Thermolysis of acyclic azoalkanes: simultaneous or stepwise carbon-nitrogen homolysis?

Engel, Paul S.; Gerth, Dale B.

doi:10.1021/ja00361a016

Cited by 38 publications

(15 citation statements)

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“…The reaction might involve the formation of diazenyl radical intermediates but these are known to fragment rapidly to the aryl radical and dinitrogen. 18 Whichever diazonium salt is selected, its reduction is the rate determining step in Scheme 3 [see Section (iii)]. Once formed, the aryl radicals 8 and 9 each undergo three competitive reactions of interest: interconversion, Sandmeyer hydroxylation to the corresponding phenols 10 and 11 and cyclisation to the respective hydrofluorenyl radicals 12 and 13.…”

Section: Results (I) Evaluation Of Relative Rate Constantsmentioning

confidence: 99%

“…For the latter, were used those given in Scheme 4, an arbitrary value of 5 × 10 4 dm 3 mol Ϫ1 s Ϫ1 for k 1 R or k 7 R (Scheme 3), and the rate constants given in Here step 1 is represented by k 1 R or k 7 R as the fragmentation of diazenyl radicals is known to be very rapid (>10 6 s Ϫ1 ). 18 Similarly, the combined cyclisation and aromatisation of 8 and 9 were represented by k 8 C and k 9 C , respectively, as the aromatisation steps k 12 A and k 13 A are also very rapid (ca. 10 7 dm 3 mol Ϫ1 s Ϫ1 ).…”

Section: (Iii) Simulation Studiesmentioning

confidence: 99%

“…119 ЊC) (Found: M ϩ 225.1145. C 15 H 15 NO requires M 225.1154); m/z 225 (M ϩ , 66%), 224 (100), 210(20), 182(11), 134(28) 119(12) and 91(18); ν max (Nujol)/cm Ϫ1 3449, 3342 (NH2 ), 1631 (C᎐ ᎐ O); δ H (270 MHz, CDCl 3 ) 2.26 (s, 3H), 2.40 (s, 3H), 6.04 (br, NH 2 ), 6.40 (ddd, J 8.2, 1.7, 0.5, 1H), 6.51 (s, 1H), 7.23 (d, J 8.0, 2H), 7.34 (d, J 8.2, 1H), and 7.53 (d, J 8.0, 2H); δ C (67.9 MHz, CDCl 3 ) 21.5, 21.7, 116.2, 116.9, 117.0, 128.7 (2C), 129.3 (2C), 134.5, 137.5, 141.3, 145.0, 151.0 and 198.4. This amine (4.0 g) was dissolved in a mixture of 50% fluoroboric acid (11 cm 3 ) and water (7 cm 3 ) at 0 ЊC and diazotised by addition of NaNO 2 (1.3 g) in a minimum of water followed by stirring at 0 ЊC for 0.5 h. The precipitate was collected, dissolved in acetone, filtered and reprecipitated by addition of diethyl ether to give 2-(4-methylbenzoyl)-5-methylbenzenediazonium tetrafluoroborate as a yellow solid (1.7 g, 30%), mp 96-98 ЊC [Found: M ϩ (FAB), 237.1026.…”

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confidence: 99%

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Sandmeyer reactions. Part 5. Estimation of the rates of 1,5-aryl/aryl radical translocation and cyclisation during Pschorr fluorenone synthesis with a comparative analysis of reaction energetics

