Dications cyclization

Condensation of an aniline with a dione with loss of water provides enamine 16. Ketone protonation and cyclization forms 18 followed by loss of water provides quinoline 4. Some have suggested the formation of dication 19 as a requirement to cyclization. Cyclization of 19 to 20 and subsequent conversion to quinoline 4 requires loss of water and acid. Another rendering of the mechanism takes into account participation of an electron-donating group (EDG), which stabilizes intermediate 21. 

Tanaka and Ohno developed the palladium(0)-catalyzed cyclization of bromoallene 222 bearing a sulfonamide for the synthesis of medium-sized heterocycle 223 (Scheme 37).48b In this reaction, bromoallene acts as an allyl dication equivalent 224, and two different nucleophiles can be introduced regioselectively. The intramolecular nucleophilic... 

Cyclic voltammetry (Section II,A) and other electroanalytical techniques9 may give answers to the following questions (i) in which stage does the cyclization occur, e.g., in the stage of a radical-cation or dication, (ii) which is the rate-determining step, (iii) what is the sequence of the reactions in the overall conversion (RH2 - cR) The second electron is very commonly transferred through a disproportionation of the intermediate radical or radical-cation. 

Anodic oxidation of enamine ketones or esters in CH30H-NaC104 at a graphite anode gives substituted pyrroles in 15-45% yield.101 Formation of the symmetrically substituted pyrroles 47 indicated radical dimerization of radical-cations formed as primary products from 46. This process leads to dications from which the pyrroles can be formed by cyclization and elimination of an amine [Eq. (44)]. 

A one-step synthesis of tricyclic diazadihydroacenaphthylenes with an isoxazoline ring has been developed from 1 -benzylamino-1 -methylthio-2-nitroethene derivatives induced by a large excess of triflic acid715 [Eq. (5.265)]. Dications 169, similar to those detected by Coustard,197 were observed by NMR spectroscopy. Quenching with water gives a reactive intermediate nitrile oxide, which undergoes an intramolecular cyclization to furnish the final products in fair yields. 

A highly regio- and stereoselective synthesis of oxepine derivatives via cyclization of bromoallenes bound with hydroxy group by a four-carbon atom tether in the presence of a palladium(O) catalyst and alcohol is developed (Equation 16). In this reaction, the bromoallenes act as an allyl dication equivalent, and the intramolecular nucleophilic attack takes place exclusively at the central carbon atom of the allene moiety <2004JA8744>. 

Bromoallene 139 was used as an allyl dication equivalent, and palladium-catalyzed cyclization leads to a mixture of regioisomers 140 (major) and 141 (minor) (Scheme 19) <2004JA8744>. 

It was reported that arylpinacols (48a) can undergo a superacid-catalyzed dehydrative cyclization to give the aryl-substituted phenan-threnes (eq 27).23 Superelectrophilic intermediates were proposed in the conversion. Tetraarylethylene dications have been studied by several methods and were observed directly by NMR as well as by UV-vis spectroscopy and X-ray crystallography.28 The low temperature oxidation of tetraaiylethylenes gives the dicationic species (50, eq 28). 

Dication 72 is estimated to have a fluorene cyclization barrier of 8.52 kcal/ mol, compared with a value of 25.04 kcal/mol for the cyclization of... 

Several lines of evidence in the studies of Ohwada and Shudo support the involvement of these gitonic superelectrophiles. An analagous dicationic species (47) has been directly observed by 1H and 13C NMR at low temperature in both CFsSOsH-SbFs and CF3SO3H solutions. With dication 47, the NMR spectra show an equivalence of the aromatic rings suggesting a perpendicular relationship between the two cationic centers. The dications having the 4-methylphenyl substituents (61-62) have likewise been directly observed by NMR at low temperature, and upon warming the solutions, the expected cyclization products are formed (eqs 11-12). 

This is reasonably close to the experimentally determined value of AFF = 11.1 kcal/mol for the superelectrophilic cyclization of 62 (eq 12). Another computational study showed that the energy barriers dramatically decrease for the electrocyclization when the monocations are protonated to form superelectrophiles. In the case of 63, cyclization provides the acetyl-substituted fluorene in 70% yield from CF3SO3H (Scheme 5). At the B3LYP/6-31 level of theory, dication 64 is estimated to have a cyclization barrier to fluorene of 8.5 kcal/mol, compared to a value of 25 kcal/mol for the cyclization of monocation 65. 

The described superelectrophilic activation and fluorene-cyclization is thought to involve a lowered energy of the LUMO and concomitant delocalization of positive charge into the aryl ring(s).32b Calculations at the 4-31G//STO-3G level on a model system (Figure 2) have shown that the amount of positive charge in the phenyl ring increases upon formation of the dication (67) when compared to the monocation (66) and the benzyl cation (calculations are based on fully planar structures). It is well known... 

Besides the fluorene cyclization, the 1,2-ethylene dications are known to undergo another type of electrocyclization reaction to produce the phenan-threne ring system (see eqs 9 and 11 and Scheme 3).32a Although derived carbenium-carboxonium dications (70) will give the phenanthrene-type products (albeit in low yield eq 15), the bis-carboxonium dication 71 does not. Moreover, the fluorene cyclization occurs readily in 1,1-dihydroxylie systems (eq 10). 

Calculations at the 4-31G level of theory found that the 1,2-bis-carboxonium ion (72) is significantly more stable than the carbenium-carboxonium dication (73). Thus, when compared with the 1,1-dihydroxylie structure 73, the 1,2-dihydroxy lie structure 72 is more effective at stabilizing the 1,2-ethylene dication. This effect leads to the stabilization of dication 71 (eq 16), and not to phenanthrene cyclization. 

Several bicyclic hydrazinium ions have been prepared and two synthetic strategies have emerged as generally useful, i.e., electrochemical oxidation and Lewis or Brpnsted acid-promoted cyclization.93 Oxidation of l,6-diazabicyclo[4.4.4]tetradecane, for example, (194) gives the intermediate radical cation (195) and the bicylic dication (196, eq 50).93a... 

There have been two reports involving gitonic superelectrophiles composed of carboxonium ions and vinylic carbocations in a 1,3-relationship. In the reaction of 3-phenylpropynoic acid (65) with benzene in superacid the novel carboxonium-vinyl dication 66 is generated, followed by reaction with benzene and then cyclization (eq 22).26a Likewise, the unsaturated amide (67) gives the cyclization product in high yields (70-97%) in very strong acids (polyphosphoric acid, CF3SO3H, Nation SAC-13, or HUSY eq 23).30... 

The carboxonium-vinylic dication (68) is considered the key intermediate leading to the cyclization product. The analogous vinylic dications (69-71) have also been generated in superacid.31 Each of the species exhibits high electrophilic reactivity. 

A number of related distonic superelectrophiles have been generated from A-heteroaromatic compounds (Table 2). Vinyl-dications (111-112) have been produced from the ethynyl pyridines,40 while /V-alkenyl A-heterocycles provide dications (113-115).42a Vinyl-substituted A-heterocycles provide access to distonic superelectrophiles such as dication 116.39 Dications 117 and 118 are generated from their precursors, and both intermediates lead to efficient cyclization reactions with the adjacent... 

The highly strained molecule of 3,4-diphenylcycloocta-l,5-diyne (165), stabilized by the coordination of Co2(CO)6, was prepared from the bis-propargylic alcohol 163. Cyclization of the propargylic dication complex 164, induced by reduction with Zn,... 

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