Cesium carboxylates

Especially for large-scale work, esters may be more safely and efficiently prepared by reaction of carboxylate salts with alkyl halides or tosylates. Carboxylate anions are not very reactive nucleophiles so the best results are obtained in polar aprotic solvents45 or with crown ether catalysts.46 The reactivity order for carboxylate salts is Na+ < K+ < Rb+ < Cs+. Cesium carboxylates are especially useful in polar aprotic solvents. The enhanced reactivity of the cesium salts is due to both high solubility and minimal ion pairing with the anion 47 Acetone is a good solvent for reaction of carboxylate anions with alkyl iodides48 Cesium fluoride in DMF is another useful... 

In rare cases, lactone formation can also be achieved by nucleophilic displacement with a cesium carboxylate onto an activated leaving group, thus circumventing the polyol-problem [30]. An alternative to the macrocyclic... 

Carboxylate promoters may also be able to coordinate to an Rh(CO)2 fragment, and therefore facilitate a process such as that shown in (45). Reactions of [Rh15(CO)27]3 with H2 in the presence of cesium carboxylates are reported (123) to be consistent with the formation of small equilibrium amounts of Rh(C0)2(02CR) by reaction (45). Carboxylates could therefore be involved in cluster growth or transformation during catalysis. 

It would be of interest to see if the more stable cesium carboxylates would be better catalysts for GTP than TBA carboxylates since Quirk showed that cesium 9-methylfluorenide works as well as TBA 9-methylfluorenide as a catalyst in his mechanism studies [6]. 

Cesium salts of substituted pyridine-3,5-dicarboxylic acids were used first by Krui-zinga and Kellogg for the synthesis of macrocyclic lactones [51]. Kellogg obtained the bis-lactone 46 in a one-pot reaction of the cesium carboxylate 44 and the dibro-mo compound 45 in 85 % yield without application of high dilution conditions [1]. By comparison with other alkali metal carbonates he proved the yield-increasing effect of the cesium ions ... 

Elucidation of rates with variation in cation, solvent and halide ion indicates that the combination of alkyl iodides and cesium salts in HMPA offers the optimum alkylation conditions for preparative use. The reactions of cesium carboxylates with alkyl halides and sulfonates will be discussed in Section 1.1.2.5 in connection with inversion processes. 

One of the crucial side reactions in the displacement reactions of alkyl halides is elimination, which can generally be neglected in the reaction of primary alkyl halides. On the other hand, a considerable amount of alkene invariably forms in the reaction of secondary alkyl halides. This undesirable side reaction can be minimized by the use of cesium carboxylates. ... 

Inversion of the configuration of an alcohol by cesium carboxylate has also been demonstrated. This Mitsunobu-type reaction is sometimes more convenient than the classical phosphine-based reaction. Although a small amount of the E2 elimination reaction also occurs, carboxylation of the mesylate of the alcohol proceeds perfectly with inversion of configuration (Scheme 2.28) [44]. 

The chief synthetic problem in an approach to (5d) would be the incorporation of the bridge thereby creating a macrocyclic ring. Numerous approaches to bridged pyridines have been described but none met our needs. We therefore developed an independent route. We had been struck by the report that cesium-carboxylates in DMF are powerfully nucleophilic. We were well rewarded on applying this report to commercially available pyridine-3,5-dicarboxylic acid (57), which could be converted into, for example, (55) in 90% yield (eq. 27)... 

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