Ortho C - H activation of haloarenes and anisole by an electron-rich iridium(I) complex

Mechanism and origin of regio- and chemoselectivity. An experimental and theoretical study

Eyal Ben-Ari, Revital Cohen, Mark Gandelman, Linda J W Shimon, Jan M L Martin, David Milstein

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Abstract

Reaction of (PNP)Ir(COE)+PF6 - (1) (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine; COE = cyclooctene) with benzene yields a stable unsaturated square pyramidal Ir(III) hydrido-aryl complex, 2, which undergoes arene exchange upon reaction with other arenes at 50 °C. Upon reaction of 1 with haloarenes (chlorobenzene and bromobenzene) and anisole at 50 °C, selective ortho C - H activation takes place. No C - halogen bond activation was observed, even in the case of the normally reactive bromobenzene and despite the steric hindrance imposed by the halo substituent. The ortho-activated complexes (8a, 9a, and 10a) exhibited a higher barrier to arene exchange; that is, no exchange took place when heating at a temperature as high as 60 °C. These complexes were more stable, both thermodynamically and kinetically, than the corresponding meta- and para-isomers (8b,c, 9b,c, and 10b,c). The observed selectivity is a result of coordination of the heteroatom to the metal center, which kinetically directs the metal to the ortho C - H bond and stabilizes the resulting complex thermodynamically. Upon reaction of complex 1 with fluorobenzene under the same conditions, no such selectivity was observed, due to low coordination ability of the fluorine substituent. Competition experiments showed that the ortho-activated complexes 8a, 9a, and 10a have similar kinetic stability, while thermodynamically the chloro and methoxy complexes 8a and 10a are more stable than the bromo complex 9a. Computational studies, using the mPW1K exchange - correlation functional and a variety of basis sets for PNP-based systems, provide mechanistic insight. The rate-determining step for the overall C - H activation process of benzene is COE dissociation to form a reactive 14e complex. This is followed by formation of a η2 c-c intermediate, which is converted into an η2 C-H complex, both being important intermediates in the C - H activation process. In the case of chlorobenzene, bromobenzene, and anisole, η1-coordination via the heteroatom to the 14e species followed by formation of the ortho η2 C-H complex leads to selective activation. The unobserved C - halide activation process was shown computationally in the case of chlorobenzene to involve the same Cl-coordinated intermediate as in the C - H activation process, but it experiences a higher activation barrier. The ortho C - H activation product is also thermodynamically more stable than the C - Cl oxidative addition complex.

Original languageEnglish
Pages (from-to)3190-3210
Number of pages21
JournalOrganometallics
Volume25
Issue number13
DOIs
Publication statusPublished - Jun 19 2006

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anisole
Iridium
Regioselectivity
iridium
Chemical activation
activation
Electrons
electrons
chlorobenzenes
Benzene
Isomers
Fluorobenzenes
isomers
selectivity
Metals
benzene
Halogens
Fluorine
metals
halogens

ASJC Scopus subject areas

  • Inorganic Chemistry
  • Organic Chemistry

Cite this

Ortho C - H activation of haloarenes and anisole by an electron-rich iridium(I) complex : Mechanism and origin of regio- and chemoselectivity. An experimental and theoretical study. / Ben-Ari, Eyal; Cohen, Revital; Gandelman, Mark; Shimon, Linda J W; Martin, Jan M L; Milstein, David.

In: Organometallics, Vol. 25, No. 13, 19.06.2006, p. 3190-3210.

Research output: Contribution to journalArticle

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abstract = "Reaction of (PNP)Ir(COE)+PF6 - (1) (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine; COE = cyclooctene) with benzene yields a stable unsaturated square pyramidal Ir(III) hydrido-aryl complex, 2, which undergoes arene exchange upon reaction with other arenes at 50 °C. Upon reaction of 1 with haloarenes (chlorobenzene and bromobenzene) and anisole at 50 °C, selective ortho C - H activation takes place. No C - halogen bond activation was observed, even in the case of the normally reactive bromobenzene and despite the steric hindrance imposed by the halo substituent. The ortho-activated complexes (8a, 9a, and 10a) exhibited a higher barrier to arene exchange; that is, no exchange took place when heating at a temperature as high as 60 °C. These complexes were more stable, both thermodynamically and kinetically, than the corresponding meta- and para-isomers (8b,c, 9b,c, and 10b,c). The observed selectivity is a result of coordination of the heteroatom to the metal center, which kinetically directs the metal to the ortho C - H bond and stabilizes the resulting complex thermodynamically. Upon reaction of complex 1 with fluorobenzene under the same conditions, no such selectivity was observed, due to low coordination ability of the fluorine substituent. Competition experiments showed that the ortho-activated complexes 8a, 9a, and 10a have similar kinetic stability, while thermodynamically the chloro and methoxy complexes 8a and 10a are more stable than the bromo complex 9a. Computational studies, using the mPW1K exchange - correlation functional and a variety of basis sets for PNP-based systems, provide mechanistic insight. The rate-determining step for the overall C - H activation process of benzene is COE dissociation to form a reactive 14e complex. This is followed by formation of a η2 c-c intermediate, which is converted into an η2 C-H complex, both being important intermediates in the C - H activation process. In the case of chlorobenzene, bromobenzene, and anisole, η1-coordination via the heteroatom to the 14e species followed by formation of the ortho η2 C-H complex leads to selective activation. The unobserved C - halide activation process was shown computationally in the case of chlorobenzene to involve the same Cl-coordinated intermediate as in the C - H activation process, but it experiences a higher activation barrier. The ortho C - H activation product is also thermodynamically more stable than the C - Cl oxidative addition complex.",
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T2 - Mechanism and origin of regio- and chemoselectivity. An experimental and theoretical study

