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[3.2.0.02,4]hept‐6‐ene complexes. The fused tricyclic phosphiranes are obtained as both the Z and the thermally less stable E isomers, the 31P 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 31P 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. Lower your shields! Novel fused tricyclic phosphiranes are reported (see figure). The Z (left) and E isomers have markedly different phosphorus chemical shieldings, which are explained computationally. The steric congestion and reactivity of these strained species has also been investigated.