Prototropic equilibrium

About 90% of all the studies on tautomerism refer to the determination of prototropic equilibrium constants in solution. Most probably in the next years there will be further studies in the remaining areas, some of them almost a desert at the present time. 

The data obtained from all the various physicochemical methods point to the predominance of the H tautomer 12a in the prototropic equilibrium over its 2H counterpart (Scheme 7). 

In the prototropic equilibrium, 3-hydroxybenzisoxazole exists exclusively as hydroxy tautomer 136a (Scheme 51) [76AHC(S1), p. 310]. 

It is worth noting, however, that the prototropic equilibrium between the N-nitrosoamine (3.7) and the diazohydroxide (3.9) has been determined semiquan-titatively for the analogous diazotization of an aliphatic amine. Fishbein and coworkers (Hovinen et al., 1992) determined an upper limit for the nitrosoamine equilibrium concentration (<1.5% see also Zollinger, 1995, Sec. 7.2). 

Imino-3p-nitrophenyl-4-phenyl-A4-selenazoiine, preparation, 229 2-Iminoseienazolidin-4-ones, benzoylation, on imino nitrogen, 270 preparation, 269 table of products, 263 2-Iminoseienazolines, prototropic equilibrium, U.V. spectra, 231 Indolenine. 2-alkylmercapto, in synthesis of monomethine cyanine bases,... 

Abstract This chapter is devoted to phosphinous amides, a particular class of tervalent aminophosphanes. First, attention is focused on their stability and synthetic procedures. Reports dealing with their prototropic equilibrium and main group chemistry are also considered. Last but not least the really important applications of these species as metal ligands in the field of catalysis are reviewed, including asymmetric variants. 

This prototropic equilibrium has been also studied in substrates bearing metals linked to the nitrogen atom in the form of CP2MCI or CpMCl2 groups [76-78], and the influence of different ligands at the metal center on the resulting equilibrium between the kinetically favored NH phosphinous amide and the thermodynamically stable tautomeric PH phosphazene form has been discussed (Scheme 11). 

Figure 5.14. Predominant fluorescent species involved in the prototropic equilibrium of SNAFL and SNARF indicators.

The reaction is thought to proceed via an excited charge transfer complex to a thermal and prototropic equilibrium mixture of cycloadducts 6 and 7, wliich decompose to the products shown. 

A fractional order in the nitrite anion was found. This differs from the observed behavior in a benzene series where it is second order. The explanation, according to the Scheme 69, is based on the presence of a prototropic equilibrium between the diazonitrite form 241 and the conjugate base 242. The nitro derivatives 243 were mainly formed from the diazonitrite 241 because the equilibrium 241 242 is shifted towards the undissociated form, as shown by the calculated value of the quantity... 

A-(Monosubstituted amino)pyridiniums (972) are in prototropic equilibrium with A-imides (973) and dications (971). For R = H or allyl, the A-imides (973) are very strong bases and cannot usually be isolated if R is an electron-withdrawing group (e.g. acyl, sulfonyl, nitro), then the imide (973) is less basic and more stable. The cations (971) are only obtained in very strongly acid media. 

Treatment of a 5-methoxy-2-furyl carbinol (106) with zinc chloride in acetone-water converts it into two products, the 4-ylidenebutenolide (111) and the 4-oxo-2-enoic acid methyl ester (112) in approximately 3 1 ratio (80T3071). The key step in this conversion is the formation of carbonium ion (107) which subsequently adds water to furnish (108). Intermediate (108) is in prototropic equilibrium between (109) and (110). The latter then break down to furnish the final products (Scheme 26). The overall scheme provides a new synthetic route to the 4-ylidenebutenolides, a class of compound that includes many natural substances of biological importance. 

Perhydropyrido[ 1,2-a]pyrimidin-2-one 305 exhibits prototropic equilibrium and ring-chain tautomerism (92AP177). In methanol -d4 hexadeuter-ated bicyclic compounds were formed. 

Alkylamino derivatives of trivalent phosphoras exist in a prototropic equilibrium with P H imtnophosphoranes. The... 

The prototropic equilibrium constant (pRa) for the equilibrium between 0 and HO radicals is 4.7. Therefore at physiological pH, the superoxide radicals exist predominantly in the form of O radicals. HO radicals are more reactive than Oj radicals, and react with substrates by hydrogen abstraction or by addition to the double bonds. 0 radicals do not exhibit these reactions but participate in a number of redox reactions with metal ions and substrates like quinone, ascorbate, etc. The rate constants for these reactions are considerably lower than diffusion-controlled limits (10 to 10 s ). Oj ... 

The data for 3 seconds, at pH values down to about 3, however, represent an instantaneous prototropic equilibrium involving unchanged COHb. Below this pH, appreciable denaturation occurs in less than 3 seconds and the color of the flowing material darkens. The data obtained in 10 minutes, on the other hand, represent darkened (denatured) material below about pH 4. When measured in 3 seconds, the data obtained down to pH 3 may be obtained by back-titration as well (if protein kept at pH above 3 for less than 3 seconds is used), and thus represent a truly reversible equilibrium. 

Photoelectron spectra of 2-, 3- and 4-mercaptopyridines and fixed structure models have been measured for evaluation of tautomeric populations. Whereas for 3- and 4-mercaptopyridine a great predominance of the mercapto form in vapor phase is demonstrated, 2-mercaptopyridine shows the existence of a prototropic equilibrium between the mercapto and thioxo forms. From the integration of band areas it follows that there is about 10% of the thioxo form in the equilibrium (77JCS(P2)1652, 79TL2585). 3-Formyl-2-mercaptopyridine was found to exist in the gas phase as a tautomeric mixture containing about 16% of the thione form (81T2663). 

First, derivatives of a-amino acids in which the amino group is part of an amide are considerably more prone to a-epimerization/racemization upon COOH activation than their carbamate congeners. Loss of stereochemical integrity is attributed to cyclization of a generic activated species, 23, to oxazolone 24, which exists in prototropic equilibrium with the aromatic tautomer 25 (Scheme 5). An amide being more nucleophilic than a carbamate, cyclization to an intermediate of type 24 is easier. Second, the tendency to cyclize is even more pronounced if the a-amido... 

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