Mechanistic Analysis of Hydroarylation Catalysts

Abstract

Recently, two organometallic systems ([Ir(μ-acac-O)(acac-O,O)(acac-C^3)]_2 and (Tp)Ru(CO)(Ph)(NCCH_3)) have been discovered that catalyze hydroarylation of unactivated olefins. Herein, we use density functional theory (B3LYP) to study the factors underlying this class of catalysts. In addition, we calculate the key steps for Rh, Pd, Os, and Pt with similar ligand sets. We previously showed there to be two key steps in the process:  (i) insertion of a phenyl into the π bond of a coordinating olefin, and (ii) C−H activation/hydrogen transfer of an unactivated benzene. An important discovery in these studies is that the barriers for these two steps are inversely correlated, complicating optimization of the overall process. However, herein we elucidate the causes of this inverse correlation, laying the foundation for the rational design of improved catalysts. Both steps are directly influenced by the accessibility of the higher 2-electron oxidation state, M^n → M^(n+2). Systems with an easily accessible M^(n+2) state activate C−H bonds easily but suffer from high energy insertions due to significant back-bonding. Conversely, systems without an easily accessible M^(n+2) state have no debilitating back-bonding which makes insertion steps facile, but cannot effectively activate the C−H bond (leading instead to polymerization). The relationship between accessibility of the M^(n+2) state and the amount of back-bonding in the coordinating olefin can be visualized by inspecting the hybridization of the coordinating olefin. Furthermore, we find a linear relation between this hybridization and the barrier to insertion. On the basis of these concepts, we suggest some modifications of the σ framework expected to improve the rates beyond this linear correlation.