Coupling of Aromatic Aldehydes with CO2Me-Substituted Tp(Me2)Ir(III) Metallacyclopentadiene

The fully CO2Me substituted aquo-iridacyclopentadiene 1 reacts with a variety of aromatic aldehydes, at 90?120 C, with the formation of bicyclic Fischer-type carbenes, generated by the transfer of the aldehydic H atom to an ? carbon of the metallacycle and concomitant bonding of the O atom to the adjacent ? carbon. These carbenes have a thermodynamically favored anti configuration of these C?H and C?O bonds but it is proposed that an unobserved syn carbene is the kinetic primary product, which then easily epimerizes by adventitious water. Milder reaction conditions (25?60 C) allow for the isolation of intermediate O-coordinated aldehyde adducts. While these reactions have been observed for a wide variety of aromatic aldehydes, 2-pyridinecarboxaldehyde behaves differently, as the reaction leads to a very stable N adduct, in spite of two isomeric O-bonded adducts being formed as kinetic products.