Arenes systems

Possible conformations of the calix[4]arene systems. (Figure adapted from Fischer S, P D J Groothenuis, roenen, PV P van Hoorn, F C j M van Geggel, D N Reinhoudt and M Karplus 1995. Pathways for mational Interconversion of Calix[4]arenes. Journal of the American Chemical Society 117 1611-1620.)... 

The best known bis-arene system, bis-benzenechromium, Cr(Bz)2, has been studied by a number of authors (94, 95, 96), but the results of Feltham, in cyclohexane solution, are typical a broad band with a maximum (e = 25) at 15.6 kK. is the only feature at lower energies, a stronger peak (e = 8000) being found at 31.25 kK. with a rising absorption... 

The overall picture for the bis-arene systems is therefore not at all satisfactory, and much more experimental work is required on these compounds, especially for complexes representing d 6 and <2 8 configurations. 

Janetka and Rich (78) have utilized the considerable stability of ruthenium-77-arene systems in the synthesis of cyclic tripeptides as analogs of the protease inhibitor K-13 (cf. 34, Scheme 28). Their approach involves the construction of linear tripeptide complexes (35) using diimide/HOBt coupling of [Ru(Cp)(Boc-p-Cl-PheOH)]PF6 (Cp = 775-C5H5) with the appropriate dipeptide ester. Cyclization of 35 affords the biphenyl ether 36, which on photolysis (350 nm) gives 34. 

Wahrend fiber weitere Reduktionen von Nitro-arenen unter Erhalt der Nitro-Funktion wenig bekannt ist, liefert die Behandlung von 10-Hydroxy-9-nitro-anthracen mit konz. Schwefelsaure unter formaler Reduktion des Aren-Systems 9,10-Anthrachinon in 95%iger Ausbeute6. 

A ditopic receptor 71, comprising a semitubular calix[4]arene system [91] for cation recognition and urea moieties for anion recognition, was reported very recently. The complexation of sodium or potassium cations into the central ethylene glycol cavity invokes substantial enhancement of the binding strength (more than one order of magnitude) towards selected anions (halides, acetate). 

Apart from the construction of phenanthrenes, carbene complexes have also been used for the synthesis of more extended polycyclic arenes. An unusual dimerization of chromium coordinated ortbo-ethynyl aryl carbenes results in the formation of chrysenes (Scheme 37) [81]. This unusual reaction course is presumably due to the rigid C2 bridge that links the carbene and alkyne moieties, and thus prevents a subsequent intramolecular alkyne insertion into the metal-carbene bond. Instead, a double intermolecular alkyne insertion favored by the weak chromium-alkyne bond is believed to occur forming a central ten-membered ring that may then rearrange to the fused arene system. For example, under typical benzannulation conditions, carbene complex 97 affords an equimolar mixture of chrysene 98a and its monochromium complex 98b. The peri-interactions between the former alkyne substituent (in the 5- and 11-positions) and the aryl hydrogen induce helicity in the chrysene skeleton. 

Bimolecular reactions of the ion-radical pair can also effectively compete with the back electron transfer if either component undergoes a rapid reaction with an additive that is present during the ET activation. In NO+/arene systems, the introduction of oxygen rapidly oxidizes even small amounts of nitric oxide to compete with back ET and thus successfully effect aromatic nitration [60]. In a related example, the CT complex of hexamethylbenzene and maleic anhydride reaches a photostationary state with no productive reaction. However, if irradiation is carried out in the presence of an acid, the anion radical in the resulting contact ion-radical pair is readily protonated, and the redox equilibrium is driven toward coupling (in competition with the back ET) to yield the photoadduct [59], i.e. ... 

Similar reaction conditions for manganese, iron and cobalt complexes lead to more stable arene systems, presumably due in part to a higher valence electron count for these metal centres. Reduction of [3,5- Pr2-2,6-Trip2C6HMX]ra (M = Mn, X = I, n= 1 M = Fe, X = C1, n= 1 M = Co, X = C1, n — 2) gives rise to the Mn(I) inverted sandwich complex [3,5- Pr2-... 

In the alkylation of arene systems with olefins, the above chemistry proceeds via a carbenium ion mechanism. Alkylation of an aromatic compound with an olefin occurs by the interaction of a Bronsted acid site of any of the catalysts with a participating olefin, creating a carbenium ion, via protonation of the double bond, and thus a polarized complex is formed, as shown below ... 

The annelated calixarene 57 may be regarded as an example for an AABC-type calix[4]arene, where the ring inversion is impossible due to the connection of the two A units by a bridge in p-position by which a second calix[4]arene system is created.117... 

Around the same time as this, Reinhoudt developed a calix[4]arene system with only two urea or thiourea functionalities attached on opposite faces 70-72 [167]. These less substituted systems display both inter- and intramolecular hydrogen bonding as a result of the calixarene adopting a pinched cone conformation (demonstrated by the use of NOESY NMR). In some spectra it is impossible for the connectivities to be made within a single molecule, so the only possibility left is that dimerisation occurs. As with the initial experiments of Rebek and Bohmer, the extent of hydrogen bonding was observed to be solvent dependent. Concentration dependant FTIR was also used, to observe the effects on the NH stretching vibrations, but no concentration dependence was observed. Of the three urea derivatives used, only 72 showed no evidence of dimerisation... 

The macroscale codeposition of PF3 has yielded a series of M-PF3 complexes. Some of these M-PF3 complexes can only be prepared by the metal vapor-ligand cocondensation technique. Very electron-rich M-phosphine and M-phosphite derivatives have been prepared by M-PMe3 and M-P(OMe)3 depositions, as shown in Table 8. A series of new homoleptic compounds have been prepared. Cocondensation of Fe and Ni with some arene systems gives stable (Ar)2Fe/Ni species, if maintained at low temperatures. 

With the corresponding dimethoxybibenzyl derivative, ring closure takes place without rearrangement to form a structure analogous to the A, B, and C ring of steroids [144], but the yield is only moderate (22%) because of the lower activation of the arene system. A remote substituent can strongly influence the occurrence of the subsequent rearrangement (Table 7, numbers 9a and b). 

However, highly oxidizable substrates such as p-methoxytoluene will consume the dissolved oxygen very quickly, which may result in total depletion of the reaction mixture from the oxidant, especially at reaction temperatures that are too high. This causes subsequent dimerization or electrophilic substitutions in the arene system. Low yields of yellow to brownish colored products result. 

McMills, M. C., Wright, D. L., Weekly, R. M. Synthesis of highly functionalized arene systems. Novel selectivities of intra- and intermolecular Friedel-Crafts reactions. Synth. Commun. 2002, 32, 2417-2425. 

The situation differs markedly however from that of the d configurations of metallocenes and bis-arene systems for which pseudo-axial symmetry also obtains. 

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