Tropylium ions—

The 1,2-dithiolylium and 1,3-dithiolylium ions (115) and (116) are iso-rr-electronic with the tropylium ion, from which they may be formally derived by replacing double bonds by sulfur atoms. Various calculations and structural data demonstrate that the rings are stabilized by tt-electron delocalization. 

Measurement of (R /R ) can be accomplished by cyclic voltammetry for relatively Stable species and by other methods for less stable cations. The values obtained for AG -range from 83kcal/mol for the aromatic tropylium ion to 130kcal/mol for destabilized betizylic cations. For stable carbocations, the results obtained by this method correlate with cation stabiUty as measured by pKf.+. Some of these data are presented in Table 5.3. 

Substituted dibenzo[6,/]thiepins can be generated from thioxanthene derivatives by the rearrangement of carbocation 1. Compared with other possible cations, the tropylium ion type 1C is favored because of its resonance energy. Depending on the reaction conditions, the thiepin cation can react to give thiepins by loss of a proton, or by trapping a nucleophile, followed by elimination. 

Tropylium salts are starting materials for the preparation of a wide range of substituted tropilidenes. The fluoborate is the salt of choice for work involving the tropylium ion because it is indefinitely stable, non-hygroscopic, and, unlike the perchlorate, non-explosive. Its preparation by this method avoids the use of... 

The first carbocationic photolysis to be investigated in detail was that of the tropylium ion (1) (van Tamelen et al., 1968, 1971), in which generation of the [3,2,0] valence bond isomer (the Dewar tropylium ion ), (2) was the dominant reaction. When irradiated for 10 minutes in 5% aqueous sulfuric acid, two major products were formed in a total... 

When irradiated in stronger acid (FHSO3) at —60°, the tropylium ion is seen to isomerize cleanly to the norbomadien-7-yl cation (7) (Childs and Taguchi, 1970 Hogeveen and Gaasbeek, 1970). The ion 7 has been shown to be photochemically stable (Cabell and Hogeveen, 1972). However, it is known to revert to the tropylium ion thermally at temperatures above 45° (Lustgarten et al., 1967). 

The most reasonable mechanism for this transformation, in accord with that suggested by van Tamelen et al. for the dilute acid photolysis, is initial photoisomerization to 2 followed in this case by thermal conversion of the Dewar tropylium ion to 7. The isomerization of 2 to 7 has been reported to be very rapid at temperatures below —60°, and it has been shown, in addition that, in nucleophilic solvents, capture of 2 competes very efficiently with isomerization (Lustgarten et al., 1967). 

A valence isomerization reaction similar to that encountered with the tropylium ion has been observed when a variety of substituted benzenes are photolyzed in strong acid. The pentamethylbenzenium ion (18) photolyzed at —78° to give a single product, the pentamethylbicyclo-[3,l,0]hexenyl cation (19), in excess of 80% conversion (Childs et cd., 1968). 

The tropylium ion has an aromatic sextet spread over seven carbon atoms. An analogous ion, with the sextet spread over eight carbon atoms, is 1,3,5,7-tetra-methylcyclooctatetraene dication (47), which is stable in solution at -50°C, is diatropic and approximately planar and is not stable above about -30°C. ... 

Homoaromatic Compounds. When cyclooctatetraene is dissolved in concentrated H2SO4, a proton adds to one of the double bonds to form the homotropylium ion (107). In this species an aromatic sextet is spread over seven carbons, as in the tropylium ion. The eighth carbon is an sp carbon and so cannot take part in the aromaticity. The NMR spectra show the presence of a diatropic ring current H/, is found at 5= - 0.3 at 5.1 6 Hj and H7 at... 

As another example, the tropylium ion [3 ], which is stabilized by virtue of the 67t electrons spread over a heptagonal sp hybridized carbon framework [Hiickel s (4n 4- 2)v rule with = 1], is also unstable in the gas phase. Its formation from toluene or the benzyl cation has been a long-standing problem in organic mass spectrometry, and the reaction mechanism and energetics have recently been exhaustively discussed (Lif-shitz, 1994). It was, however, isolated as the bromide salt by Doering and Knox (1954, 1957), and was the first non-benzenoid aromatic carbocation. 

