Ring strain ionic species

For n = 2 or 3, a large ring strain (approx. 20 kcal mole in monomers) has to be overcome in order to transform a macroester into a three — or four-membered oxonium ion. When n = 2, however, ionic species are formed in which the two last units are engaged. Thus, the attack involves the penultimate unit ... 

The interconversion of butadiene radical cations and ionized cyclobutene represents a model case for a formal pericyclic process. Much work has been invested to study not only the distinguishability of these isomers and their derivatives by mass spectrometry, but also to check the role of orbital symmetry in the ionic species. Hass has addressed the latter problem in depth in a review on pericyclic reactions in radical cations in both the gas and condensed phases and no further survey on the papers mentioned there will be given here. The topic pertains also to the ring-opening of ionized benzocyclobutene to ionized ortho-quinodimethane (cf Section V) and various otha- phenyl-, methyl- and carboxy-substituted derivatives. In this context, we restrict ourselves hwe mentioning that an upper limit of 7 kcalmol only has been detemined by CE mass spectrometry for the activation barrier of the cycloreversion of the parent cyclobutene radical cations. The energy requirement for the cycloreversion of ionized 1- and 3-substituted cyclobutenes were found, by experiment, to be markedly different. Obviously, dissociation of the (in a sense bis-allylic) strained C—C bond is much more facile when the substituent is at C-3,... 

These data indicate that the rate constants of reactions between various monomers and model active species, both ionic and covalent are the function of monomer basicity on the other hand no correlation between ring strain and reactivity was found (44). 

Prior to CRP, the only potential synthetic pathways to glycopolymers of controlled architecture were either ionic or metal-catalyzed polymerizations such as ROMP. ROMP of strained cyclic, bicyclic, or multicyclic monomers results in ring scissimi and the formation of unsaturated linear polymers. Productive cleavage of the metallacyclobutane species formed when cyclic olefins undergo [2+2] cycloaddition with metal-alkylidenes, leading to ring opening of the olefin (Scheme 7). 

Associated with this well-known propagating species equilibrium, P-lactones behave differently to other larger lactones, due to their high pwlarity and high internal strain of the four-membered ring (Table 9.1). Since 1948, when details of the P-propiolactone (PL) ionic polymerization were first pubhshed [20], much more informahon regarding the polymerization not only of P-lactones but also of substituted P-lactones, has become available [21-27]. 

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