Proton Magnetic Resonance Studies of Long-range Coupling Constants: Conformational Dependence of the 4J Coupling Between Side Chain and Ring Protons in Formylheterocycles

Abstract

The ⁴J coupling constant between side chain and ring protons is studied on particular models of planar heterocyclic aldehydes.The preferential conformation of these molecules is established by use of the "zig–zag" rule for the ⁵J coupling constant. This allows precise analysis of the ⁴J coupling across cc and ct pathways.The sign of some of these constants is determined and is found to be negative in either cc or ct pathways. The use of the "methyl replacement technique" shows a dominant σ electron contribution to the ⁴J coupling constants. In all examples studied here, the ⁴J coupling value is dependent on the pathway: ⁴J^ct (−0.30 at −0.45 Hz) > ⁴J^cc (−0.10 at −0.15 Hz). The existence of a stereospecificity for this coupling is in accordance with a preponderant σ electron mechanism and is useful in conformational analysis of planar aromatic aldehydes.