Three-bond 31P?31P coupling and 31P chemical shift effects in dimers of phospholium salts and phosphole oxides

The chemically nonequivalent ³¹P nuclei of the Diels-Alder dimers of phospholium ions and phosphole oxides are coupled through three bonds by as much as 35–45 Hz. The chemical shift for a phosphorus atom in the 2-phospholene moiety falls in the expected range; the shift for the constricted atom bridging the 6-membered ring is far downfield, making Δδ as much as 43.5 ppm for one phospholium ion dimer.