Silylium-Ion-Promoted\n(3 + 2) Annulation of Allenylsilanes\nwith Internal Alkynes Involving a Pentadienyl-to-Allyl Cation Electrocyclization

A (3 + 2) annulation\nof allenyl- and, after rapid isomerization,\npropargylsilanes with internal 1-aryl-1-alkynes to form 4-methylenecyclopentenes\nis reported. The reaction is initiated by a silylium ion, and the\ncatalytic cycle is subsequently maintained by the self-regeneration\nof the silylium-ion promoter. Unlike the well-established Danheiser\nannulation, where the allenylsilane serves as a <i>three-carbon\nsynthon</i>, the present transformation engages the allenylsilane\nas a <i>two-carbon synthon</i>. Experimental observations\nand DFT calculations unveil a reaction cascade involving various β-silicon-stabilized\ncarbocations, where a pentadienyl-to-allyl cation electrocyclization\nis the key step.