Ethers, crown nomenclature

We first confirmed the formation of these macrocycles in the polymerization of THF by using coupled gas chromatography/mass spectrometry ( 2). Macrocyclic ethers containing up to 8 THF units could be separated and identified by this method (23). The two predominant macrocyclic species found in THF polymerization mixtures are a cyclic tetramer and a cyclic pentamer. In analogy to the "crown ether" nomenclature, we proposed the name 20-crown-4 for the cyclic tetramer and 25-crown-5 for the cyclic pentamer (22). 

Equation 8.28 shows only the anionic nucleophile explicitly, since the counterion does not appear to take part in the reaction. Nevertheless, the counterion affects the solubility of a nucleophilic salt, which therefore can influence the polarity of the solvent needed for the reaction. An alternative to the use of a more polar solvent to dissolve a salt for nucleophilic substitution is to use crown ether additives. Crown ethers are cyclic polyethers that can coordinate with cations and therefore increase their solubility in organic solvents. The nomenclature provides the total number of atoms and the number of oxygen atoms in the ring. Compoimd 51 is 12-crown-4, and 52 is 18-crown-6 (Figure 8.32). Coordination of a crown ether with a cation helps to dissolve the salt in a less polar solvent and leaves the anion relatively unsolvated. The activation energy for substitution therefore does not include a large term for desolvation of the nucleophilic anion, and the reactions are fast. For example, adding dicyclohexano-18-crown-6 (53) to a solution of 1-bromobutane in dioxane was found to increase its reactivity with potassium phenoxide by a factor of 1.5 x 10. Moreover, Liotta and Harris were able to use KF solubilized with 18-crown-6 (52) to carry out Sn2 reactions on 1-bromooctane in benzene. ... 

Introduction to Ethers Nomenclature of Ethers Structure and Properties of Ethers Crown Ethers Preparation of Ethers Reactions of Ethers Nomenclature of Epoxides Preparation of Epoxides Enantioselective Epoxidation Ring-Opening Reactions of Epoxides Thiols and Sulfides... 

The need for simple descriptions of complicated organic ligands has led to the evolution of some trivial nomenclature systems, such as those for crown ethers (e.g. 76) 72AG(E)16) and cryptands 73MI10200), which have become quite elaborate 8OMII0200). These systems are intended primarily to indicate topology, and the positions of potential donor atoms, and are not particularly appropriate for general use. 

Pedersen prepared a series of macrocyclic polyethers, cyclic compounds containing four or more oxygens in a ring of 12 or more atoms. He called these compounds crown ethers, because their molecular- models resemble crowns. Systematic nomenclature of crown ethers is somewhat cumbersome, and so Pedersen devised a shorthand description whereby the word crown is preceded by the total number of atoms in the ring and is followed by the number of oxygen atoms. 

Multidentate ligands, both acyclic and cyclic, are now abundant and herein the nomenclature used will follow that recently proposed by Vogtle and Weber.33 Coronands are taken to mean macromonocyclic compounds having any heteroatoms present as potential donors, with crown ethers being reserved for compounds of the same type having only oxygen atoms present as... 

The IUPAC nomenclature for crown ethers is cumbersome and so was trivialized for common use. Dibenzo-18-crown-6 is named as follows dibenzo describes the non-ethyleneoxy content 18, the total number of heteroatoms in the crown ring and 6, the number of heteroatoms in the ring. 

Examples of various crown ethers and cryptands are shown here. The top line of compounds may be named readily enough although the problem with this semi-systematic approach is obvious. If two methylenes were added to 18-crown-6, the compound could correctly be called 20-crown-6 but in the absence of unequivocal descriptors, the positions of the 3-carbon bridges would be unclear. The more cumbersome name 1,4,7,11,13,17-hexaoxacy-cloicosane tells clearly that the longer bridges are adjacent to each other. A similar problem is apparent in the last two entries of the second line. The designations dicyclohexano and dibenzo are clear as to the substituents but not their positions. The semi-systematic nomenclature is widely used, however, because it is so much less cumbersome for most purposes. 

An additional nuance in the nomenclature of these compounds concerns their complexes. The open-chained compounds are often referred to as podands and their complexes as podates. The cyclic ethers may also be called coronands and their complexes are therefore coronates. Complexed cryptands are cryptates. The even more complicated structures known as spherands, cavitands, or carcerands are called spherates, cavitates, or carcerates, respectively, when complexed. The combination of a macrocycle (crown ether or coro-nand) and a sidechain (podand) is typically called a lariat ether. 

Although the IUPAC nomenclature is recommended in the majority of journals, it can be seen clearly that the use of jargon in respect to crown ethers and cryptands enjoys a great popularity. Not surprisingly, since their exact and complicated IUPAC names are difficult to mention frequently in the text. Common abbreviations can be found almost in all review articles 2), however, for the convenience sake, we draw attention to some of them to which is referred here. Chart 1 depicts simple examples of N,N -dimethyl diazacoronands, cryptands and more elaborated cryptands incorporating carbohydrate units. The abbreviations below each formula are easy to follow. 

Cation complexes of cryptands were called cryptates and those of spherands, spherates. The name coronand was suggested for crown ethers and their complexes would therefore be coronates <1980ICAL45>. The latter nomenclature has proved to be acceptable to the community but it has not been universally adopted. A reasonable and systematic approach, based on principles of polymer nomenclature, has appeared but has not found wide acceptance <1984JCI266>. 

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