Intermolecular reactions adducts, intramolecular trapping

The appreciable levels of asymmetric induction observed in the catalytic ARCM reactions discussed above suggest a high degree of enantio-differentiation in the association of ole-finic substrates to chiral Mo complexes. Such stereochemical induction may be exploited in asymmetric ring-opening metathesis (AROM). Catalytic ROM transformations [20] -although less explored than the related RCM processes - offer unique and powerful methods for the preparation of complex molecules in a single step [2d, 2g], The chiral Mo-alkylidenes that are products of AROM can be trapped either intramolecularly (RCM) or intermolecularly (cross metathesis, CM) to afford an assortment of optically enriched adducts. 

The most important methodology for the aliphatic C-C bond formation via radical reactions is the addition of the radical to an alkene double bond, both inter -and intramolecularly (with the 5-exo-ring cyclisation mode preferred in the latter case). This reaction leads to adduct radicals that must be converted to non-radical products before polymerisations can take place. For this reason, polymerisation is avoided either by intermolecular trapping of adduct radicals or by intramolecular, homolytic bond cleavage. Hydrogen atom donors X-H, heteroatom donors X-Z or electron donors M"+ are used as trapping agents (Scheme 7.1). 

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