Intermediates dicationic

Klumpp, D. A. Rendy, R. Zhang, Y. Gomez, A. McElrea, A. Dicationic Intermediates Involving Protonated Amides Dual Modes of Reactivity Including the Acylation of Arenes. Org. Lett. 2004, 6,1789-1792. 

Quinoline and isoquinoline react in an analogous manner with benzene in conjugated superacids (HCI-AICI3, HBr-AlBr3) to yield predominantly cis-5,1-diphenyl-5,6,7,8-tetrahydroquinoline and <7,v-6,8-diphenyl-5,6,7,8-tetrahydroisoqui-noline, respectively (75-94% yield).297 The regioselectivity observed corresponds to the most stable dicationic intermediates 73 and 74. Unsaturated imides298 exhibit similar behavior in triflic acid (Scheme 5.33). 

Alkynes bearing an adjacent Al-heterocycle, such as 5-ethynyl-1 -methyl-imidazole and 1-propargylbenzotriazole, readily alkylate benzene in the presence of triflic acid to yield diphenyl-substituted products.308 Isomeric ethynylpyridines exhibit distinct differences in reactivity 3-ethynylpyridine exhibits the highest reactivity, whereas 2-ethynylpyridine is the least reactive [Eq. (5.116)]. This is consistent with the involvement of dicationic intermediates as the de facto alkylating agents. Indeed,... 

Tricationic and dicationic intermediates were suggested to be involved in the intramolecular cyclization of 2-nitromethylene-l-phenylalkyl-substituted /V-heterocycles with the participation of superelectorophilic hydroxynitrilium cation 99 to give tricyclic products in triflic acid327 (Scheme 5.37). Likewise, the intramolecular trapping of the intermediate hydroxynitrilium cation affords the six- to nine-membered ring oximinoorthodithiolactones328 [Eq. (5.127)]. 

Nonactivated C—H bonds in imines can be selectively monofluorinated in HF-SbF5 in the presence of CC14 to yield the corresponding fluoroketones when the reaction mixture is quenched with HF-pyridine534 (Scheme 5.54). The transformation is initiated by hydride abstraction with CC13+ from the most reactive carbon farthest from the functional group and involves dicationic intermediates 134 and 135. 

A comprehensive series of ionic hydrogenation reactions have been studied by Koltunov, Repinskaya, and co-workers, and superelectrophilic intermediates have been proposed.34 Some of these intermediates have been characterized by and 13C NMR (Table 4). Many of these dicationic intermediates have been shown to react with cyclohexane by hydride abstraction, indicating superelectrophilic character. 

Evidence for the dicationic intermediates was also obtained from kinetic experiments involving the hydroxyester 57.32a The yield of the cyclization product (59) increases considerably with the acidity of the reaction media (Scheme 4). The fluorene product 59 is formed in appreciable quantities only in superacids Hq < —12). Following this observation, the kinetics of the fluorene cyclization was also studied in solutions of varying acidity. When compound 57 is reacted in solutions with acidity in the Hq — 11 to — 13 range, the cyclization rate is found to increase linearly with acidity. 

In addition to the above kinetics studies, the fluorene cyclization was studied using ab initio computational methods.323 It was found that the theoretically predicted barriers to the cyclizations for the dicationic intermediates agree well with the values obtained from the kinetic experiments. For example, geometry optimization and energy calculations at the B3LYP/6-31 level estimated that the activation energy (Ea) is 14.0 kcal/mol for the 4jt-electron conrotatory electrocyclization reaction involving compound 57 and the diprotonated intermediate (46, eq 13). 

With the monocationic species, no fluorene cyclization is observed. However upon addition of CF3SO3H, the cyclization occurs almost quantitatively. This is consistent with formation of the protonated, dicationic intermediate (46) leading to the cyclization product (59). In this same study, it is noted that other stable monocationic 1,1-diarylethyl cations (i.e., the 1,1-diphenylethyl cation) do not readily form the fluorene ring system, indicating the importance of superelectrophilic activation. 

Evidence for the involvement of the diprotonated species 40 and 42 includes 13C NMR data, which shows deshielding of the involved carbons as the acidity of the media increases from H0 —8 to —26 (mesityl oxide is estimated to be fully monoprotonated in acids of about H0 —8). Dication 42 has been shown to be capable of reacting even with very weak nucleophiles, for example, abstracting hydride from cyclohexane.19b An analogous species (43) has been proposed in interaction with excess AICI3. Protonated cyclohex-2-enone is converted to 3-methylcyclo-pent-2-enone in HF-SbFs solution at 50°C.20 The reaction mechanism is thought to involve the dicationic intermediate 44 (eq 13). Likewise, aryl-substituted indenones are converted to the dications by reaction in superacid (eq 14).21... 

The dicationic species (61) is proposed as the key intermediate in the reaction. Dicationic intermediates such as 61 have also been proposed in conversions of unsaturated amides with polyphosphoric acid, sulfated zirconia, and zeolite catalysts, such as HUSY.28 Likewise, ionic hydrogenation of 2-quinolinol (62) can be best understood by the involvement of the dicationic species (63), which is sufficiently electrophilic to react with cyclohexane (eq 21)29... 

Other diprotonated acyl-pyridines have likewise been studied.61 In studies of 5-, 6-, 7-, and 8-hydroxyquinolines and 5-hydroxyisoquinoline, dicationic intermediates like 185 (Table 4) were found to be involved in superacid catalyzed reactions with benzene and cyclohexane.59 For example, 8-hydroxyquinoline (187) reacts in CF SOsH-SbFs to generate dications (188 and 189) and undergoes ionic hydrogenation in the presence of cyclohexane (eq 64). Compound 187 also reacts with benzene in suspensions of aluminum halides (eq 65). 

Tetramethyl-2,4-pentanediol forms the oxonium dication 230 in superacid, and dehydration is followed by carbon- carbon bond cleavage. Although such cleavage reactions can occur with monocationic onium ions, in this case the cleavage reaction is likely an indication of the super-electrophilic nature of the dicationic intermediate(s). 

The polymerizations involved as a first step the formation of the monomer radical cations which underwent rapidly radical dimerization reactions to produce dicationic protonated TTF derivatives. The dicationic intermediates deprotonated slowly to stable vinylogous TTF polymers. This new syn-... 

Kinetic studies indicate a dicationic intermediate in the isomerization of a P-isocyano- to a P-cyano-azaphosphatrane monocation (Scheme 73).110... 

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