Modified Atomic Orbital Overlap: Molecular Level Proof of the Nucleophilic Cleavage Propensity of Dinitrophenol-Based Probes.

Out of six possible positional isomers of dinitrophenol, only 2,4-DNP has been used extensively by many researchers for developing reactive molecular probes. But the question remains unanswered: why has only the 2,4-isomer emerged as a labile protecting group? To answer this question, six molecular probes using available DNP isomers were developed and investigated to evaluate the effect of the extent of atomic orbital overlap on their reactivity. We have proved for the first time at the molecular level that the o-NO2 group contributes less compared to the p-NO2 group toward the reactivity of 2,4-DNP-based probes. Crystal structure analysis revealed that the 2p orbital of N atom and the 2p orbital of the adjacent ring C atom to which the o-NO2 is attached are inclined at >30° to each other, leading to substantial reduction in π overlap (as these two p-orbitals loose coplanar state) resulting in a very weak -M effect of the o-NO2 group, whereas the 2p orbitals of the N atom of the p-NO2 group and the adjacent ring C atom are almost coplanar (11° inclined to each other), leading to strong π overlap. Hence, the p-NO2 group contributes largely toward the molecular reactivity through its -M effect.