Carboxylic solvent shifts

The methyl ester (100, R = CH3), derived from this A-nor acid by treatment with diazomethane, is different from the ester (102) obtained either by Favorskii rearrangement of 2a-bromo-5a-cholestan-3-one (101) or by the action of cyanogen azide on 3-methoxy-5a-cholest-2-ene (103) followed by hydrolysis on alumina. The ketene intermediate involved in photolysis of (99) is expected to be hydrated from the less hindered a-side of the molecule to give the 2j -carboxylic acid. The reactions which afford (102) would be expected to afford the 2a-epimer. These configurational assignments are confirmed by deuteriochloroform-benzene solvent shifts in the NMR spectra of esters (100) and (102). ... 

Table III. Ring Solvent Shifts (A5) for Ethyl, l-Alkyl-3-cyano-lli-pyrazole-4-carboxylates...

In contrast to the amides, (see V-C-2) the carboxylic acids act like well-behaved Hammett bases (325) ip indicator studies. We have already seen (II-D) that this is the result of a misleading fortuity and although the relative values for the pKa s of the benzoic acids are meaningful, this is one of several groups of bases for which the real standard state is unknown. In this connection, a recent study of 2,2-diphenic acid shows a log Q vs. Hn plot of slope 0.57 (246). Unfortunately, no attempt was made to correct for the usual medium effects, but one expects that this unusually bad slope is due to activity coefficient problems over and above solvent shifts. 

NMR The H NMR signals for the hydroxyl protons of phenols are often broad and their chemical shift like their acidity lies between alcohols and carboxylic acids The range is 8 4-12 with the exact chemical shift depending on the concentration the solvent and the temperature The phenolic proton m the H NMR spectrum shown for p cresol for example appears at 8 5 1 (Figure 24 4)... 

A mixture of water/pyridine appears to be the solvent of choice to aid carbenium ion formation [246]. In the Hofer-Moest reaction the formation of alcohols is optimized by adding alkali bicarbonates, sulfates [39] or perchlorates. In methanol solution the presence of a small amount of sodium perchlorate shifts the decarboxylation totally to the carbenium ion pathway [31]. The structure of the carboxylate can also support non-Kolbe electrolysis. By comparing the products of the electrolysis of different carboxylates with the ionization potentials of the corresponding radicals one can draw the conclusion that alkyl radicals with gas phase ionization potentials smaller than 8 e V should be oxidized to carbenium ions [8 c] in the course of Kolbe electrolysis. This gives some indication in which cases preferential carbenium ion formation or radical dimerization is to be expected. Thus a-alkyl, cycloalkyl [, ... 

If one of the species is anionic and we need to transport it to the organic phase, then a phase-transfer catalyst may be employed. Consider the example of benzyl penicillin where the reaction between phenyl acetic acid and the penicillin carboxylate ion, with penicillin amidase as a catalyst, is relevant, and which at pH 4.5 - 5.0 is shifted in the desired direction. Here a catalyst like tetrabutylammonium halide works, and with chloroform as a solvent 60% yield can be realized in contrast to a yield of only 5 - 10 % in water. 

The equilibrium in this first step favors tlie shift to anhydride formation if the second mole of carboxylic acid in the second step with forms imidazole to form a salt that is insoluble in the solvent used (ether, tetrahydrofuran, benzene). 

The drum compounds are thermally quite stable. For example, the cyclopentane carboxylic acid drum, 3, was heated at 300°C for 3 h in vacuum. The material obtained was soluble in CDCI3, and Sn NMR showed a single line at -A91.A ppm, compared to the starting material -A85.8. The shift is presumably due to loss of solvent molecules present in the crystal in the starting material. 

The red shift of 3-amino-phthalate fluorescence when the solvent is changed from water to DMSO or another aprotic solvent is due to different hydrogen bonding above all, however, it is due to proton transfer from the amino group of the excited AP<—> to the neighboring carboxylate yielding the species (-)AP(-), so that the emitters are 51 a in water and 52 a in DMSO 1U> ... 

Column III shows the effect of ultrasound upon the product ratio with methanol as solvent. As can be seen there is now 53 % bibenzyl, 32 % of methyl ether and 6% of methyl ester (with a total of 5 % of other products) suggesting a slight shift towards the two-electron products, but with an overall diminuition of solvent discharge (approx. 6% ester) and side-reactions (approx. 6%). This result confirms the fact the phenyl acetate electrooxidation favours the one-electron route (to bibenzyl) in a wide range of conditions [61], and is much less sensitive to mechanistic switches by manipulation of parameters (e. g. ultrasound) than is cyclohexane carboxylate electrooxidation [54]. 

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