Dimeric dication

The absence of dimer radical cation formation by diphenyl selenide under the pulse radiolysis conditions is in contrast to bimolecular reactions believed to occur under electrochemical conditions/ In these experiments, a rotating disk electrode was used in combination with commutative voltammetry under anhydrous conditions. The results led to the conclusion that reversible one-electron oxidation is followed by disproportionation, then reaction of the resulting dication with diphenyl selenide or an external nucleophile, with the likely intermediacy of the dimer dication (Fig. 33). As expected, the dihydroxy selenane is formed when water is present. Based on the kinetics of the electrochemical reaction, the authors believe the diselenide dication, not the radical cation, to be the intermediate that reacts with the nucleophile. 

Molecular orbital calculations using PM3 were performed on the ehalcogenides, their radical cations, and the dimer dications. The radical cations were calculated to be 77 radicals, with the unpaired electron in a p orbital perpendicular to the plane containing the two carbons and the chalcogen atom (Fig. 40), as observed experimentally for the S and Se cases. Similar modeling of the corresponding... 

Intramolecular dimer radical cation and dimer dication formation... 

Other dimer dications have been prepared from dichalcogenoethers by treatment with two equivalents of the one-electron oxidant NO (Fig. 42), but these dimer dications are not sufficiently stable to be isolated and characterized by elemental and spectroscopic analyses, typically undergoing carbon-halogen bond cleavage. 

Radical cations can dimerize in a radical-radical coupling reaction (Scheme la) to afford dimer dications. An alternative pathway to form the dimer dication is a radical-substrate coupling in an electrophilic addition of the radical cation to the nucleophilic substrate. The dimer dication can lose two protons to form a bis-dehydro dimer or react with two nucleophiles to yield a disubstituted dimer. 

Olefins with electron-donating substituents as the aUcoxy, acylamino, phenyl, or vinyl group can be coupled in methanol to give 1,4-dimethoxy dimers and/or dienes (Scheme 2). The first intermediate in this coupling reaction is a radical cation, which either by electrophilic addition to the olefin and subsequent le-oxidation (path A) [49] or by radical dimerization (path B) [50, 51] leads to a dimer dication that undergoes methanolysis or deprotonation. Representative examples of this coupling reaction are summarized in Table 7. 

The sulfonium dimer can in turn be oxidized at a higher potential, which is the diffusion-limited oxidation current of Ph2S. For this reason, the concentration of the latter in the reaction layer closer to the electrode is practically zero, so a dimeric dication radical formed from [(Ph)2S-C6H4SPhj can only react with this cation. Thus, no trimers but only tetramers are obtained under such conditions (Scheme 12) [52, 54, 55]. 

Bianchini et al 1 have obtained triphos-capped tetrathio- and tetraseleno-oxalate bridged Rh dimer dications of the structure (triphos)Rh(C2E4)Rh(triphos)3, where triphos = 1,1,1-tris-(diphenylphosphinomethyl)ethane and E = S, Se. 

Oxidative dimerizations of the dithiafulvenes by both cyclic voltammetry (CV) and chemical method afford dimeric dications (Scheme 1) [6-11]. 

The thin layer cyclovoltammogram of 35a showed two independent oxidation processes (i) an irreversible wave at Epa = 1034 mV (vs. Fc/Fc+) (fcpi = -232 mV (vs. Fc/ Fc+)) and (ii) a reversible wave (E1//2 (het-ox), which corresponds to the formation of the radical cation of the dibenzodioxin subunit (Figure 28). The irreversible wave represents a two-step process involving a one-electron oxidation of the DHA subunit followed by a chemical step (EC-type mechanism) leading to a significant change in the molecular structure. Since polyenic radical cations have a preference for dimerization, 34 it is reasonable to speculate on the formation of the dimeric dication species as shown in structure 41. The chemical reversibility of this EC-type process was confirmed by multisweep thin layer experiments. 

Fig. 29 Spectroelectrogram obtained on oxidation of DHA 35a to the dimeric dication 41 (solvent acetonitrile).

S8 can undergo a SET oxidation by either 357 or Fe3 + to form a dimeric dication, 359, which upon the elimination of two protons results in the formation of the neutral dimer 362. Alternatively, 358 loses one proton, resulting in the formation of radical dimer 360, which is then oxidized to cation dimer 361. Deprotonation of 361 yields 362 [213],... 

X-ray crystallography revealed that the arsolidinium cation (50) exists as a dimeric dication. The dimmer is formed via intermolecular As- -S interactions (2.422(6) A). Addition of another equivalent of [(CH2CH2S2)AsCl] to (50) affords a bicyclic monocation, which can be viewed as the Lewis complex (52) of the corresponding chloroarsolidine [(CH2CH2S2)AsC1], and the arsolidine cation (50). ... 

For radical cations this situation is typically observed when deprotonation of the dimer dication is slow and for radical anions under conditions that are free from electrophiles, for example, acids, that otherwise would react with the dimer dianion. Most often, this type of process has been observed for radical anions derived from aromatic hydrocarbons carrying a substituent that is strongly electron withdrawing, most notably and well documented for 9-substituted anthracenes [112,113] (see also Chapter 21). Examples from the radical cation chemistry include the dimerization of the 1,5-dithiacyclooctane radical cations [114] and of the radical cations derived from a number of conjugated polyenes [115,116]. 

Under conditions that are not strictly nonaqueous, the oxidized dimer may be trapped by water, as was observed during the oxidation of 1,1-diphenylethylene catalyzed by the radical cation of dibenzo-1,4-dioxin [91]. The dimer dication upon reaction with water undergoes a 1,2-phenyl shift, resulting finally in 1,2,4,4-tetraphenyl-3-buten-l-one [Eq. (42)], reminiscent of the 1,2-shifts observed during anodic oxidation of 1-phenyl- and 1,4-diphenylnaphthalene in acidic dime thy Iformamide (DMF) [92]. 

The implications of these unexpected results for the conclusions drawn from previous studies, in particular the work based on spectroelectrochemical measurements, are not clear. The structure of the dimer dication, (ArCH )2, is not known. However, the reversible formation of dimers from hydrocarbon radical cations has been observed in other cases [203], and is probably a general phenomenon, but usually the dimers do not manifest themselves kinetically. 

---