Chandler

Hanson

Taylor

et al. 2001

J. Chem. Soc., Perkin Trans. 2

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During the Pschorr cyclisation of 2-aroylphenyl radicals, a rearrangement occurs reversibly by 1,5-hydrogen transfer to give 2-benzoylaryl radicals. Rate constants of (1.2 ± 0.2) × 10 6 s Ϫ1 at 293 K are estimated for both the forward and back reactions in the equilibrium between 2-(4-methylbenzoyl)phenyl and 2-benzoyl-5-methylphenyl radicals. Assuming an empirical estimate of 1.6 × 10 Ϫ2 dm 3 mol Ϫ1 s Ϫ1 for the hypothetical rate of abstraction of hydrogen from benzene by phenyl radicals, the radical translocation is calculated to occur with a statistically corrected effective molarity of 2.2 × 10 8 mol dm Ϫ3 . By contrast, the competing cyclisation, though occurring at a rate of (8.0 ± 0.9) × 10 5 s Ϫ1 , exhibits an effective molarity of only 5.3 mol dm Ϫ3 . The causes of these differences are analysed in terms of reaction mechanism.

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Section: Results (I) Evaluation Of Relative Rate Constantsmentioning

confidence: 99%

Section: (Iii) Simulation Studiesmentioning

confidence: 99%

mentioning

confidence: 99%

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Sandmeyer reactions. Part 5. Estimation of the rates of 1,5-aryl/aryl radical translocation and cyclisation during Pschorr fluorenone synthesis with a comparative analysis of reaction energetics

Chandler

Hanson

Taylor

et al. 2001

J. Chem. Soc., Perkin Trans. 2

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“…The main reason is the poor stability at room temperature in solution of this intermediate since it is known that loss of dinitrogen took place to give an aryl radical with a fragmentation rate constant in the 0.4–4.0×10 5 s −1 range . Nevertheless, such intermediates have been found to play a key‐role in both photochemical and thermal transformation of a wide range of azaderivatives including cyclic and acyclic azoalkanes, as well as diazenes in general . These elusive radicals are claimed to be electrophilic species due to the negatively polarized N=N group .…”

Section: Methodsmentioning

confidence: 99%

Aryldiazenyl Radicals from Arylazo Sulfones: Visible Light‐Driven Diazenylation of Enol Silyl Ethers

et al. 2020

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A versatile protocol for the diazenylation of enol silyl ethers under visible light irradiation is presented herein. The reaction is based on the underused reaction of a nitrogen-based radical (the diazenyl radical) with an enol silyl ether. Arylazo sulfones were used as photoactivatable precursors of these diazenyl radicals. The resulting azaderivatives are potentially bioactive compounds as well as starting materials for the synthesis of N-containing heterocycles.

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“…Those results, summarized in Scheme 1, can be interpreted in terms of ylide chemistry alone without invoking a diazenyl diradical intermediate (23) as the precursor of that ylide. However, in view of recent evidence for stepwise homolysis of a number of unsymmetric (28) and symmetric (29,30) acyclic azo compounds, it would be premature to dismiss the diazenyl diradical mechanism of oxadiazoline thermolysis entirely at this time.…”

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confidence: 99%

Substituent effects on rates of formation of methoxy-substituted carbonyl ylides from 2-aryl-2-methoxy- and 5-aryl-2-methoxy-Δ³-1,3,4-oxadiazolines

Bekhazi¹,

Smith²,

Warkentin³

1984

Can. J. Chem.

