Enol carboxylates

Tapia, O., Andres, J. and Safont, V. S. Enzyme catalysis and transition structures in vacuo. Transition structures for the enolization, carboxylation and oxygenation reactions in ribulose- 1,5-bisphosphate carboxylase/oxygenase enzyme (Rubisco), J.Chem.Soc.Faraday Trans., 90 (1994), 2365-2374... 

Apart from the diols examined so far there are other bis-O nucleophiles that also react with carbonyl compounds following the mechanisms discussed, as illustrated by Figure 9.21. The result is acetal analogs such as the compounds D (from hydroxy carboxylic acid A), E (from enol carboxylic acid wo-B) or F ( Meldrum s acid from malonic acid). Each of these acetal analogs is used as a reagent in organic synthesis. 

Fig. 13.10. Formation of carboxylate enolates ("carboxylic acid dianions") by reaction of carboxylic acid salts with strong bases.

In the lithium and cesium enolates of o-methoxyacetophenone, the methoxy oxygen coordinates with the smaller lithium cation but not with the cesium cation . Other examples of lithium enolate chemistry include a thermochemical analysis of the aldol reaction of lithiopinacolonate with pivalaldehyde and a comparison of the proton affinities and aggregation states of lithium alkoxides, phenolates, enolates, -dicarbonyl enolates, carboxylates and amidates. Although the lithium enolate of cyclopropanone itself remains unknown, derivatives (accompanied by their aUenoxide isomer) have been implicated in the reaction of a-(trimethylsilyl) vinyl lithium with CO. That both species are seemingly formed is surprising because cyclopropanone enolate is expected to be much less stable than its acyclic isomer cyclopropene is less stable than allene by almost 90 kJmol-. ... 

Enol Ethers, Enol Carboxylates, Enol Phosphates and Enol Sulfonates... 

Diketene, a cyclic enol carboxylate, reacts with Grignard reagents in the presence of Ni or Co salts or complexes to form 3-methylenealkanoic acids in addition to methyl, trimethylsilylmethyl (equation 62) or benzyl groups and p-H-bearing primary alkyl groups can be introduced if NiCkfDPPP) or Col (equation 63) are used as a catalyst at low temperatures. 

Transacylation. This compound, (i-PrO) 13Y5O, catalyzes acyl transfer from enol carboxylates to alcohols, and in some cases (e.g., piperidine-2-methanol), the selective O-acylation of amino alcohols. 

Condensation of alkynes with carboxylic acids. Addition of RC(X)H to alky-nes forms enol carboxylates. On the other hand, heterocyclization occurs with acetylenedicarboxylic esters. ... 

Heck-type reactions with enol carboxylates (e.g., vinyl acetate) are generally complex. Most common are reactions in which vinyl acetate is employed as an ethylene equivalent (see Scheme 24). However, an example of a preparatively useful reaction with an intact acetate function is given in entry 44.The reaction of vinyl triflates with vinyl phosphonates affords the corresponding conjugate dienylphosphonates (entry 45).f A new access to reactive nonaflates via a one-pot nonaflation-Heck reaction was recently reported (entry 46). " This reaction sequence starts from silyl enol ethers and provides functionalized 1,3-dienes in a simple manner, lodonium salts can be used as RPd precursors (entry 47). It is notable that the palladium(O) insertion preferentially occurs inbetween the iodonium ion and the vinylic, rather than the arylic sp -hybridized carbon (entry 47). Some years ago, Jeffery used acetylenic halides to achieve (JiJ-enynoates and (Ji)-enynones in fair yields at room temperature (entry 48). ... 

Aspects of the rhodium-catalysed hydroformylation of olefins have been reviewed. " Copper(ii) acetate catalyses the highly stereoselective solvolysis of -alkenylpentafluorosilicates to -alkenyl ethers under an atmosphere of air. Since the pentafluorosilicates can be obtained via hydrosilylation of acetylenes, the sequence represents a regio- and stereo-selective transformation of acetylenes into carbonyl precursors in moderate yield. The reaction of vinylmercurials and mercury carboxylates catalysed by palladium(ii) acetate provides a stereospecific route to enol carboxylates, which are valuable precursors of specific enolates. ... 

The preparation of enol esters in a regio- and stereo-specific manner is important, as they are particularly valuable synthetic intermediates. A room-temperature reaction of vinylmercurials, mercury carboxylates, and a catalytic amount of palladium acetate provides a novel and convenient stereospecific route to a variety of enol carboxylates. ... 

Variation of the electronic and steric nature of the ruthenium catalysts allow the complementary carboxylative cyclization of 1,6-diynes (Scheme 36) [150]. Lee and coworkers described how a variety of carboxylic acids condense with 1,6-terminal diynes in the presence of catalytic amounts of [Ru(p-cymene)Cl2l2. P(4-F-C6H5)3 and 4-dimethylaminopyridine to give cyclohexylidene enol carboxylates with exclusive ( )-selectivity. The proposed mechanism involves the initial formation of a ruthenium vinylidene species I followed by intramolecular cyclization induced by the nucleophilic attack of the carboxylate anion to afford a vinylmthenium species II. Final protonolysis furnished the product and turns the catalyst over. 

Scheme 36 Ru-catalyzed carboxylative cyclization of 1,6-diynes to cyclohexylidene enol carboxylates...

Stannyl enol ethers are prepared typically from the corresponding metal enolates and tin electrophiles. A convenient protocol is the reaction of enol carboxylate with tin alkoxide (Scheme 3-190). In contrast to silyl enol ethers, stannyl enol ethers usually exist as mixtures of C-Sn (ketone) and O-Sn (enol ether) forms. For example, acetone and carboxylate esters exist mostly as a C-Sn (ketone) form. In general, the 0-Sn (enol ether) tautomers are more nucleophilic to react with electrophiles faster than the C-Sn (ketone) tautomers. 

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