Catalyst-controlled regioselectivity in palladium-catalyzed carbonylation of alkenes has been a long-standing goal of homogeneous catalysis. In general, monophosphines do favor branched regioselectivity, but lead to poor enantioselectivity, while diphosphines give mainly linear products. Previously, we reported the simultaneous control of regio- A nd enantioselectivity in the hydroxy- A nd methoxycarbonylation of vinyl arenes with Pd complexes of the Phanephos ligand. Herein, we present a density functional theory study (B3PW91-D3 level of theory) of the catalytic cycle, supported by deuterium labeling studies, to understand its mechanism. Alkene coordination to a Pd-hydride species was identified as the origin of asymmetric induction and regioselectivity in both the parent Pd/Xylyl-Phanephos catalyst and electron-deficient analogue, and rationalized according to a quadrant-diagram representation. The mechanism by which the preferentially formed pro-(S) Pd-alkene complex can isomerize via rotation around the palladium-(CâC) bond was investigated. In the parent system, this process is in competition with the methanolysis step that leads to the ester product and is responsible for the overall loss of regioselectivity. On the other hand, the introduction of electron-withdrawing substituents on the catalyst framework results in the reduction of the methanolysis barriers, making the isomerization pathway energetically unfavorable and so leading simultaneously to high regiocontrol and good enantiomeric ratios.
Bibliographical noteFunding Information:
M.B. thanks the School of Chemistry and EaStCHEM for support. Calculations were performed on local computer clusters maintained by Dr. H. Früchtl. Dr. Bowen Xie (MChem University of St. Andrews, 2014), and Dr. Jamie Durrani (PhD University of St Andrews, 2014) are thanked for initial d-labelling studies. The EPSRC is thanked for funding P.D. and J.A.F. in the very early stages of this work (EP/M003868/1). The research data supporting this publication can be accessed at https://doi.org/10.17630/f9cc4c38-c55f-480a-9790-cc32899bf179
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ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry