Absolute Rate Constants for the Addition of Cyanomethyl (·CH₂CN) and (tert‐Butoxy)carbonylmethyl (·CH₂CO₂C(CH₃)₃) radicals to alkenes in solution

Abstract

Absolute rate constants and their temperature dependence were determined by time-resolved electron spin resonance for the addition of the radicals 'CH,CN and .CH,CO,C(CH,), to a variety of mono-and 1,l-disubstituted and to selected 1,2-and trisubstituted alkenes in acetonitrile solution. To alkenes CH,=CXY, .CH,CN adds at the unsubstituted C-atom with rate constants ranging from 3.3.10' M-Is- ' (ethene) to 2.4.106 M-IS-'( 1 , ldiphcnylethene) at 278 K, and the frequency factors are in the narrow range of log (A/M-'s-') = 8.7 * 0.3.. CH2C02C(CH,), shows a very similar reactivity with rate constants at 296 K ranging from 1.1 lo4 M-'s-' (ethene) to lo7 M-'s-' (],I-diphenylethene) and frequency factors log (A/M-'s-') = 8.4 f 0.1. For both radicals, the rate constants and the activation energies for addition to CH,=CXY correlate well with the overall reaction enthalpy. In contrast to the expectation of an electro-or ambiphilic behavior, polar alkene-substituent effects are not clearly expressed, but some deviations from the enthalpy correlations point to a weak electrophilicity of the radicals. The rate constants for the addition to 1,2-and to trisubstituted alkenes reveal additional steric substituent effects. Self-termination rate data for the title radicals and spectral properties of their adducts to the alkenes are also given.1. Introduction. -The addition of carbon-centered radicals to C=C bonds is one of the most useful radical reactions. However, its rate constants are far from being quantitatively understood and predictable, because they reflect a complex interplay of polar, steric, and enthalpic substituent effects [l]. To provide a larger basis for their analysis, we have developed a technique of time-resolved electron spin resonance (ESR) which yields reliable rate data for large series of reactions with moderate efforts. Hitherto 121, the addition rates of the easily oxidizable radicals tert-butyl, hydroxymethyl, and 2-hydroxy-2-propyl to various alkenes CH,=CXY were found to correlate well with the alkene electron affinities. This means that these radicals are nucleophilic, and indicates a substantial stabilization of the transition state by partial electron transfer to the substrates. The variations of the reaction enthalpy had little or no effect. On the other hand, these were found to govern the rate constants for the addition of the 2-cyano-2-propyl radical [3] which exhibits no clear polar effects, and the benzyl radical shows an intermediate, weakly nucleophilic behavior [4]. In related studies, other groups demonstrated clearcut electrophilic reactivity patterns for perfluoroalkyl [5], dicyanomethyl [6], and malonyl [7] radicals.Here, we present rate data for the addition of the cyanomethyl (. CH,CN) and the

Assessment of Nucleophilicity and Electrophilicity of Radicals, and of Polar and Enthalpy Effects on Radical Addition Reactions1

“…On the other hand, experimental studies led to the conclusion that the methyl radical is nucleophilic, whereas the cyanomethyl radical 18 and the tert -butoxycarbonylmethyl radical 19 are ambiphilic (borderline cases). Later, however, Fischer et al suggested that the latter two radicals are weakly electrophilic …”

Visible-Light-Mediated Reactions of Electrophilic Radicals with Vinyl and Allyl Trifluoroborates

“…The addition of carbon-centered radicals to olefins is one of the most used reactions for the construction of C–C bonds. , Despite its apparent simplicity, this process is mechanistically intriguing as it relies on a complex interplay between enthalpic, steric, and polar effects. − State correlation diagrams that describe these transformations involve three components: the reactants in (i) the ground, (ii) the excited state, and (iii) the polar charge transfer configuration (CTC). , As a result, the identification of polarity-matched systems might result in more facile reactions owing to the stabilization of the CTC in the transition state . A classical example is the addition of electrophilic radicals (e.g., malonyl radical) to electron-rich olefins like enol ethers and enamines (Scheme A). , …”

Organic Covalent Patterning of Nanostructured Graphene with Selectivity at the Atomic Level

“…The m / z pattern is perfectly consistent with a significant concentration of CH 2 CN • (Figure S3 and Supporting Information D for details). It is well-known that CH 2 CN • can be generated directly from CH 3 CN by homolytic cleavage of a C–H bond. − Moreover, the cyanomethyl radical (CH 2 CN • ) is known to react with C–C double bonds − and arenes forming C–C bonds.…”