Palladium(II)-Catalyzed Allylic CH Oxidation of Hindered Substrates Featuring Tunable Selectivity Over Extent of Oxidation

General Regio- and Diastereoselective Allylic C-H Oxygenation of Internal Alkenes

Branched allylic esters and carboxylates are fundamental motifs prevalent in natural products and drug molecules. The direct allylic C-H oxygenation of internal alkenes represents one of the most straightforward approaches, bypassing the requirement for an allylic leaving group as in the classical Tsuji-Trost reaction. However, current methods suffer from limited scope─often accompanied by selectivity issues─thus hampering further development. Herein we report a photocatalytic platform as a general solution to these problems, enabling the coupling of diverse internal alkenes with carboxylic acids, alcohols, and other O-nucleophiles, typically in a highly regio- and diastereoselective manner.

Chemoenzymatic Synthesis of 13-Oxoverruculogen

Verruculogens are rare fumitremorgin alkaloids that contain a highly unusual eight-membered endoperoxide. In this paper, we report a concise chemoenzymatic synthesis of 13-oxoverruculogen using enzymatic C-H peroxidation and rhodium-catalyzed C-C bond activation reactions to install the eight-membered endoperoxide and the pentacyclic core of the natural product, respectively. Our strategy involves the use of 13-epi-fumitremorgin B as a substrate analog for endoperoxidation by verruculogen synthase, FtmOx1. The resulting product, 13-epi-verruculogen, is the first unnatural endoperoxide generated by FtmOx1 and is used in the first synthesis of 13-oxoverruculogen. This strategy enables a 10-step synthesis of this natural product from commercially available starting materials and illustrates a hybrid approach utilizing biocatalytic and transition-metal-catalyzed reactions to access challenging alkaloid architectures. Moreover, this work demonstrates the use of native enzyme promiscuity as a viable strategy for the chemoenzymatic synthesis of natural products.

Transannular Functionalization of Multiple C(sp3)-H Bonds of Tropane via an Alkene-Bridged Palladium(I) Dimer

This communication describes the Pd-catalyzed C(sp3)-H functionalization of a tropane derivative to generate products with functionalization at two (β/γ) or three (β/γ/β) different sites on the alicyclic amine core. These reactions proceed via an initial dehydrogenation to generate an alkene product that can react further to form a Pd(I) alkene-bridged dimer. Functionalization of this dimer affords β/γ/β-functionalized allylic arylation and allylic acetoxylation products.

Reshuffle Bonds by Ball Milling: A Mechanochemical Protocol for Charge-Accelerated Aza-Claisen Rearrangements

This study presents the development of a mechanochemical protocol for a charge-accelerated aza-Claisen rearrangement. The protocol waives the use of commonly applied transition metals, ligands, or pyrophoric Lewis acids, e.g., AlMe₃. Based on (heterocyclic) tertiary allylamines and acyl chlorides, the desired tertiary amides were prepared in yields ranging from 17% to 84%. Moreover, the same protocol was applied for a Belluš-Claisen-type rearrangement resulting in the synthesis of a 9-membered lactam without further optimization.

Late-Stage Diversification: A Motivating Force in Organic Synthesis

Interest in therapeutic discovery typically drives the preparation of natural product analogs, but these undertakings contribute significant advances for synthetic chemistry as well. The need for a highly efficient and scalable synthetic route to a complex molecular scaffold for diversification frequently inspires new methodological development or unique application of existing methods on structurally intricate systems. Additionally, synthetic planning with an aim toward late-stage diversification can provide access to otherwise unavailable compounds or facilitate preparation of complex molecules with diverse patterns of substitution around a shared carbon framework. For these reasons among others, programs dedicated to the diversification of natural product frameworks and other complex molecular scaffolds have been increasing in popularity, a trend likely to continue given their fruitfulness and breadth of impact. In this Perspective, we discuss our experience using late-stage diversification as a guiding principle for the synthesis of natural product analogs and reflect on the impact such efforts have on the future of complex molecule synthesis.

Benzoquinone Cocatalyst Contributions to DAF/Pd(OAc)2-Catalyzed Aerobic Allylic Acetoxylation in the Absence and Presence of a Co(salophen) Cocatalyst

Palladium(II)-catalyzed allylic acetoxylation has been the focus of extensive development and investigation. Methods that use molecular oxygen (O₂) as the terminal oxidant typically benefit from the use of benzoquinone (BQ) and a transition-metal (TM) cocatalyst, such as Co(salophen), to support oxidation of Pd⁰ during catalytic turnover. We previously showed that Pd(OAc)₂ and 4,5-diazafluoren-9-one (DAF) as an ancillary ligand catalyze allylic oxidation with O₂ in the absence of cocatalysts. Herein, we show that BQ enhances DAF/Pd(OAc)₂ catalytic activity, nearly matching the performance of reactions that include both BQ and Co(salophen). These observations are complemented by mechanistic studies of DAF/Pd(OAc)₂ catalyst systems under three different oxidation conditions: (1) O₂ alone, (2) O₂ with cocatalytic BQ, and (3) O₂ with cocatalytic BQ and Co(salophen). The beneficial effect of BQ in the absence of Co(salophen) is traced to synergistic roles of O₂ and BQ, both of which are capable of oxidizing Pd⁰ to Pd^II The reaction of O₂ generates H₂O₂ as a byproduct, which can oxidize hydroquinone to quinone in the presence of Pd^II NMR spectroscopic studies, however, show that hydroquinone is the predominant redox state of the quinone cocatalyst in the absence of Co(salophen), while inclusion of Co(salophen) maintains oxidized quinone throughout the reaction, resulting in better reaction performance.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Recent advances in Wacker oxidation: from conventional to modern variants and applications

Recent developments in the well-known Wacker oxidation process from conventional to modern variants and applications to natural products' synthesis are compiled in this review.

