Dimeric carboxylate derivatives

Earlier attempted interpretations of the hydrogen bond with the help of resonance or delocalization forces, e.g. in the case of the dimeric carboxylic acids (Illa-d) or, in particular, of substances containing intramolecular hydrogen bonds such as o-nitrosophenol (IVa-d), were shown to be untenable by unambiguous experimental evidence. Thus, in the dimer carboxylic acids the proton is not in the centre of the 0. O distance and the C=0 and C—OH distances are not identical [7], and derivatives of ortho-hydroxyazobenzene and of ortho-nitrosophenol have been shown to exist as solvent-dependent tautomeric equilibria (II), in spite of the presence of the internal hydrogen bond in both tautomers [3, 4]. 

Pyridine is not polarographically reducible in aqueous solvents but in aprotic media, such as acetonitrile,211 DMF,212 or liquid ammonia,213 it is reduced at rather negative potentials to the anion-radical, which then dimerizes. Some electron-attracting substituents, notably carboxyl derivatives, render the nucleus reducible even in aqueous solvents. Quaternary derivatives are generally reducible. JV-Alkylpyridinium ions thus give a free radical, which dimerizes rapidly the radical was trapped by a-phenyl-Af-tert-butylnitrone.214 1,3-Dimethylpyridinium ion is reduced in buffered aqueous medium to a 4,4 -dimer, which undergoes further chemical reaction, possibly an addition of water to one of the double bonds of the 1,4-dihydropyridine rings.215... 

The tetracarboxylatodimolybdenum dimers that result from the reaction of molybdenum hexacarbonyl with a wide variety of carboxylic acids are the initial source of quadruply bound molybdenum units for many further chemical investigations. The initial isolation of Mo2(02CC6H5)4 (2), Mo2(02CCH3)4 (14), and the higher alkyl carboxylate analogues (235) by Wilkinson et al. established a reactivity pattern that has been successfully exploited to prepare many related molybdenum dimers. In addition to those carboxylates enumerated above, a series of substituted aromatic carboxylate derivatives has been prepared by refluxing the appropriate acid with molybdenum hexacarbonyl in diglyme (155). 

The same group also showed that mono(cyclopentadienyl) mixed hydride/ aryloxide dimer complexes of several lanthanide elements (Y, Dy, Lu) could be synthesized easily by the acid-base reaction between the mixed hydride/alkyl complexes and an aryl alcohol [144]. These complexes reacted with C02 to generate mixed formate/carboxylate derivatives, which were moderately active initiators for the copolymerization of C02 and cyclohexene oxide, without requiring a co-catalyst. The lutetium derivative 21 was the most active (at 110°C, TOF = 9.4 h ), yet despite a good selectivity (99% carbonate linkages), the molecular weight distribution remained broad (6.15) (Table 6). 

The long-chain alkanoic acids and their derivatives are polymorphic with the unit cell containing dimers formed by hydrogen bonding between carboxyl groups. 

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). 

Condensation of -pyrroline with pyrrole readily affords 2-(2-pyrro-lidyl)pyrrole (82). The dimerizations of some derivatives of A -piperideine, e.g., zl -pyrroiine and -piperideine-2-carboxylic acids, take a similar course (301). 

R2Sn(rV)] derivatives of N-Bz-Gly-Gly and N-Bz-Gly-Gly-Gly were found to involve both dicarboxylate binding to yield hexacoordinated Sn centers, and dimeric tetraorganodistannoxanes in which the carboxylates bind... 

Kolbe electrolysis is a powerful method of generating radicals for synthetic applications. These radicals can combine to symmetrical dimers (chap 4), to unsymmetrical coupling products (chap 5), or can be added to double bonds (chap 6) (Eq. 1, path a). The reaction is performed in the laboratory and in the technical scale. Depending on the reaction conditions (electrode material, pH of the electrolyte, current density, additives) and structural parameters of the carboxylates, the intermediate radical can be further oxidized to a carbocation (Eq. 1, path b). The cation can rearrange, undergo fragmentation and subsequently solvolyse or eliminate to products. This path is frequently called non-Kolbe electrolysis. In this way radical and carbenium-ion derived products can be obtained from a wide variety of carboxylic acids. 

More recent B-NMR studies at room temperature have shown that (acyloxy)diethylboranes are monomeric also in non-polar solvents like chloroform [(5( B) - 60 ppm] [46]. Under the same conditions the corresponding 9-BBN derivatives are monomeric (both types I and II) or dimeric, depending on the electron-donating or -withdrawing effect of the substituent on the carboxyl group, the temperature, and the concentration of the... 

Amides are derivatives of carboxylic acids, so that their coordination behavior to boranes might be similar to that of their parent compounds. B-NMR spectroscopic studies have shown that compounds 31 and 32 are monomeric species in solution, while compounds 33 and 34 with the more Lewis acidic 9-borabicyclo[3.3.1]nonyl unit form aggregates that may be dimeric, oligomeric, or polymeric. The grade of association could not be determined by mass spectrometric analyses, because in all cases only the monomer is liberated into the gas phase [65]. 

The acid strengths of a series of phosphonic acid derivatives in a variety of solvents have also been used to estimate Hammett constants. In contrast to carboxylic acids, the phosphonic acids are stronger in ketonic solvents than in hydroxylic solvents, which may be attributed to the dissociation of phosphonic acids without the necessity to disrupt the dimeric nature of the acid (see Scheme 3). 

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