‘Mechanochemistry strikes again’ – this time for deoxygenating phosphine oxides

Mechanochemistry can regenerate phosphine oxide waste into phosphine reagents by deoxygenating it under air, new research shows. ‘This method circumvents the need for hazardous organic solvents … and does not require complicated synthetic procedures involving inert gases,’ notes Koji Kubota from Hokkaido University in Japan, who co-led the study with Hajime Ito.

Phosphine oxide deoxygenation is usually performed in solution using highly reactive metal hydrides or low-valent reducing agents. ‘While methods have been developed for this reaction in solution, they are difficult to render operational for three main reasons: reaction length, need for oxygen free conditions and use of large amounts of solvent,’ says Audrey Moores, an expert in mechanochemistry and green chemistry at McGill University in Canada. Despite the importance of organophosphine compounds as synthetic building blocks, easy access to these compounds remains challenging. ‘I was training as an organometallic chemist in a phosphorus chemistry lab at École Polytechnique, France, and back then the question of phosphine deoxygenation was a central conundrum, affecting virtually any aspect of our work,’ adds Moores.

The mechanochemical process developed by Kubota and Ito uses ball milling and a hydrosilane in the presence of a phosphoric acid additive. It takes 30 minutes – a significant timesaving compared to traditional methods that take over 24 hours. While mechanochemistry inherently omits the need for organic solvents, this proved particularly useful for handling phosphine oxides. ‘Since the mechanochemical method does not require substrates to be dissolved in organic solvent, this approach is effective for reactions of poorly soluble compounds. Many phosphine oxides are poorly soluble in organic solvents, and thus their organic transformations in solution are often inefficient,’ explains Kubota.

Moores says she’s ‘truly amazed’ that a reaction typically known to require inert conditions, can be performed under air. ‘It’s a real advantage of this method. Here mechanochemistry strikes again.’

The Hokkaido team tested their mechanochemical deoxygenation process on phosphine oxide byproducts from a Wittig reaction. It successfully recycled the byproducts in situ into organophosphine for the Wittig reaction to use again – ‘a really interesting feature in terms of applicability,’ according to Moores.

However, as is the case for solution-based phosphine oxide deoxygenation, the mechanical process still needs heat – in this case 120°C. Kubota says the team hope to improve the mechanochemical conditions: ‘If we can find conditions that allow deoxygenation of phosphine oxides at room temperature, it would be a breakthrough.’