Interplay between steric and electronic factors in determining the strength of intramolecular N-H...O resonance-assisted hydrogen bonds in beta-enaminones

The crystal structures of five beta-enaminones are reported: (2Z)-3-(benzylamino)-1,3-diphenyl-prop-2-en-1-one, (2Z)-3-(benzylamino)-3-(2-hydroxyphenyl)-1-phenyl-prop-2-en-1-one, (2Z)-3-(benzylamino)-3-(4-methoxyphenyl)-1-(3-nitrophenyl)-prop- 2-en-1-one, 2-{1-[(4-methoxyphenyl)amino]ethylidene}cyclohexene- 1,3-dione and 2-{1-[(3-methoxyphenyl)amino]ethylidene} cyclohexene- 1,3-dione. The structures were analysed and compared with those of similar compounds in order to establish which factors determine the range (2.53-2.72 A) of (N...O) hydrogen-bond distances in intramolecularly hydrogen-bonded beta-enaminones. It has been shown that, beyond electronic resonance-assisted hydrogen-bond effects modulated by substituents, the necessary requirements to produce very short N-H...O hydrogen bonding are steric intramolecular repulsions, including the embedding of an enaminonic C-C or C-N bond in an aliphatic six-membered ring. By considering the structural features it is possible to expect the strength of N-H...O hydrogen bonds adopted by specific beta-enaminones.