Fundamental Studies on the Structures and Spectroscopic Properties of Imidazo[1,2-a]pyrazin-3(7H)-one Derivatives

The fundamental physical properties of 2-methyl and 2-phenylimidazo[1,2-a]pyrazin-3(7H)-one 1 and 2, and their N- and O-alkylated derivatives 3-6 have been investigated by X-ray crystallography, UV/vis absorption spectroscopy, NMR, and AM1-COSMO calculations. The crystal structures of 3 and 4 showed that the imidazo[1,2-a]pyrazin-3(7H)-one (imidazopyrazinone) (pi)-system has a planar ring structure and a weakened carbonyl character of the C3-O10 bond, suggesting that the imidazopyrazinone (pi)-system has the character of a zwitter-ionic resonance structure to increase the aromaticity. The data concerning the bond length alternations and the NMR chemical shifts of 1-4 also support that their imidazopyrazinone rings have small portions of aromatic character. In addition, imidazopyrazinone derivatives 1-4 showed solvatochromism originating by hydrogen-bonding interactions with hydrogen-bond donor solvent molecules; derivatives 1 and 2 prefer to be the NH form isomers in their tautomeric equilibriums. These observations were consistently evaluated by MO calculations. The physical properties of protonated species of 1-6 and anion species of 1 and 2 were also established. The fundamental properties of the imidazopyrazinone (pi)-system explain the several problems of the chemi- and bioluminescence reactivities of imidazopyrazinone derivatives and of the construction of a bioluminescent supramolecule.