Carboxylic acid to thioamide hydrogen bonding

Abstract: The lactam groups of dipyrrinones avidly engage in amide–amide hydrogen bonding to form dimeric association complexes in non-polar solvents (in CHCl3, K D ∼25,000M−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, K D ∼200M−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 K assoc ≫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. [Copyright &y& Elsevier]