Benzene trityl cation from

In 1995, Maciel and co-workers (118) synthesized the trityl cation in the supercages of zeolite HY by a clever application of Friedel-Crafts chemistry—13CC14 was reacted with an excess of benzene (Fig. 15). Maciel and co-workers carried out a number of spectroscopic and chemical manipulations that unambiguously demonstrated that the product was the trityl cation and that the cation was in the zeolite. Ab initio calculations at various levels of theory predict that the point group of isolated 16 is D3 rather than Dih. It is interesting to speculate about the extent to which the zeolite environment might force the degree of twist away from the gas-phase equilibrium value. 

Fig. 15. Ship-in-a-bottle synthesis of the trityl cation 16 from 13CCU and benzene inside the HY supercage. (Reprinted with permission from Tao and Maciel (118). Copyright 1996 American Chemical Society.)...

X = TPFPB) in benzene solution fail because the bulky mesityl groups prevent a trityl cation to come close enough to abstract a hydride ion from the silane Mes3SiH. 

There is some spectroscopic evidence that aromatic compounds complex carbenium ions [42]. For example, the complexation equilibrium constant between trityl ions and hexamethylbenzene is K = 68 mol-1 L at 0° C [43]. Complexation should be stronger with more electrophilic carbenium ions such as those derived from styrene and a-methylstyrene. On the other hand, the monoalkyl-substituted phenyl rings attached to the polymer chain are weaker nucleophiles than hexamethylbenzene. A complexation constant K = 4 mol 1 L was reported for trityl cation and styrene [43]. Similar complexes have been proposed to explain the red color observed in inifer systems based on l,4-bis(I-chIoro-l-methyl-ethyl)benzene and BCI3 in CH2C12 at low temperature [44],... 

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. 

In practice, extrapolations of p fR in water have usually used the older acidity function based method, for example, for trityl,61,62 benzhydryl,63 or cyclopropenyl (6) cations.66,67 These older data include studies of protonation of aromatic molecules, such as pKSi = —1.70 for the azulenium ion 3,59 and Kresge s extensive measurements of the protonation of hydroxy- and methoxy-substituted benzenes.68 Some of these data have been replotted as p fR or pKa against XQ with only minor changes in values.25,52 However, for more unstable carbocations such as 2,4,6-trimethylbenzyl, there is a long extrapolation from concentrated acid solutions to water and the discrepancy is greater use of an acidity function in this case gives pA 2° = —17.5,61 compared with —16.3 (and m = 1.8) based on X0. Indeed because of limitations to the acidity of concentrated solutions of perchloric or sulfuric acid pICs of more weakly nucleophilic carbocations are not accessible from equilibrium measurements in these media. 

Sommer and Bauman investigated the reaction of optically active a-naphthylphenylmethylsilyl hydride with trityl chloride and found that the stereochemical course was retention in benzene solution, inversion in methylene chloride, and full racemization in methylene chloride (93). The result in CH2C12 was explained as consistent with an exchange in which electrophilic attack on the silicon hydride by a triphenyl cation is of primary importance however, the counterion appears in the transition state [Eq. (16)]. Racemization of the unreacted R3SiH could arise from... 

Cations 1, with different substituents X, can be synthesized by hydride transfer reactions [3] from 2,5-disilaheptanes 2. The reaction of 2 with one equivalent trityl tetrakis(pentafluorophenyl) borate (TPFPB) in benzene at room temperature yields the trivalent silylium ions 3 and triphenylmethane (Scheme 1). The silylium ions 3 are only transient species and undergo intramolecular cyclization which yield the cations 1. 

---