Crown ethers Lewis acids

A lead(II) triflate-crown ether complex functions as a chiral Lewis-acid catalyst for asymmetric aldol reactions in aqueous media (Scheme 86).352 This is the first example of a chiral crown-based Lewis acid that can be successfully used in catalytic asymmetric reactions. 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

Ethers form Lewis acid Lewis base complexes with metal ions Certain cyclic polyethers called crown ethers, are particularly effective m coor dinatmg with Na" and K" and salts of these cations can be dissolved m nonpolar solvents when crown ethers are present Under these conditions the rates of many reactions that involve anions are accelerated... 

Grown Ethers. Ethylene oxide forms cycHc oligomers (crown ethers) in the presence of fluorinated Lewis acids such as boron tritiuoride, phosphoms pentafluoride, or antimony pentafluoride. Hydrogen fluoride is the preferred catalyst (47). The presence of BF , PF , or SbF salts of alkah, alkaline earth, or transition metals directs the oligomerization to the cycHc tetramer, 1,4,7,10-tetraoxacyclododecane [294-93-9] (12-crown-4), pentamer, 1,4,7,10,13-pentaoxacyclopentadecane [33100-27-6] (15-crown-6), andhexamer, 1,4,7,10,13,16-hexaoxacyclooctadecane [17455-13-9]... 

In this way hosts 141-145 with both a cr-bonded Lewis acidic boron atom for complexation of anions and a conventional multidentate ligand for cations are generated. Complexation experiments of the 21-membered crown [6] boronate 142 with different potassium salts KX (X = F, Cl, Br, I, SCN, CN, OMe) indicate that there is a high specificity for the incorporation of KF, whereby F is bound covalently to the boron atom and is complexed by the crown ether (146, Fig. 39). An X-ray study has shown that the complexation of KF is heterotopic, i.e., both ions are complexed inside the same host. Some of the salts can only be bound in a monotopic way (KI and KSCN) [237]. 

Asymmetric Lewis-Acid Catalyzed. Another important advance in aqueous Mukaiyama aldol reaction is the recent success of asymmetric catalysis.283 In aqueous ethanol, Kobayashi and co-workers achieved asymmetric inductions by using Cu(OTf)2/chiral >A(oxazoline) ligand,284 Pb(OTf)2/chiral crown ether,285 and Ln(OTf)3/chiral Mv-pyridino-18-crown-6 (Eq. 8.105).286... 

The reaction of dialkylditellurides with pressurized acetylenes in the presence of an electrophilic reagent (alkyl halide, Lewis acid), in the system KOH/crown ether/ben-zene, gives rise to alkylethynyl tellurides and 1,2-bis-alkyltelluroacetylenes in high yields. ... 

This compound represents an example of an inverse crown ether stmcture (Lewis acidic host-Lewis basic guest macrocyclic heterometallic alkoxides or amides/ . 

By analogy with cation complexing crown ethers like 47-50, attachment of a defined number and type of Lewis acids to a rigidified molecular scaffold in such a way that their electron-efficient sites are... 

An important class of alkali and alkaline earth metal amides are Mulvey s inverse crown complexes (also discussed in Chapter 2, dealing with sodium and potassium amides), in which cationic homo- or heterometallic macrocycles are hosts to anionic guest moieties.The term inverse crown indicates that the Lewis acidic/Lewis basic sites are reversed or exchanged in comparison to conventional crown ether complexes. Scheme 3.9 illustrates the range of recently published alkali and alkaline earth metal amide inverse crown complexes (for related Zn species see Chapter 7 on group 12 amides). 

Organometallic crown ethers have also been synthesized.96-98 Recently, a crown-cation group was shown to interact with an appended transition metal acyl ligand (29)." Complexes of this type have potential applications in Lewis acid-accelerated alkyl migration to coordinated carbonyls. 

One of the first ion pair receptors was the 21-membered crown ether 5.12, prepared by M. T. Reetz of the University of Marburg, Germany in 1991, which possesses a Lewis acidic boronate centre. Ligand 5.12 is capable of dissolving stoichiometric amounts of KF in dichloromethane at room temperature to give a KF adduct in which the K+ ion is bound by the crown ether ring while the I1 anion forms a dative bond with the boron centre out of the plane of the crown ether. Ligand 5.12 fails to dissolve either KC1 or KBr as a consequence of the weaker nature of the B-Cl and B-Br bonds. 

Reetz, M. T., Niemeyer, C. M., Harms, K., Crown ethers with a Lewis acidic center - a new class of heterotopic host molecules. Angew. Chem., Int. Ed. Engl. 1991, 30, 1472-1474. 

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