Effect of ring size

Strain energy and polymerizability of cyclic ethers and sulfides  

Ring size Strain energy (kcal mole ) Polymerizability  

Propagation rate coefficients and activation parameters [118, 163] of cyclic ether polymerizations  

In much of the early work basicity emerged as the most important factor relating relative reactivity of the cyclic ethers. It is still considered an important factor in many polymerizations. We have already seen that during polymerizations of thietanes, the relative basicity of the sul-phonium ion of both the monomers and the polymers plays an important role. In this case steric hindrance was also important. 

In copolymerizations of cyclic ethers, too, basicity of the monomers seems to correlate well with the reactivity ratios determined from copolymer composition. Recently, Saegusa et al. [54] have pointed out that the apparent values of the monomer reactivity ratios in cyclic ether copolymerizations may be more influenced by the exchange 

For unactivated carbocyclic systems [14], 6/5 cychzation proceeds more readily than 6/6 cychzation. For activated systems in which the electron-withdrawing group is at the end of the dienophile away from the bridge [15], there seems to be httle preference for 6/5 over 6/6. Finally, for activated systems in which the electron withdrawing group is at the end of the dienophile toward the bridge [16], or is included in the bridge [17], 6/6 cychzation is much more facile than 6/5 cychzation. 

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Electron-donating groups in the para position of the perbenzoic acid tend to decrease the rate of reaction . The reverse seems to be true when these groups are present in a 4,4 -disubstituted diaryl sulphoxide. The effect of ring size on the oxidation of cyclic sulphoxides is apparently very small in dioxane-water solution under either acidic or basic conditions . This suggests that no major hybridisation change occurs for the sulphur atom in going to the transition state. 

Fig. 2 Effect of ring size on ee in kinetic resolution of racemic allylic ethers. For n = 3, the material is 2,3-benzo-l-oxacydo-2,6-octadiene...

This review will restrict itself to boron-carbon multiple bonding in carbon-rich systems, as encountered in organic chemistry, and leave the clusters of carboranes rich in boron to the proper purview of the inorganic chemist. Insofar as such three-dimensional clusters are considered at all in these review, interest will focus on the carbon-rich carboranes and the effect of ring size and substituents, both on boron and carbon, in determining the point of equilibrium between the cyclic organoborane and the isomeric carborane cluster. A typical significant example would be the potential interconversion of the l,4-dibora-2,5-cyclohexadiene system (7) and the 2,3,4,5-tetracarbahexaborane(6) system (8) as a function of substituents R (Eq. 2). 

TABLE 1. Effect of ring size on hydration of 1,3-cycloalkadienesa... 

Effect of Ring Size (in Number of C Atoms) on the Relative Rates of Catalytic Reactions... 

This example illustrates the synthesis of cyclic compounds by intramolecular alkylation reactions. The relative rates of cyclization for ca-haloalkyl malonate esters are 650,000 1 6500 5 for formation of three-, four-, five-, and six-membered rings, respectively.28 (See Section 3.9 of Part A to review the effect of ring size on Sn2 reactions.)... 

Discuss the effect of ring size on the tendency of a cyclic monomer toward ringopening polymerization. 

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 ". ... 

Theoretical aspects of the effect of ring size on the acid-catalyzed reduction of cyclic sulfoxides by iodide ions have been studied by Tamagaki et who noticed that differences in reactivity are mainly dependent on the change in activation entropy, which is correlated to the rigidity of the transition complex 220, (Eq. 59). 

Table 5 also gives data on the effect of ring size on the rate which are consistent with observations on other reactions of cyclic compounds. This influence can be explained by the change of the strain in the ring (/ strain) in consequence of the change from sp3 to sp2 hybridisation [129]. 

Effect of ring size and substitution on dehydration of cyclic alcohols at 200°C [94]... 

Actually such sensitized dimerizations have been reported only for small ring, cyclic olefins, the reaction being reasonably efficient for cyclopropenes,262 cyclobutenes,263 and cyclopentenes, but rather inefficient for cyclohexene.264 The effects of ring size on triplet lifetimes... 

It may be of interest to compare the photochemistry of the unsubstituted cyclic ketones. The effect of ring size on the ultraviolet absorption spectrum has been discussed (4,22) although its pertinence to the photochemistry is not obvious. 

Nucleophilic substitutions of cycloalkylmethyl arenesulfonates C H2 iCH20S02CgH4Z with anilines XCgH4NH2 in methanol at 65 °C have been studied.140 The reactivity order (given Z and X) n = 4>6>7>5 largely reflects the influence on steric effects of ring size, but the behaviour for n = 5 is anomalous. Application of the Taft equation in a form involving both polar and steric terms suggests that the retardation for n = 5 may be due to an enhanced polar (p a ) term. 

An effect of the ring-size in addition to that observed in the elimination taking place in the reduction of vicinal dibromides, has been observed for medium-sized ring compounds which carry the electro-active group either in the side-chain or directly attached to the ring. The half-wave potentials of cycloalkyl bromides can be correlated with the half-wave potentials of cyclic ketones and their derivatives (Fig. 29). The observed relation indicates that the relative effect of ring-size on the reactivity is similar in all the reaction series compared. 

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