Dimerization reactions carboxylic acid reagents

Acid chlorides are useful reagents, but when the pyrazole is N- unsubstituted a dimerization occurs and the diketopiperazine (254) is isolated (Section 4.04.2.3.3(x)). However, (254) reacts with many compounds as an acid chloride would, for example with amines to yield amides (67HC(22)l). The difunctional pyrazole derivative (441) affords polymers by reaction with diphenols (69RRC763). Cyanopyrazoles can be hydrolyzed to the corresponding carboxylic acids (68CB829). 

Over the past ten years, absolute rate data have been reported on the kinetics of several bimolecular silene reactions in solution, including both head-to-tail and head-to-head dimerization the [l,2]-addition reactions of nucleophilic reagents such as water, aliphatic alcohols, alkoxysilanes, carboxylic acids and amines and the ene-addition, [2 + 2]-cycloaddition and/or [4 + 2]-cycloaddition of ketones, aldehydes, esters, alkenes, dienes and oxygen. The normal outcomes of these reactions are summarized in Scheme 1. 

The reactions are very fast, and halide ion addition always leads to 77-allylpalladium halide dimers. The cyclopentadienyl moiety has been isolated in the form of cyclopentadiene (reaction with hydrogen chloride), or as a carboxylic acid dimer (reaction with BuLi -f CO2). That part of the attacking reagent bearing positive charge always combines with the C5H5... 

It is likely that the benzenecarboxylic acids are derived from lignin units where the a-carbon of the side-chain has been oxidized to a carboxyl group (52), and different studies have interpreted the structures and distributions of the methyl esters of benzenecarboxylic acids as indicating that these structural units exist as either free or ester-bound structures in the HA 10,16,17,24)- However, it has recently become clear that certain benzenecarboxylic acids are produced from unoxidized lignin moieties by the TM AH reagent in the course of the reaction at elevated temperatures 41), Examination of the products from the TMAH thermochemolysis of a model lignin dimer, free of any carboxylic functionality, showed the production of relatively large amounts of methylated benzenecarboxylic acid derivatives. These authors... 

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