Besides the carbocations mentioned above, numerous highly stable carboca-tions have been isolated as the salts of inorganic anions. Azulene analogues of triphenylmethylium ion [ll ]-[20 ], [21 j, [22 " ] and [23 j trisubsti-tuted cyclopropenylium ions [l" ] and [24 ]-p6 ] cyclopropyl-substituted tropylium ions [27 ]-[30 ] tropylium ions annelated with bicyclic frameworks [31 ]-[36 ] and [37 ]-[39 ] fulvene-substituted cyclopropenylium [40 ] and tropylium [41 ] ions a tropylium ion condensed with acenaphthylene [42 ] and a cation containing a 147t periphery [43 ] have... 

The first observation of ionic dissociation of a carbon-carbon cr bond in a hydrocarbon was reported for 7-(2,4,6-cycloheptatrien-l-yl)-7-[3-(7//-dibenzo[c,g]fluoren-7-ylidene)-2-(7//-dibenzo[c,g]fluoren-7-ylidenemethyl)-l-propenyl]-7//-dibenzo[c,g]fluorene [3-2], The hydrocarbon [3-2] forms a green solution in DMSO, indicating the dissociation into green Kuhn s carbanion [2 j and colourless tropylium ion [3" ] as shown in (1) (Okamoto etal., 1985). 

Because of the dependence of the dissociation on the polarity of the solvent medium, in the less polar acetone solvent the dissolution of [3-2] does not give rise to the green colour of the Kuhn s carbanion [2 ] but simply the pale yellow colour of the hydrocarbon [3-2]. However, when pyrene, which forms a charge-transfer complex with the tropylium ion (Dauben and Wilson, 1968), is added to the acetone solution, it turns green, indicating that the dissociation is induced by pyrene and that the equilibrium is shifted to the ionic side (Okamoto et al., 1985). 

Following the first observation, eight substituted tropylium ions [94" ]-[98" ], [30 ], [34" ] and [35" ] (Scheme 1) were combined with Kuhn s carbanion [2 ] to make ionically dissociative hydrocarbons [94-2]-[98-2], [30-2], [34-2] and [35-2]. The structures of these hydrocarbons were determined as indicated in (23) on the basis of their and nmr spectra. Compounds [94-2], [95-2], [98-2] and [35-2] were mixtures of two positional isomers, in which [2 ] is connected to different positions of the seven-membered ring. Positions that formed a carbon-carbon bond with [2 ] are indicated by dots in Scheme 1. 

Four years after Doering s discovery of the hygroscopic bromide of the tropylium ion [3" ] (Doering and Knox, 1954), Vol pin et al. (1957) reported that the tropylium ion (pXr+ 3.88) gives a covalent compound with acetate ion (pXb 9.24) but not with isocyanate ion (pK 10.1). This suggests that an anion with pXb greater than 10.1 would give a salt with the tropylium ion. 

As the cation becomes progressively more reluctant to be reduced than [53 ], covalent bond formation is observed instead of electron transfer. Further stabilization of the cation causes formation of an ionic bond, i.e. salt formation. Thus, the course of the reaction is controlled by the electron affinity of the carbocation. However, the change from single-electron transfer to salt formation is not straightforward. As has been discussed in previous sections, steric effects are another important factor in controlling the formation of hydrocarbon salts. The significant difference in the reduction potential at which a covalent bond is switched to an ionic one -around -0.8 V for tropylium ion series and —1.6 V in the case of l-aryl-2,3-dicyclopropylcyclopropenylium ion series - may be attributed to steric factors. 

The power of this tropylium ion cycloaddition strategy for the synthesis of complex molecules can be seen in synthesis of dactylol 231 by Feldman and coworkers (Scheme 52)111. Irradiation of 229 (prepared from 4-methyltropone in two steps) afforded... 

The MS studies of carotenoids have been reviewed previously19-21. Most carotenes show a molecular ion. Some carotenes with cyclic end moieties fragment to yield the tropylium ion (mlz 91) and some yield the mlz 105 xylene fragment. Specific deuteriation... 

The spectrum of benzyl acetate is shown in Figure 9.56. The parent peak at m/z 150 is prominent (Rule 1) as is the tropylium ion peak at m/z 91 (Rule 2). The base peak at m/z 108 is due to a rearrangement (Rule 4) after cleavage of the acetyl group which itself gives a prominent peak at m/z 43... 

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