Journal
ACCOUNTS OF CHEMICAL RESEARCH
Volume 54, Issue 9, Pages 2275-2286Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.accounts.1c00122
Keywords
-
Categories
Funding
- National Natural Science Foundation of China [92061110]
- Hefei National Laboratory for Physical Sciences [KF2020102]
- Anhui University [S020118002/113]
- Swedish Research Council [2019-04042]
- Foundation Olle Engkvist Byggmastare
- Knut and Alice Wallenberg Foundation [KAW 2016.0072]
- Swedish Foundation for Strategic Environmental Research (Mistra: project Mistra SafeChem) [2018/11]
Ask authors/readers for more resources
Palladium-catalyzed oxidations involving cascade processes have challenges in achieving high palladium efficiency and selectivity, but using heterogeneous palladium catalysts immobilized on amino-functionalized support materials has shown promising results in addressing these issues. The catalysts exhibit high activity, selectivity, and efficient recycling ability, and have been applied in various oxidative cascade reactions to access important compounds for medicinal chemistry and functional materials.
CONSPECTUS: 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 Pd-0 during the catalytic cycle, while the latter solution facilitates reoxidation of Pd-0 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 gamma-lactone/gamma-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.
Authors
I am an author on this paper
Click your name to claim this paper and add it to your profile.
Reviews
Recommended
No Data Available