Theoretical studies on the nucleophilic additions of mimic compounds of pyrroloquinoline quinone with ammonia

Nucleophilic additions of a series of mimic compounds of pyrroloquinoline quinone (PQQ) and their analogues with ammonia were investigated at the B3LYP/D95(d, p) level of theory in order to study the relationship between structure and reactivity. It is demonstrated that: (1) for the monocarbonyl system, the electrophilicity of the carbonyl carbon affects significantly the reactive energy barrier; (2) for the biscarbonyl system, the hydrogen bond, formed at the transition state between the hydrogen of the nuclephilic reagent ammonia and the ortho-carbonyl oxygen is crucial for reaction activation energy; (3) while the fused ring has little effect on the reactive barrier, the reaction activation energy can be significantly reduced by the hydrogen bond between H of ammonia and N1 atom in the fused hetercycle. Quantitatively, a good linear correlation has been found between the reaction activation energy and the angle (φ(OCN)) between the attacked carbonyly and the N atom of the nucleophilic NH₃ at the transition state.