Molecular Orbital Calculations and 13C NMR Studies To Explain a Regiospecific Demethylation of 3-Alkyl-1,2-dimethoxybenzenes

Article abstract of DOI:10.1021/jo00186a010

This study was performed to explain a regiospecific demethylation of 3-alkyl-1,2-dimethoxybenzenes.PRDDO-MO calculations show that the low-energy conformation of the carbon of a methoxy group having two ortho neighbors on a benzene ring is located out of the plane of the aromatic ring, whereas a methoxy group with only one ortho neighbor executes restricted rotation in the plane of the ring.The carbon portion of the methoxy group is turned away from the neighboring substituent.These calculations also show that the atomic charge on the oxygen atom in the former caseexceeds that in the latter.The carbon of a methoxy group with two ortho neighbors yields ¹³C NMR T₁ relaxation times longer than those with only one ortho neighbor, also suggesting that the methoxy group with two ortho neighbors is crowded out of the plane of the aromatic ring. ¹³C NMR chemical shifts of these ortho-substituted methoxybenzenes did not correlate well with shifts predicted from published additive parameters; this again suggests an unusual methoxy group orientation and distribution of electrons.The forced rotation of a methoxy group out of the plane of the benzene ring diminishes the release of electrons from the methoxy group to the benzene ring.The resulting higher atomic charge on the oxygen and the orientation of the oxygen orbitals facilitate complexation with Lewis acids and methoxy group cleavage.