Palladium-Allyl Bridges Snatch CO₂

Chemists at Yale University have developed a facile route to palladium(I)-bridging allyl dimers (J. Am. Chem. Soc., DOI: 10.1021/ja110708k). Thanks to the synthetic methodology, which circumvents the use of reagents that tend to decompose, the team led by Nilay Hazari was able to study the reactivity of these unusual complexes, revealing their ability to insert carbon dioxide and form bridging carboxylates (shown). Although mononuclear palladium allyl complexes are well understood, relatively little is known about bridging metal-allyl complexes, in part because they are not trivial to prepare. Hazari and coworkers were able to create the complexes by combining commercially available allyl palladium complexes, free ligand, and an allyl Grignard reagent. With their ability to activate CO₂, the dimeric compounds behave like nucleophiles. In fact, when the researchers used a substituted N-heterocyclic carbene as the ligand, they found the dimer to be “one of the most active and stable catalysts reported to date for the carboxylation of allylstannanes and allylboranes with CO₂.” Tin carboxylates are widely used industrially to stabilize polymers made from vinyl chlorides.