Chalcogenation of the 1,4‑C₂P₄ Ring: Oxidation, Isomerization, Insertion, and Ring Contraction

The reaction of H₂C­(PCl₂)₂ with 4 equiv of ^tBuMgCl in tetrahydrofuran (THF) produces 1,4-(CH₂)₂(P^tBu)₄, 1, in about 65% yield. This six-membered ring reacts directly with elemental sulfur or selenium in toluene at low temperatures to give the mono- and dichalcogenides 1,4-(CH₂)₂(P^tBuE)­(P^tBu)₃ (E = S, 2a, E = Se, 2b) and 1,4-(CH₂)₂-2,5-(P^tBuE)₂(P^tBu)₂ (E = S, 3a, E = Se, 3b). X-ray structural determinations showed that 3a and 3b are isostructural in the solid state; the six-membered C₂P₄ ring exhibits a twist-boat geometry with chalcogen substituents in syn positions in each case. Density functional theory (DFT) calculations for the three possible isomers of disubstitution were performed to elucidate the factors that favor the 2,5-isomer. Thermal isomerism was observed in solutions of 3b or 3a in toluene at 60 and 95 °C, respectively, to give the corresponding 2,6-isomers. With an excess of chalcogen in toluene at reflux, the four-membered rings (H₂C)­(P^tBuE)₂E (E = S, 4a, E = Se, 4b) were obtained and identified by multinuclear NMR spectroscopy and single crystal X-ray crystallography, which showed the ^tBu groups in a trans orientation with respect to the CP₂E ring. With a large excess of chalcogen, the five-membered rings (H₂C)­(P^tBuE)₂E₂ (E = S, 5a, E = Se, 5b) were also observed; the X-ray structure of 5b revealed a half-envelope conformation for the CP₂Se₂ ring. The direct reaction of 4a with sulfur in boiling toluene does not produce 5a, whereas 5b is formed slowly and in low yields from 4b and selenium under similar conditions. On the basis of DFT calculations of the relative energies of likely intermediates, chalcogen insertion into the P–P bonds of 3a and 3b to give eight-membered C₂P₄E₂ rings, followed by monomerization, is proposed as a feasible pathway for the formation of the four-membered CP₂E heterocycles 4a and 4b.