Hydrogen bonds in "carboxyoximes": the case of bornane derivatives

The tendency of forming mixed carboxyl-to-oxime hydrogen bonds was tested o n the series of bornane derivatives: one with the acid function only (borna ne-2-endo-carboxylic acid), one with the oxime function (2,2'-diethylthiobo rnane-3-oxime), and one with both oxime and carboxylic functions (bornane 2 -oxime-3-endo-carboxylic acid). The crystal structures of these compounds w ere determined by means of X-ray diffraction. In bornane-2-endo-carboxylic acid and 2,2'-diethylthiobornane-3-oxime 'homogenic' hydrogen bonds were fo und, and these hydrogen bonds close eight- and six-membered rings, respecti vely. By contrast, in bornane-2-oxime-3-endo-carboxylic acid 'heterogenic' hydrogen bonds between carboxylic and oxime bonds were found. This carboxyl ic-oxime, or 'carboxyoxime' system is almost always present in compounds wh ich have both oxime and carboxylic groups; therefore it can be regarded as an element of supramolecular structures (synthon). The presence of such syn thons can break the tendency of carboxylic acids and oximes towards crystal lizing in centrosymmetric structures.