Electroreductive Recycling of Triphenylphosphine Oxide to Triphenylphosphine

Novel electrochemical recycling method for selective and scalable reduction of triphenylphosphine oxide to triphenylphosphine

The Need

Triphenylphosphine (TPP) is a widely used organic chemical, employed in everything from the pharmaceutical to the agro-chemical industries. TPP is a key reagent in many important synthetic pathways, including the Appel, Mitsunobu, Staudinger, and Wittig reactions, where it is stoichiometrically consumed and transformed into triphenylphosphine oxide (TPPO). TPPO has no industrial applications and must generally be discarded or incinerated at great financial and environmental expense. Furthermore, global phosphorous reserves are finite, so an ability to recycle TPPO would be advantageous. Methods to reduce TPPO back to the useful TPP exist but suffer from the need for super-stoichiometric amounts of reductants, high temperatures, large electrical overpotentials, and/or dangerous reaction conditions. Additionally, these methods have been difficult to scale. Thus, an inexpensive and scalable catalytic process for reducing TPPO to TPP that can be performed at ambient temperatures would provide tremendous impact.

The Technology

Researchers at The Ohio State University led by Dr. Christo Sevov have developed a direct electrochemical reduction process for recycling TPPO to TPP in high yield under mild conditions and with catalytic quantities of reductant. An electrolyte solution is created using mild overpotentials that continuously generates Al-based Lewis acid activators, thus necessitating only sub-stoichiometric quantities of activator to initiate the reduction of TPPO. Dr. Sevov’s team further demonstrated that a variety of other phosphine oxides could be reduced using this approach, underscoring its general applicability. The methodology is also compatible with simultaneous chemical reactions which utilize TPP; the researchers performed the electrochemical reduction in situ with a Wittig reaction, demonstrating an 85% yield of the Wittig product with 88% of the resulting TPPO byproduct successfully recycled to TPP. The team demonstrated the potential scalability of the methodology by employing a biphasic solvent mixture that generated TPP from TPPO solutions at as high as 1M concentrations. Finally, the reaction could be conducted at flow, allowing for 40-fold higher current densities to be used. This novel method stands poised to revolutionize phosphine-utilizing industries by taking oxidized phosphine, currently an unused noxious byproduct, and concomitantly recycling it to its useful, reduced form.

Competitive Advantages

  • Recycles useless byproduct into necessary reagent
  • Recycling occurs simultaneously with target transformation
  • Multiple phosphine oxides are compatible
  • Process is scalable

Commercial Applications

  • Any industry utilizing:
    • Appel Reaction
    • Mitsunobu Reaction
    • Staudinger Reaction
    • Wittig Reaction

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