Aromatic bridges

Annulene and dehydro[22]annulene are also diatropic. A dehydroben-zo[22]annulene has been prepared that has eight C=C units, is planar and possesses a weak induced ring current. In the latter compound there are 13 outer protons at 6.25-8.45 8 and 7 inner protons at 0.70-3.45 5. Some aromatic bridged [22]annu-lenes are also known. The [26]annulene has not yet been prepared, but several dehydro[26]annulenes are aromatic.Furthermore, the dianion of 1,3,7,9,13,15, 19,21-octadehydro[24]annulene is another 26-electron system that is aromatic. Ojima and co-workers prepared bridged dehydro derivatives of [26], [30], and [34]annulenes. All of these are diatropic. The same workers prepared a bridged tetradehydro[38]annulene, which showed no ring current. On the other hand, the dianion of the cyclophane 89 also has 38 perimeter electrons, and this species is diatropic. ... 

Bicyclams (Fig. 4) consist of two cyclam (1,4,8,11-tetraazacyclotetrade-cane) units tethered via an aliphatic (i.e., propylene, as in JM2763) or aromatic bridge [i.e., phenylenebis(methylene), as in JM3100]. While the bicyclam JM2763 inhibits HIV-1 and HIV-2 replication at a concentration of 0.1 to 1 pg/mL [24], the bicyclam JM3100 does so at a hundredfold lower concentration, that is, at a concentration that is more than... 

The composition of the complexes above depends on the metal nature and its oxidation state. Thus, divalent copper, zinc, and cadmium form complexes of the type 811a (L = py, bipy, 1,10-phen m= 1, 2) [605], tetravalent tin forms chelates 811b (X — S [606], Se [607]). The electrochemical cleavage of the S — S bond is observed not only in ligands with aromatic bridge 810, but also in those with aliphatic one, for example (CH2) in 812 [608] ... 

PMO precursors can be divided roughly into three classes bis-silylated compounds, which contain (1) short-chained alkylene or alkenylene groups as the organic bridge, (2) those with aromatic bridges, and (3) branched or cyclic multi-silylated compounds. An overview of the constitution formulae of PMO precursors is provided in Figure 3.11. 

It is still not clear how these relationships can be rationalized. However, as (hetero-)aromatic compounds are rigid and are able to form or-stacks, numerous syntheses of PMOs with aromatic bridges have been reported, thereby introducing another sort of functionality into PMOs. 

Figure 2.5 Molecular structures of zinc-iron porphyrin complexes across (a) hydrogen, (b) aliphatic and (c) aromatic bridges, and the corresponding rates of photoinduced electron transfer for each species, as reported by Rege et al. [12]...

This value is significantly smaller than the y value of 0.95 bond" for saturated hydrocarbon bridges (Eq. 23), and it reflects the lower energies of the virtual ionic states of aromatic molecules compared to saturated hydrocarbon molecules (vide supra and Eq. 6). The value of y for these systems is, however, noticeably larger than that measured for photoinduced ET in 48(1) and 48(2). These elegant studies have been extended to cover other aromatic-bridge systems [130]. 

Yet monosubstitution at C (11) does not suffice and the product is the open bridged annulene, e.g. 27. Its preparation illustrates many other cases in which [4.4.1]propellane intermediates are formed and exist as propellanes at lower oxidation states (cf. 26). But once the tetraenic one is arrived at, the product more often than not prefers to exist as the more stable aromatic bridged annulene. 

Symmetrical chromophores with the structure 94 bear as electron-rich aromatic bridge a thiophene moiety substituted at the end with electron-withdrawing benzothiazole groups. In this A-n-A chromophore, TPA significantly... 

Larsson compares t- and o-contributions of aromatic bridges and finds that both can be of the same magnitude, e.g. in pyrazin bridges between metal centers (17). 

Fig. 5 Plot of optically derived Xs in acetonitrile versus number of bonds between the hydrazines for phenyl, ter -butyl-substituted IV radical cations. The circles show saturated-bridged compounds, and the squares aromatic-bridged ones.

In recent years, a number of complexation agents have been developed that incorporate the bicyclo[3.3.1]nonane skeleton. In a series of publications, selective model receptors consisting of two substituted 3-azabicyclo [3.3.1]-nonane fragments interconnected with various aromatic bridges were described (231 -242 and references therein). The X-ray study of one of them (compound 277) revealed a chair-envelope conformation for the bicyclo[3.3.1]nonane fragments (231) a similar conformation was found in the monomeric compound 278 (231) and in the salts of compounds analogous to 277 (237,238). 

As already outlined in Section 3.2.1, several water-soluble monophosphines, such as TPPTS, have been synthesized by controlled sulfonation in oleum. In the same way several diphosphines have been sulfonated (Structures 1-5). Although the number of chiral diphosphines reported is ever increasing, the number of sulfonated achiral diphosphines remains limited and includes BINAS-8 (1) [3, 4] and BISBIS (2) [5], both of which contain an aromatic bridge between the two phosphi-... 

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