Mechanistic Analysis of Hydroarylation Catalysts

Recently two organometallic systems ([Ir(μ-acac-O)(acac-O,O)(acac-C3)]2 and (Tp)Ru(CO)(Ph)(NCCH3)) 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 pi bond of a coordinating olefin, and
II. C-H activation/hydrogen transfer of an unactivated benzene
We find that the barriers for these two steps are inversely correlated, complicating optimization of the overall process.

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. We find a linear relation between this hybridization and the barrier to insertion.

We suggest some modifications of the sigma framework that might improve the rates beyond this linear correlation. Aside from such modifications of the sigma framework, our analysis suggests that developing a commercially viable hydroarylation catalyst will likely require a significant change in mechanism.

Personnel: Dr. Jonas Oxgaard

This project is in collaboration with
Dr. Roy Periana (USC) and is being sponsored by the ChevronTexaco Energy Research Company

Recent Papers: