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Title: Mechanism of homogeneous Ir(III) catalyzed regioselective arylation of olefins.

Abstract

The mechanism of hydroarylation of olefins by a homogeneous Ph-Ir(acac){sub 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 {Delta}H(0 K){sub {+-}} = 35.1 kcal/mol and {Delta}G(298 K){sub {+-}} = 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 {Delta}H(0 K){sub {+-}} = 27.0 kcal/mol and {Delta}G(298 K){sub {+-}} = 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 charactermore » of the transition structure is seven coordinate Ir{sup 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 {beta}-hydride products is due to kinetic factors, although {beta}-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.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Sandia National Laboratories (SNL), Albuquerque, NM, and Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
970710
Report Number(s):
SAND2005-3596J
TRN: US201003%%69
DOE Contract Number:  
AC04-94AL85000
Resource Type:
Journal Article
Journal Name:
Proposed for publication in the Journal of the American Chemical Society.
Additional Journal Information:
Journal Name: Proposed for publication in the Journal of the American Chemical Society.
Country of Publication:
United States
Language:
English
Subject:
08 HYDROGEN; ACTIVATION ENERGY; ALKENES; ARYLATION; CATALYSTS; ELECTRON DENSITY; ELECTRONS; HYDROGEN; HYDROGEN TRANSFER; IRIDIUM; KINETICS; MODIFICATIONS

Citation Formats

Muller, Richard Partain, Goddard, III, William A, Periana, Roy A, and Oxgaard, Jonas. Mechanism of homogeneous Ir(III) catalyzed regioselective arylation of olefins.. United States: N. p., 2005. Web.
Muller, Richard Partain, Goddard, III, William A, Periana, Roy A, & Oxgaard, Jonas. Mechanism of homogeneous Ir(III) catalyzed regioselective arylation of olefins.. United States.
Muller, Richard Partain, Goddard, III, William A, Periana, Roy A, and Oxgaard, Jonas. 2005. "Mechanism of homogeneous Ir(III) catalyzed regioselective arylation of olefins.". United States.
@article{osti_970710,
title = {Mechanism of homogeneous Ir(III) catalyzed regioselective arylation of olefins.},
author = {Muller, Richard Partain and Goddard, III, William A and Periana, Roy A and Oxgaard, Jonas},
abstractNote = {The mechanism of hydroarylation of olefins by a homogeneous Ph-Ir(acac){sub 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 {Delta}H(0 K){sub {+-}} = 35.1 kcal/mol and {Delta}G(298 K){sub {+-}} = 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 {Delta}H(0 K){sub {+-}} = 27.0 kcal/mol and {Delta}G(298 K){sub {+-}} = 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{sup 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 {beta}-hydride products is due to kinetic factors, although {beta}-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.},
doi = {},
url = {https://www.osti.gov/biblio/970710}, journal = {Proposed for publication in the Journal of the American Chemical Society.},
number = ,
volume = ,
place = {United States},
year = {Wed Jun 01 00:00:00 EDT 2005},
month = {Wed Jun 01 00:00:00 EDT 2005}
}