An experimental and theoretical NMR study of NH-benzimidazoles in solution and in the solid state: proton transfer and tautomerism

Summary This paper reports the 1H, 13C and 15N NMR experimental study of five benzimidazoles in solution and in the solid state (13C and 15N CPMAS NMR) as well as the theoretically calculated (GIAO/DFT) chemical shifts. We have assigned unambiguously the "tautomeric positions" (C3a/C7a, C4/C7 and C5/C6) of NH-benzimidazoles that, in some solvents and in the solid state, appear different (blocked tautomerism). In the case of 1H-benzimidazole itself we have measured the prototropic rate in HMPA-d 18.


Introduction
Of almost any class of heterocycles it can be said that they have relevant biological and medicinal chemistry properties, because, for instance, over 80% of top small molecule drugs by US retail sales in 2010 contain at least one heterocyclic fragment in their structures [1]. Benzimidazoles besides being the skeleton of many relevant drugs (fungicides, anthelmintics, antiulcerative, antiviral,…) [2,3] are also part of some natural products (the most prominent benzimidazole compound in nature is N-ribosyl-5, 6-dimethylbenzimidazole, which serves as an axial ligand for cobalt in vitamin B 12 ) and have interesting ferroelectric properties [4]. Particularly relevant for the present work is their proton conducting abilities, based of the 1,3-N-H···N hydrogen bonds, not only in benzimidazole polymers but in molecular compounds [5,6].
Degenerated tautomerism (autotrope) [7,8] simultaneously simplifies and complicates the NMR spectra of molecules in solution to the point that the assignment of some signals that become magnetically equivalent (isochronous) [9,10] by fast proton exchange has been much neglected. With the advent of solid-state NMR spectroscopy and the suppression of prototropic tautomerism, the assignment problem arises anew.  In recent years, the use of very pure NMR solvents, particularly DMSO-d 6 , and highfield instruments has lead to obtain solution spectra where the prototropy has been considerably slowed down.
We present in this paper a study of four N-unsubstituted 1H-benzimidazoles ( Figure 1) including 2-methyl-1H-benzimidazole (2) that shows in the solid state ferroelectric switching in two dimensions due to its pseudo-tetragonal crystal symmetry [4], and 2-benzyl-1H-benzimidazole (4) a vessel-dilating and spasm-reducing agent known as dibazol or bendazole [3], where the tautomerism has been blocked resulting in the concomitant problem of assignment of some signals. We have selected 1-methyl-1H-benzimidazole (5) as the simplest benzimidazole without tautomerism.
Three kinds of calculations have been done: isolated molecules (gas phase), continuous model solvated molecules in DMSO, and hydrogen-bonded trimers for 1, to simulate the crystal [24]. The central benzimidazole is N-H···N hydrogen bonded to two other benzimidazoles like in the crystal chain (catemer). Two trimers, A and B, that differ in the conformation of the first benzimidazole were calculated for this compound, but the differences in energy are very small, less than 0.1 kJ·mol −1 ( Figure 2).
First, we will discuss the six carbon atoms of the benzene ring of benzimidazole (3a, 4, 5, 6, 7, 7a) and then the remaining atoms (N1, C2, N3 and those of the substituent). In NH-benzimidazoles, when prototropic tautomerism occurs (Figure 3), the signals of the benzimidazole carbons in groups of two coalesce in an average signal (the same happens with the N atoms but the chemical shifts are so different that there are no problems of assignment). Actually, this was a very common occurrence, but in the series of compounds of Table 1, only for benzimidazole itself and for 3, average signals were observed in  DMSO-d 6 , a solvent known to slow down the prototropic exchanges. For these two compounds, we recorded the spectra in HMPA-d 18 , which is better for this purpose [25] observing the signals of compounds where the prototropic exchange is blocked.
The assignment of the signals of Table 1, Table 2 and Table 3 was straightforward following these steps: i) A NOESY experiment identifies H7 (7.54 ppm) of 5 by its proximity to the N-methyl group; ii) the analysis of the ABCD system of the protons H4, H5, H6 and H7, identifies the remaining protons; iii) a series of 2D experiments assign the CH carbons (HMQC) as well as the quaternary carbons C3a and C7a (HMBC).
In the case of 3 all signals are considerably split in the solid state ( Figure 4).
The X-ray structure of compound 3 is known and reported in the Cambridge Structural Database (refcode: ZAQRIU01)       [4,26]. There are two independent molecules and in one of them, the CF 3 is disordered ( Figure 5). The most probable explanation of the splittings of Figure 4 is the existence of two independent molecules, a fact that is well documented in the literature [27][28][29][30].
We have collected in Table 4 the different equations obtained from the data of Table 1, Table 2 and Table 3.
i) The 1 H chemical shifts are much more consistent with calculations for DMSO as solvent than with those of isolated molecules (gas phase). For 26 points, the R 2 coefficient increases from 0.56 (eq. 1) to 0.80 (eq. 2). The use of the monomer or the central part of the trimer has no influence. The worse point is H7 of 4 that appears at 7.40 ppm and fitted with eq. 2 has a value of 7.14 ppm. The origin of this discrepancy is that the theoretical conformation corresponding to the X-ray structure is not stable and reverts to the minimum one, which has the benzyl group rotated. ii) The 13 C chemical shifts (eqs. 3-13) are very well reproduced by the calculations: high R 2 values, small intercepts (in several cases, not significant) and slopes close to 1. Systematically, the worse point was the carbon atom of the CF 3 substituent (halogen substituents produce effects that are not well reproduced by our calculations that not include relativistic corrections) [15], removing it does not significantly modify the regression values (compare eqs. 3-6 with eqs. 7-10). CPMAS values agree better with calculations including DMSO solvent effect (compare eqs. 9 and 10) and also better with trimer B (in turn, slightly better than with trimer A, compare intercept and slopes of eqs. 12 and 13) than with the monomer (eq. 11).
iii) Concerning 15 N NMR (eqs. 14-21), the R 2 values are lower than with 13 C NMR. Both for solution and for CPMAS, the gas phase and DMSO calculations are comparable in terms of R 2 (eqs. 14-17), however, the values of the slopes (the closer to 1, the better) and intercepts (the closer to 0, the better), clearly favored the DMSO calculations. Surprisingly, the monomer appears preferable to the trimer (eqs. 18,19 and 20,21) which is understandable for the solution but not for the solid state. More complex approaches, such as periodic calculations [31], are necessary.
Influence of the substituent at position 2 on the tautomerization rate This barrier is similar to that of pyrazole in the same solvent (58.6 kJ·mol −1 at 289 K) [33].
The effect of the substituent at position 2 on the rate (roughly, CF 3 and H, fast; CH 2 C 6 H 5 and CH 3 , slow) is probably the consequence of steric and electronic effects; many years ago, we showed that intramolecular hydrogen bonds also affect the rate [34]. The calculated electrostatic potential minima associated to the lone pair of the N3 follow the tautomerization rate ranking of the molecules (−0.085 au, CF 3 , −0.103, H, −0.104, CH 2 C 6 H 5 and −0.105, CH 3 ).

