Cycloaddition of Alkynes and Isocyanates

Ni/NHC-catalyzed reaction is highly efficient for the general construction of 5,6- (76 to 78) and 6,6-fused (75) pyrones. Notably, the reaction proceeds at 1 atm pressure of CO2 and requires relatively low catalyst loading. A sterically hindered diyne can also be used as a substrate in this cycloaddition (76). In the case of terminal diynes and diynes with phenyl substituents on the termini, oligomerization outcompetes pyrone formation. 

TMS-methyl alkyne, replacement of the alkyl group with an alkenyl group also led to selective formation of monocyclic pyridone 90 in high yields. However, a switch in the regioselectivity was observed when phenyl-methyl alkyne was used (91). This can be attributed to the electron-withdrawing nature of the aromatic groups. 

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