1,3,5-Cycloheptatriene hydride transfer

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. ... 

In addition to their reactions with alkenes and carbanions as nucleophiles benzhydryl cations react with hydride donors.282 284 These hydride transfer reactions show the same linear dependence of log k upon E as the reactions with alkenes and the same constant relative selectivity, that is with slopes of plots close to 1.0, for structures ranging from cycloheptatriene to the... 

Cycloheptatriene and derivatives thereof donate hydride readily to a variety of carbonium ion acceptors. The position of the end equilibrium depends on the thermodynamics of the exchange. " These reactions are prototypes of a broad area of carbonium ion chemistry wherein carbonium ions equilibrate via intra- and inter-molecular hydride shifts between a donor C—H bond, usually jp hybridized, and a carbonium ion acceptor. This chemistry is often achieved with heterogeneous catalysts and is of great industrial significance it lies outside the emphasis of this review, however. Excellent treatises are available, and a review has appeared on the use of carriers like adamantane to promote hydride transfer in hydrocarbons under strongly acidic conditions. ... 

Carbonium ions are Lewis acids, but most are too reactive to permit study of their effectiveness as catalysts for olefin isomerizations. An exception is tro pylium ion. The kinetics of tropylium perchlorate-catalyzed isomerization of 7-methyl-l,3,5-cycloheptatriene to a mixture of 1-, 2-, and 3-methyl-1,3,5-cycloheptatrienes in acetonitrile solutions at 25°C were studied. Isomerization rate was proportional to 7-methyltropylidene concentration and tropylium-ion concentration. The reaction probably involves reversible hydride transfer tropylium ion and the methyltropylidenes. 

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. 

Hydride transfer from cycloheptatriene to the trityi cation ( CPhs)... 

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 ... 

Initiation with Tropylium Ion. Tropylium hexachlorantimonate reacts with vinyl alkyl ethers in a manner very similar to the reactions of triphenylmethyl salts. Again, rapid initiation is followed by propagation without apparent termination. Termination can be demonstrated to be absent from experiments in which fresh samples of monomer are added to completed polymerizations, whereupon the measured reaction rates parallel those previously recorded (Table II). Molecular weights of the polymers from isobutyl vinyl ether are very similar to those obtained with triphenylmethyl salts as initiators and again give clear evidence for excessive monomer transfer. Gas chromatographic analysis of the reaction mixtures showed that cycloheptatriene (product of hydride abstraction) was not present which indicates clearly that initiation must arise via addition of the tropylium ion to the vinyl ether—i.e.,... 

Two-step hydride shifts to perchlorocarbenium ions. The expected hydride-ion transfer (150) between cycloheptatriene and PDM occurs rapidly at room temperature, a//-undecachlorodiphenylmethane (PDM—H) and tropylium hexachloroantimonate being isolated (Ballester et al., 1971c, 1982a). 

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. 

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

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