Chiral dications

A recent interesting example is the chiral dication 298 obtained by treatment of the corresponding diol with HBF4 (only one enantiomer is shown).593 It has apseudo-C2... 

As mentioned, 2-oxazolines may form a ring-opened distonic superelectrophile in reactions in superacid. These carboxonium-carbenium dications are capable of reacting with benzene and moderately deactivated substrates.2 For example, the optically active oxazoline (94) reacts in CF3SO3H to generate the chiral dication (95) and this superelectrophilic species is capable of reacting with o -dichlorobenzene in fair to modest yield and diastereoselectivity (eq 35). 

The propensity of S-S dications to undergo dealkylation was found to decrease in the order of methyl > ethyl > benzyl. This order of reactivity parallels the increase in the stability of the corresponding carbocations.94 Dealkylation of dication 77 affords thiosulfonium salt 78 in quantitative yield.95 Kinetic studies suggest SN1 mechanism of dealkylation. In addition, reaction of sulfoxide 79 with a substituent chiral at the a-carbon results in racemic amide 80 after hydrolysis. 

Chiral 2-imidazoline dianions undergo one-electron oxidation in the presence of TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) to form a radical anion that is either trapped stereoselectively by TEMPO or undergoes dimerization. Oxidation of bis-diazene oxides leads to novel (9-stabilized 4N/3e radical cations and 4N/2e dications. These were detected by ESR spectroscopy and cyclic voltammetry. B3LYR/6-31G calculations confirmed the nature of the 4N/3e and 4N/2e systems. ... 

The enantiopure BIPHEP-Pt complexes can act as chiral Lewis acids for the enantioselective Diels-Alder and carbonyl-ene reactions. The Diels-Alder products are obtained in 92-94% ee (93 7 = endo exo) and 92-94% ee (94 6 = endo exo) by (R)- and (5)-36, respectively (Scheme 8.3 la)." In the carbonyl-ene reaction catalyzed by the dication species generated from BlPHEP-PtCL 35 and AgSbFs, the (S )- and (/ )-ene products are obtained with 71% ee (99% conversion at room temperature) and 70% ee (90% conversion at room temperature) from (/ )- and (S)-35, respectively (Scheme 8.31b). 

Oxidation of TTF and its derivatives induces the transformation from neutral species into cationic ones, namely, cation radicals (TTF +) and dications (TTF2+). Moreover, TTF, TTF +, and TTF2+ exhibit different absorption spectra. Taking these advantages of TTF new TTF-based redox fluorescence switches and chiral switches have been recently reported. 

MacGillivray, L.R. Atwood, J.L. Proton-induced chirality Proton complexation in the chiral cryptand 222-2H dication isolated from a liquid clathrate medium. J. Org. Chem. 1995. 60, 4972-4973. 

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