Ethers interesting examples

In the field of nucleosides, an interesting example of a cleavage of a nonfluor-inated six-membered cyclic ether containing oxygen and nitrogen atoms to give a fluoro nucleoside has been reported [5] (equation 5)... 

An interesting example from carbohydrate chemistry is the boron trifluoride-diethyl ether complex catalyzed nucleophilic addition of silyl enol ethers to chiral imines (from n-glyceralde-hyde or D-serinal)22. This reaction yields unsaturated y-butyrolactones with predominantly the D-arabino configuration (and almost complete Cram-type erythro selectivity). 

The isomerization takes place because the excited states, both 5i and T, of many alkenes have a perpendicular instead of a planar geometry (p. 311), so cis-trans isomerism disappears upon excitation. When the excited molecule drops back to the So state, either isomer can be formed. A useful example is the photochemical conversion of c/s-cyclooctene to the much less stable trans isomer." Another interesting example of this isomerization involves azo crown ethers. The crown ether (5), in which the N=N bond is anti, preferentially binds NH4, Li, and Na, but the syn isomer preferentially binds and Rb (see p. 105). Thus, ions can be selectively put in or taken out of solution merely by turning a light source on or off." ... 

Similar formations of bisanthracenes have been studied with a variety of substituents. An interesting example of a cation-assisted intramolecular anthracene dimerization where the bisanthracene formation includes cycli-zation of a polyether chain to a crown ether is illustrated in (4.27). In the absence of Li+ the dimer reverts back to the open chein compound, but in the presence of Li+ the crown ether is stabilized so that the product itself also becomes more stable 430). 

Another very interesting example was presented by Huang et al. in 2011 [108], They used 15-crown-5-functionalized MWCNTs as dopant material (Fig. 18.6). The main incentive for their work is based on the binding Li+ ions that are present in the electrolyte by means of crown ether complexation. As a matter of fact, Li+ is expected to attract 1 ions, which would render any movement difficult within the electrolyte... 

Interesting examples of both of these processes can be seen in the reactions of a series of 2-aryl-3,3-dimethyl-4-r-butyloxetanes when treated with boron trifluoride etherate in... 

With conjugated dienes, photocycloaddition of carbonyl compounds occurs at one of the double bonds to give vinyloxetanes. An interesting example is the reaction of acetone with 2-methyl-l,3-butadiene, which gave the two oxetanes (60) and (61) in a ratio of 3 1 and a total yield of about 20% (72JA8761). Other alkenes which have been used for photosynthesis of oxetanes include enol ethers, ketene acetals, enamines, allenes and diketene, with the reaction of the last compound with benzaldehyde illustrated in equation (105) (75CPB365). 

An interesting example is the determination of complex formation constants of alkali metal ions (M+) with crown ethers (L) ... 

An interesting example of the salting out by nonelectrolytes in water was given by Grigorovich and Samoilov (69). They studied the solubility of praseodymium chloride and sulfate in water in the presence of various nonelectrolytes (methyl alcohol, ethyl alcohol, and diethyl ether.) Their results show that the decrease in solubility of praseodymium chloride is only of the order of a few percent but that this decrease is probably highly significant because praseodymium chloride is more soluble in pure methyl and ethyl alcohol than in pure water. (The... 

The cyclofunctionalization of cycloalkenyl systems where the chain containing the nucleophilic functionality is attached at one end of the double bond leads to spirocyclic structures. Cyclizations of cyclic and acyclic enol ethers to generate spiroacetals are shown in equations (66)168 and (67).169 These reactions generate the thermodynamically more stable products based on anomeric and steric factors.170 Spiroacetal products have also been obtained using isoxazolines as the nucleophilic functionality (cf. Table 14).l4lb Studies of steric and stereoelectronic control in selenoetherification reactions which form spirocyclic tetrahydrofurans have been reported.38 An interesting example of stereoelectronic control in the formation of a spirocyclic lactone has been reported in a recent mevinolin synthesis (equation 68).171... 

Examples of these processes are the oxidation of p-aminophenol at platinum electrodes in aqueous acidic solution, the reduction of dopamine at glassy carbon electrodes or that of the cation 2,6-diphenylpyrylium in acetonitrile [9]. Another interesting example arises from the facilitated ion transfer of amines from aqueous to organic media in the presence of crown ethers like the dibenzo-18-crown-6 [52, 53] (see Fig. 3.22). 

Carbon dioxide removal by reactive absorption in amine solutions is also applied on the commercial scale, for instance, in the treatment of flue gas (see later in this chapter). Another possible application field of the technique is gas desulfurization, in which H2S is removed and converted to sulfur by means of reactive absorption. Aqueous solutions of ferric chelates (160-162) as well as tetramethylene sulfone, pyridine, quinoline, and polyglycol ether solutions of S02 (163,164) have been proposed as solvents. Reactive absorption can also be used for NOx reduction and removal from flue or exhaust gases (165,166). The separation of light olefins and paraffins by means of a reversible chemical com-plexation of olefins with Ag(I) or Cu(I) compounds in aqueous and nonaqueous solutions is another very interesting example of reactive absorption, one that could possibly replace the conventional cryogenic distillation technology (167). 

However, the chemical reduction reaction does not work efficiently for the removal of two Bingel addends, whereas the electrochemical reduction process works very efficiently. One of the most interesting examples that illustrates the difference between the chemical and electrochemical methodologies involves the cyclophane type trans-1 ( -37) conjugate [76], The chemical reductive protocol fails completely with this compound, presumably due to ion complexation by the crown ether group and consequent stabilization. When the electroreductive method was used, a clean retrocyclopropanation reaction was observed (Figure 25). 

An interesting example of the deleterious effect of an ortho methyl group is to be found in the tetraphenylmethane series.16 13 While 4-aminotetraphenylmethane (III) is converted to the hydrocarbon (IV) in 46% yield, the homolog (V) gives only the ether (VI). 

There was reported an interesting example in which copper(I) complexes of the bis(bipyridyl) ether 19a are chiral (C2-symmetry) and crystallize in form of their racemate.43 The additional attachment of two chiral (S)-2-methylbutyl ether groups leads to the chiral ligand 19b which consequently can form two diastereomeric copper(I) complexes, from which the complex with the left-handed prohelical [Cu,(bpy)2] substructure is found in 30% excess.44... 

Much larger effects of this type are observed in cycloadditions of enol ethers to tetrazines (Fig. 26), a reaction shown by Sauer and co-workers to be an example of a Diels-Alder reaction with inverse electron demand 75. The rates of 3,6-di-(2 -pyridyl)-s-tetrazine to various enol ethers and styrenes are summarized in Fig. 27. These were obtained by measuring the disappearance of the 540 nm band in the absorption spectra of the tetrazine76. These results are of particular interest, since there is little or no correspondence between the electron-donor ability of the enol ether, as measured by the ir ionization potentials (Table 5), and the rate of reaction of the enol ether. For example, although the conversion of methyl vinyl ether to 1,1-dimethoxyethylene results in a 4.3 times increase in rate, in line with the 0.2 — 0.3 eV decrease in IP, the 1,2-dimethoxyethylenes are 13 to 25 times less reactive than methyl vinyl ether, even though the IPs of these molecules are much lower... 

An interesting example of a Lewis acid/base reaction between neutral reactants is the formation of tris(n-butyl)phosphonium-dithiocarboxylate, ( -Bu)3P" — 82 , from tris(n-butyl)phosphane and carbon disulfide in solution. As expected, this equilibrium is strongly shifted in favour of the dipolar zwitterion with increasing solvent polarity (diethyl ether dimethyl sulfoxide) [272, 273]. 

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