Solid State and Solution Structures of O‐Alkyl‐ and of O‐Acyl Derivatives of 1‐Hydroxypyridine‐2(1H)‐thione

Abstract

Although Barton's radical precursors, the 0-acyl derivatives of 1-hydroxypyridine-2( 1H)-thione 2, have been studied in detail for more than a decade, to date nothing has been reported on the solid state or the solution geometries of these molecules or the analogous 0x0 derivatives, the pyridones 7, 8. In view of this, selected 0-alkyl and 0-acyl derivatives of pyridinethiones and of 2-( 1H)pyridones have been prepared and investigated by X-ray diffraction and NMR (COSY, NOE) experiments. The X-ray data indicate alternating bond lengths in the carbon framework of the heterocycles 2d, 3b, 7b, and 8 and long N -0 bonds [1.384(4)-1.398(2) A], In all the compounds studied, the 0-alkyl and the 0-acyl groups are twisted out of the plane of the heterocycles, which is in accord with the VSEPR concept. Further, the thermal stabilities of thiohydroxamic acid derivatives 2, 3 and of substituted pyridones 7, 8 were studied by differential scanning calorimetry (DSC).Alkyl esters 2[l,*] of 1 -hydroxypyridine-2( 1H)-thione (l)I3I, or the mixed anhydrides 3[41, serve as useful and versatile precursors of heteroatom and of carbon centered radicals 4 and 5I5l. The convenience of the use of the heterocyclic compounds 3 in synthetic radical chemistry originates from the fact that several procedures for the eficient in situ preparation of 3 have been elaborated which do not require isolation and purification of the often thermally labile and light sensitive anhydrides 3L61. Once prepared, the radical precursors 3 can be smoothly cleaved to give their product radicals, simply by heating ( T = SOOC) or by photolyzing in inert solvents with an incandescent light bulb, in the presence of suitable radical traps.For the application of N-alkoxypyridinethiones 2 as sources of alkoxyl radicals, an additional isolation step has been necessary in order to remove sideproducts from the synthesis of 2 which would otherwise interfere with the subsequent radical rea~tion[~,'*~1. In the course of our studies on alkoxyl radicals as intermediates in the stereoselective synthesis of tetrahydrofurans, we prepared several esters 2 and were interested in a detailed analysis of the structural and spectroscopic properties of our radical precursors, especially from X-ray crystallography and NMR [NOE and (C,H) COSY experiments]. Furthermore, differential scanning calorimetry (DSC) experiments were carried out to get some appraisal of the thermal stability of the esters 2, which have hitherto simply been described as thermally We were surprised to notice that the well-known O-acylpyridinethiones 3 -the Barton anhydrides['] -as well as the corresponding oxygen derivatives, the pyridones 7 and ti[''], had not to our knowledge, been subjected to such studies previously.Results and Discussion N-Alkoxypyridine-2( 1 H)-thiones (2) were prepared from 1 -hydroxypyridine-2( 1 fQ-thione (1) via its tetraethylammonium salt[s] and the appropriate alkyl halides" 'I. The reaction of cis-4-tert-butylcyclohexyl bromide[l21 and 2-mercaptopyridine N-oxide tetraethylammonium sal...

Synthesis, structural characterization, electrochemical, catalytic, antimicrobial and thermal properties of the polymeric metal complexes

“…The structure of the 2-thiooxo-1,2-dihydropyridine-1-olate entity of the molecule is distinctively different from the one reported for the heterocyclic cores of 2-alkylsulfanyl pyridine-1-oxides [2], bis[2-thiooxo-1,2-dihydropyridine-1-olato]nickel [3] and the corresponding zink complex [4]. The correlation of data with those reported for Nalkoxypyridine-2(1H)-thiones, on the other hand, fits much closer thus pointing to a preference for the thione formula of the title compound in the solid state [5]. …”

A New Generation of Alkoxyl Radical Precursors – Preparation and Properties ofN-(Alkoxy)-4-arylthiazole-2(3H)-thiones

“…The terpreted as a structural consequence of a minimization of lone pair repulsion on the adjacent nitrogen and oxygen formation of minor amounts of bis(thiazyl) disulfide 29 [26] can be attributed to a combination of thiyl radicals 26 or atoms. [22] We feel that this picture also applies to thiazolethiones 19c, 19f, and 19k. This phenomenon should in to the addition of intermediate 26, in competition with tertbutyl radical 27, to thione 19m as part of a radical chain general lead to barriers to rotation about the NϪO bonds, [23] which can be expected to be higher in 4-aryl-sub-reaction (Scheme 3; see also Figure 1).…”

4‐ and 4,5‐Substituted N‐Methoxythiazole‐2(3H)‐thiones – Preparation,UV/Vis Spectra, and Assignment of Electronic Transitions in Comparison to N‐Methoxypyridine‐2(1H)‐thione Using Time‐Dependent Density Functional Theory Calculations

“…The computed structure of N-methoxypyridine-2(1H)-thione (1b), a labile compound that has so far not been crystallized, was compared to the corresponding subunit in N-[trans-(tert-butylcyclohexyl)-4-oxy]pyridine-2(1H)-thione, which points to a similar correlation between theory and experiment as outlined for the acid 1a. [16,30] …”

“…[22,44] UV/Vis spectra were recorded with a Varian Cary 50 spectrophotometer at 20°C using analytical grade EtOH or n-hexane as solvents in 1-cm quartz cuvettes. N-hydroxypyridine-2(1H)-thione (1a), [45] N-methoxypyridine-2(1H)-thione (1b), [16] N-hydroxy-4-methylthiazole-2(3H)-thione (3a), [20] …”

“…[14] In view of this background, it is surprising to note that a concise assignment of electronic transitions that are associated with the near UV/Vis absorption bands of heterocyclic thiohydroxamic acids and their Oalkyl or O-acyl derivatives, has so far not been reported. [15,16] Therefore it was the aim of the present investigation, to prepare 4-and 4,5-substituted N-methoxythi-azole-2(3H)-thiones 3b-7b (Figure 1), to record their electronic spectra in order to systematically probe the effect of CH 3 and aryl substituents, and to assign the experimentally observed UV/Vis bands of thiazole-2(3H)-thiones 3a-b and 5b to electronic transitions using results from time-dependent density functional theory (TD-DFT). [14,17,18] N-Methoxypyridine-2(1H)-thione (1b) and N-hydroxypyridine-2(1H)-thione (1a) have been included into the experimental and the theoretical part of the present study, in view of the significance of derivatives thereof in contemporary photochemically induced free radical reactions.…”