Long-range spin-spin coupling in 1,4-benzoquinones and some related compounds

Abstract

N.m.r, data for fifteen 1,4-benzoquinones, four 1,4-naphthoquinones, and six cyclohex-2-ene-1,4-diones are tabulated. From these, and previously available data, it is possible to obtain characteristic ranges for methyl-allylic coupling constants (c 1.3 c/s) and methyl-methyl homoallylic coupling constants (c. 1.3 c/s) for this for this series of compounds as well as values for other long-range and vicinal interactions, including a negative coupling across five bonds. On the basis of both chemical shifts and coupling constants it was concluded that 1,4-benzoquinones have little aromaticity from the N.M.R. point of view. The halogen atoms in 5,6-dihalogenocyclohex-2-ene-1,4-diones appear to be tram diaxial.