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

The chemically nonequivalent 31 P 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.