AU - Ben-Ari, Eyal

AU - Cohen, Revital

AU - Gandelman, Mark

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AU - Martin, Jan M L

AU - Milstein, David

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N2 - Reaction of (PNP)Ir(COE)+PF6 - (1) (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine; COE = cyclooctene) with benzene yields a stable unsaturated square pyramidal Ir(III) hydrido-aryl complex, 2, which undergoes arene exchange upon reaction with other arenes at 50 °C. Upon reaction of 1 with haloarenes (chlorobenzene and bromobenzene) and anisole at 50 °C, selective ortho C - H activation takes place. No C - halogen bond activation was observed, even in the case of the normally reactive bromobenzene and despite the steric hindrance imposed by the halo substituent. The ortho-activated complexes (8a, 9a, and 10a) exhibited a higher barrier to arene exchange; that is, no exchange took place when heating at a temperature as high as 60 °C. These complexes were more stable, both thermodynamically and kinetically, than the corresponding meta- and para-isomers (8b,c, 9b,c, and 10b,c). The observed selectivity is a result of coordination of the heteroatom to the metal center, which kinetically directs the metal to the ortho C - H bond and stabilizes the resulting complex thermodynamically. Upon reaction of complex 1 with fluorobenzene under the same conditions, no such selectivity was observed, due to low coordination ability of the fluorine substituent. Competition experiments showed that the ortho-activated complexes 8a, 9a, and 10a have similar kinetic stability, while thermodynamically the chloro and methoxy complexes 8a and 10a are more stable than the bromo complex 9a. Computational studies, using the mPW1K exchange - correlation functional and a variety of basis sets for PNP-based systems, provide mechanistic insight. The rate-determining step for the overall C - H activation process of benzene is COE dissociation to form a reactive 14e complex. This is followed by formation of a η2 c-c intermediate, which is converted into an η2 C-H complex, both being important intermediates in the C - H activation process. In the case of chlorobenzene, bromobenzene, and anisole, η1-coordination via the heteroatom to the 14e species followed by formation of the ortho η2 C-H complex leads to selective activation. The unobserved C - halide activation process was shown computationally in the case of chlorobenzene to involve the same Cl-coordinated intermediate as in the C - H activation process, but it experiences a higher activation barrier. The ortho C - H activation product is also thermodynamically more stable than the C - Cl oxidative addition complex.

AB - Reaction of (PNP)Ir(COE)+PF6 - (1) (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine; COE = cyclooctene) with benzene yields a stable unsaturated square pyramidal Ir(III) hydrido-aryl complex, 2, which undergoes arene exchange upon reaction with other arenes at 50 °C. Upon reaction of 1 with haloarenes (chlorobenzene and bromobenzene) and anisole at 50 °C, selective ortho C - H activation takes place. No C - halogen bond activation was observed, even in the case of the normally reactive bromobenzene and despite the steric hindrance imposed by the halo substituent. The ortho-activated complexes (8a, 9a, and 10a) exhibited a higher barrier to arene exchange; that is, no exchange took place when heating at a temperature as high as 60 °C. These complexes were more stable, both thermodynamically and kinetically, than the corresponding meta- and para-isomers (8b,c, 9b,c, and 10b,c). The observed selectivity is a result of coordination of the heteroatom to the metal center, which kinetically directs the metal to the ortho C - H bond and stabilizes the resulting complex thermodynamically. Upon reaction of complex 1 with fluorobenzene under the same conditions, no such selectivity was observed, due to low coordination ability of the fluorine substituent. Competition experiments showed that the ortho-activated complexes 8a, 9a, and 10a have similar kinetic stability, while thermodynamically the chloro and methoxy complexes 8a and 10a are more stable than the bromo complex 9a. Computational studies, using the mPW1K exchange - correlation functional and a variety of basis sets for PNP-based systems, provide mechanistic insight. The rate-determining step for the overall C - H activation process of benzene is COE dissociation to form a reactive 14e complex. This is followed by formation of a η2 c-c intermediate, which is converted into an η2 C-H complex, both being important intermediates in the C - H activation process. In the case of chlorobenzene, bromobenzene, and anisole, η1-coordination via the heteroatom to the 14e species followed by formation of the ortho η2 C-H complex leads to selective activation. The unobserved C - halide activation process was shown computationally in the case of chlorobenzene to involve the same Cl-coordinated intermediate as in the C - H activation process, but it experiences a higher activation barrier. The ortho C - H activation product is also thermodynamically more stable than the C - Cl oxidative addition complex.

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