Absolute rate expressions for hydrogen atom abstraction from molybdenum hydrides by carbon-centered radicals

James A. Franz, John Linehan, Jerome C. Birnbaum, Kenneth W. Hicks, M. S. Alnajjar

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Abstract

A new family of basis rate expressions for hydrogen atom abstraction by primary, secondary, and tertiary alkyl radicals in dodecane and benzyl radical in benzene from the molybdenum hydride Cp*Mo(CO)3H and for reactions of a primary alkyl radical with CpMo(CO)3H in dodecane are reported (Cp* = η5-pentamethylcyclopentadienyl, Cp = η5- cyclopentadienyl). Rate expressions for reaction of primary, secondary, and tertiary radical clocks with Cp*Mo(CO)3H were as follow: for hex-5-enyl, log(k/M-1 s-1) = (9.27 ± 0.13) - (1.36 ± 0.22)/θ, θ = 2.303RT kcal/mol; for hept-6-en-2-yl, log(k/M-1 s-1) = (9.12 ± 0.42) - (1.91 ± 0.74)/θ; and for 2-methylhept-6-en-2-yl, log(k/M-1 s-1) = (9.36 ± 0.18) - (3.19 ± 0.30)/θ (errors are 2σ). Hydrogen atom abstraction from CpMo(CO)3H by hex-5-enyl is described by log(k/M-1 s-1) = (9.53 ± 0.34) - (1.24 ± 0.62)/θ. Relative rate constants for 1°:2°:3°alkyl radicals were found to be 26:7:1 at 298 K. Benzyl radical was found to react 1.4 times faster than tertiary alkyl radical. The much higher selectivities for Cp*Mo(CO)3H than those observed for main group hydrides (Bu3SnH, PhSeH, PhSH) with alkyl radicals, together with the very fast benzyl hydrogen- transfer rate, suggest the relative unimportance of simple enthalpic effects and the dominance of steric effects for the early transition-state hydrogen transfers. Hydrogen abstraction from Cp*Mo(CO)3H by benzyl radicals is described by log(k/M-1 s-1) = (8.89 ± 0.22) - (2.31 ± 0.33)/θ.

Original languageEnglish
Pages (from-to)9824-9830
Number of pages7
JournalJournal of the American Chemical Society
Volume121
Issue number42
DOIs
Publication statusPublished - Oct 27 1999

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Molybdenum
Hydrides
Hydrogen
Carbon
Atoms
Benzene
Clocks
Rate constants

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Absolute rate expressions for hydrogen atom abstraction from molybdenum hydrides by carbon-centered radicals. / Franz, James A.; Linehan, John; Birnbaum, Jerome C.; Hicks, Kenneth W.; Alnajjar, M. S.

In: Journal of the American Chemical Society, Vol. 121, No. 42, 27.10.1999, p. 9824-9830.

Research output: Contribution to journalArticle

Franz, James A. ; Linehan, John ; Birnbaum, Jerome C. ; Hicks, Kenneth W. ; Alnajjar, M. S. / Absolute rate expressions for hydrogen atom abstraction from molybdenum hydrides by carbon-centered radicals. In: Journal of the American Chemical Society. 1999 ; Vol. 121, No. 42. pp. 9824-9830.
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abstract = "A new family of basis rate expressions for hydrogen atom abstraction by primary, secondary, and tertiary alkyl radicals in dodecane and benzyl radical in benzene from the molybdenum hydride Cp*Mo(CO)3H and for reactions of a primary alkyl radical with CpMo(CO)3H in dodecane are reported (Cp* = η5-pentamethylcyclopentadienyl, Cp = η5- cyclopentadienyl). Rate expressions for reaction of primary, secondary, and tertiary radical clocks with Cp*Mo(CO)3H were as follow: for hex-5-enyl, log(k/M-1 s-1) = (9.27 ± 0.13) - (1.36 ± 0.22)/θ, θ = 2.303RT kcal/mol; for hept-6-en-2-yl, log(k/M-1 s-1) = (9.12 ± 0.42) - (1.91 ± 0.74)/θ; and for 2-methylhept-6-en-2-yl, log(k/M-1 s-1) = (9.36 ± 0.18) - (3.19 ± 0.30)/θ (errors are 2σ). Hydrogen atom abstraction from CpMo(CO)3H by hex-5-enyl is described by log(k/M-1 s-1) = (9.53 ± 0.34) - (1.24 ± 0.62)/θ. Relative rate constants for 1°:2°:3°alkyl radicals were found to be 26:7:1 at 298 K. Benzyl radical was found to react 1.4 times faster than tertiary alkyl radical. The much higher selectivities for Cp*Mo(CO)3H than those observed for main group hydrides (Bu3SnH, PhSeH, PhSH) with alkyl radicals, together with the very fast benzyl hydrogen- transfer rate, suggest the relative unimportance of simple enthalpic effects and the dominance of steric effects for the early transition-state hydrogen transfers. Hydrogen abstraction from Cp*Mo(CO)3H by benzyl radicals is described by log(k/M-1 s-1) = (8.89 ± 0.22) - (2.31 ± 0.33)/θ.",
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T1 - Absolute rate expressions for hydrogen atom abstraction from molybdenum hydrides by carbon-centered radicals

AU - Franz, James A.

