Calix arenes, alkylation

Water-soluble calix[ ]arenes (62 n = 4, 6, and 8) containing trimethylammonio-methyl groups act as efficient inverse phase-transfer catalysts in the nucleophilic substitution reactions of alkyl and arylalkyl halides with nucleophiles in water.126 (Inverse phase-transfer catalysts facilitate reactions between two immiscible reactants via the transport of an organic substrate into an aqueous solution of a second substrate, in which reactions take place.)... 

A study48 of the direct upper-rim alkylation of calix[ ]arenes has shown that, with n = 8, reaction with isopropyl chloride in 1,2-dichloroethane with AICI3 gives isopropylation, whereas when n = 4 hydroxyisopropylation is observed. With n = 6 there is a mixture of products, indicating overall an increase in phenolic behaviour as n increases from 4 to 8. 

Calix[n]arenes 1-3 were used as inverse PT catalysts in the alkylation of active methylene compounds with alkyl halides in aqueous NaOH solutions,and in aldol-type eondensation and Michael addition reactions. In the aikylation of phenylacetone with octyl bromide, the IPTC procedure enhanced the alkylation versus hydrolysis and C versus O alkylation selectivities with respect to those observed xmder classical PTC reactions in the presence of tetrabutylammonium bromide (TBAB) or hexadecyltributylammonium bromide (HTPB). Moreover, the aqueous catalyst solution was easily separated from the organic phase eontaining the products, and no organic solvent was required. In the case of the aldol-type condensation of benzaldehyde with indene or acetophenone in aqueous NaOH (Fig. 9), IPTC reaetions eatalyzed by I were compared with those conducted in aqueous micelles in the presence of cetyltrimethylammonium bromide (CTAB) as the sufactant. Although selectivities and yields were similar, the IPTC proeedure avoided the formation of emulsions, thus faciUtating product separation and catalyst recovery. In the light of the results obtained, water-soluble calix[ ]arenes 1-3 were proposed... 

One of the analogues is calix arenes which are oligomeric cyclic analogues of novolac resin derived through the condensation of p-alkyl phenols with formaldehyde. Quite re-... 

Alkylation reactions of ketones, active methylene compoimds, alcohols, and phenols with alkyl halides occurred in quite good yields in aqueous NaOH solution without added organic solvent in the presence of water-soluble calix[ ]arene (n = 4,6, or 8) containing trimethy-lammoniomethyl groups acting as a reverse phase-transfer catalyst (Scheme 8.20). ° ... 

Scheme 3.2 shows per-functionalized pillar[5]arene derivatives prepared from per-hydro)ylated pillar[5]arene 3.6 as a starting compound. Applying an efficient organic reaction is required for the perfect introduction of functional groups at all 10 reaction sites. A straightforward and efficient functionalization process for per-hydroxylated pillar[5]arene is etherification, which has been used for functionalization of the lower rims of phenolic moieties in calix[ ]arene derivatives. Various functionalized pillar[5]arene derivatives can be obtained by etherification of per-hydroxylated pillar[5]-arene with an alkyl-halide in the presence of appropriate bases, such as NaH and K2CO3. Introduction of the substituents can change various physical... 

When this concept is applied to the stereochemistry of pillar[n]arene, rotation of the units triggers the planar-chiral inversion from pS to pR or from pR to pS. Pillar[5]arenes are cyclic pentamers, therefore they have eight possible conformers pS, pS, pS, pS, pS), (pR, pR, pR, pR, pR), (pR, pS, pS, pS, pS), pS, pR, pR, pR, pR), pR, pR, pS, pS, pS), pS, pS, pR, pR, pR), R, pR, pS, pR, pS) and pS, pS, pR, pS, pR), namely la/b, 2a/b, 3a/b and 4a/b, respectively. In many cases, these conformers interconvert at 25 °C. Thus, inhibition of the rotation of the units contributes to the separation of these planar-chiral pillar[5]arene enantiomers. In calix[ ]arenes, modification of alkyl substituents can slow the rotational motion of the phenolic units, and can immobilize the conformation of calix[n]arenes. Thus, we first introduced various lengths of allq l chains, such as ethyl, propyl, butyl, pentyl, hetg l and dodecyl chains to both rims of the pillar[5]arene. ... 

Alternatively, the Sn2 nucleophilic substitution reaction between alcohols (phenols) and organic halides under basic conditions is the classical Williamson ether synthesis. Recently, it was found that water-soluble calix[n]arenes (n = 4, 6, 8) containing trimethylammonium groups on the upper rim (e.g., calix[4]arene 5.2) were inverse phase-transfer catalysts for alkylation of alcohols and phenols with alkyl halides in aqueous NaOH solution to give the corresponding alkylated products in good-to-high yields.56... 

III. Metalation of Calix[4]arenes and O-Alkylated Calix[4]arenes... 

METAL ATI ON OF CALIX[4]ARENES AND O-ALKYLATED CALIX[4]ARENES USING TRANSITION METALS... 

Tert-butyl calix[4]arene possessing four antennary amine groups (7) were initially synthesized from commercially available phenolic tert-butyl calix[4]arene according to a published procedure [13]. After derivatization of the phenolic group by alkylation with ethyl bromoacetate, hydrolysis, acid... 

In an effort to impart selectivity and enhance complex stability for this class of anion receptor novel ditopic biscobaltocenium receptor molecules (Fig. 42) have been synthesized and their coordination and electrochemical properties have been studied (Beer et al., 1993f, 1995h). The two positively charged metallocene centres linked by various alkyl, aryl and calix[4]arene spacers... 

R = Alkyl, Aryl, calix[4]arene spacer R = C02Et, H... 

Thus, the characteristic potentiometric selectivity of calix[6]arene hexaester 29 for primary amine guests can be reasonably interpreted on the basis of the structural factors relevant to the nonpolar moieties of guests. Such a mode of discrimination can also be achieved by a calix[6]arene hexaester with short alkyl chains [28 (R = H)]. In addition, the selectivity was found to be essentially the same in the presence or absence of lipophilic anionic site HFPB added in the membrane (Table 3). Potentiometric discrimination of protonated amines by host 28 (R = H, Bu ) as well as of aldehydes (as the protonated hydrazones generated in situ) by hosts 26 (R = Bu ) and 28 (R = H) was also reported. Recently, quantitative estimation of optical selectivities for alkali metal ions or protonated amines were made with liquid membranes containing chromogenic derivatives of calixarenes. ... 

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