Reversal of Reactivity in Diene-Complexed o-Quinone Methide Complexes:  Insights and Explanations from ab Initio Density Functional Theory Calculations

• Published in: Organometallics 2005-08, Vol.24 (17), p.4232-4240
• Main Authors: Lev, Daniel A., Grotjahn, Douglas B., Amouri, Hani
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om0504093

The simplest o-quinone methide (o-QM, 2-methylenecyclohexa-3,5-dienone) has only been detected spectroscopically at temperatures below −100 °C, yet its derivatives have been implicated as highly reactive intermediates in organic reactions, including cycloaddition chemistry and DNA modification because of the high electrophilicity of the exocylic methylene carbon. In striking contrast, o-QM complexes of Cp*Ir and Cp*Rh can be isolated, characterized by X-ray diffraction, and show nucleophilic reactivity at the exocyclic carbon. To explain the differences in structure and reactivity as a function of metal fragment and ligand substitution pattern, we report a series of DFT calculations on Ir, Rh, and heretofore intractable Co congeners, along with a representative (arene)Ru analogue. At the B3LYP level with a variety of basis sets, the bend of the o-QM ligand is predicted to increase from Co to Rh to Ir, confirming solid-state data on complexes from the second- and third-row metals. In addition, an attentuation of nucleophilicity is predicted as substitution of the exocyclic methylene carbon is increased. These calculations are the first on o-QM complexes and are expected to guide further synthetic and reactivity studies.