Ether, lariat

Lariat ethers - from simple sidearms to supramolecular systems, Chem. Soc. Rev. 1992, 21, 39-47. 

At the borderline between the two-dimensional monocyclic polyethers and three-dimensional cryptands is a relatively new type of host ligand, the lariat ether (73,74). 

This is a term introduced by Gokel and co-workers only very recently. The compounds included in this class are those which have single macrorings like crowns but additional pendant donor groups which make them similar in some respects to crytands. The presumption from which the name derives is that a complexed cation will be bound by both the macroring and the sidechain in much the same fashion as a lasso binds an animal. The compounds are named as simple crown derivatives and are illustrated as 16 a 17 below. 

Pedersen patented a series of ftis-crowns related to the above except that the side chain was bound on each side to a crown. Cram and Sutherland have both reported similar structures. Pedersen suggested the name clams for such species but the name has not found wide acceptance. 

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Lariam [51773-92-3] Lariat ethers Laricinan Larmor frequency... 

Inclusions of Other Grown Analogues. A variety of crown analogues and hybrid modifications (24—28) with other topological features (lariat ethers (31,32), octopus molecules (33), spherands (eg, (12) (34), torands (35)) including chiral derivatives (36) have been prepared and demonstrated to show particular inclusion properties such as chiroselective inclusion (Fig. 4) (37) or formation of extremely stable complexes (K ">(LR) for (12)... 

The name lariat ethers has recently been suggested by Gokel and coworkers to describe a class of compounds which are based on crown ethers and have secondary donor groups bound to the macrormg by flexible arms (see Sect. 1.3 5). The name lariat is suggested by the analogy to the lassoes used to rope and tie animals since it is hoped that the more successful molecules of this class will be able to do likewise with sundry cationic species. 

Although the principal application for 9 has been in the synthesis of cryptands (see Chap. 8), this material has also served as precursor to a number of nitrogen based lariat ethers , sometimes referred to as crown complexanes . Binding constants for such compounds have been measured for a few examples in a few cases , but... 

Because acid-base properties on the inside of molecules will be discussed, those macrocycles (and related compounds) which possess functional groups hanging on the outside of the ring, e.g. lariat ethers or substituted polyazamacrocyles (Kaden, 1984 Gokel, 1991, 1992), will be mentioned only briefly. From the large group of azamacrocycles only a small fraction will be discussed in detail because the basic functionality rarely is located in a defined i/j-position. 

Figure 23. Chemical structures of the novel betaine lariat ethers KBC-001 (R=CH3) and KBC-002 (R=C12H25).

Typical lariat ethers are illustrated by the general structure (181). 

Cation-Binding Properties of Crown Ethers, Lariat Ethers, Bibracchial Lariat Ethers, and Poly(ethylene glycols) as Potential Phase-Transfer Catalysts... 

Although the lariat ethers (29-31) were conceived on principles related to biological activity, they are interesting candidates for study as either free phase transfer catalysts, or as polymer-bound catalysts. In the latter case, the sidearm could serve both a complexing function and as a mechanical link between macroring and polymer. Polymeric phase transfer catalyst systems have been prepared... 

The structure shown above is a representative of the class of lariat ethers we have called "nitrogen-pivot" compounds (31). The presence of invertable nitrogen as the point of sidearm attachment makes these compounds more flexible than the corresponding "carbon-pivot" lariat ethers (29,30). The carbon- and nitrogen-pivot lariat ethers have been prepared with a variety of sidechains attached. Examples of compounds which could be linked to existing polymers such as chloromethylated polystyrene or copolymerized with other monomers are shown below. 

Sodium Cation Binding by Nitrogen-pivot. Lariat Ethers... 

For lariat ethers to be effective as polymer-bound phase transfer catalysts, sidearm and macroring cooperation must be intramolecular. It is unlikely that two lariat ethers will be close enough on a polymer backbone or other support for the ring of one compound to interact with the sidearm donors of another. The mechanical attributes of lariat ethers will be independent of spacing but for any advantage in cation binding and anion activation to be realized, the macroring and its attached sidearm must cooperate to envelop the cation, solvate it, and shield it from the counteranion. 

Intramolecularity in lariat ether complexation was demonstrated in three ways. First, when solution cation binding constants were determined. Kg was shown to be independent of cation and macrocycle concentrations (within certain limits). The efficacy of complexation of a single cation by two macrorings [(ML2) complex formation] should be concentration dependent. Second, ammonium cation binding constants were determined for the series (see above) of monoaza-15-crown-5 and -18-crown-6 compounds having (CH2CH20)jiClH3 (n 0 to 8) sidearms (31). 

The complexation ability of crown-ethers has been improved by the introduction of secondary donor sites covalently bonded to the macrocyclic ring through a flexible arm, e.g. "lariat ethers" (27). It is also known that in particular conditions crown-ethers can make 2 1 sandwich complexes with the cation (8). 

Alcohol sidearms, bibracchial lariat ethers, 34 Alkenes... 

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