Carbon-13 magnetic resonance studies of azoles. Tautomerism, shift reagent effects, and solvent effects

Abstract

The effects of substitution, lanthanide-shift reagents, solvent changes, and tautomerism were investigated on the 13C chemical shifts of pyrrole, pyrazole, imidazole, s-and u-triazole, and tetrazole. It was concluded that 1SC chemical shifts are of limited value to ascertain the positions of tautomeric equilibrium for rapidly interconverting azole tautomers.Several molecules in the five-membered N-heterocycle series have been investigated by means of their 13C nuclear magnetic resonance, but not in a systematic way: pyrrole as a neat liquid3 or in acetone;4 pyrazole in water,5 in acetone,4•6 and some of its N-substituted derivatives as neat liquids;6'7 imidazole in water,5•8 acetone,4 and ethanol;9 1,2,3-triazole in acetone;4 1,2,4-triazole in acetone4 or in dimethyl sulfoxide;10 and tetrazole in dimethyl sulfoxide.4 However, in none of these researches have the 13C nmr data been used to study tautomerism. Although a great deal of work has been done on the annular tautomerism of such azoles11 by various methods, often no definite conclusions can be reached. We will give attention here to the usefulness of 13C spectroscopy in the determination of tautomeric equilibrium constants compared with 1H or 14N spectroscopy.

The synthesis and unimolecular decomposition of four novel Δ1-1,2,4-triazolines

“…Unfortunately those were also the products when the thermolyses were done in methyl acrylate or in dimethyl acetylenedicarboxylate (DMAD), two efficient dipolarophiles (8). Heterocycles 13 and 16 were identified on the basis of the spectroscopic data, which correspond well with those oi similar known aromatic heterocycles (Table 1) (12)(13)(14)(15)(16)(17)(18)(19)(20). A significant feature of the nrnr data is that the two methyl groups that are not part of the t-Bu substituent are magnetically equivalent, providing strong evidence that the 1,2,4-triazoline ring of 12 and 15 is not present in 13 and 16.…”

Acidity measurements in THF. V. Heteroaromatic compounds containing 5-membered rings

“…The shifts in tetrahydrofuran did not differ greatly from those reported in CDCI, for furan (25). thiophene (25), N-methylpyrrole, (26) N-methylimidazole (26), N-propylpyrazole (26,27), indoles (28), benzofuran (29), benzothiophene (29,30), benzoxazole (3 I), and benzothiazole (32). The "C shifts for lithium thiophenoxide, as determined in THF (0.5 M), were 148.9(CI), 134.0(C2), 128.5(C3), and 121.8(C4).…”

Substituent Effects in the 13C-NMR Spectra of Six-Membered Nitrogen Heteroaromatic Compounds

“…The same approach is applied for pyridine derivatives [3,4] and other heterocyclic compounds such as pyrrole [7,8], furan [9,10], thiophene [9,11]. This approach implies that for each type of heterocyclic ring and, moreover, for each site of substitution a separate set of parameters has to be used.…”