Paracyclophane anions

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. 

It seems reasonable to assume that the dimeric benzene ion has a sandwich conformation. A similar structure has been invoked to explain excimer spectra from neutral benzene molecules in solution where stable dimers are observed (27). Also Howarth and Fraenkel (II) have suggested that the two hydrocarbon moieties of the dimeric ion of naphthalene lie in parallel planes. Ishitani and Nagakura (12) have suggested a sandwich structure for paracyclophane anions but they also consider the possibility of a diphenyl type of structure. 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

In 2004, Bolm et al. reported the use of chiral iridium complexes with chelating phosphinyl-imidazolylidene ligands in asymmetric hydrogenation of functionalized and simple alkenes with up to 89% ee [17]. These complexes were synthesized from the planar chiral [2.2]paracyclophane-based imida-zolium salts 74a-c with an imidazolylidenyl and a diphenylphosphino substituent in pseudo ortho positions of the [2.2]paracyclophane (Scheme 48). Treatment of 74a-c with t-BuOLi or t-BuOK in THF and subsequent reaction of the in situ formed carbenes with [Ir(cod)Cl]2 followed by anion exchange with NaBARF afforded complexes (Rp)-75a-c in 54-91% yield. The chela-... 

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-... 

Anion radicals of a series of [2.2]paracyclophanes u . Chemiluminescent paracyclophanes n >. 

The first is a cyclophane-based system reported by Murakami et al. (Fig. 9.12a). [31] The sides of the host consist of tetraaza-[3.3.3.3]paracyclophane units, and its octaprotonated cation has been shown to bind anionic guests. The molecule possesses 48 asymmetric units of [(N/3)-(CH2)- (C6H4)/2 ]/2. 

Scheme 1.5 represents case f, that is, an anion-radical belonging to the borderline between moderately and completely delocalized species. Its optical spectra, along with frontier orbital analysis, testifies that in this anion-radical there is a positive overlap between C-N bonds and the psendo-geminal carbons of the opposite rings, as shown by the dashed lines in the strnctnre of Scheme 1.5 (Nelsen et al. 2005). Taken together, the experimental results considered provide direct evidence for the throngh-bond mechanism of electron transfer in these paracyclophane systems.

Twofold inter- and intramolecular PKRs have found interesting applications in synthesis of ansa-zirconocenes. An early example illustrated the use of this approach for the synthesis of cyclopentadiene anellated[2,2]para-cyclophanes 157-158. The reaction of several paracyclophanedienes (155) with alkynes gave the corresponding twofold cycloaddition products 156, which were transformed into cyclopentadienyl anions orthogonally attached to the bridges of the paracyclophane (Scheme 46) [158]. 

The aminium 24 — the synthesis is under preparation50) — interests from a similar point of view. The odd electron might skip over to the opposite arene as was demonstrated for [n.n]paracyclophane radical anions (n < 4)54). 

Paracyclophane-l,9,17,25-tetraene (42)62 can be reduced using lithium to yield a stable dianion (422 ) followed by a tetraanion (424-)63. Both anions show pronounced anisotropy effects that are manifested by their 111 NMR signals. The signals of the inner protons of the diatropic species, 422-, absorb at high field, whereas the peripheral ones are shifted to low field. 424, which is paratropic, shows an opposite effect. In contrast to 412, the lithium and potassium salts of 422 exhibit very similar ll and 13C NMR spectra. However, the potassium salt of 424 is afforded only after a long contact with the metal62. The opposite behavior is found in compound 41. While the reduction with... 

The molecular and electronic structures are discussed on the basis of B3LYP and CASPT2 quantum chemical calculations. Paracyclophanes are attractive model compounds for studying specific intramolecular interactions [146, 147]. Gerson et al. [148] showed the radical anion of [2,2] paracyclophane 71 is an unassociated specie, with the unpaired electron being equally distributed over both 7r-systems. 

Anions derived from cyclophane represent a new class of carbanions in which new modes of electron delocalization were encountered. Two main groups of cyclophane anions were studied. In the [2J (n = 2) paracyclophane 190,191 several modes of electron delocalization may occur as they have an inner and outer periphery. Another group of interesting cyclophanes is the [2.2]paracyclphanes in which through space interactions may prevail. 

The dicarbamates 254 were smoothly lithiated by using f-BuLi—TMEDA at —80°C and then allowed to warm to room temperature over 16 hours. Under these conditions a smooth anionic ortfio-Fries rearrangement gave the diamido derivatives 255 in fair yields (25-80%) (equation 117) (see also Reference 220). A similar rearrangement was also described for [2,2]-paracyclophanes. ... 

We found that radical, anionic, and coordination polymerization gave benzene-insoluble polymer or soluble polymer as shown in Table V (26). These polymers, however, did not exhibit the characteristic spectra for [3.3]paracyclophane units above mentioned. Therefore they were concluded not to be cyclopolymer. Only cationic initiators could induce cyclopolymerization of St-Cj-St (27, 28). 

Recently Yoshino and his associates (29) reported the anionic method to make [3.3]paracyclophane skeleton with malonate synthesis. Using the high dilution technique, the yield was 0.05%. 

Having established that linear networks of HCTMP" anions form as a result of templating by [Cp Ru]2(paracyclophane)]2+, we sought to examine the effect of the higher dimensionality present in [ Cp Ru( / —C6H5) 4Ep + (E = C, Si, Ge, Sn, Pb). These tetracations had several interesting structural features... 

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