| New catalytic approaches to carbocycles and heterocycles are disclosed. The long-standing challenge of using ketenes in transition-metal catalyzed cycloaddition is successfully addressed by using Ni-phosphine complexes. These complexes catalyzed the cycloaddition of various ketenes and diynes. In general, 2,4-cyclohexadienones were formed instead of products arising from decarbonylation of the ketenes. Efforts to develop the asymmetric version of this cycloaddition chemistry are also discussed. The inability of Ni-phosphine complexes to promote the oxidative coupling of alkyne and nitrile was successfully addressed using the catalytic combination of Ni(cod)2 and Xantphos. This catalyst system was used to couple a variety of diynes and unactivated nitriles to form pyridines. The reaction proceeds under ambient conditions to provide excellent yields of the products. Comparison of this catalyst with the other state-of-the-art catalysts is also provided. An easy and expeditious route to substituted piperidines is also described. A Ni-phosphine complex was used as catalyst for [4 + 2] cycloaddition of 3-azetidinone and alkynes. This unique reaction has broad substrate scope and affords piperidines in excellent yields and excellent regioselectivity. In the reaction of an enantiopure azetidinone, complete retention of stereochemistry was observed. 3-Azetidinone and a variety of diynes also undergo a cycloaddition reaction catalyzed by Ni/IPr to give [5-8] fused dihydroazocine compounds. The reaction involves a challenging C(sp2)-C(sp3) bond cleavage step, yet, surprisingly, proceeds at low temperature. Interestingly, in the case of 2,7-diynes, a spirocyclic pyran product was obtained instead of a [6-8] fused dihydroazocine. This strategy was also extended to oxetanone to access oxocines. A novel mode of reactivity of tropone is disclosed. A variety of diynes were efficiently coupled with tropone using a nickel catalyst to afford tricyclic products. Unsymmetrical diynes were successfully coupled to yield cycloadducts with high regioselectivity. Additionally, biaryls and triaryl frameworks can be easily accessed in a single, chemical operation in good yields and with excellent regiocontrol. |
