Carbocations trityl cations

Similarly, trityl cation in aromatic hydrocarbons initiates the fragmentation of simple tetraalkyl plumbanes and stannanes yielding the plumbyl or stannyl cationic species, e.g. 11, and alkenes. The reaction is thought to proceed via plumbyl-or stannyl-substituted carbocations 12, which in a second step eliminate the al-kene. This approach was used in the synthesis of norbornyl cations of the elements tin and lead, e.g. 13, (Scheme 5). ... 

One of the most stable carbocation structures is the employing all three rings. Trityl chloride ionizes read-triphenylmethyl cation (trityl cation). In this struc- ily, and can capture an available nucleophile, ture, the positive charge is stabilized by resonance... 

Arnett and Hofelich measured heats of reaction of a variety of alcohols with SbF5/FS03H in sulfuryl chloride fluoride to form their respective carbocations at constant temperature (-40 °C). In this superacid medium there were no ion-pair complications126 and hence reliable calorimetric data were obtained for various cyclopropyl and phenyl substituted cations. The heats of reaction for the formation of tricyclopropylcarbinyl cation (-59.2 kcalmol ), trityl cation (-49.0 kcalmol1) and ferr-butyl cation (-35.5 kcalmol1) show that the relative order of the stabilization of the cationic center is cyclopropyl >... 

Choride ion is considerably less reactive than the azide ion. Thus, although values of kc 1/ kn2o have been quite widely available from mass law effects of chloride ion on the solvolysis of aralkyl halides, normally the reaction of the chloride ion cannot be assumed to be diffusion controlled and the value of kn2o cannot be inferred, except for relatively unstable carbocations (p. 72). Mayr and coworkers251 have measured rate constants for reaction of chloride ion with benzhydryl cations in 80% aqueous acetonitrile and their values of logk are plotted together with a value for the trityl cation19 in Fig. 7. There is some scatter in the points, possibly because of some steric hindrance to reaction of the trityl cations. However, it can be seen that chloride ion is more... 

The interpretation of reactivities here provides a particular challenge, because differences in solvation and bond energies contribute differently to reaction rates and equilibria. Analysis in terms of the Marcus equation, in which effects on reactivity arising from changes in intrinsic barrier and equilibrium constant can be separated, is an undoubted advantage. Only rather recently, however, have equilibrium constants, essential to a Marcus analysis, become available for reactions of halide ions with relatively stable carbocations, such as the trityl cation, the bis-trifluoromethyl quinone methide (49), and the rather less stable benzhydryl cations.19,219... 

It seems clear that for reactions of carbocations with nucleophiles or bases in which the structure of the carbocation is varied, we can expect compensating changes in intrinsic barrier and thermodynamic driving force to lead to relationships between rate and equilibrium constants which have the form of extended linear plots of log k against log K. However, this will be strictly true only for structurally homogeneous groups of cations. There is ample evidence that for wider structural variations, for example, between benzyl, benzhydryl, and trityl cations, there are variations in intrinsic barrier particularly reflecting steric effects which lead to dispersion between families of cations. 

Because systematic variations in selectivity with reactivity are commonly quite mild for reactions of carbocations with n-nucleophiles, and practically absent for 71-nucleophiles or hydride donors, many nucleophiles can be characterized by constant N and s values. These are valuable in correlating and predicting reactivities toward benzhydryl cations, a wide structural variety of other electrophiles and, to a good approximation, substrates reacting by an Sn2 mechanism. There are certainly failures in extending these relationships to too wide a variation of carbocation and nucleophile structures, but there is a sufficient framework of regular behavior for the influence of additional factors such as steric effects to be rationally examined as deviations from the norm. Thus comparisons between benzhydryl and trityl cations reveal quite different steric effects for reactions with hydroxylic solvents and alkenes, or even with different halide ions... 

The first stable, long-lived carbocation observed was the triphenylmethyl (trityl) cation 135.6 y... 

In a study involving the superacid-catalyzed reaction of amino-alcohols, a chiral, dicationic electrophile was observed by low temperature 13C NMR.31 Ionization of benzylic alcohols in superacids can generate stable carbocations, such as the trityl cation. Because of the resonance stabilization of the carbocationic centers, they are fairly weak electrophiles, incapable of reacting with benzene (eq 31). However, it was shown that adjacent ammonium groups can increase the electrophilic reactivities of the diphenylethyl cations (eq 32). 

By these methods, solutions of highly stabilized (e.g., trityl cations) as well as of relatively unstable carbocations (e.g., sec-alkyl cations) have been produced. Although the precision of the calorimetric measurements is smaller than that of most equilibrium determinations, it is an advantage of Arnett s approach that very different types of carbocations can be studied by the same method (Scheme 3). Error propagations, which may be introduced when a series of equilibrium constants or overlapping scales are connected, are thus eliminated. 

A master equation has been derived from both the pA R of trimethyl- and triphenylcyclo-propenylium ions, as well as of the trityl cation, and the pA of conjugate acids of various carbanions, so that the heat of reaction can be predicted for the covalent bond formation between carbocations and carbanions. ... 

The NO ion is advantageously used to prepare a variety of stabilized carbocations such as tropylium, benzhydryl, and trityl cations. The hydride abstracting abiUty of the NO ion has been also employed to carry out such varied organic transformations as ionic fluorination (Scheme 6.21, route a), the Ritter reaction (Scheme 6.21, route Z ), and others. 

