Pseudodilution effect

One of the most striking demonstrations of the Heck reaction on solid phase was an efficient macrocyclization reaction by Hauske et al. They cyclized the resin-bound species 19 under mild conditions and obtained products with high post-cleavage purity (Scheme 19). The efficiency of these reactions for a variety of structural modifications in the ring seems to suggest that the pseudodilution effects of site isolation on resin are important. 

For a discussion of the pseudodilution effect reached with polymer-bound Pd° see [39 a, f] and references cited therein. 

Immobilisation prevents the benzyne intermediates from dimerisation and allows the alternative transformation which in solution is too slow to compete with dimerisation. Applications of functionalised polymers for monoprotection (many examples were contributed by Lezno, A"6.146-150 further examples see Refs. - SZ) or monoactivation of symmetrically difunctionalised molecules as well as their use to promote the formation of large rings are based on this site isolation or pseudodilution effect . ... 

Among the previously mentioned advantages of sohd-phase synthesis, the pseudodilution effect particularly affects the intramolecular variant of the Mizoroki-Heck reaction, commonly resulting in increased yields and leading to the most complex stractures obtained by Mizoroki-Heck reactions on solid phase. 

---