Stereoselective Radical Reactions

Thus, this first example of stereoselective radical reaction, initiated with the system based on Fe(CO)5, shows opportunities and prospects of using the metal complex initiators for obtaining the stereomerically pure adducts of bromine-containing compounds to vinyl monomers with chiral substituents. 

A stereoselective radical reaction of the ester of M-hydroxy-2-pyridinethione (see also Sect. 4.1) to chiral vinylphosphine oxides has also been described and moderate to good diastereomeric ratios have been obtained for the compound 112 [70] (Scheme 33). 

B. Giese and T. Witzel, Stereoselective radical reactions with enolones, Tetrahedron Lett. 28 2571 (1987). 

M. P. Sibi, T. R. Ternes, Stereoselective Radical Reactions, in Modem Carbonyl Chemistry, Ed. by J. Otera, Wiley-VCH, Weinheim, 2000, 507-538. 

Also for stereoselective radical reactions such as radical additions or radical cyclizations, the camphor sultam 2 is suitable as an auxiliary (Scheme 7). The acyl radical which was gener-... 

Radical reactions generally offer versatile alternatives to ionic reactions [167]. Many methods have been developed that allow chemoselective and regioselective radical transformations. More recently, stereoselective radical reactions have been reported [168]. 

Let us rehearse some principles of stereoselective radical reactions in general. Stereoselective synthesis concerns the creation of a new stereogenic (chiral) centre by conversion of a prochiral substrate (i.e. radical) into a chiral product.29 This transformation to be achieved as result of utilising ... 

Stereoselective radical reactions of carbonyl derivatives of heterocycles 00MI12. Suzuki-Miyaura cross-coupling reaction in organic synthesis (review contains many examples of reactions with participation of heterocycles) 02T9633. Tautomerism in diazoles, pyridines, and nucleic bases 02UK1120. 

Stereoselective radical reactions of carbonyl derivatives of heterocycles 00MI12. 

Stereoselective radical reactions. Gicsc el al have found that radicals can show high 1,2-slcreoinduction comparable to ionic reactions, and that the selectivity can be reversed when proceeding from cyclic to acyclic radicals. Thus the Bu-tSnH radical addition to the dioxanone 1 provides the only one isomer (2) with rran.r-selectivity. The same radical with a corresponding acyclic system (3) shows di-selectivity. 

Stereoselective radical reactions have been actively investigated. Et3B has contributed largely toward improving stereoselectivity because one can examine at lower temperatures. Many excellent diastereoselective and enantioselective reactions were reported, and most of them will be mentioned in detail later (see Chapter 4). Some examples are shown here. 

In general, free radicals are rapidly equilibrating intermediates, which makes stereoselective radical reactions extremely challenging. In ring systems that have little or no conformational bias, reactive radical intermediates can racemize either by a conformational interconversion (i.e., ring flip) or by a simple radical inversion. For simple 2-tetrahydropyranyl radicals, the barrier to radical inversion has been estimated to be <1 kcal/mol, while the barrier to ring inversion is 10 kcal/mol. Therefore, if conformational interconversion is slow relative to reaction of the radical intermediate, then non-equilibrium radical reactions are possible. Recently it has been shown that reduction of 2-tetrahydropyranyl radicals can be competitive with conformational interconversions, which allows for a new strategy for the control of stereochemistry in radical reactions [28]. 

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