Chiral Alkenes as Radical Traps

This Section is focussed on substrate-induced diastereoselectivity in reaction between achiral radicals and chiral, acyclic alkenes. The influence of chiral auxiliaries 

Acyclic alkenes with a stereogenic substituent at the carbon atom undergoing attack can react stereoselectively with radicals. Thus, the addition of R , generated by sonolysis of RI in the presence of Zn/Cu, to alkene 60 yields products 61a and 61b. With increase of the bulk in the radical trap and the radical R , the amount of product 61b increases. If both substituents are fcrf-butyl groups, then the reaction yields only one diastereomer (Table 5) [19]. 

This 1,2-stereoinduction (60 — 61b) is surprising because alkene 60 adopts the A 3-strain conformation 60A. Therefore, according to the reactions of ester sub- 

The reason for this unexpected stereochemistry is that the tetrahedral attack of the radical is shielded by the methyl- and the L substituents in the A-strain conformation to such a degree that a reaction after rotation to a conformation where hydrogen is the shielding substituent is energetically favored. Thus, the main product is formed from the less stable but more reactive conformer. This is a case, described by the Curtin-Hammett principle, where the transition states and not the ground states govern the stereoselectivity of the reaction [20]. 

If both L and R are tert-butyl groups, ab initio calculations show that the Felkin-Anh transition state 62B is much lower in energy than 62A. 

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