Exchange reactions lead enolates

Lead, like its neighbor tin, enjoys both the -t-2 and +4 oxidation state. In this case, Pb(II) is the normal metallic cation and the Pb(IV) state is rather unstable and very often oxidizing. As such, any species that might contain a bonded Pb(IV) and an enolate anion may mimic Pb(II) and an enoxyl cation. Metal-metal exchange reactions of lead enolates show both enolate and enoxyl cation behavior , depending on what is affixed to the metal. This quasi-enolate vs. enoxyl cation dichotomy is also seen for suitable mercury-and thallium-containing species. 

Color reactions Boric acid (hydroxyquinones). Dimethylaminobenzaldehyde (pyrroles). Ferric chloride (enols, phenols). Haloform test. Phenylhydrazine (Porter-Silber reaction). Sulfoacetic acid (Liebermann-Burchard test). Tetranitromethane (unsaturation). Condensation catalysts /3-Alanine. Ammonium acetate (formate). Ammonium nitrate. Benzyltrimethylammonium chloride. Boric acid. Boron trilluoride. Calcium hydride. Cesium fluoride. Glycine. Ion-exchange resins. Lead oxide. Lithium amide. Mercuric cyanide. 3-Methyl-l-ethyl-2-phosphoiene-l-oxlde. 3-Methyl-1-phenyi-3-phoipholene-1-oxide. Oxalic acid. Perchloric acid. Piperidine. Potaiaium r-butoxIde. Potassium fluoride. Potassium... 

Macromolecular transformations are also of scientific and commercial interest. They can be used for the manufacture of new compounds, particularly in cases where no monomer exists (vinyl alcohol as the enolic form of acetaldehyde) or where the monomer polymerizes with difficulty or not at all (e.g., vinyl hydroquinone). In these cases, derivatives such as vinyl acetate or vinyl hydroquinone ester are polymerized and the polymers are then saponified to poly(vinyl alcohol) and poly(vinyl hydroquinone), respectively. Other processes of industrial importance are conversions of inexpensive macromolecular compounds such as cellulose into new materials (cellulose acetate, cellulose nitrate, etc.), manufacture of ion exchange resins, and dyeing with reactive dyestuffs. All of these reactions lead to a definite product. If the degree of polymerization is retained, they are called polymer analog reactions. 

FIGURE 19.71 A high concentration of acetaldehyde results in a relatively rapid aldol reaction and little or no exchange at carhon. If, however, the concentration of acetaldehyde is low, the himolecular reaction will be slowed and the enolate can revert to acetaldehyde. In D2O, this reaction leads to exchange of hydrogens attached to carbon. The deuterium can appear in either or both of the two positions and is shown as (D). 

Certain other metal ions also exhibit catalysis in aqueous solution. Two important criteria are rate of ligand exchange and the acidity of the metal hydrate. Metal hydrates that are too acidic lead to hydrolysis of the silyl enol ether, whereas slow exchange limits the ability of catalysis to compete with other processes. Indium(III) chloride is a borderline catalysts by these criteria, but nevertheless is effective. The optimum solvent is 95 5 isopropanol-water. Under these conditions, the reaction is syn selective, suggesting a cyclic TS.63... 

LA represents Lewis acid in the catalyst, and M represents Bren sled base. In Scheme 8-49, Bronsted base functionality in the hetero-bimetalic chiral catalyst I can deprotonate a ketone to produce the corresponding enolate II, while at the same time the Lewis acid functionality activates an aldehyde to give intermediate III. Intramolecular aldol reaction then proceeds in a chelation-controlled manner to give //-keto metal alkoxide IV. Proton exchange between the metal alkoxide moiety and an aromatic hydroxy proton or an a-proton of a ketone leads to the production of an optically active aldol product and the regeneration of the catalyst I, thus finishing the catalytic cycle. 

These retro-Aldol and -Michael reactions can, obviously, follow an isomerization of the aldose to the corresponding ketose, leading thereby to different Aldol fragments or retro-Michael products. Keto-enol exchange as well as the retro-... 

Scheme 10.14 rationalizes the divergent behavior of the two catalytic systems in these selective transformations of pent-l-yn-ols. The presence of phosphine ligands promotes the formation of ruthenium vinylidene species which are key intermediates in both reactions. The more electron-rich (p-MeOC6Fl4)3P phosphine favors the formation of a cyclic oxacarbene complex which leads to the lactone after attack of the N-hydroxysuccinimide anion on the carbenic carbon. In contrast, the more labile electron-poor (p-FC6H4)3P) phosphine is exchanged with the N-hydroxysuccinimide anion and makes possible the formation of an anionic ruthenium intermediate which liberates the cyclic enol ether after protonation. 

There must never be more ketone in the mixture than base, or exchange of protons between ketone and enolate will lead to equilibration. Kinetic enolate formations with LDA must be done by adding the ketone to the LDA so that there is excess LDA present throughout the reaction. 

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