Dicationic species

Various other heteroatom-substituted earbocations were also found to be activated by superacids. a-Nitro and a-cyanocarbenium ions, R2C N02 or R2C CN, for example, undergo O- or N-protonation, respectively, to dicationic species, decreasing neighboring nitrogen participation, which greatly enhances the electrophilicity of their carbo-... 

The rhodium and iridium complexes of dibenzothiophene (L) reveal an interesting case of linkage isomerism (91IC5046). Thus, the ti S) coordinated species [MCp LCb] on thermolysis with silver tetrafluoroborate afford the Ti -coordinated dicationic species. 

In accord with the experimental findings a mechanism via a [5,5]-sigmatropic rearrangement has been formulated. In a first step the hydrazobenzene is proto-nated to the dicationic species 3, in which the phenyl groups can arrange in such a way to allow for rearrangement ... 

The rearranged dicationic species 4, which has been shown to be an intermediate, leads to the stable benzidine 2 upon deprotonation. It has been demonstrated by crossover experiments that the rearrangement does not proceed via a dissoci-ation/recombination process. From the reaction of hydrazobenzene the benzidine is obtained as the major product (up to 70% yield), together with products from side reactions—2,4 -diaminobiphenyl 5 (up to 30% yield) and small amounts of 2,2 -diaminobiphenyl 6 as well as o- and /j-semidine 7 and 8 ... 

A few electrochemical reductions of formazans to hydrazidine have been reported.370,372 Ho wever, as discussed in Section 7.4.2.6, electrochemical techniques have been widely used to study the redox chemistry of tetrazolium salts and formazans. Opinions about the reversibility of the electron transfer step, the number of electrons involved, and the identity of the rate-determining step differ widely.369- 371,656 The electrochemical oxidation of some novel formazans, e.g., 215 produces the dicationic species 216 (Eq. 28). The mechanism is not clearly understood.372,373... 

When N-ethylated-a-picoline, y-picoline, and lepidine were reacted with the ethoxy cation 75, instead of the expected cyanines of type 136 or 137 the products were dicationic species, whose structures were assigned to be bis(diphenylcyclopropenium)-monomethine cyanines, e.g. 142105 ... 

When l,2,3,4-tetrakis(4-pyridinyl)cyclobutane is treated with ethyl chloroformate in the presence of ethyldiisopropylamine, radialene 74a is formed and can be isolated as red crystals. Addition of AgBEt to a red solution of 74a results in an immediate color change to deep blue caused by the formation of the dicationic species 75a (equation 5)43. Undoubtedly, the tetrapyridiniocyclobutane 73a is an intermediate in the formation of 74a. By way of contrast, the fourfold deprotonation of the analogous tetrakis(7V-methyl-4-pyridinyl)cyclobutane (73b) did not succeed in the presence of oxygen. Treatment with NaH/EtOH in the presence of oxygen produced the dication 75b, which could be reduced to the [4]radialene only electrochemically44. Deprotonation of 73b with sodium hydride... 

A very reactive Lewis acid is obtained when the complex [(EBTHI)Zr(Me)2] is converted in situ to a dicationic species by protonation with the acid H-BARF (BARF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) in the presence of the Diels—Alder substrate oxazolidi-none [88] (Scheme 8.48). The dicationic species is stabilized through coordination by the oxazolidinone and by diethyl ether (derived from the acid etherate employed). The catalyst loading in the Diels—Alder reaction could be lowered to 1 mol% (Zr) and the reaction still... 

Acyl-transfer reactions are some of the most important conversions in organic chemistry and biochemistry. Recent work has shown that adjacent cationic groups can also activate amides in acyl-transfer reactions. Friedel-Crafts acylations are known to proceed well with carboxylic acids, acid chlorides (and other halides), and acid anhydrides, but there are virtually no examples of acylations with simple amides.19 During studies related to unsaturated amides, we observed a cyclization reaction that is essentially an intramolecular acyl-transfer reaction involving an amide (eq 15). The indanone product is formed by a cyclization involving the dicationic species (40). To examine this further, the related amides 41 and 42 were studied in superacid promoted conversions (eqs 16-17). It was found that amide 42 leads to the indanone product while 41... 

The combination of the di(l-azulenyl)methylium units produced dicationic species with high stabilities. However, transformation of the dication 212+ to the quinoid compound 21 as a Wurster type violene-cyanine hybrid was not achieved due to the instability of the reduced species. 

The oxidation of dithianes (Scheme 13) leads to a dicationic species that reacts with water affording aldehydes or ketones and 1,2-dithiolane, which undergoes further oxidation to the sulfoxide (20-74% yields) [18]. 

In the medium pH range where both the diazenium cation and the diazene exist in solution, these two species react rapidly to afford a substituted tetrazene [141], The second oxidation wave, observed for disubstituted hydrazenes in acetonitrile with a pyridine base present, is due to the further oxidation of the tetrazene to a dicationic species. 

The protonation of the methyleneiminium cation gives rise to the superelectrophilic methyleniminium dication, 19, which is a gitonic 1,2-dicationic species (carbenium ammonium species). Such a carbodication has been observed by charge stripping mass spectrometry by Schwarz and coworkers.Olah and coworkers have shown by theoretical calculations that the carbenium ammonium species, 19a, is 27.6 kcal/mol more stable than the three-center two-electron (3c-2e) bonded structure, 19b. ... 

Because singlet nitrenium ions bear a formal nonbonding electron pair, they can be regarded as Lewis bases. In principle, it should be possible to derive a dicationic species through protonation (10, Fig. 13.4) of a nitrenium ion, or by removal of two... 

Olah generated similar dicationic species upon treatment of 4-hydroxy- or 4-methoxy- nitrosobenzenes with 1 1 HSO F-SbFs in SO2 (Scheme 6) These species were stable in the superacid media and were characterized by NMR spectroscopy. No nitrenium ion species were detected. Although Olah was unable to detect simple monoarylnitrenium ions, a few stabilized diarylnitrenium ions were detected during cyclic voltammetry experiments on the corresponding diarylamine in CHsCN, For example, the di-para-anisylnitrenium ion and its conjugate acid (Scheme 7) were both observed. This ion had a lifetime of about 1 s in CH CN. ... 

Superacid dehydrative cyclization of pinacols such as (93) gives condensed aromatic compounds as shown, presumably via dicationic species like (94).137... 

Superelectrophilic dicationic species 41,42, and 43 were suggested to be involved in the alkylation steps of the above transformations. 

Protonated phenols and phenol ethers formed in superacids can be trapped by aromatics (benzene, naphthalene, tetrahydroquinoline). The products are either cyclohexenone derivatives301 [Eq. (5.112)] or aryl-substituted phenols. In the reaction of phloroglucinol with benzene, the diphenyl-substituted derivative is the main product [Eq. (5.113)], whereas 1,3,5-trimethoxybenzene gives selectively the monophenyl derivative (80% yield). Protonated dicationic species, such as 76, detected by Olah and Mo302 using NMR, were suggested to be intermediates in these processes. 

The reaction of the parent compound did not take place in trifluoroacetic acid (TFA), but did proceed on the addition of triflic acid to give the alkylated product in low yield (Table 5.24). In neat triflic acid the reaction is faster and the product is formed in much higher yield. Similar changes were observed with the other 1,3-diphenylpropanones. Substituents on the benzoyl group also induce significant differences in reactivities (compare the results for R = Me and CF3 in Table 5.24). These observations suggest that the protosolvated dicationic species 79 is involved in the rate-determining step of cyclization (Scheme 5.35). 

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