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

The TpMe2Ir­(III) complex 1-OH2 (TpMe2 = 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).