A greener alternative to Wittig

The Wittig reaction, in which chemists use phosphorus ylides to make alkenes out of aldehydes or ketones, is both well known and vastly important to industry. Generating ylides requires hazardous or toxic reagents such as n-butyllithium, however, as well as multiple solvent-thirsty purification steps. Adelina Voutchkova-Kostal and coworkers at George Washington University have now developed a greener alternative to the Wittig reaction by using a recyclable palladium catalyst (J. Am. Chem. Soc. 2019, DOI: 10.1021/jacs.9b12354). The researchers synthesized and tested multiple catalysts. The most effective one was based on a Pd-alumina catalyst and can flip through nine cycles before it begins to lose selectivity. Carbon monoxide and water are the only waste. This reaction shines when making alkenes from aliphatic aldehydes and when combining aliphatic and aromatic aldehydes, Voutchkova-Kostal says. What’s striking about the chemistry is its simplicity, she adds. “It’s combining two reactions that have been around for a long time.” The reaction performs a tandem aldol reaction, which brings the aldehyde and alcohol together, then a decarbonylation step, which spits out CO (shown). The researchers put the two together to make that C–C bond, Voutchkova-Kostal says.