Imparting Catalyst Control upon Classical Palladium-Catalyzed Alkenyl C–H Bond Functionalization Reactions

Abstract

Conspectus The functional group transformations carried out by the palladium-catalyzed Wacker and Heck reactions are radically different, but they are both alkenyl C-H bond functionalization reactions that have found extensive use in organic synthesis. The synthetic community depends heavily on these important reactions, but selectivity issues arising from control by the substrate, rather than control by the catalyst, have prevented the realization of their full potential. Because of important similarities in the respective selectivity-determining nucleopalladation and β-hydride elimination steps of these processes, we posit that the mechanistic insight garnered through the development of one of these catalytic reactions may be applied to the other. In this Account, we detail our efforts to develop catalyst-controlled variants of both the Wacker oxidation and the Heck reaction to address synthetic limitations and provide mechanistic insight into the underlying organometallic processes of these reactions. In contrast to previous reports, we discovered that electrophilic palladium catalysts with non-coordinating counterions allowed for the use of a Lewis basic ligand to efficiently promote TBHP-mediated Wacker oxidation reactions of styrenes. This discovery led to the mechanistically guided development of a Wacker reaction catalyzed by a palladium complex with a bidentate ligand. This ligation may prohibit coordination of allylic heteroatoms, thereby allowing for the application of the Wacker oxidation to substrates that were poorly behaved under classical conditions. Likewise, we unexpectedly discovered that electrophilic Pd-σ-alkyl intermediates are capable of distinguishing between electronically inequivalent C–H bonds during β-hydride elimination. As a result, we have developed E-styrenyl selective oxidative Heck reactions of previously unsuccessful electronically non-biased alkene substrates using arylboronic acid derivatives. The mechanistic insight gained from the development of this chemistry allowed for the rational design of a similarly E-styrenyl selective classical Heck reaction using aryldiazonium salts and a broad range of alkene substrates. The key mechanistic findings from the development of these reactions provide new insight into how to predictably impart catalyst control in organometallic processes that would otherwise afford complex product mixtures. Given our new understanding, we are optimistic that reactions that introduce increased complexity relative to simple classical processes may now be developed based on our ability to predict the selectivity-determining nucleopalladation and β-hydride elimination steps through catalyst design.

Palladium catalyzed aryl C–H amination with O2via in situ formation of peroxide-based oxidant(s) from dioxane

“…Addition of a more activated peroxide oxidant, tert -butylperoxybenzoate (TBPB), provided an even higher yield of carbazole (82%, entry 3). These results implicate the involvement of 1,4-dioxan-2-hydroperoxide as an oxidant in the reaction, and they resemble previous observations of Alper 14 and Sigman 15 in their studies of Wacker oxidations of alkenes in tetrahydrofuran (THF). In these reactions, oxidative decomposition of THF was observed, and TBHP was found to be effective as an oxidant.…”

Palladium‐Catalyzed anti‐Markovnikov Oxidation of Terminal Alkenes

“…[24] Indeed the use of even these simple ligands has provided more general access to AM oxidation of a-olefins (see below). Although DMF is used predominantly, the palladium-catalyzed oxidation of alkenes is remarkably tolerant to variations in solvent.…”

Übergangsmetallkatalysierte C‐C‐Kupplungen mit Ethylen im Labormaßstab

“…Wir konzentrieren uns auf die richtungsweisenden Reaktionen und stellen neuere Forschungen auf diesem Gebiet vor. Die klassischen industriellen Reaktionen des Ethylens, wie die Wacker-Oxidation, [2,5] die Hydroacylierungs- [6] und Hydroformylierungsreaktionen, [7] werden hier nicht weiter behandelt.…”