Nanometer Gold Catalysts Used In The Nitrogen Clean Synthesis Of Fine Chemicals

Nitrogen-containing compounds including amines, azo-compounds, carbamates and amides play a central role in contemporary chemistry, They are essential for industrial chemical synthesis. To date, preparation of N-containing compounds under mild and waste-free conditions using simple, inexpensive and readily available feed-stocks is still a challenging goal in catalytic and synthetic chemistry. Supported gold nanoparticles (NPs) have emerged as new versatile catalysts for a range of low-temperature transformations, in sharp contrast to the catalytic behavior experienced with those of the traditional noble metals. Apart from the clarification of catalytic origin of nano-Au catalysts, the use of supported Au as new green catalysts in fine chemical synthesis (such as liquid selective oxidation and hydrogenation reactions) under mild conditions have become one of the key aspects of the modern nano-gold catalysis. In this thesis, with the aim to improve the catalytic efficiency and reduce the complexity and pollution of the traditional reaction systems, several supported gold NPs have been developed and employed in the synthesis of various N-containing compounds. Coupled with extensive physicochemical characterizations, reaction mechanism and the specific function of gold NPs were preliminarily studied. The main conclusions are described as follows:Part1:As one of the most fundamental synthetic reactions, hydrogenation of nitroarenes is an important route for the preparation of N-containing compounds. As an alternative way to traditional hydrogenation methods, including stoichiometric reduction and catalytic reduction (hazardous H2as hydrogen source), catalytic transfer hydrogenation (CTH) has the advantages of high selectivity, diverse hydrogen source and environmentally friendly, which has reached broad consensus. Based on the above understandings, this part of work focused on the most recent research development of Au catalysis, and further explored the CTH of nitro-compounds to the synthesis of unstable azocompounds. The results showed that Au nanoparticles supported on meso-Ceria with large BET area (Au/CeO2-meso) can successfully reduce nitroarenes into unstable intermediate-azocompounds, employing iso-propanol as both the hydrogen source and solvent. Furthermore, addition of KOH and H2O can significantly improve the activity and selectivity of azobenzene. Besides, optimization of reaction temperature, addition of KOH and H2O, reaction time can selectively switch the main products to azoxy-compounds or anilines. The unique nature of support and alkali additives played a critical role in this catalytic system.Part2:Based on our previous research work on Au-catalyzed reduction of nitro-compounds and amine N-alkylations, the direct reductive N-alkylation process using nitrobenzene with benzyl alcohol as the model substrates has been invesigated. In this catalytic system, the alcohol may conceivably serve two possible functions:as the hydrogen source for nitro reduction and as the alkylating reagent based on the catalytic hydrogen transfer. The results showed that the TiO2turned out to be the support of choice, and the very small Au-NPs supported on TiO2(noted as Au/TiO2-VS) gave the best secondary amine selectivity when the reaction taken place under the condition of120℃,1:8ratio of nitrobenzene and benzyl alcohol (nphNO2/nPhCH2OH=1:8). Further improving the reaction temperature at prolonged reaction time, structurally complex tertiary amines could be obtained. Under optimized conditions, the use of five equivalents of benzyl alcohol (nphNO2/nphCH20H=1:5) can also afford the selective formation of N-benzylideneaniline. Au/TiO2-VS can also be applied to the reductive coupling of diverse nitro-compounds and various alcohols, illustrating the high applicability of the present protocol to the reductive N-alkylation system, especially for aliphatic alcohols and nitro-compounds which contain reducible functional groups. Moreover, this catalyst can be recycled for several times without obvious loss in activity. Mechanistic studies have revealed that the dehydrogenation step and transfer hydrogenation of nitro group proceeded rapidly, and the subsequent N-alkylation step turned out to be the rate-determining step.Part3:Besides secondary and tertiary amines, other amines containing active carbonyl groups, such as carbamates and amides, are also important synthetic intermediates in agrochemicals, pharmaceuticals and organic synthesis. Based on the previous understandings on "Hydrogen-borrowing" coupling reaction, studies in this part further applied supported Au-catalysts to the "borrowing hydrogen" N-alkylation of carbamates or amides with alcohols. The results showed that meso-ceria supported Au catalyst (Au/CeO2-meso), acting as an efficient multi-functional catalyst, can smoothly catalyze the N-alkylation reaction of simple carbamates or amides with alcohols to prepare complicated compounds under mild conditions.