Baldwin-Beckwith rules

The additions of thiyl radical to alkenes and alkynes can also occur intra-molecularly, thus leading to sulfur heterocycles. This subject has been explored by Surzur s group [32-33]. Based on steric and stereoelectronic factors, the Baldwin-Beckwith rules [34] predict the 5-exo ring closure as the favored pathway. Scheme 6 shows that the pent-4-enylthiyl radical, generated from the allyl sulfide, yields both five- and six-membered rings, in the ratio of 1 19. The predominance of the larger cycle, in contrast with the above-mentioned rules, is due to the reversibility of the reaction. The product distribution reflects a thermodynamic control. 

In addition to nucleophilic reactions, Baldwin s rules also apply to homo-lytic and cationic processes. Table 21 lists rate constants for ring closure of lower -alkenyl radicals (71), in which intramolecular addition to the double bond occurs in the exo-mode (Beckwith, 1981). It is unfortunate that EM-... 

For a detailed discussion of the rules for radical ring-closure, see A.L.J. Beckwith, C.J. Easton and A.K. Serelis, 1980, J. Chem. Soc., Chem. Commun., 482. For Baldwin s earlier rules for ring-closure, see J.E. Baldwin, 1976, J. Chem. Soc., Chem. Commun., 734. 

Generation of a radical in a molecule that contains a site of unsaturation presents an opportunity for (yclization. These types of radical cyclizations have been developed into very useful synthetic reactions. The synthetic utility of these cyclizations is enhanced by the abhity to predict the regiochemistry of cyclization by applying the Baldwin rules. The Baldwin rules cover the formation of three- to seven-membered rings by various reactions and are based on consideration of both kinetic and thermodynamic factors. (See Beckwith, A. L. J. Easton, D. J. Serelis, A. K. J. Chem. Soc., Chem. Commun. 1980, 482-483, and Baldwin, J. E. J. Chem. Soc., Chem. Commun. 1976, 734-736.)... 

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