Crown ethers, catalysis

Both phase transfer and crown ether catalysis have been used to promote a-elimination reactions of chloroform and other haloalkanes.153 The carbene can be trapped by alkenes to form dichlorocyclopropanes. 

Scheme 11.11 gives some representative preparative reactions based on these methods. Entry 1 is an example of the classical procedure. Entry 2 uses crown-ether catalysis. These reactions were conducted in the aromatic reactant as the solvent. In the study cited for Entry 2, it was found that substituted aromatic reactants such as toluene, anisole, and benzonitrile tended to give more ortho substitution product than expected on a statistical basis.180 The nature of this directive effect does not seem to have been studied extensively. Entries 3 and 4 involve in situ decomposition of A-nitrosoamides. Entry 5 is a case of in situ nitrosation. 

The characteristics of homogeneous crown-ether catalysis were nicely demonstrated by Thomassen et al. (1971) who studied the rate of alkylation of potassium phenoxide with 1-bromobutane in dioxan at 25°C. By measuring the initial consumption of phenoxide (r, in M s-1), any effect of the bromide ion was neglected. The results (Table 21) show hardly any effect of tetraglyme... 

Increasing the hydrophobicity of quaternary ammonium salts increases the apparent extraction constants for the ion pair and therefore leads to a higher catalytic activity (Brandstrom, 1977). The same phenomenon has been observed by Cinquini and Tundo (1976) for crown ether catalysis (Table 35). The catalytic activity of 18-crown-6 [3] and alkyl-substituted derivatives [117]—[ 119] in the reaction of n-CgH17Br with aqueous KI follows the order [117], [118] > [119] s> [3]. The alkyl-substituted [2.2]-cryptand derivatives are also much more efficient than the parent compound [86]. Increasing the hydrophobicity of [2.2.2]-cryptand (Cinquini et al., 1975) and even of polypode ligands (Fornasier et al., 1976) leads to higher catalytic activity. The tetradecyl-substituted compounds show the reactivity sequence [2.2.2]-cryptand at 18-crown-6 > [2.2]-cryptand on the reactivity scale that can be distilled from Table 35. 

The chemical reactivity of crown-ether complexes with neutral molecules has received little attention. Nakabayashi et al. (1976) have reported crown-ether catalysis in the reaction of thiols with l-chloro-2,4-dinitrobenzene. The catalytic activity was attributed to deprotonation of thiols by dicyclohexyl-18-crown-6 in acetonitrile solution. Blackmer et al. (1978) found that the rate of aquation of the cobalt(III) complex [333] increases on addition of... 

Crown ether catalysis of the reduction is also useful (Table I), with potassium hydroxide as the base and 18-crown-6 as the catalyst (18). 

The two-phase reduction of nitro compounds to anilines by Fe3(CO)i2, potassium hydroxide, and 18-crown-6 has previously been described (18). Crown ether catalysis (18-crown-6 or dicyclohexyl-18-crown-6) is also useful for reducing the double bond of the a,/3-unsaturated ketone, ben-zylideneacetone. Either of the following reaction systems can be employed Fe(CO)5/KOH/crown ether/benzene or Fe(C0)5/RNH2/Fl20/ crown ether/benzene (26). 

As indicated in Table 12, the application of process (100) is straightforward what seems to be needed is an active or electrophilic alkyne and usually, but not inevitably, an aprotic medium. It is probable that phase transfer and/or crown ether catalysis will broaden the utility of this reaction. 

Zubrick, S. W., B. I. Dunbar, and H. D. Durst, Crown Ether Catalysis II. Cyanide and Nitrite as Naked Anions, Tetr.Lett, 71 (1975). 

The long known Reissert reaction involves the kinetic trapping by cyanide of an -acylquinolinium or -isoquinolinium salt in the classical process the acylating agent is benzoyl chloride. Reissert compounds are usually prepared using a dichloromethane/water two-phase medium recent improvements include utilising phase-transfer catalysts with ultrasound or crown ether catalysis. 

Crown-ether catalysis has been used to prepare 5 -0-alkyl-derivatives of 2 ,3 -Q-isopropylidene uridine. 58... 

Pedersen reported in 1967 that the dicyclohexyl-18-crown-6 complex of potassium hydroxide was soluble in toluene and in this medium could readily hydrolyze the very sterically hindered ester methyl mesitoate [33]. He later reported that this complex could also hydrolyze the tertiary-butyl ester [34]. Similarly, Lehn found that the [2.2.2]-cryptate complex of potassium hydroxide was even more effective in this saponification reaction under directly comparable conditions [35]. The hydrolysis reaction is formulated in equation 9.15. This method has recently been applied in the hydrolysis of C-labelled methyl tetradecanoate [36]. Starks found that tetra-decanoate anion acted as a catalyst poison and impeded further hydrolysis [7]. This is therefore one of the few examples where crown ether catalysis is clearly superior to quaternary ammonium ion catalysis. 

Full details have now appeared of the crown-ether catalysis of hydrolysis and decarboxylation of substituted malonates and related compounds (Scheme 50) (2,139), a useful alternative to classical procedures. The lactonization of... 

Especially for large-scale work, esters may be more safely and efficiently prepared by reaction of carboxylate salts with alkyl bromides, iodides or tosylates. The carboxylate anion is not a particularly reactive nucleophile so reaction proceeds best in polar aprotic solvents or with crown ether catalysis. Acetone has also been found to be a good solvent for reactions with alkyl iodides. Carboxylate alkylation procedures have been particularly advantageous for preparation of hindered esters which can be relatively difficult to prepare by the acid-catalyzed esterification reactions to be discussed in Section 3.4. Sections F and G of Scheme 3.2 give some specific examples of ester alkylation by both diazoalkanes and other alkylating agents. 

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