Stereoelectronic and Conformational Effects in Meisenheimer Complexes. Intrinsic Reactivities of Spiro vs. 1,1-Dimethoxy and 1-Methoxy-1-phenoxy Complexes

Article abstract of DOI:10.1021/ja00389a057

Rate and equilibrium constants of formation of spiro Meisenheimer complexes derived from 1-(2-hydroxyethoxy)-2,6-dinitro-4-X-benzenes in aqueous solution are compared with those for 1,1-dimethoxy Meisenheimer complexes derived from 2,4-dinitro-4-X-anisoles.Hammett ρ values for the equilibrium constants are 8.2 for the dimethoxy and 5.9 for the spiro complexes, while the normalized ρ values for the rate constants of complex formation (cleavage) are 0.44 (-0.56) for the dimethoxy and 0.58 (-0.42) for the spiro complexes.The large difference in the equilibrium ρ values is attributed to a conformation of a dimethoxy complexes in which there is repulsion between the lone pairs on the ketal and the o-nitro group oxygens.A procedure for making the reactions in the two families thermodynamically comparable by correcting the rate and equilibrium constants for the pK_a difference in the nucleophile and for intramolecularity of spiro complex formation was applied.The corrected parameters show that the spiro complexes form and decompose much faster than the dimethoxy complexes.This enhanced reactivity, ΔΔG^<*>, is attributed to a stereoelectronic effect or p-? overlap between a lone pair orbital of the nonreacting oxygen and the benzene ring, which is only feasible in the transition state of the spiro complex reaction (14) but not in the thransition state of the dimethoxy complex reaction (15).This interpretation is strongly supported by the fact that ΔΔG^<*> increases with increasing electron-withdrawing strength of X substituent.The enhanced rate of formation and decomposition of the spiro complex from catehol 2,4,6-trinitrophenyl ether compared to that of the corresponding 1-methoxy-1-phenoxy complex can be explained in a similar way.