Vinyl dications

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. 

The formation of distonic superelectrophile 104 provides a high-yield route to the antispasmodic dmg, fenpiprane 105. The 1,4-dication (108), 1,5-dication (106) and 1,6-dication (107) have likewise been shown to react with benzene in high yields.39 The superelectrophilic vinyl-dications (109-110) have also been studied.40,41... 

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 a-(diphenylmethylsilyl) esters have been shown to be vinyl dication equivalents 3, and as such are precursors to terminal olefins and deuterated olefins, 1,1-dlsubstituted olefins and tri- and tetrasubstituted olefins. ... 

The a-(diphenylmethylsilyl) esters have been shown to be vinyl dication equivalents 3, and as such are precursors to terminal olefins and deuterated olefins, 1,1-disubstituted olefins and tri- and tetrasubstituted olefins. They are precursors to B-ketosilanes and ketones, wherein the overall transformation results in an ester to ketone conversion. They can also be deprotonated and the enolate anion condensed with aldehydes and ketones to give a,6-unsaturated esters, in particular a-alkylated-a,B-unsaturated esters. Their y-lactone counterparts, o-(diphenylmethy1snyl )-y-butyrolactone 4a and a-(diphenylmethylsilyl)-y-valerolactone 4b, are precursors to 4-oxo acids,1,4-diketones and a-ylidene-y-lactones. ... 

Ph3Si ethylene oxide catalytically adds PhSH to give a- and -adducts in 6 1 ratio, and serves as a vinyl dication equivalent with RCu to give alkenes stereoselectively, while epoxides rearrange in the presence of silylated nucleophiles Diphenylmethylsilylation of cyclopropane carboxylates gives the C-silylated ester which then form C-silylated cyclopropyl ketones, P-silyl cyclopropyl carbinols, and alkylidene cyclopropancs, which ring expand to the... 

Ethyl trimethysilylacetate is converted into 1,1-disubstituted olefins in high yield ( 80%) by the addition of two equivalents of Grignard reagent, followed by treatment of the resulting (3-hydroxysilanes with acid thus (43) can be considered as a vinyl dication synthon (Scheme 12). ... 

Interaction of simple acyclic dications with acetylenes proceeds anomalously. Instead of unsaturated disulfonium salts, the reaction leads to the corresponding vinyltriflates 96.107 It was shown earlier108 that interaction of trifluoromethylsulfonyloxysulfonium triflate (the dimethylsulfide ditriflate complex) with certain acetylenes leads to analogous vinyl triflates. The drastic differences between the reactivity of cyclic and acyclic dications toward acetylenes provide another evidence for a stepwise mechanism of this process (Scheme 37). 

Olefins with electron-donating substituents as the aUcoxy, acylamino, phenyl, or vinyl group can be coupled in methanol to give 1,4-dimethoxy dimers and/or dienes (Scheme 2). The first intermediate in this coupling reaction is a radical cation, which either by electrophilic addition to the olefin and subsequent le-oxidation (path A) [49] or by radical dimerization (path B) [50, 51] leads to a dimer dication that undergoes methanolysis or deprotonation. Representative examples of this coupling reaction are summarized in Table 7. 

In the 40 years since Olah s original publications, an impressive body of work has appeared studying carbocations under what are frequently termed stable ion conditions. Problems such as local overheating and polymerization that were encountered in some of the initial studies were eliminated by improvements introduced by Ahlberg and Ek and Saunders et al. In addition to the solution-phase studies in superacids, Myhre and Yannoni have been able to obtain NMR spectra of carbocations at very low temperatures (down to 5 K) in solid-state matrices of antimony pentafluoride. Sunko et al. employed a similar matrix deposition technique to obtain low-temperature IR spectra. It is probably fair to say that nowadays most common carbocations that one could imagine have been studied. The structures shown below are a hmited set of examples. Included are aromatically stabilized cations, vinyl cations, acylium ions, halonium ions, and dications. There is even a recent report of the very unstable phenyl cation (CellJ)... 

If a carbocation or a dication at the same time is also a Hiickeloid An + 2)jt aromatic system, resonance can result in substantial stabilization. The simplest 2jt aromatic system is the Breslow s cyclopropenium ion 206.434 439 Recently, electronic and infrared spectra of the parent ion cyclo-C3H3+ (206, R = H) in neon matrices440 and the X-ray characterization of the tris(trimethylsilyl) derivative were reported.441 The destabilizing effect of the silyl groups was found to be significantly smaller than in vinyl cations. The ion was computed to be more stable than the parent cyclopropenium ion by 31.4 kcal mol1 [MP3(fc)/6-31 lG //6-31G +ZPVE level]. The alkynylcy-clopropenylium ions 207 have been reported recently.442... 

Ammonium-carbenium dications and related species are also generated readily from olefinic precursors.77 For example, the tetrahydropyridine (226) leads to the 1,3-dication (227) and vinyl-substituted /V-heteroaromatics can give dications (i.e., 228) in superacid, both of which show high electrophilic reactivities (eqs 70-71). 

The other possibility is the nucleophilic substitution or addition to the radical cation or the dication (path b) in the presence of external or internal nucleophiles. Thus, electrochemical oxidation of morpholino cycloalkenes in methanol/methoxide leads to the formation of a compound with an allyl or vinyl methoxy substituent [Eq. (26)] [152]. 

The mechanism of this reaction is not known. It is interesting that in some cases, the reaction takes place without the template ion. A hydrogen bonding network may act as the template for this latter reaction and may also serve to stabilize the macrocycle once it is formed. The hydrogen bromide salt of ethylenediamine reacted with acetone, methyl vinyl ketone, or 4-methyl-3-penten-2-one to form a macrocyclic dication (Hay et al., 1975 Kolinski and Korybut-Daszkiewicz, 1969 Sadasivan and Endicott, 1966 Tait and Busch, 1978a). In this case, the trani-ligand was isolated in a yield of 80% or greater... 

The radical cations and radical anions may homodimerize to give dications and dianions or heterodimerize to give diradicals. Homodimerization presumably occurs in the copolymerization of vinyl ethers, CH2=CHOR, with vinylidene cyanide, CH2=C(CN)2. On mixing these two monomers, a mixture of poly(vinyl ether) and poly(vinylidene cyanide) is produced, which can be explained in terms of the formation of dications and dianions, which then polymerize the respective monomers ... 

Whether macrodications actually exist is still disputed. In the polymerization of iV-vinyl carbazole induced by a one-electron transfer [see (18-14)], it is possible that diradicals, and not dications, are formed from the radical ions primarily formed ... 

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