Stereochemistry, radical reactions

There have been many studies aimed at deducing the geometiy of radical sites by examining the stereochemistry of radical reactions. The most direct kind of study involves the generation of a radical at a carbon which is a stereogenic center. A planar or rapidly inverting radical would lead to racemization, whereas a rigid pyramidal structure should... 

Scheme 12.2. Stereochemistry of Radical Reactions at Chiral Carbon...

The first three chapters discuss fundamental bonding theory, stereochemistry, and conformation, respectively. Chapter 4 discusses the means of study and description of reaction mechanisms. Chapter 9 focuses on aromaticity and aromatic stabilization and can be used at an earlier stage of a course if an instructor desires to do so. The other chapters discuss specific mechanistic types, including nucleophilic substitution, polar additions and eliminations, carbon acids and enolates, carbonyl chemistry, aromatic substitution, concerted reactions, free-radical reactions, and photochemistry. 

The course of each of the free radical reactions shown in equations 19-21, where fluorine substitution alters the normal trans stereochemistry of addition, is ascnbed to endo fluonne stenc effects... 

Stereochemistry of Radical Reactions - Concepts, Guidelines and Synthetic Applications... 

Curran, D. R Rorter, N. A. Giese, B. Stereochemistry of Radical Reactions Concepts, Guidelines, and Synthetic Applications, VCH, Weinheim, 1996. 

It is sometimes difficult to distinguish clearly between these mechanisms, but determination of reaction stereochemistry provides one approach. The tme one-step insertion must occur with complete retention of configuration. The results for the two-step process will depend on the rate of recombination in competition with stereorandomization of the radical pair intermediate. 

Curran DP, Porter NA, Giese B (1996) Stereochemistry of Radical Reactions. Wiley-VCH, Weinheim... 

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

By and large, the solute exerts pressure on the solvent. Internal pressure can affect the liquid-phase ion-radicals reactions and requires special study. For instance, such pressure can determine the selectivity and even the stereochemistry of these reactions (Okamoto et al. 1998, Adam and Trofimov 2003). 

Our approach was to use the unsaturated bromodeoxylactones in an intramolecular radical reaction, since these compounds possess both the radical precursor and the radical trap within the same molecule. Thus, reacting the unsaturated bromodeoxyheptonolactone 20 (Scheme 14) with tributyltin hydride and a radical initiator, the bicyclic lactone 65 a was obtained in a quantitative yield within 1 h. The stereocontrol in the reaction was determined by the structure of the product, since the compound obtained has two fused cyclopentane rings which can only be cis anellated. The radical A, which is the intermediate, was trapped by the tin hydride. The stereochemistry of the newly formed chiral center is determined by the configuration at C-4 in the educt 20 [45]. 

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