1. High Oxidation State, Late Transition Metals Featuring Metal-Ligand Multiple Bonds: Strategies for Sequential C-H Bond Activation and Functionalization
Theodore A Betley, Chemistry & Chemical Biology, Harvard University
In the field of homogenous catalysis, few reports have appeared offering general, chemically mild methods for the introduction of nitrogen- or oxygen-containing functional groups into simple hydrocarbon substrates. The combination of small-molecule activation with C—H bond functionalization represents a significant advance in converting inexpensive chemical feed stocks (e.g. hydrocarbons) to valuable functional molecules with minimal or complete absence of waste generation. We are developing multi-electron redox transformations by mid-to-late, first row transition metal complexes targeting methods for the functionalization of C–H bonds. We have synthesized a new class of electrophilic complexes featuring transiently-formed, or metastable metal-ligand multiple bonds capable of mediating C–H functionalization. The coordination chemistry was investigated for the redox-active dipyrromethane and tris(pyrrolyl)ethane supported transition metal complexes to establish the electronic structure for the pyrrole-based ligand donors. Dipyrromethene (or semi-porphyrin) supported ferrous complexes were found to effect a range of intra- and intermolecular C–H bond functionalization reactions - allowing for the construction of new C–N, C–C and C–O bonds from unactivated C–H bonds, culminating in catalytic amination reactions. While electron-releasing ligands (phosphines, carbenes, and β-diketiminates) have been elegantly employed to stabilize metal-ligand multiple bond formation on mid-late transition metal platforms, we posit that the weak-field ligand environment is critical to the overall efficacy of the dipyrromethene-supported catalyst system.