Recent advances in transition-metal-catalyzed [2+2+2] cycloaddition in aqueous phase
Transition metal-catalyzed [2+2+2] cycloaddition has become an evitable route for the synthetic community in the past decades owing to its application to straightforward formation of aromatic or hetero-aromatic compounds. In this minireview, [2+2+2] cycloaddition for synthesis of functionalized benzenes and pyridine derivates in environmentally friendly media is reviewed. A variety of transition metal such as Rh, Ru, and Co-complexes in aqueous phase have contributed significantly to the development of these annulations. These transformation, carried out in pure water or biphase system, exhibited superior advantages because of avoidance of utilization of toxic organic solvent to abbreviate waste emission. On the other hand, high dilution condition can be realized under this condition to suppress the dimerization and trimerization of diynes. From the prospect of sustainability and environment-friendness, investigation of the cycloadditon in depth is highly demand.
Functionalized carbo- and heterocycles play a pivotal role in different areas, including pharmaceuticals industry, noval materials and nature products (1).
Figure 1 provides examples of nature products and top 200 pharmaceutical products by US prescription in 2012 that possess six-membered rings, and the special structure has been confirmed to have antibacterial, anticancer, antiviral activity and can be used as proton pump inhibitors to treat with gastric ulcer.
Transition metal-catalyzed [2+2+2] reactions of alkynes and nitriles have emerged as established methodologies for the formation of carbon-carbon and carbon-heteroatom bonds over the past decades (2). The previous designation of [2+2+2] cycloaddition reaction was first descripted by Reppe using a mixture of NiBr2 and CaC2 in the 19th century (3). Cyclotrimerisation has undeniably attracted scientists’ attention in the coming years. Notably, significant attention and elegant contributions have also been focused on exploring catalyst systems involving various modes of transition-metals, including Co (4), Ru (5), Rh (6), Ni (7), Fe (8), Ir (9), Z ...