Trimerization and cyclization of reactive P-functionalities confined within OCO pincers

In order to stabilize a 10–P–3 species with C_2v symmetry and two lone pairs on the central phosphorus atom, a specialized ligand is required. Using an NCN pincer, previous efforts to enforce this planarized geometry at P resulted in the formation of a C_s-symmetric, 10π-electron benzazaphosphole that existed as a dynamic “bell-clapper” in solution. Here, OCO pincers 1 and 2 were synthesized, operating under the hypothesis that the more electron-withdrawing oxygen donors would better stabilize the 3-center, 4-electron O–P–O bond of the 10–P–3 target and the sp³-hybridized benzylic carbon atoms would prevent the formation of aromatic P-heterocycles. However, subjecting 1 to a metalation/phosphination/reduction sequence afforded cyclotriphosphane 3, resulting from trimerization of the P(i) center unbound by its oxygen donors. Pincer 2 featuring four benzylic CF₃ groups was expected to strengthen the O–P–O bond of the target, but after metal–halogen exchange and quenching with PCl₃, unexpected cyclization with loss of CH₃Cl was observed to give monochlorinated 5. Treatment of 5 with (p-CH₃)C₆H₄MgBr generated crystalline P-(p-Tol) derivative 6, which was characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The complex ¹⁹F NMR spectra of 5 and 6 observed experimentally, were reproduced by simulations with MestreNova.

Attempted synthesis of OCO-supported 10–P–3 species led to trimerization or cyclization.