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).