Steric Effects in the Addition of Alkyl Radicals to Alkenes

Mechanistic Perspectives on Stereocontrol in Lewis Acid-Mediated Radical Polymerization

“…73 In contrast, the addition of the cyclohexyl radical to MMA is also slower than comparable addition to methyl acrylate, but only by around 30%. 73 While the presence of a CH 3 group slows the rate of cyclohexyl radical addition to both alkenes, a much more dramatic rate reduction is observed for methyl crotonate because the substituent is much closer to the adding radical. Unsurprisingly, as the steric bulk of the substituent increases, the rate of addition decreases.…”

“…At 20 C, the rate of addition of the cyclohexyl radical to methyl (E)-3-isopropylacrylate is slower than comparable addition to methyl acrylate by a factor of around 600. 73 For comparison, the addition of the cyclohexyl radical to methyl 2-isopropylacrylate is slower than comparable addition to methyl acrylate but only by 60%. 73 Regioselectivity in radical addition reactions to substituted alkenes is determined by a combination of steric, polar, and thermodynamic effects.…”

“…This importance of steric effects is nicely illustrated by considering the relative rate coefficients for the addition of cyclohexyl radicals to methyl acrylate, methyl crotonate, methyl methacrylate (MMA), methyl (E)-3-isopropylacrylate, and methyl 2-isopropylacrylate ( Figure 4). 73 At 20 C, the rate of addition of the cyclohexyl radical to methyl crotonate is nearly two orders of magnitude slower than comparable addition to methyl acrylate. 73 In contrast, the addition of the cyclohexyl radical to MMA is also slower than comparable addition to methyl acrylate, but only by around 30%.…”

“…73 At 20 C, the rate of addition of the cyclohexyl radical to methyl crotonate is nearly two orders of magnitude slower than comparable addition to methyl acrylate. 73 In contrast, the addition of the cyclohexyl radical to MMA is also slower than comparable addition to methyl acrylate, but only by around 30%. 73 While the presence of a CH 3 group slows the rate of cyclohexyl radical addition to both alkenes, a much more dramatic rate reduction is observed for methyl crotonate because the substituent is much closer to the adding radical.…”

Synthetic applications of radical C-C bond forming reactions

“…The influence of the bulk of the substituent on the alkene reactivity is shown in Figure 2. The reactivity of acrylic esters, having alkyl substituents at the m-position, is nearly independent of the bulk of the alkyl group (by a factor of 2.5 for the range CH 3 to t-C4H9; and 4 for H to t-C4H9) [6]. In contrast, substituents at the ~-carbon atom exert large steric effects on the rate (by a factor of 200 for the range CH 3 to t-C4Hg; 20000 for H to t-C4H9) [6].…”