Cryptand nomenclature

A number of bi- or polycyclic structures have been prepared over the years which are somewhat difficult to classify within the narrow confines of crown and cryptand nomenclature. Nevertheless, these molecules deserve mention and are noted here. 

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. 

For the purposes of nomenclature, the simple cryptands are assumed to be macro-bicyclic and nitrogen is assumed to be the bridgehead atom. The different cryptands are designated by assigning numbers according to the number of heteroatoms in each ethylenoxy chain. The three cryptands shown below are designated [ 1.1.1 ]-cryptand (9), [2.2.1 ]-cryptand (10) and [3.2.2]-cryptand (11), respectively. 

An even more complicated nomenclature problem arises with the closely related all-oxygen cryptands. These compounds do not utilize nitrogen as the three-chain junction. Most examples of this class of compounds have utilized pentaerythritol or glycerol as the junction. This naturally imparts a somewhat lower flexibility to the molecule than would be present in the nitrogen-containing cases. Structures of two such molecules are illustrated below. 

The disadvantages of these names are manifest, but one shudders to think about naming them systematically. It seems likely that for the foreseeable future, nomenclature of crowns, cryptands and related substances will bear a semblance to heiro-glyphics. 

There is some evidence that Cs + can be formed by cyclic voltammetry of Cs+[OTeF5] in pure MeCN at the extremely high oxidizing potential of 3 V, and that Cs + might be stabilized by 18-crown-6 and cryptand (see pp. 96 and 97 for nomenclature). However, the isolation of pure compounds containing Cs + has so far not been reported. 

Cryptands, 42 122-124, 46 175 nomenclature, 27 2-3 topological requirements, 27 3-4 Cryptate, see also Macrobicyclic cryptate 12.2.2], 27 7-10 applications of, 27 19-22 cylindrical dinuclear, 27 18-19 kinetics of formation in water, 27 14, 15 nomenclature, 27 2-3 spherical, 27 18 stability constants, 27 16, 17 Crystal faces, effect, ionic crystals, in water, 39 416... 

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>. 

Dinitroazobenzene was used as the fluorescent residue in a chiral receptor molecule that incorporated both a crown ether and calixarene, the structure of which is shown in Figure 15. Formally, this comprises a ditopic cryptand but such a simple nomenclature is clearly inadequate <2004CH1174>. 

As Pedersen had done, Lehn offered a simplified nomenclature to deal with the new family of extremely comphcated molecular structures. Simplified cryptand names assume that all spans contain CH2CH2O units attached at either end by nitrogen. It is necessary only to specify how many ethyleneoxy units are present in each of the three chains. The compound N[(CH2CH20)2CH2CH2]3N contains three molecular threads, each of which possesses two oxygen atoms. The simplified name for this compound, using a variant of the nomenclature standard for bicyclic structures, is [2.2.2]-... 

The spherical cryptands have a somewhat different nomenclature. Each host is denoted by a series... 

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