Intramolecular tautomerizations

For TPBC no intramolecular tautomerism could be observed up to 140 °C, indicating that the sum of E +E is larger than about 86 kj mokk assuming a rate constant smaller than 100 s at 140 °C as estimated from the linewidths of the iH signal of the inner protons. 

Next, Ch. 11 by Lochbrunner, Schriever and Riedle deals with excited electronic state intramolecular tautomerization proton transfers in nonpolar, rather than polar, solvents. But there is a connection to the previous chapter the ultrafast optical experiments discussed here emphasize evidence that the proton is not the reaction coordinate. The proton transfer is controlled by low vibrational modes of the photo-acids, rather than by the proton motion itself, an interpretation supported by separate vibrational spectroscopic studies and theoretical calculations The key role of modes reducing the donor-acceptor distance for proton transfer is highlighted, and for the featured molecule of this chapter, the proton adiabatically follows the low frequency modes, and no tunneling or barrier for the proton occurs. (See also Ch. 15 by Elsaesser for direct ultrafast vibrational studies on these issues). 

Transformations of this type play an important role in the mechanism of quite a few reactions, in particular, it is they which lie at the root of the intramolecular tautomerism [52]. It is therefore not surprising that the theoretical investigation of the intramolecular proton transfers is represented by a wide range of calculations of methodological importance. Particularly thoroughly was studied the reaction of the 1,5-proton shift in 3-oxy-2-propenal (cis-enol of malonal-dehyde) XI which affords one of the most significant examples of the degenerate intrachelate tautomerism. 

Examples of Intramolecular Tautomerizations without Requiring Reorganization of the Environment... 

In this section we will discuss some typical examples exhibiting intramolecular tautomerizations whose kinetics have been studied by dynamic NMR in various environments. Often, the gas phase and liquid state degeneracies were found to be slightly disturbed in the solid state. In most cases, nonlinear Arrhenius curves were observed, which indicated the role of vibrationally assisted tunneling, supported in almost all cases by the observation ofnonclassical kinetic isotope effects. 

Figure 14.8). (c) The tautomerism of bis-aryltriazene 26 [50, 51] is quenched in the liquid state without catalyst as they are not able to form cyclic dimers. A base B catalyzed intramolecular tautomerization which occurs with bases such as trimethylamine. The proton is transferred in the complex 27 temporarily to the base and then back to the original nitrogen or to the adjacent nitrogen without intermolecular proton exchange [51]. 

Tautomerism has been discussed in Section 4.04.1.5.2. It concerns prototropic tautomerism and the decreasing order of stability is (hydrazone) >A (azo)> A (enehydrazine). The isomerization A -> A occurs via a A -pyrazoline (65BSF769). Pyrazolidones and amino-A -pyrazolines exist as such. The only example of non-prototropic tautomerism deals with the isomerization (403) —> (404) (74CJC3474). This intramolecular process is another example (Section 4.04.1.5) of the thermodynamic analogy between prototropy and metallotropy. 

C NMR, 6, 398 molecular dimensions, 6, 397 tautomerism, 6, 404 Furoxan-3-carbohydra2ide intramolecular hydrogen bonding, 6, 396 Furoxan-3-carboxamide intramolecular hydrogen bonding, 6, 396 Furoxancarboxylic acids reactions, 6, 413 with nucleophiles, 6, 406 Furoxan-3,4-dicarbaldoxime synthesis, 6, 409 Furoxanoaaines fused... 

DFT STUDY OF 8-MERCAPTOQUINOLINE INTRAMOLECULAR HYDROGEN BOND, SINGLE PROTON TRANSFER AND WATER-ASSISTED TAUTOMERIZATION... 

The above value for R = H corresponds to an intermolecular process, either assisted by the solvent (in solution) or by other NH-pyrazole molecules (in solution and in the solid state) while metal migrations are probably intramolecular (the bigger the metal, the easier) and those of COR correspond (for R = NHR ) to a dissociation-recombination mechanism. Minkin [quoted in 96MI(15)339] suggests that a tautomeric process should... 

Numerous data on kinetics of annular tautomerism have been obtaifled for the degenerate rearrangements of pyrazole derivatives. Since all these rearrangements were found to be intramolecular, we can compare kinetic measurements carried out in different media. The following order of increase in migration ability of various groups is established ... 

The amino form 66a of 5-pyridylamino-l,2,4-triazole stabilized by two intramolecular hydrogen bonds is the only tautomer observed by the X-ray study of the crystal (90KGS1632). However in DMSO-dg, DMF-d7, and HMPA-di8 solutions, the equilibrium involves three tautomeric forms 66a-66c (Scheme 31) (90KGS1632). 

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