Aromaticity dications

One could expect diazadiboretidines to be converted into Huckel aromatic systems either by adding or by subtracting one pair of n-electrons. The addition of two electrons to diazadiboretidines of the type (RBNalkali metals. The dianions [(RBNiBu)2] are stable in solution and can be reconverted into the diazadiboretidines by oxidants. Because they contain six n-electrons, aromatic character may be attributed to the dianions (19). Cyclodimers of the type (R BNR)2 are also readily oxidized, but the adoption of an aromatic dication [(R BNR)2] as a product would be mere speculation at present. 

The preparation of the 8C-6jt aromatic dication 236 of the unknown pentalene 235 has proven to be unusually difficult. However, the dibenzoannulated derivatives have been prepared by Rabinowitz and co-workers.497,498... 

There are several other carbodications that may be represented as 1,3-dicationic systems (or nonclassical, highly delocalized systems), including Schleyer s l,3-dehydro-5,7-adamantanediyl dication (28),11 Hogeveen s pyramidal dication (29),12 diallyl dications (i.e., 30),13 dicationic ethers (i.e., 31),14 diprotonated pyrones (32),15 as well as various aromatic dications. 

The aromatic dication, dipyrido[l,2-a 2, T-c]pyrazinium (94), has been shown to undergo reduction to a cation-radical which is reoxidized back to 94 by oxygen in high yield. Consistent with this behavior and its structural relationship with Diquat, 94 is a potent herbicide. 

The 20 7i-electron macrocycle 3.44 was found to undergo ready two-electron oxidation when treated with Br2 and perchloric acid. This afforded the 18 Ji-elec-tron aromatic dication 3.45. This species was originally isolated in the form of its bisperchlorate salt (3.45a). It was found, however, that this latter could be converted into the corresponding nitrate salt (3,45b) by treating with 65% nitric acid. Both salts... 

First, monocation then aromatic dication with 2 tt electrons. 29.2 (a) Aromatic, with 2 IT electrons ... 

The aprotic solvent, liq SOj, is inert to oxidation and is promising for anodic pro-cesses , such as for electrolytic preparation of stable solutions of aromatic dications that are labile under normal nonaqueous conditions. 

The structure analysis of the dilithium pentalenediide 45 reveals a C2-sym-metric ion triplet with the two lithium cations located on opposite sides of the two different rings. The structural parameters were extremely well reproduced by ab initio calculations [35] (see Fig. 3). Both the experimental structural parameters and calculated magnetic susceptibility exaltation classify the 107r-elec-tron species 45 as an aromatic compound. Apparently, the lithium counterions in 45 do not exert any significant effect on the bond lenghts of the dianion 22. On the other hand, the antiaromatic pentalene 2 and its aromatic dication 22+ show the characteristic bond length alternation (Fig. 3) [35]. 

Brendle JJ, Outlaw A, Kumar A, Boykin DW, Patrick DA, Tidwell RR, Werbovetz KA. Antileishmanial activities of several classes of aromatic dications. Antimicrobial Agents and Chemotherapy 2002 46797-46807. Doi 10.1128/AAC.46.3.797-807.2002. 

The covalent capsules 227-230 are reported in [162] to form host-guest complexes with encapsulated aromatic dications 92 and 93 by Scheme 2.149. In solution and gas phase, only 1 1 cage complexes have been detected by H NMR and ESI-MS methods. In the case of 228 its 2 1 cage complex with paraquat dicationic guest 92 has been characterized by X-ray diffraction... 

A caging ligand 248 encapsulates aromatic dications in its two asymmetrical 24-crown-8 cavities, and the caged species undergo reversible... 

SO2CIF (eq 28). In these systems, separation of the two cation centers by at least two methylene groups is necessary for the ions to be observable. Aromatic dications are usually prepared by oxidizing the corresponding aromatic compounds with SbFs (eq 29). In the case of pagodane containing a planar cyclobutane ring, oxidation leads to the formation of cyclobutane dication which was characterized as a frozen two-electron Woodward-Hoffmann transition state model (eq 30). ... 

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