Synthesis of Cyclopentadienes for Cyclopentadienyl Ligands via Cp*Rh-Catalyzed Formal sp C-H Activation/Spiroannulations.

An efficient Cp*Rh-catalyzed formal C(sp)-H activation/spiroannulation of alkylidene Meldrum's acids with alkynes has been developed using catalytical Cu(OAc) and air as the oxidant. This reaction demonstrates a new and straightforward approach to spirocyclopentadienes with Meldrum's acid moieties in good to excellent yields under mild reaction conditions with a broad substrate scope. Notably, this protocol provides a novel and straightforward approach to cyclopentadienes with various substitution patterns and the corresponding cyclopentadienyl-type ligands from simple substrates.