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AU - Birnbaum, Jerome C.

AU - Hicks, Kenneth W.

AU - Alnajjar, M. S.

PY - 1999/10/27

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N2 - A new family of basis rate expressions for hydrogen atom abstraction by primary, secondary, and tertiary alkyl radicals in dodecane and benzyl radical in benzene from the molybdenum hydride Cp*Mo(CO)3H and for reactions of a primary alkyl radical with CpMo(CO)3H in dodecane are reported (Cp* = η5-pentamethylcyclopentadienyl, Cp = η5- cyclopentadienyl). Rate expressions for reaction of primary, secondary, and tertiary radical clocks with Cp*Mo(CO)3H were as follow: for hex-5-enyl, log(k/M-1 s-1) = (9.27 ± 0.13) - (1.36 ± 0.22)/θ, θ = 2.303RT kcal/mol; for hept-6-en-2-yl, log(k/M-1 s-1) = (9.12 ± 0.42) - (1.91 ± 0.74)/θ; and for 2-methylhept-6-en-2-yl, log(k/M-1 s-1) = (9.36 ± 0.18) - (3.19 ± 0.30)/θ (errors are 2σ). Hydrogen atom abstraction from CpMo(CO)3H by hex-5-enyl is described by log(k/M-1 s-1) = (9.53 ± 0.34) - (1.24 ± 0.62)/θ. Relative rate constants for 1°:2°:3°alkyl radicals were found to be 26:7:1 at 298 K. Benzyl radical was found to react 1.4 times faster than tertiary alkyl radical. The much higher selectivities for Cp*Mo(CO)3H than those observed for main group hydrides (Bu3SnH, PhSeH, PhSH) with alkyl radicals, together with the very fast benzyl hydrogen- transfer rate, suggest the relative unimportance of simple enthalpic effects and the dominance of steric effects for the early transition-state hydrogen transfers. Hydrogen abstraction from Cp*Mo(CO)3H by benzyl radicals is described by log(k/M-1 s-1) = (8.89 ± 0.22) - (2.31 ± 0.33)/θ.

AB - A new family of basis rate expressions for hydrogen atom abstraction by primary, secondary, and tertiary alkyl radicals in dodecane and benzyl radical in benzene from the molybdenum hydride Cp*Mo(CO)3H and for reactions of a primary alkyl radical with CpMo(CO)3H in dodecane are reported (Cp* = η5-pentamethylcyclopentadienyl, Cp = η5- cyclopentadienyl). Rate expressions for reaction of primary, secondary, and tertiary radical clocks with Cp*Mo(CO)3H were as follow: for hex-5-enyl, log(k/M-1 s-1) = (9.27 ± 0.13) - (1.36 ± 0.22)/θ, θ = 2.303RT kcal/mol; for hept-6-en-2-yl, log(k/M-1 s-1) = (9.12 ± 0.42) - (1.91 ± 0.74)/θ; and for 2-methylhept-6-en-2-yl, log(k/M-1 s-1) = (9.36 ± 0.18) - (3.19 ± 0.30)/θ (errors are 2σ). Hydrogen atom abstraction from CpMo(CO)3H by hex-5-enyl is described by log(k/M-1 s-1) = (9.53 ± 0.34) - (1.24 ± 0.62)/θ. Relative rate constants for 1°:2°:3°alkyl radicals were found to be 26:7:1 at 298 K. Benzyl radical was found to react 1.4 times faster than tertiary alkyl radical. The much higher selectivities for Cp*Mo(CO)3H than those observed for main group hydrides (Bu3SnH, PhSeH, PhSH) with alkyl radicals, together with the very fast benzyl hydrogen- transfer rate, suggest the relative unimportance of simple enthalpic effects and the dominance of steric effects for the early transition-state hydrogen transfers. Hydrogen abstraction from Cp*Mo(CO)3H by benzyl radicals is described by log(k/M-1 s-1) = (8.89 ± 0.22) - (2.31 ± 0.33)/θ.

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