Photochemical Methoxide Exchange in Some Nitromethoxybenzenes. The Role of the Nitro Group in SN2Ar* Reactions

Article abstract of DOI:10.1021/ja00414a036

Photoinduced methoxide exchange has been measured for o-, m-, and p-nitro<14C>methoxybenzene as well as for the <14C>methoxy-labeled 4-nitroveratroles.In these reactions (of the S_N2³Ar^* type), where the substituting and leaving groups are the same, the origin of the meta activation found cannot be otherwise than in the steps leading to the symmetrical ? complex.Next to considerations on the basis of charge distributions of the excited aromatic compounds and of electron densities in the HOMO and LUMO, a rationalization for the regioselectivity and activation is offered on the basis of the energy gap between the ground-state and the excited-state surfaces.Since the level of the ground-state ? complex corresponding to meta substitution as compared to that for ortho/para substitution is highest in energy and since the trajectory from the (triplet) excited state leading to the ? complex for meta substitution is lowest in energy, the energy gap between the excited-state and ground-state hypersurface is considerable smaller for the geometry that leads to the meta ? complex and thus to meta substitution.Meta direction and activation of photosubstitutions will therefore be outspoken in the case of a substituent that gives difficult or no substitution at the meta position in the ground state and brings with it low triplet energy.The NO2 group conforms nicely to these criteria.