Free radical addition to branched allylsilanes: stereoselective formation and elimination of β bromosilanes

Allylsilanes in Organic Synthesis − Recent Developments

“…Scheme 44 Porter also extended this process to chiral allylsilanes 162, demonstrating that an excellent level of 1,2-stereocontrol could be attained, with a diastereomeric ratio of up to 94:6 in favour of the syn isomer (Scheme 45). [89] A FelkinϪAnh-type model XVIIa was proposed to account for this 1,2-stereocontrol. β-Elimination of the β-selenoor -halosilane 164a,b led stereospecifically to olefin 165 having a (Z) configuration.…”

“…As mentioned above, fluoride-mediated elimination of the major isomers led exclusively to the (Z)-olefins. Based on a study on the stereochemistry of fluoridemediated elimination of β-azidosilanes, [90b] we were able to demonstrate that, similarly to Porter's atom-transfer reaction, [89] the carboazidation led to the syn isomer as the major product. Scheme 46 We rationalized our results by invoking a pyramidalization of the transition state into a quasi-staggered conformation XVIIb to avoid gauche interactions between large groups (here between SiR 3 and CH 2 CH 2 CO 2 Et groups; Figure 11).…”

Carboazidation of Chiral Allylsilanes: Experimental and Theoretical Investigations

“…[13] Porter et al first demonstrated that an allylic silicon group could control the stereochemistry of a new stereogenic center created during the radical addition (atom transfer) of a-bromo and a-iodo amides onto a chiral allylsilane. [14] Based on these premises, we started an investigation on the 1,2-stereocontrol arising from free-radical mediated carboazidation of chiral allylsilanes (Scheme 1). [15] We report here a full account of our studies, including the scope and limitations of the process, and an EPR study on the bsilyl radical species issued from the addition of the carbon radical onto chiral allylsilanes.…”

Mechanism of β-Silyl Diacyl Peroxide Decomposition: A Mild and Stereoselective Synthesis of β-Silyl Esters

“…This mechanism can be discounted since a regioisomeric mixture of β-silyl esters is produced and there is no evidence from previous studies that β-silyl radicals undergo silyl rearrangement. 13,14 Mechanism B also makes use of intermediate 3 but differs from mechanism A in that the initially formed radical pair undergoes electron transfer, a step that is reasonable on the basis both that decarboxylation of the aryl carboxyl radical is slow and that the ion pair may be energetically more stable than the radical pair (a β-silyl cation has a stabilization energy of ∼38 kcal/mol, 15 whereas a β-silyl radical has a stabilization energy of only 3-4 kcal/mol 16,17 ). Following electron transfer, the result- ing β-silyl cation could rearrange, 18 giving the observed regioisomeric mixture of ester products (6 and 7).…”