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
- Oxgaard, J.; Bhalla, G.; Periana, R. A.; Goddard, W. A. III "Mechanistic Investigation of Ir-Catalyzed Hydrovinylation of Olefins" Organometallics 2006, ASAP
- Bhalla, G.; Oxgaard, J.; Periana, R. A.; Goddard, W. A. III "Hydrovinylation of Olefins Catalyzed by an Iridium Complex via C-H Activation" Organometallics 2005, 24, 5499 - 5502.
- Bhalla, G.; Liu, X. Y.; Oxgaard, J.; Goddard, W. A. III; Periana, R. A. "Synthesis, Structure and Reactivity of O-Donor Ir(III) Complexes: C-H Activation Studies with Benzene" J. Am. Chem. Soc. 2005, 127, 11372 - 11389.
- Bhalla, G.; Oxgaard, J.; Periana, R. A.; Goddard, W. A. III "Anti-Markovnikov Hydroarylation of Unactivated Olefins Catalyzed by a Bis-tropolonato Iridium(III) Organometallic Complex" Organometallics, 2005, 24, 3229 - 3232.
- Oxgaard, J.; Periana, R. A.; Goddard, W.A. III "Mechanistic Analysis of Hydroarylation Catalysts" J. Am. Chem. Soc. 2004, 126, 11658 - 11665.
- Oxgaard, J.; Goddard, W.A. III "Mechanism of Ru(II)-Catalyzed Olefin Insertion and C-H Activation from Quantum Chemical Studies" J. Am. Chem. Soc. 2004, 126, 442 - 443.
- Oxgaard, J.; Muller, R.P.; Periana, R.A.; Goddard, W.A. III "The Mechanism of Homogeneous Ir(III) Catalyzed Regioselective Arylation of Olefins" J. Am. Chem. Soc. 2004, 126, 352 - 363.