Captodative substituent effects in radical chemistry

Viehe, H. G.; Merényi, R.; Janousek, Z.

doi:10.1351/pac198860111635

Cited by 63 publications

(42 citation statements)

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“…(34) Radical thiol−ene polymerizations limited by chain transfer, such as the thiol−allyl ether polymerization, generally involve a relatively stable radical that is slow to abstract a thiol hydrogen. Sulfide substituents are believed to have a stabilizing effect on neighboring radicals, 35,36 which may explain the rate-limiting chain transfer step in the thiol−yne polymerization.…”

Section: Resultsmentioning

confidence: 99%

Thiol−Yne Photopolymerizations: Novel Mechanism, Kinetics, and Step-Growth Formation of Highly Cross-Linked Networks

et al. 2008

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Radical-mediated thiol−yne step-growth photopolymerizations are utilized to form highly cross-linked polymer networks. This reaction mechanism is shown to be analogous to the thiol−ene photopolymerization; however, each alkyne functional group is capable of consecutive reaction with two thiol functional groups. The thiol−yne reaction involves the sequential propagation of a thiyl radical with either an alkyne or a vinyl functional group followed by chain transfer of the radical to another thiol. The rate of thiyl radical addition to the alkyne was determined to be approximately one-third of that to the vinyl. Chain-growth polymerization of alkyne and vinyl functionalities was only observed for reactions in which the alkyne was originally in excess. Analysis of initial polymerization rates demonstrated a near first-order dependence on thiol concentration, indicating that chain transfer is the rate-determining step. Further analysis revealed that the polymerization rate scaled with the initiation rate to an exponent of 0.65, deviating from classical square root dependence predicted for termination occurring exclusively by bimolecular reactions. A tetrafunctional thiol was photopolymerized with a difunctional alkyne, forming an inherently higher cross-link density than an analogous thiol−ene resin, displaying a higher glass transition temperature (48.9 vs −22.3 °C) and rubbery modulus (80 vs 13 MPa). Additionally, the versatile nature of this chemistry facilitates postpolymerization modification of residual reactive groups to produce materials with unique physical and chemical properties.

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Section: Resultsmentioning

confidence: 99%

Thiol−Yne Photopolymerizations: Novel Mechanism, Kinetics, and Step-Growth Formation of Highly Cross-Linked Networks

et al. 2008

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“…Several previous studies addressed the change of BDEs of the C α H bond in toluene upon the presence of para substituents . It was found that the benzyl radical is stabilized upon introduction of a substituent group, irrespective of whether the Z group is an electron donor or acceptor.…”

Section: Resultsmentioning

confidence: 99%

Group additive modeling of substituent effects in monocyclic aromatic hydrocarbon radicals

et al. 2016

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The thermodynamic properties of the unsubstituted and substituted phenyl, phenoxy, anisyl, benzoyl, styryl and benzyl radicals with six substituents (hydroxy, methoxy, formyl, vinyl, methyl, and ethyl) are calculated with the bond additivity corrected (BAC) post‐Hartree‐Fock G4 method. Bond dissociation energies of monocyclic aromatic hydrocarbons are calculated and used to identify substituent interactions in these radicals. Benson's Group Additivity (GA) scheme is extended to aromatic radicals by defining 6 GAV and 29 NNI parameters through least squares regression to a database of thermodynamic properties of 369 radicals. Comparison between G4/BAC and GA calculated thermodynamic values shows that the standard enthalpies of formation generally agree within 4 kJ mol−1, whereas the entropies and the heat capacities deviate less than 4 J mol−1 K−1. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2089–2106, 2017

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“…Free‐radical processes are known to be sensitive to both radical and polar effects (28, 29), which could explain the observations. One argument against a HAT process is the fact that the calculated bond dissociation energies in substituted acetophenone oximes were reported to be relatively independent of the substituent (30, 31).…”

Section: Introductionmentioning

confidence: 86%

Quinone-sensitized Steady-state Photolysis of Acetophenone Oximes Under Aerobic Conditions: Kinetics and Product Studies†

Park

Kosareff

Kim

et al. 2006

Photochem Photobiol

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Oxidation of oximes via photosensitized electron transfer (PET) results in the formation of the corresponding ketones as the major product via oxime radical cations and iminoxyl radicals. The influence of electron-releasing and electron-accepting substituents on these reactions was studied. The observed substituent effect strongly supports formation of iminoxyl radicals from the oximes via an electron transfer-proton transfer sequence rather than direct hydrogen atom abstraction. Correlation of the relative conversion of the oximes with Hammett parameters shows that radical effects dominate for the meta-substituted acetophenone oximes (rho(rad)/rho(pol) = 5.4; r2 = 0.93), whereas the para-substituted oximes are influenced almost equally by radical and ionic effects (rho(rad)/rho(pol) = -1.1; r2 = 0.98). From these data sets we conclude that the follow-up reactions proceed through a number of intermediates with both radical and ionic character. This was confirmed by product studies with the use of an isotopically labeled nucleophile. In addition to the major oxidation product (ketone), a chlorine-containing product was often identified as well. Studies on the formation of this product show that the most likely pathway is either via a direct nucleophilic addition in a complex formed between the oxime radical cation and the chloranil radical anion or via a radical substitution (SH2) mechanism. These studies show that with the increasing use of oximes as drugs and pesticides, intake of these chemicals followed by enzymatic oxidation may result in the formation of a variety of reactive intermediates, which may lead to cell and tissue damage.

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Captodative substituent effects in radical chemistry

Abstract: Abstract

Cited by 63 publications

References 0 publications

Thiol−Yne Photopolymerizations: Novel Mechanism, Kinetics, and Step-Growth Formation of Highly Cross-Linked Networks

Thiol−Yne Photopolymerizations: Novel Mechanism, Kinetics, and Step-Growth Formation of Highly Cross-Linked Networks

Group additive modeling of substituent effects in monocyclic aromatic hydrocarbon radicals

Quinone-sensitized Steady-state Photolysis of Acetophenone Oximes Under Aerobic Conditions: Kinetics and Product Studies†

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