Ketoenolates

Metal Complex. Complexation gas chromatography was first introduced by V. Schurig in 1980 (118) and employs transition metals (eg, nickel, cobalt, manganese or rhodium) complexed with chiral terpenoid ketoenolate ligands such as 3-ttifluoroacetyl-lR-camphorate (6),... 

Oxygen-Gontaining Organics. Neutral and anionic oxygen-containing organic molecules form complexes with thorium. Recent work has focused on alkoxides (72), aryloxides, and carboxylates however, complexes with alcohols, ethers, esters, ketones, aldehydes, ketoenolates, and carbamates are also well known. 

A synthesis of 2-alkyl-2,3-dihydro-y-pyrones (187) from methoxybutenyne and aldehydes has been described (83TL4551). The condensation of lithiomethoxy-butenyne (184) with aldehydes at -78°C leads to the secondary alcohols 185, which form the dihydropyrones 187 via hydration of the acetylenic bond and hydrolysis of the methoxyethenyl group to the ketoenol 186 (0°C, p-TSA, THF, H2O or 30% HCIO4, 20 min) folowed by intramolecular cycloaddition. 

Metal P-ketoenolate complexes. J. P. Fackler, Prog. Inorg. Chem., 1966, 7, 361-425 (414). 

Although this synthesis is extremely simple, its mechanism is not yet clearly understood. It has been proposed that such reactions can be rationalized348 in terms of 1,4-additions, where the initial attack by a proton occurs at the carbonyl oxygen, giving rise to a very stable carbocation that is not only allylic but also stabilized by the mesomeric donating oxygen. This step is followed by the nucleophilic addition of SnCl3( f and by a ketoenol tautomerism (Scheme 27). 

NMR spectroscopy has been utilized in the quantitative analysis of the racemic mixtures. The important condition is that the proton signals of the components are well separated. Ketoenol tautomerism has been studied in this manner. This is illustrated by acetyl acetone. 

In further support of the notion of electronic tautomerism, Fry cited the 1905 paper by E. C. C. Baly and C. H. Desch in which they described ketoenol tautomerism as the result of the motions of a labile hydrogen atom that functions as a potential ion, "inasmuch as the bond of attraction or Faraday tube of force must be considered to be lengthened sufficiently to allow the interchange of the atom from the one position to the other within the molecule." Thus, the difference between "electrolytes" (electrical molecules) and nonelectrolytes (chemical molecules) has to do with conditions that determine the varying lengths of the Faraday tubes. 125 Baly and Desch s hypothesis was a result of their rumination on Wislicenus s earlier hypothesis (i.e., that the influence of "electrochemical polarization" explains the acidic properties of acetoacetic ester) in combination with thinking about J. J. Thomson s ideas. 126... 

In a mixture of liquid ammonia with alcohol, ketoenols and pinacols are still formed along with secondary alcohols. Process selectivity was enhanced on the basis of mechanistic studies (Rautenstrauch et al. 1981). The initial stages of the reaction include the formation of ketone anion-radicals and their dimerization with a metal cation participation. This dimerization results in pina-col formation as shown in Scheme 7.6. 

To prevent the dimerization and by-product formations, Rautenstrauch et al. (1981) proposed to protonate the ketone anion-radicals just at the moment of their formation. These anion-radicals contain the negatively charged oxygen atom. They protonate faster and then undergo dimerization. The resulting hydroxyl-containing carboradicals accept electrons faster and then undergo disproportionation or recombination. This leads to the suppression of ketoenol and pinacol formation. 

One of the most sophisticated methods is the use of chiral gas chromatographic capillary columns for the direct separation of volatile enantiomers. Complexation gas chromatography with enantioselec-tive transition metal fl-ketoenolates permits the stereochemical analysis of volatile oxygenated compounds in the nanogram range with high 44,45... 

Up to six nitric oxides readily add to ketoenolates when they are treated with strong base in methanol. This sequential addition is stereospecific, generating Z-configured products. The tris(diazene-iV-oxide-iV -hydroxylates) are an intriguing type of molecular propeller (Figure 21) with relatively low energy barriers for rotation of the three N2O2 blades around the C—N bond. 

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