Paracyclophane dimer

Poly-paraxylylenes are deposited by a thermal CVD process popular in the microelectronics industry as the "Gorham process"." The starting material for this deposition process (precursor) is usually a paracyclophane dimer (DPX). The dimer dissociates to form reactive monomer species, which then undergoes nucleation and growth leading to polymer thin film formation. The following reaction ensues ... 

Most PPX polymers are prepared by a unique chemical vapor deposition (CVD) polymerization method that was developed for coating components. In the CVD process the paracyclophane dimer is vaporized and the coating forms from a gaseous monomer without an intermediate liquid stage. As a result. 

In the commercial Gorham process (2), the extremely reactive PX is conveniendy generated by the thermal cleavage of its stable dimer, Vo-di- -xyljIene (DPX), a [2.2]paracyclophane [1633-22-3] (3). In many instances, substituents attached to the paracyclophane framework are carried through the process unchanged, ultimately becorning substituents of the polymer in the coating. 

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). 

An example is the synthesis of substituted [2.2]paracyclophanes as reported by Hopfet al When hexa-l,2,4,5-tetraene 23 is treated with dimethyl acetylenedi-carboxylate 24 (an electron-poor acetylenic dienophile), the initially formed reactive intermediate 25 dimerizes to yield the [2.2]paracyclophane 26 ... 

Diels-Alder additions with numerous dienophiles 283 lead to the conjugated tri-enes 320 [133], whereas the use of activated triple bonded dienophiles 321 (R1 = R2 = C02CH3i CN, CF3i etc. R1 = CHO, CN, COOCH3, etc., R2= H) constitutes the simplest route to functionalized [2.2]paracyclophanes, 322, the primary p-xyly-lene adducts dimerizing under the reaction conditions [134,135]. 

Tetramethyl[2.2]paracyclophane (20a), in which the methyl groups are not in an "eclipsed position, occurs as main product in the dimerization of 2,5-dimethyl-p-xylylene (2/)48> the isomer (20b) was discovered much later 49>. 

A similar 1,6-elimination of 3, prepared as shown, induced by fluoride ion results inp-quinodimelhane (a), which dimerizes to [2.2] paracyclophane (4).2... 

Covitz in the same company followed the study and clearly established the presence of -xylylene as an intermediate by means of the electrochemical reduction of a,a -dibromo-/>-xylene (29). Half-wave potentials were observed at — 0.80 and —1.72 V vs. SCE. in polarography. A dimer, [2,2]-paracyclophane, was also obtained as a minor product of electrolysis as well as polymer. 

Finely powdered monomer crystals were dispersed in distilled water containing a few drops of surfactant and irradiated, with vigorous stirring, by a 500-W super-high-pressure mercury lamp set outside of the flask. Dimers 2 were prepared by the irradiation of crystals 1 with /.>410 nrn, and by successive purification by preparative TLC. [2.2]Paracyclophane 3a was obtained by the photoirradiation of la crystals with X >300 nm at -40 °C, followed by successive washing with ethyl acetate. 

Treating 113 (easily available from 45 by reaction of its dilithio derivative with formaldehyde and treatment of the resulting diol with phosphorus dibromide) with the so-called Mori reagent [72], a trimethylsilyl(tri-n-butyl)stannane/cesium fluoride combination that has been used very successfully in many debromination reactions [73], results in the production of the [6.6]paracyclophane 115 in acceptable yield. Since p-xylylene (p-quinodimethane) is known to dimerize to [2.2]paracyclophane, it may well be that the bis(cumulene) 114 is formed as an intermediate in this reaction. Since a stepwise process is also conceivable, further clarification of the mechanism is obviously desirable (Scheme 25). 

The first approach has been pioneered by Hopf and coworkers by covalently attaching polyene chains onto a rigid molecular fragment, a [2.2]-paracyclophane. " The paracyclophane scaffold serves as a template to hold the two polyenes in the vicinity for a sequential [2 + 2] photodimerization. The covalent attachment of the polyene chains onto the cyclophane effectively makes the dimerization an intramolecular process (Figure 15). Upon UV-irradiation... 

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.

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. ... 

A.B. Smith and co-workers have devised an efficient strategy for the synthesis of the cylindrocyclophane family of natural products. Olefin ring-closing metathesis was used for the assembly of the [7,7]-paracyclophane skeleton. During their investigations they discovered a remarkably efficient CM dimerization process, that culminated in the total synthesis of both (-)-cylindrocyclophane A and (-)-cylindrocyclophane F. They established that the cross metathesis dimerization process selectively led to the thermodynamically most stable member of a set of structurally related isomers. Out of three commonly used ROM catalysts, Schrock s catalyst proved to be the most efficient for this transformation. 

Matrix-isolated diene (96) undergoes photochemical transformation into the isomeric compounds (97) and (98) as the primary photoproducts. Maier and his coworkers have observed the photoisomerization of the cyclobutadiene dimer (99) into the pentacyclic compound (100). The u.v. irradiation of the Dewar benzene (101) a.R ords the paracyclophane (102, 13%). The prismane (103) can be produced by the irradiation of the Dewar benzene (104). The intermediacy of the benzene (105) in the transformation is likely. ... 

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