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Allylic Coupling. 4-Bond coupling of vinyl to allylic hydrogens is usually easily observable. We can think of the coupling as having two components, the usual W-coupling transmitted through the σ-system, which is positive and is maximized for the trans proton when the allylic C-H bond is in the plane of the vinyl C-H group (Θ = 0 °, J > 0), and a π-component, which is negative, and whose magnitude is maximized when the allylic C-H bond is perpendicular to the double bond (Θ = 90 °, J < 0) (Garbisch, J. Am. Chem. Soc. 1964, 86, 5561). The positive σ-contribution added to the larger negative π-contribution normally results in a numerically slightly smaller (negative) coupling to the trans vinyl proton, but the effect is small, and not reliable enough for the unambiguous determination of double bond stereochemistry (note the marked entry below in which Jtrans > Jcis) (Barfield, M.; Chakrabarti, B.Chem. Rev. 1969, 69, 757).
