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Mechanism of Homogeneous Ir(III) Catalyzed Regioselective Arylation of Olefins

Oxgaard, Jonas and Muller, Richard P. and Goddard, William A., III and Periana, Roy A. (2004) Mechanism of Homogeneous Ir(III) Catalyzed Regioselective Arylation of Olefins. Journal of the American Chemical Society, 126 (1). pp. 352-363. ISSN 0002-7863. doi:10.1021/ja034126i.

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The mechanism of hydroarylation of olefins by a homogeneous Ph-Ir(acac)_2(L) catalyst is elucidated by first principles quantum mechanical methods (DFT), with particular emphasis on activation of the catalyst, catalytic cycle, and interpretation of experimental observations. On the basis of this mechanism, we suggest new catalysts expected to have improved activity. Initiation of the catalyst from the inert trans-form into the active cis-form occurs through a dissociative pathway with a calculated ΔH(0 K)^⧧ = 35.1 kcal/mol and ΔG(298 K)^⧧ = 26.1 kcal/mol. The catalytic cycle features two key steps, 1,2-olefin insertion and C−H activation via a novel mechanism, oxidative hydrogen migration. The olefin insertion is found to be rate determining, with a calculated ΔH(0 K)^⧧ = 27.0 kcal/mol and ΔG(298 K)^⧧ = 29.3 kcal/mol. The activation energy increases with increased electron density on the coordinating olefin, as well as increased electron-donating character in the ligand system. The regioselectivity is shown to depend on the electronic and steric characteristics of the olefin, with steric bulk and electron withdrawing character favoring linear product formation. Activation of the C−H bond occurs in a concerted fashion through a novel transition structure best described as an oxidative hydrogen migration. The character of the transition structure is seven coordinate Ir^V, with a full bond formed between the migrating hydrogen and iridium. Several experimental observations are investigated and explained:  (a) The nature of L influences the rate of the reaction through a ground-state effect. (b) The lack of β-hydride products is due to kinetic factors, although β-hydride elimination is calculated to be facile, all further reactions are kinetically inaccessible. (c) Inhibition by excess olefin is caused by competitive binding of olefin and aryl starting materials during the catalytic cycle in a statistical fashion. On the basis of this insertion-oxidative hydrogen transfer mechanism we suggest that electron-withdrawing substituents on the acac ligands, such as trifluoromethyl groups, are good modifications for catalysts with higher activity.

Item Type:Article
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URLURL TypeDescription Information
Goddard, William A., III0000-0003-0097-5716
Periana, Roy A.0000-0001-7838-257X
Additional Information:© 2004 American Chemical Society. Received 11 January 2003. Published online 12 December 2003. Published in print 1 January 2004. We gratefully acknowledge financial support of this research by the Chevron Texaco Energy Research and Technology Co., and thank Dr. William Schinski of Chevron Texaco for helpful discussions. We also wish to thank Guarav Bhalla and Dr. Xiang Yang Liu for suggestions and insights.
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Chevron Texaco Energy Research and Technology Co.UNSPECIFIED
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Record Number:CaltechAUTHORS:20170419-095432581
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Official Citation:Mechanism of Homogeneous Ir(III) Catalyzed Regioselective Arylation of Olefins Jonas Oxgaard, Richard P. Muller, William A. Goddard, III, and Roy A. Periana Journal of the American Chemical Society 2004 126 (1), 352-363 DOI: 10.1021/ja034126i
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76669
Deposited By: Ruth Sustaita
Deposited On:19 Apr 2017 18:04
Last Modified:15 Nov 2021 17:01

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