Transmission of the substituent effects in 2‐substituted benzimidazoles studied by ¹H and ¹³C nuclear magnetic resonance

Abstract

The substituent influence on the 'H and "C NMR chemical shifts in 2-substituted benzimidazoles and their anions and cations has been investigated. The transmission of the electronic effects of substituents from C-2 to C-5 (6) is approximately 20% less effective than that in the opposite direction. The solvent influence on the chemical s h i i of protons and transmission effects in the charged forms of 2-substituted benzimidazoles has been studied.The Hamett-Taft equation is widely applied in the investigation of the substituent influence in heterocyclic compounds. At the present time various sets of o-constants, with subsequent comparison of correlation coefficients, are often used. Such a formal approach does not foresee essential distinctions in mechanisms of transmission of the substituent effects across the heterocyclic and benzene systems, although such can exist.It is possible to obtain more detailed information on the influence of the inductive and resonance effects of substituents on the basis of two-and three-parameter correlation^.'-^ The I3C NMR chemical shifts in benzothiazoles were studied in this way576 and it was shown that the transmission of the substituent effects from C-2 to C-6 was approximately one-third less effective than that from position 6 to 2.6The transmission of the electronic effects in benzimidazoles tis insufficiently studied, in spite of the great interest in these compounds owing to their wide use as drugs, herbicides and dye-stuffs.* The substituent influence on the acid-basic properties of benzimidazoles has been studied in Refs 9-1 1. Excellent correlations of AS,_, with up and u6 were observed byIn the present paper the data on the substituent effects on 'H and 13CNMR chemical shifts of 2-substituted benzimidazoles (I), their anions (11) and cations (111) are reported. I3c NMR?I H I11 I I1* Authors to whom correspondence should be addressed. RESULTS AND DISCUSSIONThe 'H and I3CNMR data are presented in Tables 1-3. Protonation of the benzimidazole ring results in downfield shifts of H-4,7, H-5,6 and C-5,6 along with upfield shifts, in most cases, of C-2, C-4,7 and C-8,9, in agreement with the data reported in Ref. 12. Deprotonation, on the contrary, causes upfield shifts of H-4,7, H-5,6 and C-5,6 along with downfield shifts of C-2, C-4,7 and C-8,9. Table 4 shows the results of three-parameter correlations of the 'H and 13C chemical shifts with the Swain-Lupton parameters (9 and 22) and the semiempirical parameter Q which,13 together with the electronic factors, account for the paramagnetic shielding. The choice of the number of points was determined by the limited number of substituents for which the Q values are reliable. As can be seen from Table 4, the correlation coefficient for C-2 is lower than that for carbons and protons in positions 4,7 and 5,6. The contribution from the effects described by the parameter Q is significantly higher in the correlation of the 13C than the 'H chemical shifts. Nevertheless, despite the comparatively high correlation coefficients of the 'H...

A carbon-13 nuclear magnetic resonance study of a series of 5-substituted benzimidazoles

“…[I] indicate that these results compare favorably with those obtained for other benzazole systems (4,5,6,10). For the ipso carbon, C-5, the correlation is not quite as good; this is consistent with the results observed for the Z-substituted benzimidazoles (4).…”

Synthesis and NMR spectral assignments of novel nitrogen and sulfur heterocyclic compounds

“…The experiments were optimized for a 13 C, 1 H coupling of 8 Hz. The acquisition data size was 2048 points, and the number of increments for evolution was 256.…”

Product Class 4: Benzimidazoles

“…In the cationic benzimidazolium species H2 appears as a triplet with a coupling constant (J 1,2 = J 2,3 ) of 2.5 Hz. [34] Table 4 lists 1 H NMR data for 1H-benzimidazole, protonated benzimidazolium, and 1-methyl-1H-benzimidazole. The methyl signal (ä~3.63) is similarly little affected.…”

“…[33] Signals for H4 and H7 usually appear at lower field than the H5 and H6 signals. [34] b TFA added. A strongly electron-withdrawing group may, however, shift the signals of adjacent protons to lower field despite this general rule.…”