Conclusion
The data reported here for NH-benzimidazoles when the prototropy is blocked should be useful to determine the tautomeric composition when there are substituents at positions 4(7) or 5(6), for instance, in the case of omeprazole, a 5(6)methoxy-1H-benzimidazole derivative [35,36]. Besides, solid state results as well as GIAO calculations provide new data to characterize this important family of compounds. Finally, by means of DNMR experiments it was possible to determine the barrier to proton transfer of benzimidazole itself in HMPA-d 18 , thus providing a missing value in heterocyclic tautomerism of azoles and benzazoles [33].

Experimental
Four of the compounds reported in this paper are commercial (Sigma-Aldrich): 1, 2, 3 and 5. We reported the synthesis of the fifth one, 4, in [37].

NMR spectroscopy
Solution NMR spectra were recorded on a Bruker DRX 400 (9.4 Tesla, 400.13 MHz for 1 H, 100.62 MHz for 13 C and 40.54 MHz for 15 N) spectrometer with a 5 mm inverse-detection H-X probe equipped with a z-gradient coil, at 300 K. Chemical shifts (δ) are given from internal solvent, DMSO-d 6 2.49 for 1 H and 39.5 for 13 C. Typical parameters for 1 H NMR spectra were spectral width 4800 Hz and pulse width 8.3 μs at an attenuation level of 0 dB. Typical parameters for 13 C NMR spectra were spectral width 21 kHz, pulse width 12.5 μs at an attenuation level of −6 dB and relaxation delay 2s, WALTZ-16 was used for broadband proton decoupling; the FIDS were multiplied by an exponential weighting (lb = 1Hz) before Fourier transformation.
Inverse proton detected heteronuclear shift correlation spectra, ( 1 H, 13 C) gs-HMQC and ( 1 H, 13 C) gs-HMBC, were acquired and processed using standard Bruker NMR software and in nonphase-sensitive mode. Gradient selection was achieved through a 5% sine truncated shaped pulse gradient of 1 ms.

Supporting Information
Supporting Information File 1 Optimized geometry of the systems, and chemical shifts in gas phase and PCM/DMSO environment.