Carboxylic acid general features

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

The kinetics of reactions between aroylpyruvic acids, ArC0CH2C0C02H, and arylamines in toluene show evidence of several mechanistic features intramolecular carboxyl catalysis, and catalysis by a second molecule of nucleophile, either on its own, or in concert with an (external) carboxylic acid. An extended solvent study shows an increase in the efficiency of the aforementioned intramolecular carboxyl catalysis with decreasing polarity of the solvent.Hydrolysis of the related /3-keto esters, methyl 4-aryl-2-arylamino-4-oxobut-2-enoates [ArC0CH=C(NHAr)C02Me] in aqueous dioxane is subject to general acid catalysis. " ... 

Initially progress in the direct investigation of tetrahedral intermediates derived from carboxylic acids was made by studying species of general structure [2] which possess some special structural feature which makes them particularly stable with respect to the carbonyl derivative to which they are related. Four structural features may be identified which favour the tetrahedral addition structure [2] over the carbonyl form ... 

As in the case of dipeptide esters in solution, the rate of piperazine-2,5-dione formation in SPPS strongly depends on steric and chiral features of the original dipeptide structure.11501 The rate of the cyclization increases upon catalysis by weak carboxylic acids.11761 However, it was also reported that piperazine-2,5-diones are formed under basic conditions, e.g. removal of the Fmoc protecting group. Formation of piperazine-2,5-diones is generally overcome by... 

Cyclizations conducted by the thiohydroxamate method are even less common than those conducted by the fragmentation method, but there is every reason to believe that thiohydroxamate cyclizations should be generally applicable.119 Precursors are formed from carboxylic acids, and require the inclusion of an extra carbon since decarboxylation occurs. This feature may be very useful when normal precursors containing carbon-heteroatom bonds are unstable or difficult to prepare. The thiohydroxamate... 

The extremely stable and general sp conformation of esters and carboxylic acids (3a) is one of the remarkable features of stereochemistry. It was observed uniformly2 in all other molecules with the group O—C=0 and also with their heteroatom analogues with S, Se or Te. In contrast with the rigid conformation, the conjugation in the ester molecule, expressed by the formulas 69, is relatively weak and can be just detected in the dipole moment values164. 

It is obvious from the above results that adsorption of acetic acid, and, of course, presumably other carboxylic acids, is different in detail from one metal oxide to another and is perhaps also somewhat a function of whether adsorption occurs from gas or solution phase. However, in all cases acetate ions are formed and differences presumably reflect more subtle features of surface structure and chemistry. In general, there seems to be a correspondence between the frequencies reported by IR and IETS for IR active modes although intensity patterns are not similar, as one should expect based on the different mechanisms of vibrational excitation. Further work is obviously needed to define the differences between the two spectroscopies more exactly. 

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