Dication special

Cation-radicals containing three electrons in the field of four atoms or anion-radicals with five electrons retained by four atoms represent a special group of multicentered ion-radicals. Thus, nonclassical, cyclically delocalized 3e/4C cation-radicals and 2e/4C (dication)-radicals of substituted cyclobutadienes are known (Allen and Tidwell 2001). The 3e/4N cation-radical and the 5e/4N anion-radical have also been discovered (Exner et al. 1998, 1999, 2000). The reactions in Scheme 3.31 illustrate structures of the 5e/4N anion-radical as well as the corresponding dianion and acety-lated products of the latter. 

Fig. 4.23 Hiickel s rule says that cyclic n systems with An + 2 n electrons ( = 0, 1, 2,. .. An + 2 = 2, 6, 10,. ..) should be especially stable, since they have all bonding levels full and all antibonding levels empty. The special stability is usually equated with aromaticity. Shown here are the cyclopropenyl cation, the cyclobutadiene dication, the cyclopentadienyl anion, and benzene formal structures are given for these species - the actual molecules do not have single and double bonds, but rather electron delocalization makes all C/C bonds the same...

The dimeric Mo dication XXII is a special case that shows reactivity similar to the complexes mentioned earlier and adds nucleophiles such as cyanide and methoxide anions to give mono-jy -cyclohexadienyl complexes (XXIII). Electro oxidation of XXIII (Nuc = CN ) induces loss of the endo hydrogen and generates the -benzonitrile complex, XXIV ... 

Whereas carotenoids in general are often considered by non-specialists as unstable compounds, they may easily be studied when particular precautions are made [1], This is also true for most subtypes with charged functional groups. The instability of carotenoid oxonium ions, radical cations, mono- and dications, however, represent special challenges. This review appears to be the first compilation on charged carotenoid species. 

It is difficult to choose a reference compound against which to judge the stability of the dication. That it can be formed at all, however, is suggestive of special stabilization associated with the two TT-electron system. The dianion formed by adding two electrons to the TT system of cyclobutadiene also meets the 4 - -2 criterion. In this case, however, four of the six electrons would occupy HMO nonbonding orbitals, so high reactivity could be expected. There is some evidence that this species may have a finite existence. " Reaction of 3,4-dichlorocyclobutene with sodium naphthalenide, followed a few minutes later by methanol-0-<7 gives a low yield of 3,4-di-deutero-cyclobutene. The inference is that the dianion [ 4114 ] is present, but there has not yet been direct experimental observation of this species. Cyclooctatetraene is reduced by alkali metals to a dianion. 

Since the double bond at the homoallylic position, as shown above, decelerates solvolysis, the rate ratio of solvolysis of tosylates 63 and 194 is 3.37 but this fact requires a special discussion. Two explanations can be proposed. One of them assumes diester 193 to detatch simultaneously both tosylate groups to form a dication 195 which further reacts with the solvent to form a homoallylic ion 196 and finally — diacetates. 

The described dications represent nonclassical ions of a special kind — pyramidal mono- and dications the latter can be regarded as a link to the elementoorganic chemistry e.g., boranes, by the peculiarities of their stereochemistry and of their bond nature. Thus, the pyramidal structure of pentaborane 509 is similar to that of monocation (CH)j 510 with a pentacoordinated carbon while the pyramidal dication (CH) 511 is similar to carborane 512. By the peculiarities of their stereochemistry pyramidal cations are similar to metalloorganic sandwich compounds, e. g., ferrocene 513 the dication 511 is regarded as a semi-sandwich . 

We have chosen to divide the porphyrinoids into two famihes the peripherally functionalized porphyrinoids (e.g., N-confiised porphyrins, jr-extended porphyrins, oligo-porphyrins) and the coie-fimctionalized (nonplanar) por-phyrinoids (e.g., porphyrinogens, alkylidene porphyrins, oxoporphyrinogens, and some oligopyrroles). The former contains mostly molecnles that have complex elaboration at their periphery, which leads to special snpramolecular properties. Core-snbstituted porphyrinoids inclnde N-snbstituted oxoporphyrinogens and even some metalloporphyrins and porphyrin dications. 

A special class of ionic liquids have recently been synthetized using geminal imidazolium dications (Anderson et al., 2005 Payagala et al., 2007 Sun et al., 2009). This class of compounds represents an interesting variation of the cationic partner and may present several advantages over the traditional monocationic ionic liquid in their application as lubricants (Jin et al., 2006), solvents (Han Armstrong, 2005 Xiao Shreeve, 2005) and separation media (Anderson et al., 2005). 

---