Putting a new twist on an old favorite of organic chemistry, researchers have unveiled the phospha-bora-Wittig reaction. It offers a relatively straightforward way to prepare phosphaalkenes, which are used as ligands in transition-metal catalysts and as polymer building blocks. But its inventors, led by Michael J. Cowley of the University of Edinburgh, say that it might also help to expand the scope of the original Wittig reaction so that it works with a wider range of molecules (J. Am. Chem. Soc. 2021, DOI: 10.1021/jacs.1c06228).
The classic Wittig reaction is a mainstay of synthesis. It can convert an aldehyde or ketone to an alkene, using a phosphonium ylide that reacts with the carbonyl group to form a carbon-carbon double bond. A phosphine oxide is released as a by-product, and the formation of this strong phosphorus-oxygen double bond provides the thermodynamic driving force for the reaction.
Phosphaalkenes bearing a carbon-phosphorus double bond can be made from carbonyl compounds in a similar way, using phospha-Wittig reagents that contain a pair of phosphorus atoms. But these reagents can be unstable and difficult to prepare, and their reactions are limited to a few types of carbonyl compounds.
Cowley’s team has now developed a phosphaborene reagent that can form phosphaalkenes from a much broader group of carbonyl compounds in what they call the phospha-bora-Wittig reaction. “I was impressed that this reaction can be applied not just to ketones, but also esters and amides,” says Florence J. Williams of the University of Iowa, who works on boron-mediated chemistry and was not involved in the research.
The phosphaborene can be prepared as a stable dimer in large batches of 30 g or more. “You can keep it in a jar quite well,” Cowley says. “I would say it’s probably better equipped [than phospha-Wittig reagents] to be used as a kind of standard reagent.”
The team used nuclear magnetic resonance spectroscopy and density functional theory calculations to confirm that the reaction’s mechanism is analogous to the standard Wittig. The phosphaborene dimer reacts with carbonyl compounds to form a phosphaboraoxetane intermediate, a 4-membered ring reminiscent of the oxaphosphetane intermediate formed in the Wittig mechanism. Treatment with aluminum tribromide, followed by pyridine, breaks up the ring to produce phosphaalkenes in yields of 53–95%. The by-product of the reaction, a boroxine, contains newly formed boron-oxygen bonds that are even stronger than phosphorus-oxygen bonds. “I thought it was really smart to use the driving force of that boron-oxygen bond to push this reaction,” Williams says.
In one example, the researchers prepared a fluorenylidene phosphaalkene that could be used to make materials with potentially useful optical and electronic properties.
In the future, Cowley thinks that understanding the factors behind the broad reactivity of the phospha-bora-Wittig reaction might help to improve its iconic progenitor. “I think it gives you a good insight into how to develop the organic Wittig reaction in a useful direction, which is to expand its scope with less-reactive carbonyl compounds,” Cowley says.