Ni-Catalyzed Cycloaddition Approach to Coumarins by C-C σ-Bond Cleavage

Significance

Reported is the Ni-catalyzed cycloaddition of o-arylcarboxybenzonitriles 1 with alkynes 2 to give coumarins 3. Interestingly, the reaction proceeds with the formal cleavage of two C-C σ-bonds (C-CN and C-CO) alongside the insertion of the alkyne to form two new C-C bonds. A new benzonitrile (ArCN) is formed as a byproduct of the reaction, potentially giving an insight into the mechanism. Both EDGs and EWGs at R^¹ are ­tolerated. When unsymmetrical alkynes were used as substrates, mixtures of regioisomers (1:1 to 3:1) were obtained, with the more sterically ­demanding group (R^²) placed at C-4. Terminal alkynes failed to participate in the reaction due to rapid oligomerization. In a valuable extension to the methodology, an o-arylamide substituted benzonitrile gave the corresponding 2-quinolone in 83% yield.