Absolute Rate Constants for the Reactions of Primary Alkyl Radicals with Aromatic Amines<sup>1</sup>

Burton, A. G.; Ingold, K. U.; Walton, John C.

doi:10.1021/jo952162x

Cited by 21 publications

(13 citation statements)

References 17 publications

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“…Very minor side products, identified on the basis of their mass spectra as 1-(1-naphthyl)-2- methylpropene and 1-(2-naphthyl)-2-methylpropene, respectively, were also detected. These most likely arise from disproportionation of the rearranged radicals 1b and 2b , similarly to what reported for the related neophyl radical 3b …”

Section: Resultssupporting

confidence: 69%

“…On the basis of EPR measurements, Maillard and Ingold estimated that the neophyl-like rearrangement of the 2-methyl-2-(2-naphthyl)-1-propyl radical ( 2a ) would have a log A value of 11.75 and an activation energy, E a , of 11.3 kcal/mol, which yield a rate of rearrangement k r = 2.9 × 10 3 s -1 at 25 °C. Since in the same paper the rate constant for the neophyl rearrangement was given as k r = 59 s -1 at 25 °C, which has proven to be much lower than the currently accepted value of 1.1 × 10 3 s -1 , a value of ca. 5 × 10 4 s -1 would be expected for radical 2a if relative measurements were correct.…”

mentioning

confidence: 85%

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Synthesis and Calibration of Two Radical Timing Devices: 2-Methyl-2-(1-naphthyl)- and 2-Methyl-2-(2-naphthyl)- 1-bromopropane

et al. 1999

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Radical reactions have gained increasing importance over the past decades in several fields of chemistry and biology, and the measurement of their rates is of great importance both for theoretical and practical purposes. 1 Several direct and indirect methods have been devised for the determination of radical kinetic parameters. Although, in principle, experimental techniques based on the time-resolved detection of the reacting species are more reliable, indirect measurements may become the method of choice in several cases either for the lack of suitable direct techniques or simply because they allow accurate measurements to be made even when expensive and sophisticated instrumentation is not available.A very popular indirect method makes use of competing unimolecular radical reactions as timing devices to investigate the rates of radical-molecule reactions. In this method, called the "radical clock" technique, 2 a unimolecular rearrangement whose kinetic parameters are well-known is set to compete with the bimolecular reaction to be calibrated, as shown in Scheme 1.The ratio, k A /k r , of the rate for reaction of the unrearranged radical U • with the atom donating substrate AB over the known rate for rearrangement to R • can be easily obtained from the analysis of the reaction products ([UA]/ [RA]). This method has proven to be especially valuable in the case of the hydrogen atom abstraction by alkyl radicals from any hydrogen-donating substrate (eq 1).A convenient timing device should satisfy the following requirements: (i) the alkyl radical should be easily generated from a suitable precursor directly in the reaction mixture; (ii) the rate of radical rearrangement should be accurately known and comparable to the pseudo-first-order rate of the bimolecular reaction to be timed, to simultaneously detect products arising from both competing reactions; (iii) the overall system should be sufficiently simple to be represented by Scheme 1; that is, no reaction other than hydrogen abstraction and the unimolecular rearrangement should take place in the same time scale; and (iv) the reaction products should be stable and detectable by means of a simple analytical method such as gas chromatography. Several radical rearrangements, complying with the above requirements, have been studied over the past few years. The slower ones, for which accurate calibrations have been reported over a wide range of temperatures, are collected in Chart 1.It can be seen that no radical clock, rearranging at room temperature with a rate constant in the range between 10 3 s -1 (neophyl and cyclobutylmethyl radicals) and 10 5 s -1 (1-hexenyl radical), is available for common laboratory practice, thus leaving uncovered a gap of 2 orders of magnitude. Incidentally, several substrates of wide interest, such as phenols, react with alkyl radicals with rates that, to be accurately measured, would require a unimolecular competing process falling in this range. 6 On the basis of EPR measurements, Maillard and Ingold 7 estimated that the neophyl-like...

