Formation of Six-Membered and Larger Rings

Julia Perez-Prieto and Miguel Angel Miranda 

---

The formation of six-membered or larger rings by intramolecular C-H bond insertion normally requires the attacked position to be especially activated towards electrophilic attack [1157,1158]. Electron-rich arenes or heteroarenes [1159-1162] and donor-substituted methylene groups can react intramolecularly with electrophilic carbene complexes to yield six- or seven-membered rings. Representative examples are given in Table 4.8. 

An important aspect in the carbomagnesation of six-membered and larger heterocyclic rings is the exclusive intermediacy of metallacyclopentanes where the C-Zr bond is formed a to the heterocycle C-X bond. Whether the regioselec-tivity in the zirconacycle formation is kinetically nonselective and rapidly reversible, where it is only one regioisomer that is active in the catalytic cycle, or... 

The intramolecular hydrogen atom abstraction by a polymer peroxy radical (POO ) can take place unless six-membered (or larger) rings are formed in the transition state. It is obvious that polymer segment conformation will determine the probability of formation of a transition complex of the optimum structure. Since structural relaxation is slow in a polymer matrix and the local mobility depends on the local conformation of a segment of macromolecule, the kinetics of intramolecular reactions will be influenced by the segment conformation of the macromolecule. 

---