Efficient Synthesis of Unsaturated δ- and ε-Lactones/Lactams by Catalytic Cycloisomerization: When Pt Outperforms Pd

Platinum pincer complexes featuring an SCS indenediide backbone have been prepared and evaluated in the catalytic cycloisomerization of alkynoic acids and N-tosyl alkynylamides. One of the platinum complexes significantly outperforms its palladium analogue for the formation of 6- and 7-membered rings. The catalytic system takes advantage of the alkynophilicity of Pt and of the non-innocent character of the indenediide framework. Like for Pd, the catalytic performance is significantly improved by using an H-bond donor additive such as pyrogallol. For the first time, a large variety of omega-unsaturated delta- and epsilon-lactones/lactams could be prepared with high selectivities and in very good yields.