Yang, Qingning; (2023) Organic coupling reactions by heterogeneous photocatalysis. Doctoral thesis (Ph.D), UCL (University College London).

Preview

Text Qingning thesis.pdf - Accepted Version Download (9MB) | Preview

Abstract

Carbon−carbon (C−C) bond formation is one of the most important research areas in modern organic synthesis. Different cross−coupling reactions, e.g. Suzuki Miyaura coupling (SMC) reaction, Sonogashira reaction, Heck reaction and Stille reaction, have been intensively studied and recognised as the very useful processes for constructing C−C bonds. In conventional methods, transition metal catalysts are often used to promote these coupling reactions, and in order to achieve the redox process and overcome the energy barrier, high temperature is always required. Recent research trend has moved to develop a green, sustainable and efficient process for C−C coupling reactions. Photocatalytic technology has been considered as an emerging technique for organic synthesis because the light energy can trigger the electron transfer process, hence carrying out the C−C coupling reaction at ambient temperature with potential high efficiency. In this thesis, three coupling reactions have been investigated, including pinacol coupling reaction, homo-coupling of benzyl chloride and Suzuki-Miyaura cross-coupling reaction. I first reported pinacol coupling of benzaldehyde, which has been successfully carried out with a 96% selectivity. When adjusting experimental conditions, such as pH, light source, and selection of a catalyst, benzaldehyde can be transformed into benzaldehyde dimethyl acetal or benzyl alcohol with 92% and 85% selectivity, respectively. The reaction mechanism was then studied using isotopic labelling and it reveals the difference between the tuning selectivity of hydrogenation and pinacol coupling in the presence/absence of titanium dioxide (P25). I then investigated bibenzyl synthesis using copper-loaded zinc oxide (ZnO) via a homo−coupling route. Here, copper (Cu) exhibits the best performance to promote the coupling of aromatic halides over other transition metals, and Cu−modified ZnO shows a remarkable 92% selectivity compared to a previous report (57% using Cu/TiO2). The low−cost photocatalyst has great stability that there is no obvious decay after 9 runs. It also shows that selective coupling of a series of benzyl chloride derivatives can be achieved using this Cu/ZnO catalyst, demonstrating the heterogeneous photocatalytic C−C coupling as an attractive process for applications. The high selectivity is attributed to the enhanced adsorption of reactants, stabilisation of intermediates (benzyl radicals) for the selective coupling by the Cu loading and the moderate oxidation ability of the ZnO support, besides the promoted charge separation and transfer by Cu species. Finally, Suzuki-Miyaura coupling of aryl bromides was investigated using low concentration palladium (Pd) supported catalyst. Covalent triazine framework-1 (CTF-1) was chosen as the photocatalyst, and a second co-catalyst, copper, was introduced. The concentration of the bimetallic Pd Cu nanoparticles (NPs) was then optimised separately. Compared to the monometallic sample Pd0.2/CTF-1, a 16-fold increase in yield was observed over Pd0.2Cu1/CTF-1, and this activity is comparable against 1.5% Pd loading, providing a new strategy to achieve Suzuki-Miyaura coupling with less cost photocatalyst. The high performance is attributed to the synergistic between PdO and CuOx. Palladium functions as a reduction site to activate the carbon-bromine (C−Br) bonds, while copper species accelerate the oxidative half-reaction during the photocatalytic process and further improve the charge separation and transfer in the system.

Download activity - last month

Export as CSVJSONXML

Download activity - last 12 months

Export as CSVJSONXML

Downloads by country - last 12 months

Export as XMLJSONCSV

Archive Staff Only

View Item