Operando Spectroscopic and Kinetic Characterization of Aerobic Allylic C-H Acetoxylation Catalyzed by Pd(OAc)2/4,5-Diazafluoren-9-one

Allylic C-H acetoxylations are among the most widely studied palladium(II)-catalyzed C-H oxidation reactions. While the principal reaction steps are well established, key features of the catalytic mechanisms are poorly characterized, including the identity of the turnover-limiting step and the catalyst resting state. Here, we report a mechanistic study of aerobic allylic acetoxylation of allylbenzene with a catalyst system composed of Pd(OAc)₂ and 4,5-diazafluoren-9-one (DAF). The DAF ligand is unique in its ability to support aerobic catalytic turnover, even in the absence of benzoquinone or other co-catalysts. Herein, we describe operando spectroscopic analysis of the catalytic reaction using X-ray absorption and NMR spectroscopic methods that allow direct observation of the formation and decay of a palladium(I) species during the reaction. Kinetic studies reveal the presence of two distinct kinetic phases: (1) a burst phase, involving rapid formation of the allylic acetoxylation product and formation of the dimeric Pd^I complex [Pd^I(DAF)(OAc)]₂, followed by (2) a post-burst phase that coincides with evolution of the catalyst resting state from the Pd^I dimer into a π-allyl-Pd^II species. The data provide unprecedented insights into the role of ancillary ligands in supporting catalytic turnover with O₂ as the stoichiometric oxidant and establish an important foundation for the development of improved catalysts for allylic oxidation reactions.

Palladium-catalyzed allylic C-H oxidation under simple operation and mild conditions

We discovered an effective and simple system (Pd/BQ/air/r.t.) for making allylic alcohols through Pd-catalyzed allylic C-H bond functionalization. This approach exhibits advantages due to its simple operation, mild conditions, and environmentally benign features. By modifying reaction conditions, it can be suitable for preparing unsaturated aldehydes, allylic esters, ethers, and amines.

Catalytic allylic functionalization via π-allyl palladium chemistry

This review highlights the palladium-catalyzed allylic C–H functionalizations via π-allyl palladium reported from early 2014 to present date.

Efficient synthesis of esters through oxone-catalyzed dehydrogenation of carboxylic acids and alcohols

An environmentally friendly oxone (20 mol%) catalyzed esterification of carboxylic acids with alcohols has been developed, providing an attractive alternative to the construction of valuable carbonyl esters.

Palladium-Catalyzed Aerobic Oxygenation of Allylarenes

An efficient and practical protocol for the synthesis of (E)-allylethers from readily available olefins with alcohols or phenols was developed. This aerobic oxidative allylic C-H oxygenation protocol features mild conditions, broad substrate scope, and high atom and step economy, making it a valuable and convenient synthetic method. Notably, molecular oxygen is the sole oxidant in this novel transformation.

Enantioselective Total Synthesis of Nigelladine A via Late-Stage C-H Oxidation Enabled by an Engineered P450 Enzyme

An enantioselective total synthesis of the norditerpenoid alkaloid nigelladine A is described. Strategically, the synthesis relies on a late-stage C-H oxidation of an advanced intermediate. While traditional chemical methods failed to deliver the desired outcome, an engineered cytochrome P450 enzyme was employed to effect a chemo- and regioselective allylic C-H oxidation in the presence of four oxidizable positions. The enzyme variant was readily identified from a focused library of three enzymes, allowing for completion of the synthesis without the need for extensive screening.

Oxidation of Hindered Allylic C-H Bonds with Applications to the Functionalization of Complex Molecules

We report the palladium-catalyzed oxidation of hindered alkenes to form linear allylic esters. The combination of palladium(II) benzoate, 4,5-diazafluoren-9-one, and benzoquinone catalyzes the mild oxidation of terminal alkenes with tert-butyl benzoyl peroxide as an oxidant in the presence of diverse functional groups. Selective oxidation of terminal alkenes in the presence of trisubstituted and disubstituted alkenes has been achieved, and the ability to conduct the reaction on a gram scale has been demonstrated. The mild conditions and high tolerance for auxiliary functionality make this method suitable for the synthesis and derivatization of complex molecules.

Catalytic Multisite-Selective Acetoxylation Reactions at sp2 vs sp3 C-H Bonds in Cyclic Olefins

The first Pd-catalyzed multisite-selective acetoxylation reactions are disclosed at an unactivated alkene sp² C-H bond versus secondary allylic sp³ C-H bond in cyclic olefins via the modulation of directing groups. The different directing groups overcome the key challenge in differentiating C-H bonds and provide a new controlling approach for site-specific C-H activation. A wide variety of substrates are readily acetoxylated under operationally simple conditions. Mechanistic studies suggest that different Pd (IV) intermediates were involved in the multisite-selective acetoxylation reactions.