Carboxylic Acid to Thioamide Hydrogen Bonding.

The lactam groups of dipyrrinones avidly engage in amide-amide hydrogen bonding to form dimeric association complexes in nonpolar solvents (in CHCl(3), K(D) ~25,000 M(-1) at 22 degrees C). The corresponding thioamides (dipyrrinthiones), prepared from dipyrrinones by reaction with Lawesson's reagent, also form intermolecularly hydrogen-bonded dimers in nonpolar solvents, albeit with much weaker association constants (in CHCl(3), K(D) ~200 M(-1) at 22 degrees C). When a carboxylic acid group is tethered to C(9) of the dipyrrinone, as in the hexanoic acid of [6]-semirubin, tight intramolecular hydrogen bonding between the carboxylic acid group and the lactam moiety (intramolecular K(assoc) >>25,000) is found in CHCl(3) with no evidence of dimers. In contrast, the analogous dipyrrinthione, [6]-thiosemirubin, eschews intramolecular hydrogen bonds, as determined using NMR spectroscopy and vapor pressure osmometry, preferring to form intermolecularly hydrogen-bonded dimers of the thioamide-thioamide type.