Synthesis of palladium nanoparticles under visible light assisted by silicones and its application in Suzuki miyaara coupling reaction

In recent years, chemists have developed the preparation technology of many kinds of metal nanoparticles by light reaction In 2006, scaiano first reported a scheme 1a (J am Chem SOC 2006, 28, 15980) method for the preparation of metal nanoparticles (such as Ag, Au, Cu) using ultraviolet (UV) combined photoinitiator such as i-2959 (1) In the excited state i-2959, Norrish type I C-C bond breaks and carbonyl radicals are produced, which can reduce metal salts to corresponding metal nanoparticles I-2959 can also be fixed to the polymer to prepare Au and Pd nanoparticles of polymer coating In addition, photoinitiator bis (acyl) phosphine oxide (Bapo) has also been successfully applied to the synthesis of UV mediated AG, Au and Pd nanoparticles Although this kind of method is very interesting, expensive special equipment (such as UV reactor, quartz glass cuvette) and harmful UV light limit the universality of this kind of technology Therefore, it will be a great progress to develop the method of preparing metal nanoparticles under visible light (source: org Lett.) recently, a new method (scheme 1b, c) (DOI: 10.1021 / ACS Orglett 7b03892) for the preparation of palladium nanoparticles (pdnps) under visible light with silicone as photoinitiator was reported by the student group of the University of Minster, Germany Silicones have excellent Norrish I-type reactivity (c-Si or C-C bond cleavage) in the visible range, and have been widely used in polymer chemistry In this paper, the author first used it in the synthesis of metal nanoparticles The pdnps obtained by this method can be coated by PVP, and can catalyze Suzuki miyaara coupling reaction efficiently and circularly Palladium nanoparticles were prepared by mixing silicone and Pd (OAC) 2 in a ratio of 40:1 in DMF under inert atmosphere When phenyl (trimethylsilyl) ketone (4) is used as photoinitiator, the brown reaction solution is obtained by irradiating the blue light at λ = 420 nm for 30 minutes The average diameter of PD @ 4 nanoparticles was 5.2 ± 0.9 nm (Figure 1a) The characteristic binding energy of PD 0 can be shown by X-ray photoelectron spectroscopy (XPS), which proves that PD ion is completely reduced to PD 0 When bis (methylsilyl) ketone (5) is used as photoinitiator, dark purple reaction mixture is produced after irradiation with λ = 462 (blue light) or 520 nm (green light) for 1 hour TEM results show that they all form small palladium nanoparticles (PD @ 5, figure 1b) The average diameter of nanoparticles was 1.9 ± 0.6 nm under blue light irradiation and 1.9 ± 0.5 nm under green light irradiation The control experiment shows that photoinitiator and photopair reaction are necessary By adding 2,2,6,6-tetramethylpiperidine-1-oxy (tempo) into the reaction system, the benzoyl-tempo complex was isolated and obtained, which indicated that Norrish I-type c-Si bond homocleavage occurred in silicone (source: org Lett.) it is worth noting that although the exposed pdnp shows high stability in DMF, the nanoparticles converge and integrate into larger particles when the solvent evaporates Pdnp can be further stabilized by adding PVP (molar mass = 10000 g / mol) The polymer PD @ 4pvp and PD @ 5pvp of pdnp and PvP can not aggregate when the solvent evaporates Dynamic light scattering (DLS) tests show that these pdnps are coated with very thin PVP layers (source: org Lett.) in order to test the catalytic activity of these pdnps, the authors used them as catalysts for Suzuki miyaara coupling reaction of phenylboric acid (6a) and iodobenzene (7a) Surprisingly, the larger PD @ 4 PVP (5.2 ± 0.9 nm) showed higher catalytic activity, and the yield of 8AA was 93% (Table 1, entry 2) after 1 hour It is suggested that the low reactivity of PD @ 5 PVP may be due to the strong adsorption of the active intermediate on the surface of NP or the deactivation of the catalyst by 5-derived fragments It is worth noting that PD @ 4 PVP can be easily recycled, and the recycled PD @ 4 PVP still has high catalytic activity (Table 1, entry 3-5) 8 AA (Table 1, entry 6) can be obtained in 94% yield when the reaction is carried out in 1 mmol scale At a lower catalyst loading (0.1 mol%), the coupling reaction was also well carried out, providing 8 AA at 92% yield, but the reaction time must be extended to 4 hours (Table 1, entry 7) However, these pdnp catalysts can not catalyze the reaction of bromobenzene and Chlorobenzene with 6a to form 8AA (source: org Lett.) next, the universality of the substrate was studied with 0.5 mol% PD @ 4 PVP as catalyst The electrical properties of substituents and the position of substituents on aryl iodide 7b - H have little effect on the reaction, and the coupling products can be obtained in excellent yield The reaction of 4-bromophenyl boric acid (6b) with 7a resulted in 4-bromobiphenyl (8ba) with a very low yield (29%) However, the corresponding biphenyl 8ca (99%) and 8da (94%) were obtained in excellent yields from chlorofluoroborates 6C and 6D, indicating that the brominated substituents in 6B were not favorable for the reaction Benzoyl and acyl substituted boric acid 6e and 6F can be converted into biphenyl 8ea (98%) and 8fa (94%) in excellent yields 2-tolyl boric acid (6g) can provide 8 GA biphenyl derivatives in almost quantitative yield The biphenyl 8ha can also be obtained by coupling 2,6-dimethylphenylboric acid (6h) with 7a, but the reaction is slow and the catalyst load must be increased to 1mol%, the yield is low (66%) Conclusion: a new method for the preparation of pdnps with silicone as photoinitiator has been developed by the student group of the University of Minster, Germany The diameter of pdnps can be adjusted to 1.9 or 5.2 nm by adjusting photoinitiator and light source Polyvinylpyrrolidone can be combined with pdnps to form composite materials, which were characterized by TEM, XPS, DLS, NMR and IRJ PD @ PVP composite can be used as a catalyst for the coupling of various boric acids with a series of Suzuki miyarura aryliodides.