Fused Tricyclic Phosphiranes—Analysis of Phosphorus Chemical Shieldings

1,2‐Addition of transient W(CO) 5 ‐complexed phosphinidenes exo to hexamethyl Dewar benzene affords the novel 3‐phosphatricyclo[3.2.0.0 2,4 ]hept‐6‐ene complexes. The fused tricyclic phosphiranes are obtained as both the Z and the thermally less stable E isomers, the 31 P NMR chemical shifts of which differ by about 60 ppm. A computational investigation shows that the phosphorus pyramidalization and the presence of the γ double bond are responsible for this effect. The semiquantitative results contribute to a more systematic understanding of the structural influences on 31 P chemical shieldings. The congested double bond of the Z isomer can be epoxidized with m ‐chloroperbenzoic acid (MCPBA) to afford a fused tetracyclic P,O bis‐adduct.