Cyclic structure ring size, control

In summary, the five-membered ring formation, i.e., the occurrence of intramolecular hh addition was favored with the increase of the polarity of solvent, the elevation of polymerization temperature, and the decrease of monomer concentration thus, the ring size of cyclic structure could be controlled freely by choosing appropriate polymerization conditions. In addition to these results the rate of polymerization and the molecular weight of the polymer were reduced under polymerization conditions favorable to five-membered ring formation(23). 

For the photoadducts derived from cyclic enones and alkenes, stereochemistry at the ring junction is influenced by the structure and especially by the ring size of the starting reagents. For steric reasons, only cis-fused cycloadducts can be formed on photocycloaddition of cyclopentenones (n = 0). From cyclohexenone derivatives (n = 1), cis- and transfused adducts can be isolated, even if the cis-fused structure is thermodynamically more stable. This indicates that trans-fused cycloadducts result from a kinetic rather than a thermodynamic control. Fortunately, trans-fused cycloadducts can be epimerized easily to the more stable cis stereoisomers (Scheme 11). 

The hydrides of the heavier congeners of the Group 14 to 16 elements have weak E-H bonds and they can be decomposed under mild conditions to yield the pure element or a low-oxidation-state hydride (in many cases of ill-defined chemical composition and structure). This tendency, which also applies to the E-C bonds, underlies the usefulness of hydrides in many gas and vapor phase deposition methods.3 There is still, however, a need for catalysts, particularly to control the specificity of dehydrocoupling for example, the ability to make rings of a particular size or isomeric composition, or the ability to avoid cyclic products altogether. In addition, it is desirable to control homo- vs hetero-dehydrocoupling selectivity, something difficult to do by noncatalytic methods. 

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