Benzhydryl cation reactivities

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... 

Compared with chloride, the bromide ion is more reactive by rather less than a factor of 10 with respect to the benzhydryl cations and by less than a factor of 2 for the trityl cation. Fluoride and acetate ions are a little less reactive than chloride ion toward the trityl cation (less than a factor of 10) but their equilibrium affinities for the ion are more than 106-fold greater.19,219... 

Just as N for a nucleophile can be determined from a plot of log k against E for a series of electrophiles, in principle, the value of E for an electrophile can be determined from the intercept (at E + N = 0) of a plot of log (k/s) versus N for a series of nucleophiles (or indeed, if need be, from the measurement for a single nucleophile). In this way E values have been determined for many electrophiles other than benzhydryl cations, including metal-coordinated cations,186 BF3-coordinated aldehydes,274 tropylium ions, and many benzylic-and heteroatom-substituted carbocations. In the low reactivity range... 

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... 

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... 

Dinitrobenzofuroxan (DNBF) is known as a superelectrophile due to its high reactivity both as an electrophile and in its pericyclic addition reactions. NMR studies show that reaction with 2-aminothiazole and its 4-methyl derivative yield anionic carbon-bonded adducts such as (11) by reaction at the 5-position, whereas the 4,5-dimethyl derivative reacts via the exocyclic amino group. Kinetic studies of the first two compounds, both in acetonitrile and in 70 30 (v/v) water-DMSO, have been used to assess their carbon nucleophilicities and place them on the Mayr nucleophilicity scale.55 In a related study, the nucleophilic reactivity, in acetonitrile, of a series of indoles with both DNBF and with benzhydryl cations have been compared and used to determine nucleophilicity parameters for the indoles.56... 

Allyl halides heterolyze just as easily as benzyl halides because they also produce a resonance-stabilized carbenium ion. Even faster heterolyses are possible when the charge of the resulting carbenium ion can be delocalized by more than one unsaturated substituent and can thereby be stabilized especially well. This explains the remarkably high SN1 reactivities of the benzhydryl halides (via the benzhydryl cation) and especially of the triphenylmethyl halides (via the trityl cation) ... 

Ruasse (1993) pointed out that substituent effects (Mindl and Vecera, 1971, 1972 Mindl, 1972) on the equilibria for forming benzhydryl cations, by analogy with those obtained in the solvolyses of chlorides (Nishida, 1967), can be analysed in terms of the selectivity-reactivity relationship (3). The values of the selectivity coefficient S = 0.34 (thermodynamic) and S = 0.52 (for the solvolysis), show elegantly that the transition-state shift is significant in this process. 

The generally observed identity of the r value for solvolysis reactivity and gas-phase stability AAG(c+> of the corresponding carbocation leads to an important prediction concerning the solvolysis transition state. In a typical (limiting) two-step SnI mechanism with a single dominant transition state, the r values of transition states for the various nucleophile-cation reactions should be essentially controlled by the intrinsic resonance demand of the intermediate cation the substituent effect should be described by a single scale of substituent constants (a) with an r value characteristic of this cation. In a recent laser flash-photolysis study (Das, 1993) on the recombination of stable trityl and benzhydryl cations with nucleophiles and solvents, McClelland et al. (1986, 1989) have treated the substituent effects on solvent-recombination processes by (2). 

The dissociation constant KD, which is often derived from spectral properties in carbanion chemistry, therefore includes a covalent term that corresponds to Kx in carbocationic chemistry. As one would not expect equal reactivity of benzhydryl chloride and benzhydryl cations, one also should not expect equal reactivity for benzhydryl lithium and benzhydryl anions. As one realizes that the terms contact ion-pair and dissociation have a different meaning in carbocation and carbanion chemistry, the apparent discrepancies quoted above, will disappear. 

Figure 6 Correlation of the reactivities of benzhydryl cations (parasubstituents in graph) toward methylenecyclopentane at 20° C with the corresponding reactivities toward H2O (kn2o in FLO/acetonitrile = 2/1 sec-1) log k2 = 1.397 log h3o -4.55. (Reprinted with permission from Ref. 136. Copyright 1991 American Chemical Society.)...

The same line of arguments rationalizes why the reactivity ratio of two electrophiles is growing with increasing difference of their standard ionization free enthalpies. Under conditions of almost complete ionization, the p-phenoxy-substituted benzhydryl cation is 5400 times more reactive than the bis(p-methoxy)-substituted analog, whereas in presence of catalytic amounts of Lewis acid, the relative reactivity of the two compounds is 0.016 (Fig. 18). 

Figure 21 Relationship between the basicity of arenes and their reactivity IL mol-1 sec" ] to benzhydryl cations (CH2CI2, -70° C). (From Ref. 215.)...

More reliable estimates of monomer reactivity are available from reactions of model compounds (Chapter 2). For example, the rate constants of addition of the same standard benzhydryl carbenium ion to various substituted styrenes correlate very well to Hammett s + = 4.9 [193]. Addition of various p-substituted benzhydryl cations to the same standard alkene yielded p(Acr = — 5.1 [193]. These results demonstrate that carbocationic polymerizations are extremely sensitive to even small changes in the monomer structure. They also demonstrate that the reactivity of carbenium ions scales nearly perfectly to the... 

