Cesium enolates

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-. ... 

Metal enolates are expected to have individual selectivities depending on the character of metal atom. The structure of cesium enolate, and its reaction, have been well studied kinetically and theoretically by Streitwieser [46]. 

Davis and coworkers carried out a Wittig-cyclization sequence with the partially protected 2-deoxyamino sugar 191 using an amino acid based phosphonate. Reaction of 191 with the cesium enolate 192 gave a 53% yield of epimers 193 and 194 (1 1 ratio) in which epimerization of the 2-amino group had taken place. The maimo isomer 194 was then epimerized to the more stable gluco 193 derivative by simple treatment with t-BuOLi in methanol (Scheme 38) [54]. 

Krom, J.A. and Streitwieser, A. (1996) Cesium ion pair acidities of some iVW-dialkylacetamides and aggregation of their cesium enolates in tetrahydrofuran. Journal of Organic Chemistry, 61,6354-6359. 

Both enol acetates and enols are fluormated by cesium fluoroxysulfate at room temperature in either acetonitnie or methylene chloride solutions (Table 2, entries 1,2 and 6)... 

The enolate of dimethyl trifluoromethylmalonate, formed by the action of cesium fluonde [II9] or of an electrolytically generated pyrrolidone anion [120], can be alkylated with methyl iodide (equation 103)... 

The 1,4-reduction of a,/3-unsaturated aldehydes is best carried out with diphenylsilane in the presence of zinc chloride and tetrakis(triphenylphosphine) palladium436 or a combination of triethylsilane and tris(triphenylphosphine) chlororhodium 437 Other practical approaches use phenylsilane with nickel (0) and triphenylphosphine438 and diphenylsilane with cesium fluoride.83 It is possible to isolate the initial silyl enol ether intermediate from the 1,4-hydrosilylation of o, /3-unsaturated aldehydes (Eq. 264).73,411 The silyl enol ethers are produced as a mixture of E and Z isomers. 

Cesium fluoroxysulfate reacts with ketones, diketones and enol acetates to give monofluoro products,27 28 e.g. formation of 22, 23 and 24. but barbituric acid is converted into 5.5-di-fluorobarbituric acid.29... 

Enol sulfonates. Sulfonylation of enolate anions with 1 is markedly affected by the gegenion. Lithium enolates undergo mainly C-sulfonylation. Cesium or quaternary ammonium enolates undergo regioselective O-sulfonylation. The same behavior is observed with nonailuorobutanesulfonyl fluoride.1... 

In a related case, Maier and coworker [153] have recently also demonstrated that vinylogous enolates can enter into a cross-coupling step. However, the final products of Pd-catalyzed reactions of the hexahydro naph-thalenone and aryl bromides or iodides in the presence of cesium carbonate and tetraalkylammonium bromide in DMF at room temperature are 7-aryltetralones 208 (Scheme 85). This process can be readily interpreted as a Pd-catalyzed /-selective arylation of the thermodynamic enone enolate with a concomitant dehydrogenation-aromatization of the initial crosscoupling product. 

Ion-pair acidity is an important parameter to probe highly diluted solutions, as demonstrated by the sophisticated visible or near-UV absorption methods developed mainly by Streitwieser and coworkers. This analytical approach afforded the pXa values and aggregation states of a large set of cesium and lithium enolates. Its principle relies... 

Interestingly, no correlation could be observed from their monomer ion-pair acidities (pAT0 in THF) and the second-order rate constant for the monomer in their reaction with m-chlorobenzyl bromide (Table 2, right), a linear relationship occurs when the corresponding cesium salts are alkylated with methyl tosylate. On the other hand according to the authors, this accounts for the fact that the lithium cation is as important as the basicity of the enolate. 

Imidazole-containing compounds have been utilized as reagents for various synthetic transformations. A convenient access to substituted allyl enol carbonates was established through the reaction of ketone enolates with the complex of allyl l//-imidazole-l-carboxylates 74 and boron trifluoride etherate <07JOC9372>. Relatively mild and highly efficient Cul-catalyzed /V-arylation procedures for imidazoles with aryl and heteroaryl bromides or chlorides have been developed in the presence of ligands 75 and cesium carbonate <07JOC2737>. a, -Unsaturated 2-acyl imidazoles 76 are an alternative and practical class of dienophiles for the DNA-based catalytic asymmetric Diels-Alder reaction in... 

Lithium enolates have been actively investigated, including reaction rates and equilibria. Sodium enolates, more ionic than their lithium analogs, more covalent than their heavier alkali metal counterparts, have been studied less while potassium appears frequently. Rubidium enolates have been almost totally ignored. Cesium and lithium enolate chemistry are often compared. We know of no francium enolate chemistry. 

---