A Radical Way To Make Phosphines

By starting with elemental phosphorus, MIT chemists have developed a radical-based route to versatile organophosphorus compounds that avoids the use of toxic and hard-to-handle chlorine (New J. Chem. 2010, 34, 1533). If the method can be upgraded to a catalytic process and successfully scaled up, it could become a primary industrial route for producing phosphorus compounds.

Phosphorus is at the heart of many chemicals such as pharmaceuticals, fertilizers, and pesticides. Trialkyl- and triarylphosphines used as reagents and catalyst ligands to make these various products are currently prepared by chlorinating white phosphorus (P₄) to yield PCl₃, which is then treated with a Grignard or lithium reagent or an organohalide with a harsh reducing agent. Chemists have been searching for a way to streamline this synthesis by making phosphines directly from P₄ and bypassing Cl₂.

Brandi M. Cossairt and Christopher C. Cummins of MIT might have an answer in their general route to PR₃ compounds that employs a titanium amide reducing reagent, Ti(NRRʹ)₃, where R is tert-butyl and Rʹ is di­methylphenyl. The titanium complex selectively extracts a halogen from an organohalide such as PhBr or CyBr (where Ph is phenyl and Cy is cyclohexyl) to generate carbon-centered radicals that P₄ readily traps.

In one experiment, Cossairt and Cummins added PhBr to a mixture of P₄ and the titanium amide in benzene solvent. The stoichiometric process generated triphenylphosphine in less than a minute at room temperature, leading to 72% isolated yield. By comparison, the industrial method for making PPh₃ involves the high-temperature reaction of PhCl with PCl₃ using molten sodium as a reducing agent.

“A chlorine-free process that avoids the use of organometallic reagents would be a highly desirable green approach for large-scale production of phosphorus derivatives,” says Armido Studer of the University of Münster, in Germany, whose group has carried out radical-based organophosphorus chemistry. “This novel process has the potential to become the favored route for large-scale production of PR₃ compounds.”