Conjugated Crown Ether

SCHEME 33 A slerically regular chiral conjugated crown ether polymer (S)-78. 

Fluorescent chemosensor 25 [53] (Fig. 28.6) with calix[4]arene in the 1,3-altemate conformation, a conjugated crown ether unit, and an anthracenyl amine showed strong fluorescence under acidic conditions but very weak emission in neutral and basic media. The strong fluorescence is due to the inhibition of PET to the anthracene moiety from the amine units. The addition of K ions leads to the formation of complexes with the crown ether moiety, which induce deprotonation of ammonium ion, causing resumption of the PET process. This metal ion-induced deprotonation is interesting for the design of improved receptors for specific cations. 

Inspired by the separation ability of cyclic selectors such as cyclodextrins and crown ethers, Malouk s group studied the synthesis of chiral cyclophanes and their intercalation by cation exchange into a lamellar solid acid, a-zirconium phosphate aiming at the preparation of separation media based on solid inorganic-organic conjugates for simple single-plate batch enantioseparations [77-80]. 

Quinolizinium and other fused pyridinium salts are formed when a-methylheterocycles react with 2,4,6-triphenylpyrylium, which thus behaves as a C3-synthon <96MC99>. Pyrylium salts also feature in a stereocontroUed route to conjugated dienynes which has led to a synthesis of Carduusyne A, a marine metabolic fatty acid <96TL1913> and in the formation of pyridinium containing crown ethers <96LA9S9>. 

A methanofullerene derivative possessing an ammonium subunit has been prepared and subsequently shown to form a supramolecular complex with a porphyrin-crown ether conjugate <06T1979>. The synthesis and study of these fullerene-containing supramolecular photoactive devices have also been reported <06CRC1022>. 

Importantly, the tethers in these bisadduds can be readily removed, making them true templating units. This point is illustrated by the cleavage of the crown ether tether in conjugate 24 to give untethered trans-1 fundionalized fullerenes that are of interest for use as further molecular scaffoldings (Scheme 5).156 60 611... 

In 1990 we reported the synthesis of new redox-responsive crown ether molecules that contain a conjugated link between the crown ether unit and a ferrocene redox-active centre (Beer et al., 1990a). Examples of some of the species synthesized are shown in Fig. 5. The electrochemical behaviour of these species was investigated and also the electrochemical behaviour of their analogues with a saturated link between the ferrocene unit and the crown ether. The changes in the CVs of [2a] upon addition of magnesium cations are shown in Fig. 6. The metal cation-induced anodic shifts of [2a], [2b] and also their saturated analogue [3] and vinyl derivatives [4a], [4b] are shown in Table 1. 

B. Iron Complex of a Crown Ether-Porphyrin Conjugate Suitable for... 

Transesterification of the fullerene crown ether conjugate (+)-89 allowed for the removal of the tether, leading to the D2fj-symmetrical bisadduct 94 (34%) [93], which is a very useful building block for further addition chemistry. 

The conjugation in the molecular wire may be disrupted or modulated to create systems with different properties. For example, a porphyrin Ceo donor-acceptor system linked with a conjugated binaphthyl unit, has a preference for the atropi-somer where the fullerene unit is closer to the porphyrin system, thus increasing the through space interactions [82]. The charge transfer process on a dyad containing a crown ether in the linker structure can be modulated by complexation/ decomplexation of sodium cations [83] but even more interesting is the construction of supramolecular systems where the donor and acceptor moieties are... 

A few other interesting molecular architectures exhibiting uncharacteristic electrochemical behavior have been constructed on the basis of the methanofullerene building block. These include two methanofullerene-substituted bipyridine ligands complexed to rhenium and ruthenium ((35) and (36) in Fig. 17), which were prepared as possible candidates for photoinduced electron transfer processes [138]. In addition, three fullerene crown ether conjugates ((38), (39), and (40) in Fig. 18) have... 

Electrochemical studies of fullerene crown ether conjugates (38), (39), and (40) yielded fascinating results. The three compounds differ only in the points of attachment of the tangential crown... 

An extensive spectroscopic and mechanistic study on the enantioselective Cu/ferrocenyl bisphosphine-catalyzed conjugate addition has been performed. Several parameters such as solvent, nature of the halide present in the Grignard reagent and Cu(I) source, and additives (i.e. dioxane and crown ethers) were identified. These factors directly affect the formation and nature of the intermediate active species, and therefore the selectivity, rate and overall outcome of the reaction. Importantly, the presence of and Br ions in the reaction are essential in order to achieve high selectivity and efficiency. 

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