A survey of the lone pair effect on the ring geometry of 1,2,4-triazoles and analogous 1,2,3-triazoles and imidazoles

A simple N-alkylation method of highly insoluble cyclic amides based on the high solubility of their easily isolable tetraalkylammonium and tetraalkylphosphonium salts was elaborated. The method has a rather wide scope, it is not influenced by the identity of the different rings attached to the 1,2,4-triazolo[1,5-a]-pyrimidinone moiety of isomers 1 and 2, nor the presence of the triazole substituents. It proceeds smoothly without any unwanted by-products, at relatively low temperatures, and is not sensitive to moisture. The method allows an easy isolation of all possible N-alkylated derivatives 3, 7, and 8. Spectral analysis of isomers 3, 7, and 8 showed that our previous results concerning the formation of 3 type N-alkylated derivatives as main products of the N-alkylations as well as the tautomeric structure of the non-alkylated isomers 1 and 2 is correct. However, it also showed that the isolation of a single N-alkylated isomer 3 and its comparison with the corresponding non-alkylated derivative to prove its tautomeric structure may lead to mistakes. [4,5], and the corresponding nitrogen (1 and 2, R 1 + R 2 = (CH 2 ) n -NR-(CH 2 ) m ) [6][7][8] and sulphur (1 and 2, R 1 + R 2 = (CH 2 ) n -S-(CH 2 ) m ) [9,10] containing hetero analogues required model compounds of "fixed" tautomeric structures, like that of derivatives 3 and 4 (Scheme 1). They were synthesized by the N-alkylation of the corresponding sodium salts 5 and 6 (Scheme 2) prepared either in situ with sodium hydride in dimethylformamide [2,4,[6][7][8][9] or in advance in hot sodium hydroxide solution [3,6,10]. However, the yields of the above alkylations strongly depended on the solubility of the sodium salts in dimethylformamide, which can lead to difficulty during isolation of the products from the very diluted dimethylformamide containing solutions. Moreover the above methods enabled the isolation of the main Nalkylated products only. Luckily the isolated N-alkylated products of type 3 and 4 (Scheme 1) contain the same chromophore systems as derivatives 1 and 2, respectively, making their structure elucidation possible [2][3][4][5][6][7][8][9][10]. However, in spite of their similarities, doubts still remained concerning the validity of identifying the above structural pairs by comparing the spectral data. To solve this problem unambiguously we wanted to synthesize all possible N-alkylated isomers of type 1 and that of their O-alkylated analogues, i.e. derivatives 7-9 as well as compounds 3 (Scheme 3).It was known [11] that the N-alkylation of different cyclic amides can be performed in two-phase (liquidliquid or solid-liquid) systems using phase transfer catalysts (quaternary ammonium or phosphonium salts).Brändström [12] successfully isolated the stoichiometric crystalline tetrabutylammonium salts (10) of weak CH-acids like methyl 2,4-dioxovalerate (10z, X = COCH 3 , Y = COOMe) and methyl cyanoacetate (10w, X = CN,

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On triazoles. IV—NMR study of 5‐amino‐1,2,4‐triazole isomers

Dvortsak¹,

Reiter²,

Somorai³

et al. 1985

Magnetic Reson in Chemistry

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A 'H NMR and % ! NMR study of all possible isomers of monoalkylated 3-methylthio-5-amino-1,2,4-triazoles was carried out. The %NMR spectra, including the corresponding proton coupled measurements, proved unambiguously the structures of all derivatives studied.In the preceding papers of this series'32 we reported the syntheses of different A-D type 1,2,4-triazole derivatives. It was shown that neither the MS method proposed for the differentiation between A and B3 nor the study of the IR and UV spectra of derivatives A-D was suitable for a complete structural determination of these isomers. We also showed that previous 'H NMR investigation^^,^ of derivatives 2A and 2B led to erroneous results. ' In order to establish a general rule for the differentiation between derivatives A-D, systematic NMR investigations were initiated. EXPERIMENTALThe 'HNMR spectra were recorded on Varian EM-390, Bruker WM-250, or WP-80SY instruments at 90, 250 or 80 MHz, respectively, in the CW or FT mode, at ambient temperature. The 13CNMR spectra were recorded on Varian XL-100, Bruker WM-250, or WP-80SY F T spectrometers. Typical measuring conditions were as follows: temperature, ca 40 OC; pulse width, 15, 5 and 3.5 p s , respectively (30-45"); spectral width, 5, 16 and 5 kHz, respectively; data points, 16, 32 and 16K, respectively; acquisition time, 0.8, 1.02 and 1.64 s, respectively; relaxation delay, 0-1.0 s, using broad band proton decoupling. The proton coupled spectra were recorded using the gated decoupling technique, with the same parameters except that of the repetition time (3.0 s). The transform sizes were 8, 16 and 8K, respectively. To attain better digital resolution, some proton coupled spectra were also taken with a sweep width of 500 or 1000 Hz.The solvent (except when stated otherwise) was DMSO-d6, with DSS as internal reference. Chemical shifts are quoted relative to TMS in CDCl, solution, and relative to DSS in DMSO solutions, for concentrations of approximately 10%. RESULTS 'H NMR spectroscopyThe 'HNMR spectra (Table 1)

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A survey of the lone pair effect on the ring geometry of 1,2,4-triazoles and analogous 1,2,3-triazoles and imidazoles

Cited by 13 publications

References 18 publications

Structural Chemistry

Structural Chemistry

On triazoles XLII . A new convenient method for the N‐alkylation of highly insoluble cyclic amides

On triazoles. IV—NMR study of 5‐amino‐1,2,4‐triazole isomers

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