Paracyclophane, pyrolysis

DPXC ndDPXD. The economic pressure to control dimer costs has had an important effect on what is in use today (ca 1997). Attaching substituents to the ring positions of a [2.2]paracyclophane does not proceed with isomeric exclusivity. Indeed, isomeric purity in the dimer is not an essential requirement for the obtaining of isomeric purity, eg, monosubstituted monomer, in the pyrolysis. Any mixture of the four possible heteronucleady disubstituted dichloro[2.2]paracyclophanes, will, after all, if pyrolyzed produce the same monomer molecule, chloro- -xyljIene [10366-09-3] (16) (Fig. 4). 

The high strain energy of [2.2]paracyclophane (see Section 2.1.) facilitates ring-opening of the molecule via cleavage of the benzyl-benzyl bonds. Pyrolysis at 400 °C affords p,p -dimethylbibenzyl (155) and p,p -di-methylstilbene 109h At 600 °C, p-xylylene (156) is formed it polymerizes spontaneously to the linear poly-p-xylylene (10) on condensation 110>. 

An oxo atom spans two Ru atoms and bonds to the facially coordinated [2.2]paracyclophane ring. It has been proposed that the carbide and oxo atoms are both derived from the same CO ligand. This seems reasonable given that pyrolysis of 204 affords 205 and C02 in quantitative yield. In contrast to the generation of 117 from 202 the transformation here appears to involve an intramolecular process.115... 

A reversible vinylidene insertion was proposed to explain die formation of (55) on flash vacuum pyrolysis of the anthracene derivative (56) at 1100 °C.65 The expected loss of HC1 followed by 1,2-H shift and 1,5-CH insertion of the resulting vinylidene species would give rise to the strained paracyclophane (57). This is proposed to ring open to the alternative alkylidene (58) before proceeding to the observed product (55). 

Gorham 43 45 synthesized poly-p-xylylene(poly-QM) by a new technique involving vacuum pyrolysis of [2,2]-paracyclophane, cyclo-di-xylene (cyclo-di-QM) at 600 °C. When the pyrolyzed gas of cyclo-di-QM, namely QM, is condensed on glass or... 

Fig. 3. Reaction scheme of the pyrolysis of 2-acetyl-[2,2]-paracyclophane and its fractional polymerization...

Most examples of pyrolyses of one-sided non-benzylic disulfones come from the synthesis of [3.3]- and [4.4]phanes. The parent compound [3.3]paracyclophane 205 so far only was obtained by ring-enlarging methods in low yields. Haenel et al. succeeded in synthesizing this compound by sulfone pyrolysis in 75% starting from the readily available corresponding sulfide and sulfone, respectively [69]. 

Fig. 18 Reaction scheme for PPV by thermal dehalohydrogenation of (2) film. (2) is formed by the CVP of (1), the dimer (1,9-dichloro-2, 2-paracyclophane). Tp pyrolysis temperature, Tg. sublimation temperature of precursor, T substrate temperature during CVD.

Vaeth and Jensen indicated that, fragmentation of precursors during pyrolysis was the major source of aliphatic hydrocarbon incorporation. This fragmentation was also observed in parylenes, which prompted the introduction of (2,2) paracyclophanes" and a,a -... 

Paracyclophane has been isolated from the pyrolysis of p-xylene and by dimerization of / -xylylene. Paracydophanes have been synthesized by intramolecular Wurtz reactions at high dilution. ... 

Vapor phase thermolysis of [6]paracydophane (la) (Structures 1) yielded spiro triene 77 (Structures 10) as the major product via homolytic cleavage of one of the benzylic bonds [5b, 10]. FVP of [5.2.2]propelladiene (21a) (Structures 3) afforded spiro compound 78 (Structures 10) by similar homolysis of intermediate [5]paracyclophane (2a) (Structures 1) [61]. In contrast, the reverse reaction, FVP of spiro trienes, was successfully used for the preparation of [n]paracy-clophanes with n = 7 and 8 [62]. Pyrolysis of spiro tetraene 79, however, did not give [7]paracycloph-3-ene (80) (Structures 10) [63]. Evidently, [7]paracy-clophane is the borderline case with regard to the thermodynamic stabilities between the bridged aromatic compound and the spiro triene isomer. 

Organometallic derivatives of poly-p-xylylene (PPX) with Ge and Sn atoms covalently bonded to polymer chains have been synthesized by a vapor deposition technique using bridged [2,2]-paracyclophanes with corresponding organometallic groups [32, 33]. Pyrolysis of these cyclophanes, along with polymerization of the /7-xylylene monomer mixture, is shown in Scheme 2.2. 

The Gorham process uses [2.2]paracyclophanes as a precursor for deposition. The precursor is evaporated at 150°C in vacuo. In the next stage, the vapors are conducted into a pyrolysis chamber at 700°C. Here the monomeric diradical is formed. Then the reactive vapor reaches a deposition chamber at an ambient temperature. The vapor condenses and polymerizes at the cold surfaces. The technique of CVD for polymer has been reviewed by several authors. ... 

A synthesis of [6]paracyclophane has been reported which involves flash pyrolysis of the lithium salt of tosylhydrazone (119). Authentic samples of diketones (120) have been prepared by lead tetra-acetate oxidation of the corresponding tricyclic diols. ... 

The second evidence confirming the structure of the pyrolysis product was furnished shortly after publication of the above results. The pyrolysis of p -xylene yields, in addition to the poly-p -xylelene, some minor products. From their mixture Brown and Farthing isolated an interesting hydrocarbon and identified this product by x-ray analysis as paracyclophane,... 

Tetrafluoro-p-xylylene 23 has been matrix isolated by flash vacuum pyrolysis of the corresponding octafluoro[2.2]paracyclophane followed by condensation at 30 K with a large excess of argon. This is a further example of the use of matrix isolation to stabilize a highly reactive species produced in a thermal reaction, which then becomes the starting material for photochemical studies. Irradiation of 23 at 248 nm resulted in partial conversion to a new species, which was identified as the tetrafluoroheptafulvene 24 by comparison of the experimental and computed IR transitions. Several possible reaction mechanisms for this rearrangement were proposed, but no intermediate species was detected in the matrix photolysis experiments, so a final conclusion as to the mechanism could not be drawn. 

The general synthetic methods for [2.2] CPs have been already estabhshed. Most of them employ the ring contraction reactions of the 2,11-diheteroatom-substituted [3.3]CPs. Photochemical ehmination of sulfur or selenium atoms from [3.3] CP-2,11-disulfide or diselenide and flash vacuum pyrolysis of sulfur dioxide obtained by the oxidation of the corresponding sulfides are the most general methods. 2,11-Diaza[3.3]CPs are converted to the corresponding [2.2]CPs via their nitroso derivatives by reductive elimination of nitrogen. " Photoextrusion reactions of COj from cychc diesters can also be appHed to the synthesis of [2.2]paracyclophanes (PCPs)"- and [2.2]heterophanes, as described below. "... 

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