From carboxylic acids tautomers

That such a delicate balance exists between tautomers is not an obvious prediction based solely on ApK s. The 10 pK unit difference in 9 benzoate favors the amidine-carboxylic acid form by 0.6 eV. However, a simple electrostatic calculation [123] for a positive and negative charge at a salt bridge distance of 3.8 A translates into a stabilization energy of-0.50 eV in the solvent THF, which nearly offsets the stabilization of the amidine-carboxylic acid tautomer derived from the ApfQ. Electron-rich carboxylates such as benzoate are sufficiently basic that the amidine-carboxylic acid hydrogen bond interaction prevails while the interface retains its ionic nature for more acidic carboxylic acids and various sulfonic acids. 

Trihydroxypteridine exists predominantly in the dioxo-mono-hydroxy form 191(R = H), its ultraviolet spectrum closely resembling those of both the 1- and the 3-methyl derivatives and that of l,3-dimethyl-7-methoxypteridine-2,4-dione (191, R = Me). These spectra are quite different from those of 8-methyl- (192, R = H) and l,3,8-trimethyl-pteridine-2,4,7-trione (192, R = Me), which are similar to each other and to those of other 8-substituted pteridine-2,4,7-triones. However, the ultraviolet spectrum of 2,4,7-trihydroxypteri-dine does, indeed, show that a small proportion of the trioxo form is present at equilibrium. A somewhat larger proportion of the 6-methyl derivative exists in the trioxo form, although structure 193 predominates. The trioxo form (194) of 2,4,7 trihydroxy-l,3,6-trimethyl-pteridine is the most important tautomer, but the corresponding 6-carboxylic acid exists entirely in the monohydroxy-dioxo form 195. 

The first example of a stable geminal enediol derivative of a carboxylic acid, isolated as its platinum(II) complex, has been reported.324 Treatment of [Pt(OH2)2(en)]2+ with /V,/V-bis(phos-phonomethyl)aminoacetic acid yields zwitterionic [Pt(bpmaa)(en)] (bpmaaH = 7V,7V-bis(phosphono-methyl)aminoacetic add), which upon crystallization from water (pH 1) affords the corresponding enol tautomer (130), as determined by X-ray crystallography and IR spectroscopy. 

The stereochemical consequences of the cyclization of some 3-(2,5-dihydroxyphenyl)pro-pan-l-ols (247) have been investigated, with a view to optimizing the chiral economy of a tocopherol synthesis from (S)-chroman-2-carboxylic acid (81JOC2445). It was observed that acid-catalyzed dehydration occurred with retention of configuration and it was proposed (79JA6710) that the process involved the formation of a hemiketal through nucleophilic attack by the side-chain hydroxy group on the keto tautomer. 

Figure 6.10 The two tautomer forms of a carboxylic acid dimer (upper drawing) and of formamidine H-bonded to two HjO molecules (bottom). One passes from one form to the other one by H-atom transfers.

Scheme 8.102. The formation of furfural (furan 2-carboxaldehyde) from pentoses (typical examples, D-ribose and its proton tautomer D-ribulose, are shown) on treatment with sulfuric acid (Scheme 8.75).The conversion to furan 2-carboxylic acid,furan (oxa-2,4-cyclopentadiene), and tetrahydrofuran (THF, oxacyclopentane, oxolane) follows.

Ni-alkyne bonding consists of contributions from both the 77, 7t- and cr,diyl tautomers. This bonding picture helps visualize the insertion reactions with alkynes, alkenes, and CO that result in the formation of metallacycles. Thanks to such insertion reactions, Ni-alkyne species are active intermediates in a number of catalytic applications such as alkyne oligomerization, carbonylation, and insertion of heterocumulenes such as CS2 and GO2. For example, a recent example of a C02-fixation reaction involved the stoichiometric, alkylative or arylative carboxylation of alkynes to give a,(3- and / ,/ -unsaturated carboxylic acids. Ni(0)-alkyne complexes have also been used as pre-catalysts in the addition of hydrosilanes to alkynes. In most cases, monoalkynes react to give the products of m-addition, whereas diynes produce enynes (1,2-addition), allenes (1,4-addition), or 1,3-butadienes (1,2,3,4-addition). ... 

These oscillatory changes were observed not only for the optically pure enantiomers but also with the racemic 2-phenyl propionic acid sample. The molecular mechanism was attributed to keto-enol tautomerism that is acid catalyzed as well. All the profens are carboxylic acids with relatively well-pronounced electrolytic dissociation. Thus, the keto-enol transenantiomerization of profens in aqueous media was considered justified because of the self-catalytic effect of the protons originating from the dissociated carboxyl groups. Since reducing the temperature stabilizes the short-lived tautomers there was greater amplitude of oscillation at 6°C than at 22- C. 

Further elucidations on the mechanism of alkynes hydration arise from the isolation of both enol and keto tautomers of organometallic intermediates, starting from an alkyne-carboxylic acid ester as tetrolic acid ethyl ester [43]. 

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