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A Diels–Alder Reaction Triggered by a [4 + 3] Metallacycloaddition

IIQ

Ángela Vivancos, Florencia Vattier, Joaquín López-Serrano, Margarita Paneque, Manuel L. Poveda, and Eleuterio Álvarez Journal of the American Chemical Society 2015, Vol. 137, 4074- 4077

The Tp^Me2Ir(III) complex 1-OH₂ (Tp^Me2 = hydrotris(3,5-dimethylpyrazolyl)borate), which contains a labile molecule of water and an iridium-bonded alkenyl moiety (−C(R)═C(R)–(R=CO₂Me)) as part of a benzo-annulated five-membered iridacycle, reacts readily with the conjugated dienes butadiene and 2,3-dimethylbutadiene to afford the corresponding Diels–Alder products. Experimental and DFT studies are in accordance with an initial [4 + 3] cyclometalation reaction between the diene and the five-coordinated 16-electron organometallic fragment 1 (generated from 1-OH₂ by facile water dissociation). The reaction can be extended to a related TpIr(III) complex (Tp = hydrotris(pyrazolyl)borate) that also features a labile ligand (i.e., 2-THF).