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Section: Resultssupporting

confidence: 69%

mentioning

confidence: 85%

Synthesis and Calibration of Two Radical Timing Devices: 2-Methyl-2-(1-naphthyl)- and 2-Methyl-2-(2-naphthyl)- 1-bromopropane

et al. 1999

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“…Newcomb has reported rate constants for the reactions of the n -octyl radical with both THF and diethyl ether at 22 °C (4.9 × 10 2 and 1.15 × 10 2 M -1 s -1 , respectively). , Comparing these numbers with those given in Table indicates that the n -C 4 F 9 • radical is 63 times more reactive toward THF and 191 times more reactive toward diethyl ether than is the n -octyl radical. Recently, a rate constant for the reaction of the primary, 2-methyl-2-phenylpropyl radical with dodecane at 100 °C was reported (3.5 × 10 3 M -1 s -1 ) . When temperature-corrected using the “standard” value of log A = 8.5 for the preexponential factor recommended for H-abstractions, and normalized for the number of hydrogens, it provides relative rates of 76 and 78 ( n -C 4 F 9 • vs 2-methyl-2-phenylpropyl radical), using our Table k H values for n -heptane and cyclohexane, respectively.…”

Section: Resultsmentioning

confidence: 99%

Absolute Rates of Intermolecular Carbon−Hydrogen Abstraction Reactions by Fluorinated Radicals

et al. 1999

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Using competition kinetic methodology, absolute rate constants for bimolecular hydrogen abstraction from a variety of organic substrates in solution have been obtained for the n-C4H9CF2CF2 •, n-C4F9 •, and i-C3F7 • radicals. Fluorine substitution substantially increases the reactivity of alkyl radicals with respect to C−H abstraction, with the secondary radical being most reactive. A wide range of substrate reactivities (5200-fold) was observed, with the results being discussed in terms of an interplay of thermodynamic, polar, steric, stereoelectronic, and electrostatic/field effects on the various C−H abstraction transition states. Representative carbon−hydrogen bond dissociation energies of a number of ethers and alcohols have been calculated using DFT methodology.

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“…The relative rates of these non-aryl migrations were obtained based on product analysis and the neophyl rearrangement was used as a radical clock to determine the absolute rate constant of the competition reaction, which in turn served as a second radical clock in determining the absolute rate constants of other nonaryl migrations. The same group further revised the rate constant of neophyl rearrangement to be k r1,2 = 4.3 Â 10 4 s À1 at 373 K. 14 In the latest report, Fischer and Weber re-determined the absolute rate constant of neophyl rearrangement with the aid of time-resolved electron spin resonance (ESR) and finally refined the constant to be k r1,2 = 402 s À1 at 298 K. 15 The initial synthetic applications of neophyl rearrangements were rare except for some examples involved in radical ring closure reactions. Parker and co-workers discovered that when bromides 22 were treated with Bu 3 SnH, the resulting radicals 24 underwent cyclization to afford products 23 (Scheme 5).…”

Section: Neophyl Rearrangementmentioning

confidence: 99%

Radical aryl migration reactions and synthetic applications

2015

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Radical aryl migration reactions are of particular interest to the chemical community due to their potential application in radical chemistry and organic synthesis. The neophyl rearrangements used as radical clocks for examining the radical-molecular reactions have been known for decades. The combinations of these migrations with other radical reactions have provided a wide range of novel synthetic methodologies that are complementary to nucleophilic rearrangements. This review will give an overview of various types of radical aryl migrations, with an emphasis on their mechanistic studies from a historical point of view, as well as their application in tandem radical reactions.

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Absolute Rate Constants for the Reactions of Primary Alkyl Radicals with Aromatic Amines¹

Cited by 21 publications

References 17 publications

Synthesis and Calibration of Two Radical Timing Devices: 2-Methyl-2-(1-naphthyl)- and 2-Methyl-2-(2-naphthyl)- 1-bromopropane

Synthesis and Calibration of Two Radical Timing Devices: 2-Methyl-2-(1-naphthyl)- and 2-Methyl-2-(2-naphthyl)- 1-bromopropane

Absolute Rates of Intermolecular Carbon−Hydrogen Abstraction Reactions by Fluorinated Radicals

Radical aryl migration reactions and synthetic applications

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Absolute Rate Constants for the Reactions of Primary Alkyl Radicals with Aromatic Amines1

Cited by 21 publications

References 17 publications

Synthesis and Calibration of Two Radical Timing Devices: 2-Methyl-2-(1-naphthyl)- and 2-Methyl-2-(2-naphthyl)- 1-bromopropane

Synthesis and Calibration of Two Radical Timing Devices: 2-Methyl-2-(1-naphthyl)- and 2-Methyl-2-(2-naphthyl)- 1-bromopropane

Absolute Rates of Intermolecular Carbon−Hydrogen Abstraction Reactions by Fluorinated Radicals

Radical aryl migration reactions and synthetic applications

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Absolute Rate Constants for the Reactions of Primary Alkyl Radicals with Aromatic Amines¹