Radical Stabilization and Ground State Polar Substituent Effects in the Thermal Decomposition of Azoalkanes

Nau, Werner M.; Harrer, Heinrich M.; Adam, Waldemar

doi:10.1021/ja00103a012

Cited by 40 publications

(32 citation statements)

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“…This is in accordance with the previously observed Evans-Polanyi relationship. 7 Evidence for a polar effect in this reaction, which had been previously detected for the related deazatization of phenylazomethanes, 26 was not obtained.…”

Section: Discussionmentioning

confidence: 68%

“…Rather, the observed dependence is diagnostic for a radical-stabilizing effect, since both electron-donating and -withdrawing para substituents are known to stabilize the benzylic radicals, 24 which are being produced by decarbonylation (Scheme 2). Indeed, the Creary scale for radical substituent effects, 25 which we have employed in the quantitative analysis of the deazatization rate constants of phenylazomethanes (right-hand in Scheme 3), 26 predicts the selected aryl substituents to be radical stabilizing (cf. positive s rad values in Table 1); the p-CF 3 group displays the weakest radical-stabilizing effect.…”

Section: Substituent Effectsmentioning

confidence: 99%

“…r rad /r pol , was 3.6. 26 This polar effect was attributed to a ground-state stabilization, 26 which arises from the action of the substituents on the strength of the polar bond. 27,28 A similar polar effect is expected for the decarbonylation, but the data set is too small and displays insufficient variation to separate a potential polar effect from the larger radical effect.…”

Section: Substituent Effectsmentioning

confidence: 99%

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Aryl substituent effects and solvent effects on the decarbonylation of phenacetyl radicals

Zhang

Nau

2000

J. Phys. Org. Chem.

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Section: Discussionmentioning

confidence: 68%

Section: Substituent Effectsmentioning

confidence: 99%

Section: Substituent Effectsmentioning

confidence: 99%

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Aryl substituent effects and solvent effects on the decarbonylation of phenacetyl radicals

Zhang

Nau

2000

J. Phys. Org. Chem.

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“…[1][2][3][4][5][6][7][8][9][10][11][12][13]38,41,48,53,54 DBO and its derivatives are special examples, because many of them are photochemically "persistent" azoalkanes, which show fascinating photophysical properties instead. 3,38,52,55 Besides an unexpectedly short phosphorescence lifetime, 41 DBO exhibits a very broad fluorescence spectrum 52 and an exceedingly long fluorescence lifetime (up to 1 µs in the gas phase).…”

Section: Photophysical Characterizationmentioning

confidence: 99%

“…[1][2][3] They have remained key intermediates in the synthesis of strained compounds 1,4 and in the mechanistic investigation of the behavior of biradicals, [5][6][7][8][9][10][11] with some seemingly never-ending mechanistic disputes like the double-inversion peculiarity of 2,3-diazabicyclo[2.2.1]hept-2-enes. 12,13 Derivatives of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO, 1) have recently become valuable structural and dynamic probes 14 as well as sensors for supramolecular [15][16][17][18][19][20][21][22][23] and biomolecular assemblies.…”

Section: Introductionmentioning

confidence: 99%

Bridgehead carboxy-substituted 2,3-diazabicyclo[2.2.2]oct-2-enes: synthesis, fluorescent properties, and host-guest complexation

Hennig¹,

Schwarzlose²,

Nau³

2007

Arkivoc

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Two novel derivatives of 2,3-diazabicyclo[2.2.2]oct-2-ene were synthesized, carrying a carboxyl (4) and a methylcarboxyl (5) substituent at the bridgehead position. The photodecomposition quantum yields (51% for 4 and 2.9% for 5) and fluorescence lifetimes (29 ns for 4 and 345 ns for 5) in water were determined. The higher photoreactivity and fluorescence quenching for 4 was attributed to its higher propensity to undergo photochemical elimination of nitrogen as a consequence of the presence of the radical-stabilizing carboxyl group in the α-position. The absolute photodecomposition rate constants of 4 became faster upon protonation (e.g., at pH 2), which contrasted anticipated substituent effects on the C-N bonds strengths. ) which was attributed to reduced Coulomb attractions as a consequence of the 1,3-alternate conformation which this host adopts in water. The binding constants towards β-cyclodextrin were also low at pH 7.0 (< 120 M -1 ), which was attributed to the low hydrophobicity of the anionic form of the guests; in line with this interpretation, the binding constants with β-cyclodextrin increased at pH 2.0, by about one order of magnitude.

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An Electronegativity Model for Polar Ground-State Effects on Bond Dissociation Energies

Nau¹

1997

J. Phys. Org. Chem.

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Homolytic bond dissociation energies are a composite of the radical stabilization energies (RSE) of the product radicals and the polar ground-state stabilization energies (PSE) of the reactant molecules. Substituent effects on the PSE are rationalized in terms of changes in the difference of group electronegativities. Thus, the PSE is composed of a bond polarity term, which measures the contribution due to the change in the electronegativity difference between the atoms in the bond, which is broken, and a polar relaxation term, which accounts for the substituent-dependent group electronegativity changes in the remaining bonds. A semi-quantitative model based on Pauling's bonding theory is suitable to assess the direction and relative magnitude of such effects. For the cleavage of benzylic and related bonds, the polar relaxation energy can be neglected (one-bond approximation) to allow the use of correlation analyses and substituent parameters for the interpretation of aryl substituent effects on the PSE. Accordingly, the plots of the PSEs versus substituent parameters should be linear, curved or parabolic depending on the electronegativity difference of the atoms in the bond being broken (⌬EN); moreover, the slopes ( values) should increase linearly with ⌬EN. The predicted dependences of the PSEs on aryl substituents are compared with known experimental results and with the data obtained from semiempirical calculations.

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Radical Stabilization and Ground State Polar Substituent Effects in the Thermal Decomposition of Azoalkanes

Cited by 40 publications

References 0 publications

Aryl substituent effects and solvent effects on the decarbonylation of phenacetyl radicals

Aryl substituent effects and solvent effects on the decarbonylation of phenacetyl radicals

Bridgehead carboxy-substituted 2,3-diazabicyclo[2.2.2]oct-2-enes: synthesis, fluorescent properties, and host-guest complexation

An Electronegativity Model for Polar Ground-State Effects on Bond Dissociation Energies

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