Tropylium cation, from 1,3,5-cycloheptatriene

The tropylium cation (274) first observed 1891 and rediscovered in 1957 is perfectly stable and isolable. Cyclopropenyl cations have been observed in solution a long time ago, but 273 remained elusive until very recently. Benzocyclo-propene (1) reacts with triphenylfluoroborate via hydride transfer some 5 times less rapidly than cycloheptatriene. The reaction of deuterated 1 exhibits a kinetic isotope effect of 7.0. However, only a low yield of benzaldehyde (277), the expected hydrolysis product of 273, could be isolated from the reaction mixture. ... 

The tropylium cation is prepared easily by transfer of a hydride ion from cycloheptatriene to triphenylmethyl cation in sulfur dioxide solution. This reaction is related to the hydride ion transfer, (CH3)3C + RH —> (CH3)3CH + R , discussed in Section 10-9 ... 

The highly crowded tris( 1 -naphthyl)methyl cation 150 and tris(2-naphthyl)methyl cation 151 were prepared and used to abstract hydride ion from cycloheptatriene to generate tropylium ion.363 Hydride abstraction, however, could be performed only with the less crowded cation 150. 

Phenyl-1,3-dithiolium or 1,3-benzodithiolium cations abstract hydride from cycloheptatriene, giving the thermodynamically more stable tropylium salt and 4-phenyl-l,3-dithiole or 1,3-benzodithiole. ... 

In 1945 Michael J. S. Dewar suggested that the tropylium ion (the cation derived from cycloheptatriene) should also be aromatic (Figure 9). This was confirmed in 1954 since then, the dianion of butadiene and the dication of cyclooctatetraene have also been shown to be aromatic. Like benzene, all four of these ions are planar rings with six tt electrons. According to Hiickel s rule the cyclopropene cation should also exhibit aromaticity, and it does. (In this case n = 0, and 4n + 2 = 2.) The planar anion of cyclononatetraene and the dianion of cyclooctatetraene should also be aromatic (n = 2, and 4n + 2 = 10), and both of them are. 

Electrochemical studies suggest that the pX of cycloheptatriene is about 36 [385-387]. Two waves are observable, reflecting successive reduction to the radical and anion. Both before and after the second wave the major product is bis(cycloheptatrienyl) and it is suggested that after the second wave this arises from rapid electron transfer from a cycloheptatrienide anion to a tropylium cation, followed by combination of the resultant radicals [386,388]. 

Cycloheptatriene does not react with Fe(CO)s to give the expected dicarbonyl complex 7i -C7H8Fe(CO)2, but forms instead a mixture of tricarbonyl complexes derived from cycloheptatriene and cyclohepta-1,3-diene (20, 35). This work, together with protonation and related reactions of the free double bond of 7r-C7HsFe(CO)3, have been summarized by Pettit and Emerson (118). More recently, Pettit and co-workers (101) have shown that (7-methoxycycloheptatriene)Fe(CO)3 is protonated by fluoro-boric acid with loss of methanol to give the ir-tropylium-iron tricarbonyl cation. 

When cycloheptatriene is treated with a reagent that can abstract a hydride ion, it is converted to the cycloheptatrienyl (or tropylium) cation. The loss of a hydride ion from cycloheptatriene occurs with unexpected ease, and the cycloheptatrienyl cation is found to be unusually stable. The NMR spectrum of the cycloheptatrienyl cation indicates that all seven hydrogen atoms are equivalent. If we look closely at Fig. 14.12, we see how we can account for these observations. 

Cycloheptatriene forms an aromatic cation by conversion of its CH2 group to a CH+ group with this sp hybridized carbon having a vacant 2p atomic orbital. The cycloheptatrienyl (tropylium) cation is planar and has six tt electrons in seven 2p orbitals, one from each atom of the ring. It can be drawn as a resonance hybrid of seven equivalent contributing structures (Figure 21.13). 

We have now discussed the lack of aromaticity in An molecules and we have seen a few examples of aromaticity (benzene and some of the molecules in Problem 13.7) it would seem to be time to look at some other potentially aromatic 4m + 2 molecules. It is in the stability of certain ions that the most spectacular examples have appeared. As we have stressed over and over, small carbocations are most unstable. But there are a few exceptions to this generality. One of them is the cyclohep-tatrienylium ion, or tropylium ion (CyHy ), the ion derived from the loss of hydride (H ) from 1,3,5-cycloheptatriene, also called tropilidene (Fig. 13.25). Hydride cannot be simply lost from cycloheptatriene, but it can be transferred to the trityl cation, a carbocation attached to three benzenes, which is itself a quite stable carbocation. 

If triphenylmethyl chloride in ether is treated with sodium, a yellow colour is produced due to the presence of the anionic spiecies PhsC". Alternatively, if triphenylmethyl chloride is treated with silver perchlorate in a solvent such as THF, the triphenylmethyl cation is obtained. More conveniently, triphenylmethyl salts, PhsC X", can be obtained as orange-red crystalline solids from the action of the appropriate strong acid on triphenylcarbinol in ethanoic or propanoic anhydride solution. The perchlorate, fluoroborate and hexafluoro-phosphate salts are most commonly used for hydride ion abstraction from organic compounds (e.g. cycloheptatriene gives tropylium salts). The salts are rather easily hydrolysed to triphenylcarbinol. 

In contrast, it is the resonance-stabili/ed cation derived from cyclo-hep tatriene that possesses the aromatic sextet of Jt-electrons. Tropylium bromide is formed by the addition of bromine to cycloheptatriene and... 

The cation (26) was obtained by protonation of the corresponding azulene in dichloromethane. A tropylium ion-mediated a-cyanation of amines was described. The key step is a hydride transfer from the amine to the cation, resulting in cycloheptatriene and an iminium ion, the latter then reacting with cyanide to give the aminonitrile. The dehydrofropylium-Co2(CO)6 ion has been prepared as a BF4 salt. Various measures suggest that the ion is weakly aromatic, with about 25% of the aromaticity of the tropylium ion. Computational analysis of a number of annulenes predicts that the Mobius dication (CH)i4+ should be stable under persistent ion conditions. In particular, this dication is stable towards reactions such as cis-trans isomerization and electrocyclic rearrangement that limit the lifetime of other Mobius annulenes. 

FIGURE 13.25 The tropylium ion (CyHy ) can be made through transfer of hydride (H ) from 1,3,5-cycloheptatriene to the trityl cation. 

Note first that the tropylium ion has one important difference from its parent, 1,3,5-cycloheptatriene. The ion is fully conjugated (Fig. 13.26). The removal of a hydride from the cyclic triene generates a 2p orbital at the 7-position, which once insulated the two ends of the n system from each other. In the cation they are now connected and this molecule is fully conjugated. 

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