P-Cyclophanes

In the synthesis of l,3-dithiolan-2-ones from spirocyclic intermediates, via episulfides, substituted tetrathiacyclododecane and the related pentathiacyclopentadecane were isolated in good yields <96JCS(P1)289>. Preparation and molecular dynamics studies of 2,5,8,17,20,23-hexathia[9.9]-p-cyclophane have been reported <96P4203>. The syntheses and properties of thiocrowned l,3-dithiole-2-thiones and their conversion to tetrathiafiilvenes via treatment with triethylphosphine have been described <96LA551>. 

Polycarbogermane and polycarbostannane block copolymers 56134 were prepared by pyrolysis of the organogermanium and organotin bridged p-cyclophanes 55 followed by deposition polymerization of the p-xylene monomers thus produced (Scheme 29). 

Another possibility that may lead us to chiral dendrimers lies in the axial chirality of substituted [2.2]p-cyclophanes. The salicylaldehyde-containing cyclophane (80) of this type was presented by Belokon et al.28 In contrast to the experiments mentioned above, we tried to bring elements of chirality into the periphery of the molecule by connecting it through an imine bridge (Section IV.A.). The synthesis of 4-hydroxy [2.2]p-cyclophane (79), via the bromide29 and the final formyla-... 

The solid [2.2]-p-cyclophane was sublimated at 383 K, and the vapors were passed through a pyrolitic quartz tube at 873 K. This compound converts into PX without formation of detectable side products under these conditions [29, 30]. As stated in reference 22, solid PX most probably contains polymer chain precursors in the form of monomer stacks transforming into polymer chains as a result of the rather small rotational displacement of molecules without their translation movement. The assumed pattern of the solid-state PX polymerization is presented in Figure 2.1. 

It was suggested that the 1,4-isomer ring opens to [5]-/7-cyclophane which then undergoes benzylic bond fission and recombination to the spiro compound. However, formation of the cyclophane by photolysis of the 1,4-Dewar benzene isomer at — 20°C led to polymer. A rationalization for the disparate behavior of the system is the reversible formation of the [5]-p-cyclophane at all temperatures with... 

A solution-based two-step process for the fabrication of titania-parylene composite films for materials for bioimplant applications has been described [101]. In the first step, a ligand that can bind titania, such as phenylphosphonic acid, is adsorbed onto a nanos-tructured poly(p-xylylene) thin film on a surface. This film is prepared by a vapor phase pyrolysis of [2.2]-p-cyclophanes. 

It should be borne in mind that each activated monomer and polymer should only react with nonactivated monomer (addition polymerization) in both of these chemical examples. Reaction between an activated monomer and another activated monomer or a polymer (polycondensation) should not occur. With cyclophane, the mathematical treatment of the consecutive equilibria yields different expressions to those given in Table 16-1, since the /7-cyclophane, as initial monomer unit, yields two activated monomer molecules, and each reaction with activated monomer and its successive products yields only species with uneven numbers of structural elements. In addition, the polymerization of p-cyclophane is no longer a living polymerization when the degrees of polymerization are low, since, in this case, monomolecular (that is, intramolecular) termination reactions leading to the formation of inactive rings can occur. 

Poly(p-xylylenes) are obtained from (2,2)-p-cyclophane, also known as di-p-xylylene ... 

Typical examples for strong coupling are the dimeric aggregates which exist in more concentrated aqueous solutions of many dyes such as thionine or the rhodamines " . Their absorption spectra are much broader than those of the monomers and, in several cases, consist of two distinct components with peak-to-peak separations of 1000 cm" or more, and comparable to the monomer band width. Similar but somewhat smaller splittings occur in the spectra of some double molecules, such as the p-cyclophanes and hydrogen-bonded dimers . 

Chiral Homo-Quadridentate Ligands. Chiral porphirines (PORPH) com-plexed to manganese(III) have been published recently as efficient epoxidation catalysts (97). The synthesis of the ligand is based on the condensation of enan-tiomerically pure [2.2]-p-cyclophane-4-carbaldehyde and pyrrol (Fig. 11). 

Much related work has occxured based on organized catalysis in natural and synthetic macrocycles, polymers, and modified surfaces, which can only be hinted at briefly in such a review. We highlight Lehn s application of crown-ethers (83) and (84) in the catalysis of hydrolysis of chiral dipeptide esters (S R up to 50 1) and hydride transfer to ketones, respectively, Knowles s successful preparation of specifically substituted a-cyclodextrins (85) and their involvement in phosphate ester hydrolysis, and Murakami s syntheses of catalytically active p-cyclophanes. " The long-standing contribution of... 

A much better method for preparing poly-jpj-quinodimethane was subsequently developed by Gorham who found that low-pressure ( 0.1 torr) pyrolysis of p-cyclophane at 600 C produces linear poly-p-quinodimethane free of low molecular weight by-products. 

A question arises concerning the pj-cyclophane pyrolysis as to whether p quinodimethane is actually formed. For example only one bond in the p-cyclophane may break to produce... 

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