Triflates crown ether

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. 

Aryl, heteroaryl, and alkenyl cyanides are prepared by the reaction of halides[656-658] or triflates[659,660] with KCN or LiCN in DMF, HMPA, and THF. Addition of crown ethers[661] and alumina[662] promotes efficient aryl and alkenyl cyanation. lodobenzene is converted into benzonitrile (794) by the reaction of trimethylsiiyl cyanide in EtiN as a solvent. No reaction takes place with aryl bromides and chlorides[663]. The reaction was employed in an estradiol synthesis. The 3-hydroxy group in 796 was derived from the iodide 795 by converting it into a cyano group[664]. 

Most interesting are the effects of salt complexation on the mesomorphic behavior of liquid crystalline crown ethers and liquid crystalline crown ether polymers. Sodium triflate was added to poly(17) [34] and poly(25) (Scheme 14) [39]. The enantiotropic nematic and smectic phases of poly(17) were changed dramatically [40]. With increasing amounts of salt, the clearing temperatures are shifted to higher values while the melting transition increases only slightly. 

Ligand 131 contains both cryptate and crown ether binding units. It reacts with neodymium triflate to give a 1 1 cryptate [Nd c 131]3+ exhibiting NIR luminescence upon excitation at 355 nm. The lifetime of the Nd(4F3/2) level is 347 and 711 ns in methanol and methanol-, respectively, which translates to MeOH 0 using eq. (10b). When barium ions are added to a solution of this complex in acetonitrile, the intensity of the 1.06 pm emission band is reduced substantially, while the lifetime of the Nd(4F3/2) level remains unchanged at about 470 ns. The Ndm ion is therefore not displaced from the cryptand cavity, while its luminescence is modulated by the presence of the Ba2+ ions bound by the crown ether (Coldwell et al., 2006). 

Recently, Torisawa reported that the addition of 18-crown-6 promotes the reactions of aryl triflates when Cs2C03 is used as the base [121]. This study focused on the reactions of N-Boc piperazine. In the absence of the crown ether, bis-N-Boc piperazine was produced as a byproduct, and reaction times were long. Yields for the reaction of 3,4-dichlorophenyl triflate with N-Boc piperazine were 35 % and 65 %, respectively, in the absence and presence of the crown ether. A similar effect was also observed for the reaction of the analogous dichloro bromoarene. 

Treatment of the triflate 9 with the sodio derivative of r-butyl dimethoxphosphorylacetate in dimethylformamide (DMF), at room temperature for 20 h in the presence of a crown ether, gives the epimeric phosphonates 10 in 81% yield [9]. Cleavage of the glycosidic bond by hydrogenolysis then affords the hemiacetals 11 which, with sodium hydride in tetra-hydrofuran (THF), give the C-6 phosphonium ylid, which reacts with C-1 in the aldehydo form to afford the alkene 12 in 73% (from the glycoside 10 Scheme 2). The cyclization... 

A novel scandium complex formed from scandium triflate in the presence of 18-crown-6 and p-sulfonatocalix[4]arene has been investigated.224 The crown ether resides in cavities created by two calixarenes from adjacent polymeric sheets. There are two types of scandium ions in the complex, one type is bound to the phenolic oxygen of the calixarene and the second exists as the hydroxide bound dimer ion [Sc(OH)2(H2O)10]4+. A similar complex may also be prepared without the crown ether. 

There are several examples of crown ether complexes where yttrium is not bound to the crown ether. [Y(H20)8]C13 (15-crown-5) has a [Y(H20)8]3+ cation (distorted dodecahedron) with the crown ether and chloride hydrogen bonded to the coordinated water molecules.58,225 An aqueous solution containing yttrium triflate and 18-crown-6 produces crystals of [Y(CF3-COO)2(H20)6](CF3COO),(18-crown-6) which have the crown ether hydrogen bonded to the coordinated water molecules.226 The complex [Y(N03)3(H20)3] (dibenzo-24-crown-8) has a nine-coordinate yttrium formed by bonding to three bidentate nitrate groups and three water molecules. The crown ether is hydrogen bonded to the water molecules.227... 

A study on the reaction profile of the asymmetric aldol reaction catalyzed by Pr(OTf)3 with 4 revealed that his crown ether-type chiral ligand did not significantly reduce the activity of the metal triflates. This retention of the activity even in the presence of the crown ether containing oxygen and nitrogen atoms in a key to realize the asymmetric induction in this asymmetric aldol reaction in aqueous me-... 

Lanthanide triflates are stable Lewis acids in water and are successfully used in several carbon-carbon bond-forming reactions in aqueous solutions. The reactions proceed smoothly in the presence of a catalytic amount of the triflate under mild conditions. Moreover, the catalysts can be recovered after the reactions are completed and can be re-used. Lewis acid catalysis in micellar systems will lead to clean and environmentally friendly processes, and it will become a more important topic in the future. Finally, catalytic asymmetric aldol reactions in aqueous media have been attained using Ln(OTf)3-chiral crown ether complex as a catalyst. 

Silylvinyl triflates such as 3 are decomposed by fluoride ion even more readily. Thus treatment of 3 at -20° with KF, and a crown ether or with R4N F" at — 20° also gives the carbene (a) in essentially quantitative yield. 

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