Radicals ring size effects

Work towards a quantitative analysis of these effects was initiated in our laboratory in the early seventies9) when an unusually large ring size effects on the thermal cleavage reaction of 1,1 -diphenylbicycloalkyls 3 into 1-phenylcycloalkyl radicals 4 was observed this was inexplicable by i-strain effects alone and pointed to the importance of f-strain in this phenomenon 10). 

Undoubtedly, further work will be necessary in order to get a solid background for silyl radical cyclizations. In particular, further investigations on the nature of the pendant alkenyl substituent as well as on the ring size and substituent effects on the silicon in the ring expansion can be expected in the future. 

The mass spectrometry of four-membered heterocycles has been considerably investigated by Scala and Colon. Holmes et have provided structures associated with metastabile ion peaks. The effect of ring size on the fragmentation process of cyclic sulfoxides was studied by Tamagaki and Oae. Thietane 1-oxide appears as a strong radical ion at mje 90. At 80 eV no appreciable deoxygenation takes place. Instead, the loss of OH and/or SOH is observed, giving the peak at mje a value of 73 for CaHjS" and the base peak at mje 41 for CaH ". ... 

This interesting reaction is especially useful for the synthesis of medium- and large-ring compounds from dicarboxylic esters, and is effective for ring sizes that cannot be made by the Dieckmann condensation or decarboxylation (Section 18-1 OB). Radical anions formed by addition of sodium to the ester... 

The formation of ring sizes other than five- and six-membered rings has been reported, in particular the formation of three- and four-membered rings by 5-exo and 4-exo radical cyclization, respectively.33,34 High-yielding 5-exo cyclizations have been reported when the cyclized radical contains a radical-stabilizing group.33 The effects of... 

The regioselectivity in diene addition reactions can also be influenced by ring strain effects in cyclization reactions. The regioselectivity is highly predictable in those cases, in which addition to the preferred diene center forms the preferred ring size. Thus, the cyclization of radical 15 proceeds readily to form the ct s-disubstituted cyclopentyhnethyl radical 16 with high selectivity. Similarly, cyclization of 17 affords exclusively bicyclic radical 18, in which the additional cyclopentane ring has been formed by addition to the terminal position of the butadiene subunit. This preference for 5-exo cyclizations onto dienes is not even dismpted by substiments at the C1 or C4 positions of the diene system, as seen for radical 19, which cyclizes to 20 (equation lO). This is in contrast to alkyl radical cyclizations to alkenes, in which major amounts of 6-endo cyclization is observed for 5-substituted systems. ... 

In another analysis of structure-reactivity relationships in free radical addition reactions of cycloalkenes with ring size 5-8, it was concluded that strain effects as well as the electronegativity of the attacking radical affect the reactivity (259). The rate constants for the addition reactions of the p-chlorophenylthiyl radical to cycloalkenes of ring size 5-8 and 10 have been discussed in terms of released strain energy and the polar nature of the transition state (260). 

On the other hand, we reexamined in detail the ring size of the cyclic structural units of poly-AA s by means of IR, 1H-NMR, and C-NMR spectroscopy these analytical procedures were applied to the structural analysis of poly-AA, the poly(acrylic acid) derived from hydrolysis of the poly-AA, and the poly(methyl acrylate) obtained by subsequent esterification of the poly(acryl-ic acid) in comparison with the corresponding model polymers of five- or six-membered ring structure. Then, we investigated in detail the effects of polymerization conditions on the ring size of poly-AA s, i.e., on the intramolecular addition modes in the cyclopolymerization of AA since five- or six-membered ring anhydride structure can be formed via intramolecular hh or ht addition of the uncyclized radical to the internal double bond(22,23). 

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