Research On Palladium-catalyzed Aerobic Oxidation Of Alkenes With Electron-withdrawing Groups

Petroleum feedstocks serve as the primary source of the world's industrial organic chemicals, ranging from pharmaceuticals and agrochemicals to large-scale commodity products. Petroleum primarily consists of reduced hydrocarbons, and their selective oxidation chemistry remains one of the foremost challenges in the chemical industry. Since the discovery of the notably PdCl₂/CuCl₂/O₂ catalyzed Wacker oxidation which transfers terminal alkenes to corresponding aldehyde in the late 1950s, enthusiasm for the Pd catalyzed aerobic oxidation reactions has never stopped.Pd catalyzed aerobic oxidation is generally divided into two steps: 1) selective oxidation of the organic substrate by the oxidized catalyst and 2) efficient dioxygen-coupled reoxidation of the reduced catalyst. The key to the success of Pd catalyzed aerobic oxidation is to solve the fundamentally key problem: how to effectively get the reduced catalyst spieces Pd (0) oxidized to Pd (II) by molecular oxygen. Thermodynamically, molecular oxygen is capable of oxidizing reduced Pd (0) spieces to active Pd (II) catalyst, however, direct aerobic oxidation of palladium often cannot compete with kinetically aggregation of the Pd (0) spieces into inactive bulk metal. How to solve such a contradiction derivativats the two strategies for palladium catalyzed aerobic oxidation. One is to add cocatalyst systems to break down the thermodynamic energy barrier of oxidation of Pd (0) by molecular oxygen into smaller ones; The other is to add a Ligand system which will stable the Pd(0) spieces and hold back its aggregation.In this contest, we have developed two different Palladium catalyzed aerobic oxidation systems with molecular oxygen as the sole oxidatant: PdII/O₂/scCO₂(MeOH) system and PdII/DMF/O₂ system, and product control of palladium-catalyzed aerobic oxidation of terminal olefins with electron-withdrawing groups can be achieved through modifying the catalytic system conditions:1, PdII/O₂/scCO₂ (MeOH) system, when using excess MeOH, higher pressure of oxygen and lower temperature, acetalization of olefin with electron withdrawing groups can be achieved. Compared with previous catalytic system with CuCl₂ as a cocatalyst, it successfully avoids the presence of a large number of chloride ions, which is beneficial to the process ofβ-hydride elimination, thereby facilitating the formation of acetals, and also effectively inhibited the Michael addition product.2, PdII/O₂/scCO₂ (MeOH) system, when using appropriate dosage of MeOH, lower pressure of oxygen and higher temperature, cyclotrimerization of terminal olefin with electron -withdrawing groups can be achieved. This approach is of great synthetic significance, it broke through the regioselectivity and substrate limitations for cyclotrimerization of alkynes. It only uses the cheap and readily available olefins as raw materials and posess a higher potential value in industry application.3, PdII/DMF/O₂ system, when the reaction system was strictly dryed, cyclotrimerization ofα-carbonyl terminal olefin can be achieved. Both the alkyl and aryl olefins can be cyclotrimerized to 1,3,5-trisubstituted benzene derivatives. Compared to the PdII/O₂/scCO₂ (MeOH) system, the lack of MeOH avoids the acetalization and transesterification. These 1,3,5-trisubstituted benzene derivatives are very important initiators in the field of polymer chemistry to the synthesis of well-defined star polymers.4, PdII/DMF/O₂ system, when the reaction system was not dried, we have established an in situ DMF hydrolysis producing PdCl₂(HNMe₂)₂ to catalyze highly selective 2: 1 cross [2+2 +2] cycloaddition of alkynoates and alkenes with electron-withdrawing groups under molecular oxygen to synthesize pentakis-substituted benzenes. This methodology not only makes an extension of alkyne surrogates from special alkenes with leaving groups to simple alkenes under molecular oxygen, but also makes the olefins which can not cyclotrimerize in the the previous one-component system, including acrylonitrile, acrylic acid, and acrylamide derivatives, oxidatively coupled with alkynoates to smoothly yield aromatic products in good to excellent yields.