Highly Diastereoselective Palladium-Catalyzed Oxidative Cascade Carbonylative Carbocyclization of Enallenols

Iridium-catalyzed stereoselective [3+2] annulation of α-oxocarboxylic acids with 1,3-dienes

The stereoselective annulation of α-oxocarboxylic acids with 1,3-dienes proceeded in the presence of a hydroxoiridium catalyst to give α-hydroxy-γ-lactones in good yields with high 3,5-trans relative stereochemistry. The use of a chiral diene ligand for a cationic iridium complex enabled asymmetric annulation with high enantioselectivity.

Efficient Heterogeneous Palladium Catalysts in Oxidative Cascade Reactions

Palladium-catalyzed oxidations involving cascade processes provide a versatile platform for streamlined conversion of simple feedstocks into functional molecules with high atom and step economy. However, the achievement of high palladium efficiency and selectivity in Pd-catalyzed oxidative cascade reactions is still challenging in many cases, as a result of the aggregation of active palladium species to Pd black and the possible side reactions during each bond-forming step. The two current solutions for addressing these issues are either to utilize oxidant-stable ligands or to use electron transfer mediators (ETMs). The former solution, which includes the use of amines, pyridines, sulfoxides, and carbene derivatives, inhibits aggregation of Pd⁰ during the catalytic cycle, while the latter solution facilitates reoxidation of Pd⁰ to Pd^II to improve the activity and selectivity. Following our long-standing interest in Pd-catalyzed oxidations, very recently we developed heterogeneous catalysts to resolve the issues mentioned above in oxidative cascade reactions. The heterogeneous palladium catalysts (Pd-AmP-MCF or Pd-AmP-CNC) comprise palladium nanoclusters (1–2 nm) immobilized on amino-functionalized siliceous mesocellular foam (MCF) or on crystalline nanocellulose (CNC), exhibiting high activity, selectivity as well as excellent recycling ability.

In this Account, we will discuss the synthesis and characterizations of the heterogeneous palladium catalysts, as well as their catalytic behaviors, and the mechanisms involved in their reactions. An important aspect of these catalysts in oxidation reactions is the generation of active Pd(II) species within the heterogeneous phase. Typical oxidative cascade reactions of our recent research on this topic include oxidative carbocyclization-carbonylation, oxidative carbocyclization-borylation, oxidative alkynylation-cyclization, oxidative carbonylation-cyclization, and oxidative carbocyclization-alkynylation. These reactions provide access to important compounds attractive in medicinal chemistry and functional materials, such as γ-lactone/γ-lactam-based poly rings, cyclobutenols, highly substituted furans, and oxaboroles. During these processes, the heterogeneous catalysts exhibited much higher turnover numbers (TONs) than their homogeneous counterparts (e.g., Pd(OAc)₂) as well as unique selectivity that cannot be achieved by homogeneous palladium catalysts. The origin of the high efficiency and unique selectivity of the heterogeneous catalysts was also investigated. Asymmetric syntheses for the construction of optically pure compounds were realized based on the excellent selectivity in these heterogeneous processes. Kinetic studies revealed that the rate and yield of the reactions were essentially maintained during recycling, which demonstrates that Pd-AmP-MCF and Pd-AmP-CNC are robust and highly active in these oxidative cascade reactions. In addition, inductively coupled plasma optical emisson spectroscopy (ICP-OES) analysis and hot filtration test suggest that these processes most likely proceed via a heterogeneous pathway.

Recent progress in our group has shown that the activity of Pd-AmP-MCF and Pd-AmP-CNC could be improved even further by the addition of Ag⁺ to generate cationic Pd(II). Furthermore, intriguing solvent effects were observed in a Pd-AmP-MCF-catalyzed oxidative cascade process, and solvent-controlled chemoselective transformations were developed based on this property of the catalyst. The heterogeneous strategy of this Account provides solutions to palladium deactivation and selectivity issues in Pd(II)-catalyzed oxidative cascade reactions and enables efficient catalyst recycling, which will open up new opportunities in oxidative cascade reactions.

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

The allene functionality has participated in one of the most exciting voyages in organic chemistry, from chemical curiosities to a recurring building block in modern organic chemistry. In the last decades, a special kind of allene, namely, allenol, has emerged. Allenols, formed by an allene moiety and a hydroxyl functional group with diverse connectivity, have become common building blocks for the synthesis of a wide range of structures and frequent motif in naturally occurring systems. The synergistic effect of the allene and hydroxyl functional groups enables allenols to be considered as a unique and sole functionality exhibiting a special reactivity. This Review summarizes the most significant contributions to the chemistry of allenols that appeared during the past decade, with emphasis on their synthesis, reactivity, and occurrence in natural products.

Polycyclic Compounds from Allenes via Palladium-Mediated Intramolecular Carbopalladation/Nucleophilic Substitution Cascade Processes

In recent decades transition metals have made a substantial contribution to the development of novel synthetic processes, with palladium catalysis being, arguably, at the forefront of this research. The efficiency of Pd-promoted C–C or C–X bond formation along with a variety of other transformations renders this metal an indispensable tool in synthetic organic chemistry. Of particular interest are Pd-catalysed multicomponent cascade reactions as they often allow the creation of complex structures from relatively simple starting materials, mimicking in this sense biochemical processes. Allenes as partners in Pd-promoted cascades involving carbopalladation/nucleophilic substitutions have been extensively studied in recent years. Many tactical variants have been explored showing a high level of efficiency and chemoselectivity with predictable outcomes. This short review is focused on intramolecular processes of this type because they provide access to relatively complex polycyclic products, possessing structural features often found in natural products and related compounds. Various approaches are discussed with the intention to demonstrate their applicability and synthetic potential.

1 Introduction

2 Intramolecular Palladium-Promoted Cascades of Allenes

3 Class I Cyclisations

4 Class II Cyclisations

5 Class III Cyclisations

6 Class IV Cyclisations

7 Conclusion

Amino-Supported Palladium Catalyst for Chemo- and Stereoselective Domino Reactions

A solid amino-supported palladium catalyst is used in an oxidative domino reaction for the diastereoselective construction of alkyne-substituted cyclopentenol compounds. This heterogeneous catalyst exhibits high efficiency and excellent chemoselectivity, as well as good recyclability. The chemoselectivity of the domino reactions was readily controlled by switching the solvent and catalyst. Asymmetric syntheses and an oxidative carbocyclization-borylation reaction have also been developed based on the heterogeneous palladium catalyst.