Paracyclophanes 2,2 -paracyclophane

SJ paracyclophane etc.] refers to the benzene rings in the structure, i.e. [8]-paracyclophane, [2,2,2]-para-cyclophane. Systems based upon heterocyclic molecules are also known. 

Figure Bl.11.7. chemical shifts in [10]-paracyclophane. They have values on either side of the 1.38 ppm found for large polymethylene rings and, thus, map the local shielding and deshielding near the aromatic moiety, as depicted in the upper part of the figure.

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. 

The unsubstituted C-16 hydrocarbon, [2.2]paracyclophane (3), is DPXN. Both DPXC and DPXD are prepared from DPXN by aromatic chlorination and differ only in the extent of chlorination DPXC has an average of one chlorine atom per aromatic ring and DPXD has an average of two. 

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). 

Fig. 4. Isomeric dichloro[2.2]paracyclophanes produce the same xylylene.

Macrocyclic polyethers containing the 2.2-paracyclophane unit are interesting structures and several such compounds have been prepared . Despite the diverse structural possibilities, the syntheses of these molecules have generally been accomplished by straightforward Williamson ether syntheses. The only unusual aspect of the syntheses appears to be a novel approach to certain paracyclophanes developed by Helgeson (see footnote 7a in Ref. 91). The first step of Eq. (3.28) illustrates the formation of the required tetrol, which is then treated with base (KOH or KO-t-Bu) and the appropriate diol dito-sylate to afford the macrocycle. 

As an example of ring systems which are accessible through this reaction, the formation of [ ]paracyclophanes like 8 with n >9 shall be outlined ... 

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 ... 

As an illustrating example for the application of the Friedel-Crafts acylation in the synthesis of complex molecules, its use in the synthesis of [2.2.2]cyclophane 13 by Cram and Truesdale shall be outlined. The reaction of [2.2]paracyclo-phane 10 with acetyl chloride gives the acetyl-[2.2]paracyclophane 11, which is converted into the pseudo-geminal disubstituted phane 12 by a Blanc reaction, and further to the triple bridge hydrocarbon 13 ... 

A variant, the 1,6-Hofmann elimination, has become a standard method for the synthesis of [2.2]paracyclophanes 17 although it often gives low yields. 

Schneider and Busch have showed that tetraazafS 1 8 l paracyclophane catalyzes the nitration of alkyl bromides with sodiiun nitrite In dioxane-water d l at 30 C, the reaction of 2-bromomethylnaphthalene with sodiiun nitrite is accelerated by a factor of 20 in the presence of the catalyst Concomitantly, the product ratio of [R-ONO [RNO-, changes from 0 50 1 to 016 1 Thus, an acciuruiladon of nitrite ions at the positively charged cyclophanes or IRA-900-nitrite form provides a new method for selective nitration of alkyl halides... 

Thermolyses of 3-oxaquadricyclanes with different substituents at C1(C5) and C6(C7) such as carboxy and phenyl groups showed that the reaction generally gives oxepins with the carboxy functions in the 4- and 5-position.24 This is also true when the substituents in the 6- and 7-positions form a bridge of six carbon atoms, e.g. formation of 9.129131 The rearrangement of these 3-oxaquadricyclanes gives access to the [6]paracyclophane system. 

A novel example of a 3,5-disubstituted 1//-azepine is methyl 12-azabicyclo[8.3.2]penta-deca-l0,l3,14-triene- 2-carboxylate (6) obtained, albeit in low yield, by the action of basegenerated (methoxycarbonyl)nitrene on [8]paracyclophane (5).147... 

The reaction fails with the thermally generated nitrene on account of the thermal instability of the paracyclophane under the reaction conditions. 

In this case, the benzene dioxide 5 is obtained from a [2,2]paracyclophane diene by photooxygenation and rearrangement of the derived endoperoxide. 

Optical Activity Caused by Restricted Rotation of Other Types. Substituted paracyclophanes may be optically active and 25, for example, has been resolved. In this case, chirality results because the benzene ring cannot rotate in such a way that the carboxyl group goes through the alicyclic ring. Many chiral layered cyclophanes (e.g., 26) have been prepared. ... 

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