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, 1646 (1984). 2-Aryl-2-methoxy-5.5-dimethyl-A" I ,3,4-oxadiazolines (4) and 5-aryl-2-methoxy-2,5-dimethyl-A3-1,3,4-oxadiazolines (5) were synthesized. Compounds 4 decompose in solution with first order kinetics. Rate constants are correlated with Hammett substituent constants (a-) with p(49.2"C) = 0.74 and 0.89 for CCI, and CD30D, respectively. The final products from 4 indicate that thermolysis involves the cleavage of both C-N bonds, to form Nz and, initially, a carbonyl ylide. Compounds 5, which were obtained as mixtures of cisltrans isomers containing several impurities, and which therefore gave poorer kinetic data, decomposed in CDCI, solution with ~( 4 5°C ) --1.1. Carbonyl ylide intermediates, similar to those from the closelyrelated compounds 4, were assumed on the basis of analogy and on the basis of partial identification of products. The effects of para substituents in the aryl groups of 4 and 5 show that the transition states have greater electron density at C-2 of 4 and at C-5 of 5 than do the starting materials. In spite of the increase in electron density at C-2 of 4, the transition state must be less polar, overall, than the ground state because rate constants for thermolysis of 4 in methanol are smaller than those for CCI, solvent. A plausible explanation for the substituent effects and the solvent effects is that the loss of N2 is concerted, with a transition state resembling more closely a spin paired 1,3-diradical than a I ,3-dipole. Alternative stepwise mechanisms, in which C2-N3 bond scission of 4 and C5-N4 bond scission of 5 are the rate-determining steps, leading to 1,5-diradical intermediates, can not be excluded on the basis of the evidence. intermtdiaires semblables a ceux observCs dans le cas des composts 4 qui leur sont trbs voisins. Les effets des substituants en position para des groupes aryles des composts 4 et 5 montrent que, par rapport aux produits de dtpart, les ttats de transition ont une plus grande densitt Clectronique en C-2 dans les composes 4 et en C-5 dans les composts 5. En dtpit d'une augmentation de la densit6 tlectronique en C-2, dans les composts 4, l'ttat de transition doit Ctre, dans I'ensemble, moins polaire que I'ttat fondamental puisque les constantes de vitesse de la thermolyse des composts 4 dans le mtthanol sont plus faibles que celles dans le CCI,. Une perte de N2 concertte, avec un Ctat de transition ressemblant de trbs prbs un diradical-1.3 de spin pairt plutbt qu'a un dipole, constitue une explication plausible des effets de substituant et de solvant. Les preuves obtenues n'excluent pas la possibilitt d'un micanisme par Ctapes dans lequel la scission de la liaison C2-N3 des composts 4 et de la liaison C5-N4 des composCs 5 constituerait l'ttape dtterminante et conduirait a la formation de diradicaux-1,5 intermtdiaires.[Traduit par le journal]

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Thermolysis of acyclic azoalkanes: simultaneous or stepwise carbon-nitrogen homolysis?

Cited by 38 publications

References 0 publications

Sandmeyer reactions. Part 5. Estimation of the rates of 1,5-aryl/aryl radical translocation and cyclisation during Pschorr fluorenone synthesis with a comparative analysis of reaction energetics

Sandmeyer reactions. Part 5. Estimation of the rates of 1,5-aryl/aryl radical translocation and cyclisation during Pschorr fluorenone synthesis with a comparative analysis of reaction energetics

Aryldiazenyl Radicals from Arylazo Sulfones: Visible Light‐Driven Diazenylation of Enol Silyl Ethers

Substituent effects on rates of formation of methoxy-substituted carbonyl ylides from 2-aryl-2-methoxy- and 5-aryl-2-methoxy-Δ³-1,3,4-oxadiazolines

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Thermolysis of acyclic azoalkanes: simultaneous or stepwise carbon-nitrogen homolysis?

Cited by 38 publications

References 0 publications

Sandmeyer reactions. Part 5. Estimation of the rates of 1,5-aryl/aryl radical translocation and cyclisation during Pschorr fluorenone synthesis with a comparative analysis of reaction energetics

Sandmeyer reactions. Part 5. Estimation of the rates of 1,5-aryl/aryl radical translocation and cyclisation during Pschorr fluorenone synthesis with a comparative analysis of reaction energetics

Aryldiazenyl Radicals from Arylazo Sulfones: Visible Light‐Driven Diazenylation of Enol Silyl Ethers

Substituent effects on rates of formation of methoxy-substituted carbonyl ylides from 2-aryl-2-methoxy- and 5-aryl-2-methoxy-Δ3-1,3,4-oxadiazolines

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Substituent effects on rates of formation of methoxy-substituted carbonyl ylides from 2-aryl-2-methoxy- and 5-aryl-2-methoxy-Δ³-1,3,4-oxadiazolines