Aromatic compounds paracyclophanes

To date the structure and reactivity of numerous complexes derived from aromatic compounds and nitrosonium cation have been studied (5, 56-63). However, relatively few studies are available on the nitrosonium complexes of cyclophanes (5, 57, 59, 61, 62), cf ref. (63). The interaction of [2.2]paracyclophane with nitrosonium tetrachloroaluminate was studied by H and 13C NMR spectroscopy using deuterium isotope perturbation technique (64). It was found that the resulting nitrosonium complexes containing one (25) or two NO groups (26) are involved in fast interconversion (on the NMR time scale) (Scheme 17). 

Nomenclature of [m][n]paracyclophane Smith, B. H. Bridged Aromatic Compounds, Academic Press, New York 1964 p. 13... 

Vogtle, F., and Hohner, G. Stereochemistry of Multibridged, Multilayered, and Multistepped Aromatic Compounds. Transanular Steric and Electronic Effects. 74, 1-29 (1978). Vogtle, F., and Neumann, P. 12.2) Paracyclophanes, Structure and Dynamics. 48, 67-129 (1974). 

Paracyclophane is an unusual aromatic compound with a bridge connecting two para carbons. Explain why the labeled protons absorb in different regions of the H NMR spectrum, even though both are bonded to sp hybridized C atoms. 

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. 

The first complex where the central atom is bound exclusively by 7t-orbital interactions was produced by Pierre et al. [19]. They treated [2.2.2]paracyclophane (18) with silver triflate and observed an interaction of the silver cation with the Tt-prismand which was one hundred times stronger than with open-chained aromatic compounds. 

Figure 1.7. Axially chiral molecules (a)-(d) and a molecule with a chiral plane (e). (a) Dichlorallen, (b) twisted biphenyl, (c) helicene, (d) highly twisted aromatic compound [25], and (e) paracyclophane.

Investigations on the electron spin resonance of the radical anions 29 31> of [2.2]-and higher [n.n]paracyclophanes have shown that delocalization of the unpaired electrons over both aromatic nuclei is scarcely possible until the number of bridge members n >3 31>. In open-chain compounds of the type Ar-[CH2] -Ar the corresponding condition is n> 1. This would suggest that the mechanism responsible for the transfer of elec-... 

The UV spectra of 4,5,7,8-tetrafluoro[2.2]paracyclophane (26) 18> and of the octafluoro compound 27 54> reveal the close relationship of these compounds to unsubstituted [2.2]paracyclophane (2). The absorption bands occurring between 286 and 291 nm, like those in the spectra of 2 can be attributed to deformation away from planarity of the aromatic rings. Compared with the fluorine-substituted open-chain analogs, these absorption bands are likewise bathochromically shifted by about 25 nm. 

Multilayered cyclophanes having three aromatic rings fixed in parallel planes above one another exhibit properties intermediate between those of the [2.2]paracyclophanes and the above-mentioned compounds 51 and 52. A cyclic compound of this type, (53), has apparently been isolated by Hubert 77>. The tetracyanoethylene complex of... 

A similar mixed ox a- thia macrocycle incorporating a rigid horseshoeshaped aromatic moiety, l,ll,21-trioxa-8,14-dithia[2,9,2]paracyclophane, L40, has been synthesized and reacted with copper iodide in MeCN solution (Scheme 19).160 Its polymeric copper iodide complex [Cu4I4(L40)2] 40 crystallizes, in a 2 1 metal to ligand ratio, as an ID infinite array of cubane-like units consisting of four copper atoms, four i3-iodine atoms, and four sulfur atoms, stemming from four different macrocycles. The Cu—Cu distances are about 2.731 A. Unfortunately, the photophysics of this compound have not been studied. 

Apart from the role of cyclophanes as a model system for studying the electronic interaction between the aromatic moieties, chiral [2.2]paracyclophanes have also been utilized as planar chiral ligands in asymmetric catalysis. Recent advances and applications in this area have been reviewed [5, 6]. The synthesis of heterocyclic compounds based on [2.2]paracyclophane architecture, where the long-distance electronic communication and the planar chirality play significant roles in their application, has also been reported recently [7]. Although the preparation and application of chiral cyclophanes in asymmetric synthesis has attracted much attention for a long time, their chiroptical properties, especially the CD spectra, have rarely been paid attention or even completely ignored. 

Paracyclophanes are aromatic hydrocarbons with unique steric and electronic properties. The two 7T-systems in such compounds interpenetrate, so that they behave as one 7r-systcrn50. Paracyclophane anions are of interest in view of the proximity of the two anionic moieties and their mutual interaction. 

Dibenzo[2.2]paracyclophane-1,9-diene (2), a unique molecule with two parallel sets of mutually orthogonal aromatic rings, was first synthesized in 1985 by Wong et al. [18]. The reported five-step synthesis gave the strained hydrocarbon in only 0.5% yield. Later a more versatile and economic approach to dibenzoannelated [2.2]paracyclophanedienes 11 including the parent compound 2 was developed by de Meijere et al. [19]. The dibromo[2.2]paracyclo-phane-l-ene (6) and the tetrabromo[2.2]paracyclophane-1,9-diene (7) were obtained from commercially available [2.2]paracyclophane (1) in a bromination-dehydrobromination reaction sequence. With compounds 6 and 7 available in large quantities, the palladium catalyzed coupling with alkenes (Heck reaction)... 

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