Although the ESCA spectrum of the f-butyl carbocation shows C Is levels for both the carbenium carbon atom and the three methyl carbon atoms, the ESCA spectrum of the trityl cation (PhsC ) suggests that all of the carbon Is electrons have the same binding energy. Explain this result. 

Isodesmic hydride exchanges (48) show that the primary carbocations MeHgCHjCHf and Me3SnCH2CHf are kinetically more stable than trityl cation (49). These species can be considered as three-centered bound ethylene metal ions (50) in which a considerable amount of the positive charge is carried by the metal. Only the relatively soft metals can be expected to interact so strongly with the moderately soft carbenium center. 

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. 

Among the new carbocations generated, the first example was reported for a triazolyl-based, triaryl methylcation (41). This heterocyclic analogue of the trityl cation was prepared by ionization of the tris(3-benzyl-triazolyl-5-yl)methanol with trifluoroacetic anhydride. Cation (41) was trapped with varied carbon, nitrogen, oxygen, and sulfur nucleophiles. 

An organocatalytic method has been developed which utilizes the Lewis acidic character of the trityl cation. For example, -naphthol condenses with benzaldehyde and acetamide to give the l-amidoalkyl-2-naphthol product (215). The synthesis is done under solvent-free conditions at 70 °C. The trityl chloride ionizes in the mixture to form the carbocation and this coordinates to the benzaldehyde to initiate the conversion. 

An essential characteristic of ferrocene chemistry is the stabilization of ferrocenyl carbonium ions. These carbocations are mesomers of the corresponding hexahapto fulvene complexes [FeCp(r -fulvene)]+. They are even more stable than the trityl cation PhsC". The stabilization of the a-ferrocenyl carbonium ions explains the acetolysis of vinylferrocene, the hydrolysis of the acetate formed, the ease of nucleophilic substitution in a position, and the OH" abstraction from the a-ferrocenyl alcohols. This stabilization is still enhanced by going down in the iron column of the periodic table, because the size of the d orbital increases, which facilitates their insertion with the carbocation and accelerates the solvolysis of acetates (Os > Ru > Fe). 

Ionization rate constants in aqueous acetonitrile were obtained for trityl chlorides, bromides, and acetates covering 21 units in the pA1r+ of the trityl cation. This study observed solvolyses with and without common ion return, solvolyses where the trityl cation could be observed to form and then decay, solvolyses where water addition occurs before complete formation of the cation, and at the other extreme, solvolyses that yield persistent carbocations. Mayr and coworkers showed how electrophilicity and nucleophilicity parameters rationalize reactivity patterns and resolve mechanistic controversies in organocatalytic cyclizations. The thermodynamic affinities of a large number of Lewis bases were computed for addition to the methyl, diphenylmethyl, and triphenyl-methyl cations. ... 

When using a cation source in conjunction with a Friedel-Crafts acid the concentration of growing centers is most often difficult to measure and remains unknown. By the use of stable carbocation salts (for instance trityl and tropyhum hexachloroantimonate) the uncertainty of the concentration of initiating cations is eliminated. Due to the highly reproducible rates, stable carbocation salts have been used in kinetic studies. Their use, however, is limited to cationicaHy fairly reactive monomers (eg, A/-vinylcarbazole, -methoxystyrene, alkyl vinyl ethers) since they are too stable and therefore ineffective initiators of less reactive monomers, such as isobutylene, styrene, and dienes. 

The initiation of the cationic polymerisation of alkenes is examined in detail by means of simple thermodynamic concepts. From a consideration of the kinetic requirements it is shown that the ideal initiator will yield a stable, singly charged anion and a cation with a high reactivity towards the monomer by simple, well defined reactions. It must also be adequately soluble in the solvent of choice and for the experimental method to be used. The calculations are applied to carbocation salts as initiators and a method of predicting their relative solubilities is described. From established and predicted data for a variety of carbocation salts the position of their ion molecule equilibria and their reactivity towards alkenes are examined by means of Born-Haber cycles. This treatment established the relative stabilities of a number of anions and the reason for dityl, but not trityl salts initiating the polymerisation of isobutene. 

Preformed or in situ-prepared carbocation salts (tropylium, trityl, etc.) are also active in transforming alkenes to carbocations.119,138,140 Preformed carbocation salts are the simplest initiators in cationic polymerization and ideal if the cation is identical to the one derived from the momomer (e.g., fert-butyl cation in the polymerization of isobutylene). 

Advantage has been taken of the ready accessibility of eleven para-substituted trityl and 9-phenylxanthyl cations, radicals, and carbanions in a study of the quantitative relationship between their stabilities under similar conditions.2 Hammett-type correlations have also been demonstrated for each series. Heats and free energies of deprotonation and the first and second oxidation potentials of the resulting carbanions were compared. The first and second reduction potentials and the p/CR values of the cations in aqueous sulfuric acid were compared, as were calorimetric heats of hydride transfer from cyanoborohydride ion. For radicals, consistent results were obtained for bond dissociation energies derived, alternatively, from the carbocation and its reduction potential or from the carbanion and its oxidation potential. 

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. 

---