Model studies discussed in previous chapters show that the reactivity of cations and alkenes are very strongly affected by inductive and resonance effects in the substituents. Correlation of the rate constants of addition of benzhydryl cation to various styrenes with Hammett substituted benzhydryl cations to a standard alkene (2-methyl-2-pentene) gave also good correlation and p+ = 5.1 [28]. The large p value signals difficult copolymerizations between alkenes, even of similar structures. Thus, in contrast to radical copolymerization which easily provides random copolymers, cationic systems have a tendency to form either mixtures of two homopolymers or block copolymer (if the cross-over reaction is possible). 

Reactions of 2-trialkylsilyl- and trialkylstannyl-substituted furans with benzhydryl cations provided 2,5-disubstituted furans and ipso-substituted furans. Kinetic investigations of the reactions revealed that the monosubstituted product was produced from the protonolysis of the 2,5-disubstituted furylstannane, while the 2,5-disubstituted furan was derived from an electrophilic substitution of the mono-substituted furan. Introduction of a trialkylsilyl and a trialkylstannyl group to the 2-position of furan hardly affected the reactivity of this position towards carbenium ions ipso attack), while the 5-position is somewhat activated <01OL1629,01OL1633>. 

A,A/-dialkylhydrazones, can act as C- or A/-nucleophiles. Their reactivities have been measured by reaction with a range of benzhydryl cations, Ar2CH+, as reference electrophiles with known E values. Kinetic reaction of the carbocations at the (terminal)... 

A series of azolium enolates (99 Ar = phenyl, mesityl) have been synthesized and characterized. Their ambident reactivities have been measured by studying their reactions with benzhydryl cations, Ar2CH , in r/ -acetonitrile, using known electrophilicity parameters for the latter. NMR shows predominantly 0-attack initially, with a switch to C-product over 1-2 days, with second-order rate constants for the two processes calculable. The azolium enolate reactivities have been compared with those of the corresponding free carbenes, and deoxy-Breslow intermediates. 

These benzhydrilium carbocations were also used to develop a comprehensive scale of hydride donor strength. Both Si-H and C-H donors were evaluated by their second-order rate constants of hydride transfers to the carbocation electrophiles. The respective nucleophilicity parameter, N and %, were then determined. The benzhydryl cation series was also used to evaluate the nucleophilic reactivities of azolium enolates (1). ... 

It is not intended to extend this discussion of reactions of carbocations with water to consideration of the alcoholic solvents trifluoroethanol (TFE) and hexa-fluoroisopropanol (HFIP), which have been extensively studied and reviewed by McClelland and Steenken.3 However, an important point of interest of these solvents is that their reactivities toward carbocations are greatly reduced compared with water (by up to a factor of 104 in TFE and 108 in HFIP) and that differences in rate constants can be observed between cations which would react indiscriminately at the solvent relaxation limit in water. The following comparisons of rate constants for carbocations with similar pAR values reacting with hexafluoroiso-propanol241,242 reinforces the conclusion that structural variations in the cation lead to changes in intrinsic barrier and, for example, that phenyl substitution is probably associated with such an increase in going from benzyl to benzhydryl (although the benzyl cation itself is not shown). 

The O-alkylation of carboxylates is a useful alternative to the acid-catalyzed esterification of carboxylic acids with alcohols. Carboxylates are weak, hard nucleophiles which are alkylated quickly by carbocations and by highly reactive, carbocation-like electrophiles (e.g. trityl or some benzhydryl halides). Suitable procedures include treatment of carboxylic acids with alcohols under the conditions of the Mitsunobu reaction [122], or with diazoalkanes. With soft electrophiles, such as alkyl iodides, alkylation of carboxylic acid salts proceeds more slowly, but in polar aprotic solvents, such as DMF, or with non-coordinating cations acceptable rates can still be achieved. Alkylating agents with a high tendency to O-alkylate carboxylates include a-halo ketones [42], dimethyl sulfate [100,123], and benzyl halides (Scheme 6.31). 

The reactivity sequence shown above corresponds well to Mayr s [18] model reactions of the electrophilic addition of benzhydryl carbenium ions to substituted alkenes. Table 2 lists the second-order rate constants for the addition of a diarylcarbenium ion to various alkenes and dienes [36]. One alkyl group offers little activation of the double bond a-olefins therefore form only oligomers with isomerized repeat units in low conversions under cationic polymerization conditions. One vinyl group activates the double bond slightly more than alkyl groups do. Table 2 also demon-... 

If concentrations of carbenium ions are too low to be observed directly, they must be detected indirectly in kinetic studies of the racemiza-tion of optically active dormant species, ligand exchange and/or detailed studies of the effect of substituents, solvent and salts. Some of the most convincing and elegant work in this area was presented in Chapter 2 using primarily benzhydryl derivatives. As discussed in the next section, correlations between ionization rates and equilibrium constants, rates of solvolysis and rate constants of electrophilic addition can be interpolated and in some cases extrapolated to cationic polymerizations of alkenes to evaluate the reactivities of various active species and the dynamics of their isomerization. 

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