Dication-like

Asymmetric Diels—Alder catalysis was more successful with dication-like versions of the Zr-EBTHI system, and using conformationally better defined acyl-oxazoline dieno-philes. The bis(triflate) [Zr(EBTHI)(OTf)2] (Scheme 8.47) induced high levels of ee (>90%) in the cycloaddition to cyclopentadiene at low temperatures, especially in the polar solvent 2-nitropropane [87]. 

It was also noted that dications like 63 may only be discrete intermediates in reactions in very strong liquid superacids, while protosolvated species like 64 may be more likely intermediates in reactions involving weaker superacids or zeolites. 

Besides carbo-ammonium dicationic systems, there have been studies related to carbo-phosphonium dication systems. Some of the reported chemistry suggests that superelectrophilic activation is involved. Olefinic phosphonium salts are protonated in superacid to generate dications like 140 and these species have been shown to react with benzene in good... 

Recently, the classical nitrenium-based mechanism for this reaction (Scheme 18.19) was contested when DPT studies showed a dication-like transition state may be taking part [69]. In this work, it was calculated that the nitrenium ion, C Hj—NH+, often suggested as an intermediate, was absent owing to the high nucleophilicity of the water cluster around it. Furthermore, investigation showed that a diprotonated system, Ph—N(0H)H + (H30+)2(Hj0)j3, was evidenced, a system that possesses an activation energy similar to the experimental one. Based on this evidence, a new mechanism for the rearrangement was proposed [69]. 

Substituents on the methine chain can stabilize the dye radical cation if the substituent (like methyl) is located on the high electron density carbons. However, no significant stabilization occurs when alkyl groups are on the alternate positions (like 9, 11 for the dication in Fig. 9). Current results for several dyes including die arbo cyanines and carbocyanines indicate that electronic stabilization of the dication radical lengthens the radical lifetime and also enhances the reversibiUty of the dimerization process (37). 

Comproportionation equilibrium constants for Equation 9.3 between dications and neutral molecules of carotenoids were determined from the SEEPR measurements. It was confirmed that the oxidation of the carotenoids produced n-radical cations (Equations 9.1 and 9.3), dications (Equation 9.2), cations (Equation 9.4), and neutral ir-radicals (Equations 9.5 and 9.6) upon reduction of the cations. It was found that carotenoids with strong electron acceptor substituents like canthaxanthin exhibit large values of Kcom, on the order of 103, while carotenoids with electron donor substituents like (J-carotene exhibit Kcom, on the order of 1. Thus, upon oxidation 96% radical cations are formed for canthaxanthin, while 99.7% dications are formed for P-carotene. This is the reason that strong EPR signals in solution are observed during the electrochemical oxidation of canthaxanthin. 

The generalized Woodward-Hoffmann rule suggests that a synchronous addition of disulfonium dications at the double C=C bond of alkenes would be a thermally forbidden process and so would be hardly probable. Simulation of the frontal attack by ethylene on l,4-dithioniabicyclo[2.2.0]hexane 115 gave no optimal structure of an intermediate complex. On the other hand in the lateral approach of the reactants, orbital factors favor attack of the double bond by one of the sulfonium sulfur atoms of the dication. This pattern corresponds to SN2-like substitution at sulfur atom as depicted in Figure 5. Using such a reactant orientation, the structure of intermediate jc-complex was successfully optimized. The distances between the reaction centers in the complex, that is, between the carbon atoms of the ethylene fragment and the nearest sulfur atom of the dication, are 2.74 and 2.96 A, respectively. 

E = S, Se X = I, Br) cation.20 Whether this cation really forms in solutions, especially in low-polar solvents, is difficult to prove. It has been shown, however, that the calculated NBO charge distribution on [LE-X]+ can be of great help in predicting the most likely product, at least among [LE-EL]2+ dications, C.-T. spoke , and T-shaped hypervalent adducts.21,22 In fact, these three types of products can formally derive from a nucleophilic attack of the appropriate nucleophile on the [LE-X]+ cation at the chalcogen or the halogen site. 

The dication [(oep)Pb(IV)]2+ is unstable. Its absorption spectrum differs from that of doubly oxidized porphyrins of other metals like Zn or Mg, where the ligand is doubly oxidized. Spectrometry and voltammetry indicate that PblV binds weakly to the unoxidized ligand (the Soret and adjacent lines are those of the unoxidized ligand). Another unique aspect of the lead complex is its instability, which stands in contrast to the stable Sn(IV) analog171, implying the inability of PblV to accommodate into the central cavity of the porphyrin plane. 

