Electronic properties of furyl substituents at phosphorus and their influence on 31P NMR chemical shifts

The electronic properties of 2-furyl and 3-furyl substituents attached to phosphanes and phosphonium salts were studied by means of IR spectroscopy and experimental and computational P-31 NMR spectroscopy. The heteroaromatic systems proved to be electron withdrawing with respect to phenyl substituents. However, phosphorus atoms with attached furyl substituents are strongly shielded in NMR. The reason for this phenomenon was studied by solid state P-31 MAS NMR experiments. The chemical shift tensor was extracted, and the orientation within the molecules was determined. The tensor component sigma(33), which is effected the most by furyl systems, is oriented perpendicular to the P-C bonds of the substituents. P-furyl bonds are shorter than P-phenyl bonds. We assume therefore a lower ground-state energy of the molecules, because of the electron withdrawing properties of the 2-furyl systems. The sigma(para) component of the P-31 NMR magnetic shielding is therefore smaller, which results in an overall increase of the magnetic shielding.