Pyridinethiones, tautomerism

As far as one can tell from the rather limited data, and as we shall see later, much the same factors as listed above appear to be at work in deciding the amount of thiol form in 2- and 4-pyridinethione tautomerism. 

Pyridinethiones acylation, 2, 357 alkylation, 2, 357 aromaticity, 2, 148 protonation, 2, 357 tautomerism, 2, 356 Pyridine-2-thiones aromaticity, 2, 156 basicity, 2, 157 oxidation, 2, 357 N-oxide, sodium salt biocide, 1, 399 synthesis, 2, 360... 

Application of pulsed ion gas-phase cyclotron resonance (ICR) spectroscopy to proton affinities of the derivatives 2-methoxypyridine and N- methylpyridin-2-one confirm previous deductions on the enthalpy of 2-hydroxypyridine-pyridin-2-one tautomerism (76JA6048), provided that the difference between the influences of O-methylation on 2-hydroxypyridine and A-me thy lat ion in pyridin-2-one are taken into account. These measurements have been further clarified and extended to other gas phase basicity measurements (79JA1361). A similar estimation of the gas phase basicities of 2- and 4-pyridinethiols and 2- and 4-pyridinethiones confirms that the thiol form is predominant in the gas phase (77TL1777), in line with previous studies involving mass spectrometric deuterium isotope studies (75BSB465). Photoelectron spectroscopy has also been employed in such studies (see Section 2.04.3.6 and Figure 21 for details) <77JCS(P2)1652>. 

The tautomerism between 2-pyridinethione and 2-pyridine thiol has also been examined using variable temperature IR spectroscopy <2002JOC9061>. No evidence for the S-H stretch was observed in a range of solvents and this was determined computationally to be a solvent effect the thiol form is more stable in the gas phase but the thione is more stable in solution. (The effect of phase on the tautomers of 2-hydroxy, 2-amino-, and 2-thiopyridine has also been studied by infrared spectroscopy <2001SAA2659>.) Dimerization is also observed, with the indication that the thione dimer predominates, in contrast with the computational studies described above. 

Pyridinethiol is in equilibrium with 2-pyridinethione and it is been previously thought that the thiol is more stable in nonpolar solvents while the thione is more stable in polar solvents. Recent variable temperature Fourier transform IR experiments and computational studies calculated at the B3LYP/6-311 level of theory indicate that the thione is more thermodynamically stable than the thiol in nonpolar solvents and that tautomerism occurs via the dimer <2002JOC9061> (Equation 82). 

The photochemistry of ethyl esters of 2-oxo-carboxylic acids has the participation of both singlet and triplet excited states o. Triplet state lifetimes have been measured and the occurrence of Norrish type II splits in these molecules established. Other flash photolysis studies reported deal with the photoinduced tautomerism of 2-hydroxyphenazine °, 3-methylisoxazolo[5,4-b]pyridine °, and the photolysis of 4,4 -biphenylbiazide °. The tautomerism and phototautomerization of 4(3H)-pyridinethione has been examined theoretically by the infra red isolation technique . The effect of pressure on the photoinduced abstraction reaction of azanaphthalenes in mixed crystals of durene has also been studied. Photosolvolysis of arylmethanols also occurs in aqueous solutions of sulphuric acid . 

Tautomeric constants of several 2-substituted 5-methoxy-4(l//)-pyridinethiones were determined by comparing their pKa values with those of their 4-methylthio- and V-methyl analogs, measured spectrophotometrically in water at 25 °C. The KT values are affected by the substituent at the position 2 and are decreased by electron-withdrawing groups. The thione forms predominate in aqueous solution by factors of ca. 105 for 5-methoxy-2-methyl-4(l//)-pyridinethione and ca. 105 for 5-methoxy-2-hydroxymethyl-4(l//)-pyridinethione (77BCJ3295). 

Related to the reaction of thiols are pyridine thiones since they exist in an apparent tautomeric equilibrium shown in Equation 10.59 for the 2-pyridinethione/2-pyridi-nethiol system. 

The relative stabilities of these two sulfur analogs should be compared with the tautomeric equilibrium of 2-thiopyridine where the pyridinethione tautomer predominates in aqueous solution. These observations demonstrate that tautomeric equilibria in protic systems cannot be used reliably to predict the relative stabilities of the methylated isomers. It has been shown recently that 0-mesyloxypyridines (MI-506)are more stable than A -mesyl-2-pyridones. ... 

Recently, however, there has been increased interest in N-oxides for the generation of hydroxyl radicals in aqueous solution. This process has been reported for some nitrated N-oxides but is most often observed for tautomeric N-oxides, where the N-hydroxy form is significant. " "" The main application is in photobiology, as specific sources of hydroxyl radicals for DNA damage, both strand break and base modifications. N-Hydroxy-2-pyridone and 2-pyridinethione, - N-hydroxy-4-(4-chlorophenyl)thiazole-2(3H)-thione, " and pyrimido[5,4-g ]pteridinetetraone N-oxide (for which the path followed depends on conditions) " and related N-oxides " have been used for this purpose. 

Alternate Name l-hydroxy-2-(17/)-pyridinethione the tautomeric W-oxide form [1121-31-9], although the minor component, is often the source of alternate names for this compound, which include 2-pyridinethiol 1-oxide, 2-mercaptopyridine N-oxide, 2-mercaptopyridine 1-oxide, and the abbreviated from pyrithione . 

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