In summary, we have shown that stable cationic charge centers can significantly enhance the reactivities of adjacent electrophilic centers. Most of the studied systems involve reactive dicationic electrophiles. A number of the reactive dications have been directly observed by low temperature NMR. Along with their clear structural similarities to superelectrophiles, these dicationic systems are likewise capable of reacting with very weak nucleophiles. Utilization of these reactive intermediates has led to the development of several new synthetic methodologies, while studies of their reactivities have revealed interesting structure-activity relationships. Based on the results from our work and that of others, it seems likely that similar modes of activation will be discovered in biochemical systems (perhaps in biocatalytic roles) in the years to come. 

The dication 212+ composed of two methylium units connected to a p-phenylene spacer would be a candidate for new Wurster type violene-cyanine hybrid (Figure 12) (15). The reaction of four molar amounts of azulene 6b with terephthalaldehyde yielded the hydro precursor. Synthesis of the dication 212+ was accomplished by hydride abstraction with DDQ in almost quantitative yield. The dication 212+ was expected to show destabilization, but instead it exhibited high thermodynamic stability just like the corresponding monocation 3b+. 

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. 

In order to vary the electronic situation at the carbene carbon atom a number of carbo- and heterocycle-annulated imidazolin-2-ylidenes like the benzobis(imida-zolin-2-ylidenes) [58-60] and the singly or doubly pyrido-annulated A -heterocyclic carbenes [61-63] have been prepared and studied. Additional carbenes derived from a five-membered heterocycle like triazolin-5-ylidenes 10 [36], which reveals properties similar to the imidazolin-2-ylidenes 5 and thiazolin-2-ylidene 11 [37] exhibiting characteristic properties comparable to the saturated imidazolidin-2ylidenes 7 have also been prepared. Bertrand reported the 1,2,4-triazolium dication 12 [64]. Although all attempts to isolate the free dicarbene species from this dication have failed so far, silver complexes [65] as well as homo- and heterobimetallic iridium and rhodium complexes of the triazolin-3,5-diylidene have been prepared [66]. The 1,2,4-triazolium salts and the thiazolium salts have been used successfully as precatalysts for inter- [67] and intramolecular benzoin condensations [68]. 

Iminio-phosphonio dications have been obtained by N- and F-methylation of (imidazolylphenyl)diphenylphosphine. These dications are suitable precursors for C,C-chelating ligands. Deprotonation of the N-heterocycle leads to an NHC carbene which can be coordinated to a suitable metal center like Pd The subsequent deprotonation of the phosphonio moiety gave the complex with a bidentate chelating coordinated NHC-ylide ligand [207, 208]. 

Diamines with a cage-like structure in which the nitrogen atoms are separated by a three carbon chain, form radical-cations where the non-bonding orbital from one nitrogen atom interacts with the radical-cation on the other nitrogen atom to form a three-electron bond [76]. Cyclic voltammetry of 16 in acetonitrile shows two reversible waves with E° = 0.11 and 0.72 V vs. see. The second wave is due to the formation of a dication with a tw o-electron bond between the nitrogen atoms. 

The mono- and diprotonated carbocations and the two-electron oxidation dications derived from parent pyrene and its nonalternant isomers azupyrene (dicyclo-penta[e/, /]heptalene) (DCPH) 72 and dicyclohepta[eJ,g /z]pentalene (DCHP) 73 were studied at the B3LYP/6-31G(d) level (Fig. 29). The most likely site(s) for mono-and diprotonation were determined based on relative arenium ion energies and the structures of the energetically most favored carbocations were determined by... 

The similarity of the results obtained with phenyl azide and N-phenyl-hydroxylamine in benzene/TFA indicates that both reactions proceed by similar mechanisms, but N-phenylhydroxylamine in benzene/TFSA produces a higher yield of the C-substitution products 27 and 28 As previously suggested (Scheme 5), N-phenylhydroxylamine can be doubly protonated to yield the dication 11 in strong acids, but the more weakly basic phenyl azide is less likely to be doubly protonated. The differences observed between the behavior of N-phenylhydroxylamine and phenyl azide in TFSA may be due to the inability of phenyl azide to directly generate 11. 

Phosphamethin-cyanines, like methin-cyanines, can be protonated by strong acids, forming colorless dications. These can be converted back to the original phosphamethin-cyanines by careful addition of weak bases such as tert.-butanol. This acid-base reaction is least successful in the case of the weakly basic bis-benzthiazole-phosphamethin-cyanine 6. For bis-quinoline-phosphamethin-cyanine 7 b, we obtained vnth perchloric acid in glacial acetic acid the absorption spectrum of the N-ethyl-quinolinium salt. 

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