons can be used however, toluene is relatively easily lithiated, and benzene is toxic and cannot be used at low temperatures. The obvious trade-off of using a hydrocarbon as solvent is the lower solubility of starting materials as well as of the organolithium intermediate. The presence of excess 

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Capillary Electrophoresis. Capillary electrophoresis (ce) is an analytical technique that can achieve rapid high resolution separation of water-soluble components present in small sample volumes. The separations are generally based on the principle of electrically driven ions in solution. Selectivity can be varied by the alteration of pH, ionic strength, electrolyte composition, or by incorporation of additives. Typical examples of additives include organic solvents, surfactants (qv), and complexation agents (see Chelating agents). 

Cosolvents and Complexing Agents Many nonvolatile polar substances cannot be dissmved at moderate temperatures in nonpolar fluids such as CO2. Cosolvents (also called entrainers) such as alcohols and acetone have been added to fluids to raise the solvent strength for organic solutes and even metals. The addition of only 2 mol % of the complexing agent tri-n-butyl phosphate (TBP) to CO2 increases the solubility of hydroquinone by a factor of 250 due to Lewis acid-base interactions. 

Caubere, P. Complex Bases and Complex Reducing Agents. New Tools in Organic Synthesis. 73, 49-124(1978). 

Because metal ions bind to and modify the reactivity and structure of enzymes and substrates, a wide spectrum of techniques has been developed to examine the nature of metal ions which serve as templates, redox-active cofactors, Lewis acids/bases, ion-complexing agents, etc. 

The choice of mobile phase is largely empirical but general rules can be formulated. A mixture of an organic solvent and water with the addition of acid, base or complexing agent to optimize the solubility of the components of a mixture can be used. For example, good separations of polar or ionic solutes can be achieved with a mixture of water and n-butanol. Addition of acetic acid to the mixture allows more water to be incorporated and increases the solubility of basic materials, whilst the addition of ammonia... 

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