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The adduct formed upon addition of water to an aldehyde or ketone is called a hydrate or germinal diol (gem-diol). These compounds are rarely stable (and only in aqueous solution), i.e., the equilibrium is greatly dependent on the structure of the hydrate. Thus, formaldehyde in water at 20ºC exists 99.99% in the hydrated form, while for acetaldehyde this figure is 58%, and for acetone the hydrate concentration is negligible. For that reason, the few stable crystalline hydrates known (such as polychlorinated aldehydes) are those that have a strongly electronegative group associated with the carbonyl group since, in general, the hydrates can seldom be isolated because they readily revert to the parent aldehyde. The aim of the present work was to study the existence and stability of the aldehyde-hydrate form of some pyridine or imidazole carboxaldehyde derivatives commonly employed as reagents in the synthesis of active compounds (formylimidazoles and formylpyridines), using solution- and solid-state NMR experiments (13C CP-MAS and 1H-13C HETCOR). In addition, some of the derivatives presented here were studied through single-crystal X-ray crystallography. In particular, the 2-formylimidazole hydrate is a stable crystalline substance because, in order to revert to the aldehyde-form, a water molecule must be left out, and this is difficult by the electron-withdrawing character of the imidazolium cation. That was probed since the 2-formylimidazole in D2O-TFA solution evolved completed to its hydrated form (100%), however, the 4-formylimidazole and 4-methyl-5-formylimidazole existed 40% and 10% in the hydrated form, respectively. Interestingly, single crystal for the hemiacetal derivative was obtained from N-methyl-2-formylimidazole in TFA solution and confirmed by X-ray diffraction techniques. In addition, the hydrate and hemiacetal form coexisted in the solid, after the completely removal of the TFA, according with the 13C CP-MAS results.