Bismuth(III)-catalysed hydroalkylation of styrene with acetylacetone: a DFT-Based mechanistic study

Abstract

Density functional theory (DFT) has been used to investigate the mechanism of the experimentally efficient hydroalkylation of styrene with acetylacetone in the presence of a bismuth catalyst. It is shown that the mechanism is fundamentally different to that of the analogous gold-catalysed reaction, even though it leads to the same product. Whereas gold prefers to coordinate to the π-bond of the enol isomer of acetylacetone, bismuth coordinates to the two oxygens to form a chelated complex. Furthermore, the overall reaction with bismuth via the enol isomer of acetylacetone occurs with a much lower activation energy compared to the ketone isomer. In addition, several bismuth catalysts were considered and two of these were shown to have no activity. All of these results have been rationalised in terms of the strength of binding of the metal centres to the acetylacetone. The stronger the binding, the greater the acidity of a proton on acetylacetone, and thus the lower the activation energy for the protonation of styrene, which turns out to be the rate-determining step in the overall reaction. In this way, good agreement is obtained with all the experimental data.

GRAPHICAL ABSTRACT

Keywords:

• Density functional theory (DFT)
• bismuth(III) catalysis
• reaction mechanism
• hydroalkylation
• styrene

Acknowledgments

We thank the Australian Research Council (ARC) for project funding (DP180100904) and the Australian National Computational Infrastructure and the University of Tasmania for the generous allocation of computing time. A.C.B.’s contributions were supported by an ARC Future Fellowship (FT200100049). B.F.Y. wishes to acknowledge the friendship, inspiring conversations since PhD days and the many significant contributions to computational chemistry by Peter Gill, and wishes him all the best for his 60th birthday.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Supplemental Material (free energy reaction profiles, Cartesian coordinates and calculated energies for all species discussed in this work) is available via the Molecular Physics journal website.

Statement of article significance

The significance of this article is that it shows that two reactions that lead to the same product actually follow quite different mechanisms. One reaction is catalysed by a gold complex, while the other is catalysed by a bismuth compound. The gold species prefers to bind to a π-bond in the substrate while the bismuth prefers to bind to the oxygen atoms in the substrate. A comprehensive exploration of the two reactions with DFT allows us to understand and make sense of all the experimental observations.

Additional information

Funding

This work was supported by Australian Research Council: [Grant Number DP180100904]; National Computational Infrastructure: [Grant Number d43].