A Theoretical Approach to Substituent Effects in Radical Chemistry

Dechlorination of substituted trichloromethanes by an iron(II) porphyrin

“…Figure 5 shows that the heme oxidation rate follows a strong relationship with R electronegativity, or χ R . In agreement with the results of previous studies [29–31], the electronegativity of a substituent appears to be an important variable to predict a trend in the reactivity of homologous compounds in a reaction involving radical species (Table 1). The good correlation of ln k ii and χ R ( r 2 = 0.88; Eqn.…”

“…5), and this supports the hypothesis that profile II is a particular case of profile I. These results confirm that the electronegativity of a substituent is a helpful property in predicting the reduction rate of a homologous series of substrates by a metal center [28–30]: where r 2 is 0.88. For the particular case in which τ −1 ≈ k ii (profiles I and II), Equations 13 and 14 may be combined and τ written as a function of χ R according to …”

“…Presumably, the relative formation rate of the Fe‐Cl bond is a function of (χ Fe − χ c ), and the rate of formation of chlorohemin and 'CCl 2 R radical depends on the electronic contribution of R to stabilize the radical. For instance, π acceptor groups (NO 2 , CN, COO − ) are substituents that possess high electronegativity to promote formation of the Fe‐Cl bond, but they can also provide a higher CCl 2 R radical stability via odd‐electron delocalization and form a three‐electron bond [29]. On the other hand, σ acceptor groups (Cl, CH 3 , H) have a moderate (χ Fe − χ c ) value and cannot contribute to CCl 2 R radical stability, thus experiencing a slower dechlorination than the π acceptor groups.…”

Le radical méthylènecyclopropyle: étude théorique. Chloration par t-BuOCl et réduction par (nBu)3SnH de dérivés cyclopropéniques

“…Cependant, les angles a ainsi obtenus semblent sous-estimts par rapport aux calculs ab initio: pour le cyclopropyle, a = 3" en MNDO (28) et a = 40" en ab initio (38); -CH2F, a = 0" en MNDO (27,28) et a = 32" en ab initio (39). I1 semble donc que la mCthode sous-estime en rkgle gtnCrale la tendance B la pyramidalitt du centre radicalaire.…”

ESR studies of acyclic delocalised radicals

“…studies of the addition of alkyl radicals to C-D substituted alkenes yielded ambiguous results [157]. Some theoretical treatments provided supporting evidence [158], others have indicated negligible C-D stabilisation [159].…”