Strain rings other than

Other radical reactions not covered in this chapter are mentioned in the chapters that follow. These include additions to systems other than carbon-carbon double bonds [e.g. additions to aromatic systems (Section 3.4.2.2.1) and strained ring systems (Section 4.4.2)], transfer of heteroatoms [eg. chain transfer to disulfides (Section 6.2.2.2) and halocarbons (Section 6.2.2.4)] or groups of atoms [eg. in RAFT polymerization (Section 9.5.3)], and radical-radical reactions involving heteroatom-centered radicals or metal complexes [e g. in inhibition (Sections 3.5.2 and 5.3), NMP (Section 9.3.6) and ATRP (Section 9.4)]. 

In order to account for the nonvolatility, infusibility, and limited solubility, Leuchs postulated polymerization of the ground type cyclic compound, as indicated by the subscript x in his formula given above. It is now well established that linear polypeptides are produced on decarboxylation of the N-carboxyanhydrides of a-amino acids, and under favorable conditions the chain length may be fairly large. Leuchs favored the view that strained rings, i.e., those of other than five or six... 

Extensions of the strain theory have been made, with varying success, to other hydrocarbon ring structures, saturated and unsaturated, to ring compounds in which the hydrogen atoms have been variously substituted, and to rings containing atoms other than carbon, as well as to bieyclic and polycyclic systems,... 

The choice of mercury salt for this reaction appears to be less critical than intermolecular variants and many salts (X = Cl, OAc, NO3, SO4, CIO4, CF3CO2) have been successfully utilized. The electrophilicity of the salt can effect the regio- and stereo-chemistry of addition.434-438 The regioselectivity is also determined by the substitution pattern of the unsaturated alcohol. For cycloalkenes, steric factors, ring strain, and other reaction variables become important (equation 264). High regio- and stereo-selectivity are often encountered in these intramolecular processes. 

The relative energies at HF/6-31G //HF/6-31G of various isomers of monosilacyclobu-tadiene are given in Figure 22161. The global minimum on the C3SiFl4 PES is silylene 127, which is stabilized by the interaction of the vacant p-orbital on silicon with the C=C jr-bond to form a 27T-aromatic system. Four other silylenes 128-131 follow 127. These silylenes are all lower in energy than the isomeric structures which possess a C=Si double bond or strained rings, such as 132-137. This stability order contrasts with... 

This conclusion raises the question whether the radical cation of 107 may be localized on a single cyclopropane entity rather than involving both strained rings. The CIDNP method does not lend itself to decide this issue, because the two cyclopropane groups are magnetically equivalent in the substrate/product. The ESR method, on the other hand, is well suited to elucidate the precise nature of... 

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