Behind the Science: Recycling Phosphine Oxides

Behind the Science: Recycling Phosphine Oxides

Author: J. Chris Slootweg, Sabrina Turba

Dr. Sabrina Turba, Zeitschrift für anorganische und allgemeine Chemie (ZAAC), talked to J. Chris Slootweg, University of Amsterdam, The Netherlands, about his recently published article on the Brønsted-acid-promoted reduction of tertiary phosphine oxides with hydrosiloxanes.

Dr. Slootweg, you have investigated the reduction of tertiary phosphine oxides. What was the inspiration behind this study?

The Wittig reaction is undoubtedly one of the most important conversions in organic chemistry, yet the atom economy of this C=C bond forming reaction is poor due to the production of triphenylphosphine oxide as a byproduct. In industry, each year more than thousand tonnes of phosphine oxides are produced and considered as waste; this material is often not recycled. The development of cheap, versatile reduction protocols for phosphine oxides to the corresponding phosphines is therefore important.

The concept of the circular economy motivates me to design circular chemistry, which highlights the need to reduce the use of chemicals, promote the recovery of chemicals, and advance the recycling of chemicals, preferably in all chemical processes that are operating at the industrial scale. Phosphorus is a critical, irreplaceable raw material that is not widely recovered and recycled, which is why I am focusing on circular phosphorus chemistry.

What is the main significance of your results?

Recently, Brønsted acids, such as phosphoric acids, carboxylic acids, and triflic acid, were found to catalyze the reduction of phosphine oxides to the corresponding phosphines. In our study, we have found that these additives actually do not activate the phosphine oxide, but activate the silane reducing agent instead. E. J. Corey has published the reaction of iPr3SiH with triflic acid resulting in the formation of iPr3SiOTf and dihydrogen [1]. We have applied this knowledge to the reduction of phosphine oxides, and the results indicate that silylated phosphine oxides are likely intermediates in this process.

What is the broader impact of this paper for the scientific community?

Mechanistic insight into new and known reactions is key. This study highlights that the role of additives can influence the activity and selectivity of a reaction dramatically. If one understands what the role of an additive is, this can lead to the development of better processes, and ultimately can lead to the recycling of key chemicals in industry.

How long did this investigation take?

We have been working on the reduction of phosphine oxides for about ten years now and are still targeting an economically viable reduction protocol for the conversion of phosphine oxides into the corresponding phosphines. For specialty phosphines, this ambition is within reach. Triphenylphosphine, however, is produced on a large scale and it is currently cheaper to make a new batch of triphenylphosphine than to recycle the triphenylphosphine oxide waste into triphenylphosphine.

What methods do you use in your work?

I am very much interested in transition-metal-free chemistry and catalysis. So typically in my group, we develop novel synthetic methodologies using main-group chemistry. We use the full gamut of physical organic chemistry tools, in particular density functional theory (DFT) calculations, to elucidate the reaction mechanisms and to generate a transferable understanding which will allow us to design even nicer chemistry.

The article they talked about



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