Crown ethers, leaving groups

Table 3.10 Dependence of reaction rate with A = CH3C02 on added crown ether (leaving group, X = Cl).

Zollinger and coworkers (Nakazumi et al., 1983) therefore supposed that the diazonium ion and the crown ether are in a rapid equilibrium with two complexes as in Scheme 11-2. One of these is the charge-transfer complex (CT), whose stability is based on the interaction between the acceptor (ArNj) and donor components (Crown). The acceptor center of the diazonium ion is either the (3-nitrogen atom or the combined 7r-electron system of the aryl part and the diazonio group, while the donor centers are one or more of the ether oxygen atoms. The other partner in the equilibrium is the insertion complex (IC), as shown in structure 11.5. Scheme 11-2 is intended to leave the question open as to whether the CT and IC complexes are formed competitively or consecutively from the components. ... 

As outlined in (1) at the beginning of Section 2, a solution containing a metal salt (MX) and a crown ether (Cr) may contain three different anionic species, namely, the free anion X-, the tight ion pair M+.X and the crown ether-separated ion pair M+.Cr.X. All of these species may react with a substrate SY with different rate constants. As the reaction proceeds, any leaving group Y may affect the rate of the reaction by competing with M+. X-for crown ether as shown in (11). It is evident from (1) and (11) that the effect... 

The changes in product distribution brought about by crown ethers depend not only on the base but also on the type of solvent, leaving group, and... 

By saturating the co-ordination sites of the cation with a crown ether, the relative contribution of the syn-pathway decreases, as was shown by Zavada et ai (1972) for t-BuOK-promoted eliminations from 5-decyl tosylate. Furthermore, trans/cis ratios in both anti- and syn-eliminations were affected by the presence of dicyclohexyl- 18-crown-6 and the nature of the leaving group (Zavada et ai, 1976). Fiandanese et ai (1973) observed that elimination from fluorosulphonylethanes [177] promoted by potassium phenoxide in dioxan... 

As an example of ion pairing, consider the elimination of meso-l,2-dichloro-l,2-diphe-nylethane in Eq. 10.87. Here, syn elimination occurs 13% of the time. Addition of 18-crown-6 completely wipes out any syn elimination. Apparently the small percent of syn elimination arises from an ion pair where the potassium cation bridges the base and the leaving group (see margin). Addition of the crown ether negates the ability of the K to act in this manner. 

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