The Study On The Allylic Oxidation With Oxygen Catalyzed By NHPI Combined With G-C₃N₄or MOFs

Abatract:Selective catalytic allylic oxidation of alkenes into value-added α,β-unsaturated carbonyl compound or alkenyl alcohol compound is a fundamental transformation in organic chemistry that has numerous industrial applications. The resulting product is widely used as the important intermediate of medicines, spices, food additives, agricultural chemicals, etc. Molecular oxygen is the ideal green oxidant of allylic oxidation reaction since it is free, forms water as the only by-product. However, molecular oxygen is paramagnetic with the triplet ground state, which cannot react with common compound with the single ground state. In order to make the reaction conduct smoothly, we should changed the electronic configuration of molecular oxygen. Currently, one of the popular way to activate molecular oxygen is radical method. The nitroxyl radical organocatalysts, such as N-hydroxyphthalimide, has shown excellent catalytic performance for activing molecular oxygen, and attracted much attention. However, the excellent catalytic activity of NHPI should be achieved with transition metals or organic compounds as cocatalyst. To achieve green oxidation process, we conducted the allylic oxidation reaction of a-isophorone (α-IP) to ketoisophorone (KIP) catalyzed by NHPI assited by heterogeneous catalysts with molecular oxygen.Firstly, NHPI combined with graphtic carbon nitride (g-C3N4) was untilized for allylic oxidation of a-IP. The effects of reaction time, temperature, pressure of oxygen and amount of catalyst were investigated extensively. The optimual conditions are:10mmol a-IP,200mg g-C3N4,10mol%NHPI, O21.2MPa, CH3CN as solvent,130℃,5h affording74.8%conversion of a-IP with44.4%selectivity to ketoisophorone. Repeated runs demonstrated that g-C3N4was stable for at least three cycles without noticeable loss of its catalytic activity. This process avoids the use of metal completely, and the reaction condition is mild, apart that, the energy consumption is low. Secondly, we carried out the reaction of pyridine-2,6-dicarboxylic (H2PDA),1,3,5-benzenetricarboxylic acid (H3BTC), pyridine-2,4,6-tricarboxylate (H3ptc) and Cu(NO3)2-3H2O, Co(COOCH3)2-4H2O or Mn(COOCH3)2-4H2O giving rise to nine metal-organic frameworks{M(PDA), M(BTC), M(ptc), M=Cu, Co, Mn}. Then, we presented a heterogeneous protocol by adsorbing NHPI inside the pores of MOFs forming NHPI/MOFs heterogeneous system. The allylic oxidation of a-IP utilizing the NHPI/MOFs as heterogeneous catalyst with moleular oxygen as primary terminal oxidant was conducted. The effects of different catalytic system, reaction time, temperature and catalyst amount were investigated. The optimual conditions are:10mmol a-IP,200mg NHPI/MOFs,110℃,5h. The NHPI/Co(BTC) exhibits optimal catalytic activity affording36.6%conversion of a-IP with81.7%selectivity to KIP. The catalyst could be recycled for at least three cycles without noticeable loss of its catalytic activity and the reaction condition is mild, which make it a green oxidation process.