Photochemical Formation of Dibenzosilacyclohept-4-yne for Cu-Free Click Chemistry with Azides and 1,2,4,5-Tetrazines

Photochemical generation of dibenzosilacyclohept-4-yne <b>3</b> from the corresponding cyclopropenone <b>1</b> and its copper-free click reactions are reported. Steady-state irradiation, kinetic, and transient absorption spectroscopy studies revealed that strained alkyne <b>3</b> is rapidly (<5 ns) and efficiently (Φ = 0.58–0.71) photoreleased from <b>1</b> and undergoes remarkably fast, selective, and high-yielding 1,3-dipolar cycloaddition with benzyl azide (∼20 M<sup>–1</sup> s<sup>–1</sup>) or [4 + 2] inverse-electron-demand Diels–Alder reaction with 1,2,4,5-tetrazines (∼260 M<sup>–1</sup> s<sup>–1</sup>) in both methanol and acetonitrile.