Small rings, definition

Pyrazine-2,3-dicarboxylic anhydride gave no products of the reaction of the corresponding heteroaryne with pyridine however, it gave a small but definite amount of phenylpyrazine with benzene, suggesting the formation of pyrazyne. This anhydride is readily converted by ring opening and rearrangement to succinonitrile (Brown et al., 1966). 

Although it is not surprising that Wiberg extended to tricyclic systems (propellanes) his studies on hybridization of the bridgehead carbons in small-ring bicyclic compounds and the chemistry resulting therefrom, we shall perhaps extend somewhat the realm of the propellane substrates. This is done pragmatically (every definition is, by definition, arbitrary) in order to permit inclusion of certain piquant results. 

Kubisa et al. [64] have been exploring the use of chiral ionic liquids in polymer synthesis. Using ionic liquids with a chiral substituent on the imidazolium ring for the ATRP of methyl acrylate gave a small but definite effect on polymer tacticity, with more isotactic polymer formed than in simple [BMIM][PF6]. They also found that the use of ionic liquids led to fewer side reactions. Ionic liquids have been used as solvents in biphasic ATRP to facilitate the separation of the products from the catalysts [65]. 

As with petroleum asphaltene (Speight, 2007), it is difficult (if not impossible) to accurately depict the structure of coal-derived asphaltene constituents. A simple assessment should include the statement small-ring polynuclear aromatic systems, basic and nonbasic nitrogen as well as phenolic and etheric oxygen. How these types enjoy an intramolecular existence is another matter which is difficult and whose definition is left to future research. 

The effective molarity (EM) is formally the concentration of the catalytic group (RCOO- in [5]) required to make the intermolecular reaction go at the observed rate of the intramolecular process. In practice many measured EM s represent physically unattainable concentrations, and the formal definition is probably relevant only in reactions (which will generally involve very large cyclic transition states) where the formation of the ring or cyclic transition state per se is enthalpically neutral, or in diffusion-controlled processes. For the formation of small and medium-sized rings and cyclic transition states the EM as defined above contains, and may indeed be dominated by, the enthalpy of formation of the cyclic form. This topic has been discussed briefly by Illuminati et al. (1977) and will be treated at greater length in a future volume in this series. 

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