| The formations of carbon-carbon bond are key steps in organic synthesis. The Csp2-Csp2 coupling reaction is one of the most important carbon-carbon bond forming reactions and has become an indispensable tool in modern organic synthesis due to its high efficiency and wide applicability to substrates.In this paper, a ligand-free in situ-generated palladium nanoparticles catalyzed in situ system has been developed, that is, the palladium nanoparticles were formed under coupling reaction conditions. The palladium nanoparticles were formed fast and exhibited stable and highly efficient. It was worth noting that PEG-400 [molecular formula: H(OCH2CH2)nOH; average molecular weight: 400] played versatile roles: green reactive medium, reductant and stabilizer. The spectra of 1H NMR and UV confirmed the reduction of Pd(Ⅱ) to Pd(0) by PEG-400 . This is the first example of in situ catalytic system [Pd(OAc)2/PEG-400] applied in Suzuki reaction, Heck reaction and Ullmann-type reaction.In the catalyzed in situ Suzuki reaction, control experiments have been designed in air, oxygen and nitrogen, respectively. The reactions carried out in air and oxygen had the same highly efficient catalytic activity, but much quicker than those in nitrogen. Thus, it could be concluded that it was the oxygen that promoted the Suzuki reaction and the reaction could be efficiently proceeded in air. Further, the Suzuki reaction between 4-chloronitrobenzene and phenylboronic acid was selected as a model reaction to optimize the reaction conditions. Under the optimized reaction conditions of Pd(OAc)2 as precatalyst, K2CO3 as base, at room temperature in air, a range of aryl chlorides were coupled effectively with arylboronic acids, affording the desired products in 84-98 % isolated yields. Meantime, the effect of the substituents of arylboronic acids on the Suzuki coupling reaction was studied and the roles of the arylboronic acids were disclosed by UV-vis and TEM analysis. Control experiments illustrated that the reactivity of the in situ-generated palladium nanoparticles was much higher than that of the in ex situ-generated ones, corresponding explanation provided on the basis of the TEM study. At last, a proposed mechanism of the oxygen-promoted Suzuki reaction in the in situ catalytic system was given.In the Heck reaction, coupling between 4-bromonitrobenzene and styrene was chosen as a model reaction for screening the conditions. The in situ catalytic system effectively catalyzed the Heck reactions of aryl iodides and aryl bromides under mild conditions and the high yields of products were obtained in most of cases. Excitingly, 1H NMR analysis showed that the catalytic system was exclusively stereoselective for the formation of E-geometrical products. Furthermore, control experiments illustrated that the reactivity of the palladium nanoparticles generated under the Heck reaction conditions was much higher than that of the in ex situ-generated ones. Based on TEM analysis, a proposed mechanism of a ligand-free Heck reaction catalyzed in situ by palladium nanoparticles in PEG-400 was presented.In the Pd(OAc)2/PEG-400-catalyzed Ullmann-type reaction, screening conditions of the reaction were from homocoupling p-bromoanisole as a model reaction. Under the optimized reaction conditions (NaOH as base at 80℃), Ullmann-type reactions of aryl bromides were effectively catalyzed. Moreover, it is noteworthy that the Ullmann-type reaction could be proceeded smoothly without additional reducing agent in the catalytic system.
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