Energy Transfer from CdS QDs to a Photogenerated Pd Complex Enhances the Rate and Selectivity of a Pd-Photocatalyzed Heck Reaction

This Article describes the design of a colloidal quantum dot (QD) photosensitizer for the Pd-photocatalyzed Heck coupling of styrene and iodocyclohexane to form 2-cyclohexylstyrene. In the presence of 0.05 mol % CdS QDs, which have an emission spectrum that overlaps the absorption spectrum of a key Pd(II)alkyl iodide intermediate, the reaction proceeds with 82% yield for the Heck product at 0.5 mol % loading of Pd catalyst; no product forms at this loading without a sensitizer. A radical trapping experiment and steady-state and transient optical spectroscopies indicate that the QDs transfer energy to a Pd(II)alkyl iodide intermediate, pushing the reaction toward a Pd(I) alkyl radical species that leads to the Heck coupled product, and suppressing undesired beta-hydride elimination directly from the Pd(II)alkyl iodide. Functionalization of the surfaces of the QDs with isonicotinic acid increases the rate constant of this reaction by a factor of 2.4 by colocalizing the QD and the Pd-complex. The modularity and tunability of the QD core and surface make it a convenient and effective chromophore for this alternative mode of cooperative photocatalysis.