An otherwise undesired by-product acts as a ligand for the reaction catalyst, resulting in improved reactivity and selectivity in the oxidation of dienes
The palladium-catalysed 1,4-oxidation of 1,3-dienes has been known for over 20 years. It is thought to proceed through the complexation of the diene to a Pd(II) species and sequential addition of two nucleophiles. This is followed by reductive elimination to give a Pd(0) species, which is then reoxidized to complete the catalytic cycle.
Now, an in-depth study into the reaction1 by Martin Eastgate and Frederic Buono from Bristol-Myers Squibb has provided a deeper insight into the reaction mechanism. This led them to develop a more selective reaction, and opened the way to study a new class of ligands. Studying the kinetics of the reaction, Eastgate and Buono observed an induction period before the maximum reaction rate was obtained, and found that the diastereoselectivity of the reaction changed over the course of the reaction. These two observations suggested that a new catalytic species was formed in the course of the reaction. They isolated a Pd(0) complex bearing two bicyclic diene ligands — shown to be the true catalyst — that was both more reactive and selective.
The diene had been identified previously as a major by-product of the reaction. The ease with which such complexes are formed and the modular nature of the ligands should open the way to further improvements in this useful reaction.
Eastgate, M. D. & Buono, F. G. Mechanistic insight into the palladium-catalyzed 1,4-oxidation of 1,3-dienes to 1,4-dicarboxyalk-2-enes. Angew. Chem. Int. Ed. 48, 5958–5961 (2009).