Phosphorus garners an organic starring role

Over time, chemists have synthesized an incredible array of molecules that combine carbon, hydrogen, nitrogen, and oxygen. Second row periodic table elements such as phosphorus and sulfur have made important guest appearances in compounds, but they tend not to form heavy analogs of common CHNO organic molecules. A research team based at ETH Zurich has reported a rare exception by preparing the phosphorus analog of cyanuric acid.

A six-membered heterocyclic ring, cyanuric acid, C₃N₃(OH)₃, and its derivatives are often used as polymer cross-linking agents and to make industrial chemicals such as herbicides and dyes. Riccardo Suter, Zoltan Benkő, and Hansjörg Grützmacher, who led the discovery team, believe the phosphorus analog, the triphosphabenzene 2,4,6-tri(hydroxy)-1,3,5-triphosphinine, C₃P₃(OH)₃, could play a similar role to make phosphorus-containing plastics and could serve as a valuable ligand for metal catalysts (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201610156).

CHNO compounds were a part of the birthing of modern organic chemistry when Friedrich Wöhler first prepared urea, H₂NC(O)NH₂, in 1828. The next year, Wöhler prepared cyanuric acid (a trimer of isocyanic acid, HNCO), which is made industrially by the pyrolysis of urea. These discoveries have long inspired chemists to explore heavier analogs in which phosphorus takes the place of nitrogen.

“To the best of our knowledge, the phosphorus analogs of cyanuric acid, and its keto form isocyanuric acid, have neither been synthesized nor observed by spectroscopic methods,” Grützmacher and his colleagues note.

The researchers thought they might be able to make the phosphorus analog of cyanuric acid by first generating an analog of isocyanic acid, but efforts to make HPCO failed. After some trial-and-error, they figured out that by using Na(OCP) in conjunction with an organoborane they could generate a boryl-substituted phosphaalkyne. Trimerization of this molecule yielded the needed C₃P₃ ring on a multigram scale. Subsequent treatment of this borylated intermediate with tert-butyl alcohol led to the target C₃P₃(OH)₃.

“Grützmacher and coworkers have made a significant breakthrough,” says Jose M. Goicoechea of the University of Oxford, whose group first synthesized a phosphorus analog of urea, H₂PC(O)NH₂, in 2013. “Their notable finding promises to give rise to a lot of fascinating follow-up chemistry.”

Grützmacher says there is no immediate plan to commercially develop the phosphorus analog. For now, his group is focusing on using the aromatic C₃P₃(OH)₃ and its borylated and silylated derivatives as π–accepting ligands to prepare transition-metal complexes. “We believe they may act as remarkable cooperating ligands and stabilize unusual low-valent metal centers,” he says.