Carboxylic acid to thioamide hydrogen bonding

The lactam groups of dipyrrinones avidly engage in amide-amide hydrogen bonding to form dimeric association complexes in non-polar solvents (in CHCl3, KD ∼25,000 M−1 at 22 °C). The corresponding thioamides (dipyrrinthiones), prepared from dipyrrinones by reaction with Lawesson's reagent, also form intermolecularly hydrogen-bonded dimers in non-polar solvents, albeit with much weaker association constants (in CHCl3, KD ∼200 M−1 at 22 °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 Kassoc ≫25,000) is found in CHCl3 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.

[6]-Semirubin (X=O) is an intramolecularly hydrogen-bonded monomer in non-polar solvents that is favored over the intermolecularly hydrogen-bonded dimer, with Kassoc ∼25,000. [6]-Thiosemirubin (X=S) is a dimer, with Kassoc ∼200.