Paracyclophanes formation

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 ... 

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. 

We used all of the Cis hydrocarbons in Pedley and derived their enthalpies of sublimation by subtracting the recommended enthalpies of formation of the solid and the corresponding gaseous species. There was considerable variation in the sublimation enthalpies, as seemingly befits the diverse choice of compounds (and associated crystal packing) including such species as naphthacene, 6,6-diphenylfulvene, 3,4,5,6-tetramethylphenanthrene, [3.3]paracyclophane and n-octadecane. 

They have also examined the formation of the ir-complex between the silacalixarenes and silver cation by FAB mass spectrometry. Similar 77-complex formation with silver cation was observed for hexasila [2,2,2]paracyclophanes 37 (41). 

The resonance structures 65 suggest that the pseudo-meta and pseudo-geminal positions are the preferred sites for substitution however, it has been observed (e.g. in the bromination and acetylation of 4-bromo[2.2]paracyclophane (67)) that there is predominant formation of pseudo-ortho and pseudo-para products 8 3b>8 5> (see Table 3) ... 

The ion 96 carries a positive charge which can be distributed over both rings formation of 96 involves compensation of bond angle strain, leading to facilitation of n-n repulsion strain between the neutral benzene nuclei in the starting compound. The twisting of the system in 96 corresponds to the somewhat twisted crystal structure of the [2.2]-paracyclophane molecule. 

Intramolecular Diels—Alder reactions without prior 1,4-addition of oxygen (cf. previous section) have similarly been postulated for a number of [2.2]paracyclophane analogs. When [2](2,5)furano[2](l,4)naphthalen-ophane (42) is heated in excess dimethyl acetylenedicarboxylate at 100 °C, a polycyclic compound of structure 134 is formed. The mechanism of formation of 134 is most probably as follows 101> the furan moiety reacts as active diene component in an intermolecular Diels—Alder reaction to give 135. This is followed by further intramolecular 1,4-addition with the unsubstituted naphthalene ring as diene component to give the product 133, which has been isolated. 

Both of the tetraaza[3.3.3.3]paracyclophane (1) and tetraaza[n.l.n.l]paracyclo-phane (n = 6, 7, 8 cf. 2) rings have frequently been used as fundamental molecular skeletons for preparation of functionalized macrocyclic hosts [24-36]. Formation of three-dimensionally extended hydrophobic cavities was approached by introducing multiple hydrocarbon branches into the macrocyclic skeletons. Multiple hydrophobic chains thus placed in a macrocycle must be extended in the same direction and undergo mutual association to attain their optimal hydrophobic interactions in aqueous media due to thermodynamic reasons, while in nonaqueous media they presumably assume a free and separated configuration to minimize their mutual steric interactions. Consequently, such hydrophobic branches may provide a large hydrophobic cavity in aqueous media. 

The inter-ring separation in [4.4] paracyclophane has been calculated to be 3.73 A, assuming normal bond angles and planar benzene rings. At this distance, there is no ground-state overlap, and the UV absorbance does not extend past 280 nm. Nevertheless, the peak of the excimer fluorescence intensity of [4.4] paracyclophane is red-shifted 1900 cm"1 relative to the peak of the solution excimer of toluene at 31,300 cm-1. Neither the excimer lifetime nor the excimer fluorescence response function have been reported for any of the exrimer-forming paracyclophanes, so little is known about the kinetics of excimer formation in these compounds. 

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). 

The first observation of the thermal transformation of a strained paracyclophane into its Dewar isomer has been reported.56 Hexahalobispropellane (41), on treatment with potassium t-butoxide, has been shown to afford the phenol (43) along with (44). The formation of both these compounds has been rationalized57 by invoking the intermediacy of (42) (see Scheme 10). 

IV. RING FORMATION BY WURTZ AND DIYNE COUPLING REACTIONS A. Heptasila[7]paracyclophane... 

The ROMP of [2.2]paracyclophane-l,9-diene (128) yields poly(p-phenylenevinylene) (129) as an insoluble yellow fluorescent powder. Soluble copolymers can be made by the ROMP of 128 in the presence of an excess of cyclopentene387, cycloocta-1,5-diene388 or cyclooctene389. The UV/vis absorption spectra of the copolymers with cyclooctene show separate peaks for sequences of one, two and three p-phenylene-vinylene units at 290, 345 and about 390 nm respectively, with a Bernoullian distribution. The formation of the odd members of this series must involve dissection of the two halves of the original monomer units by secondary metathesis reactions. 

The use of cesium compounds in C—C bond formation reactions has so far been limited to a few cases. Vogtle and KiUener [21] observed that the use of cesium metal in the Mtiller-Rdscheisen procedure [22] of the Wurtz coupling reaction of p-xylylene dibromide, intended to yield [2 ]paracyclophanes 1, leads to the [23]paracyclophane lb with the highest yield compared to other alkali metals. The yields of the other cyclic compounds Ic-e thereby are much lower compared to the application of sodium metal (Fig. 1) [23]. 

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 ... 

---