Diarylcarbenium ions

Ohal, A. R. Ohkubo, K. Fukuzumi, S. Lucius, R. Mayr, H. Role of electron-transfer processes in reactions of diarylcarbenium ions and related quinone methides with nucleophiles. J. Am. Chem. Soc. 2003, 125, 10960-10912. 

Alkenes are scavengers that are able to differentiate between carbenes (cycloaddition) and carbocations (electrophilic addition). The reactions of phenyl-carbene (117) with equimolar mixtures of methanol and alkenes afforded phenylcyclopropanes (120) and benzyl methyl ether (121) as the major products (Scheme 24).51 Electrophilic addition of the benzyl cation (118) to alkenes, leading to 122 and 123 by way of 119, was a minor route (ca. 6%). Isobutene and enol ethers gave similar results. The overall contribution of 118 must be more than 6% as (part of) the ether 121 also originates from 118. Alcohols and enol ethers react with diarylcarbenium ions at about the same rates (ca. 109 M-1 s-1), somewhat faster than alkenes (ca. 108 M-1 s-1).52 By extrapolation, diffusion-controlled rates and indiscriminate reactions are expected for the free (solvated) benzyl cation (118). In support of this notion, the product distributions in Scheme 24 only respond slightly to the nature of the n bond (alkene vs. enol ether). The formation of free benzyl cations from phenylcarbene and methanol is thus estimated to be in the range of 10-15%. However, the major route to the benzyl ether 121, whether by ion-pair collapse or by way of an ylide, cannot be identified. 

Competition experiments by Mayr and Pock74 showed that allylsilane 153 is at —78 °C 30700 times more reactive towards diarylcarbenium ions than propene 154. Thus, in the... 

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-... 

The relative chloride affinities of the benzhydryl cations from Scheme 5 are now graphically displayed in Scheme 7 (bottom, right). The correlation between the chloride affinities of diarylcarbenium ions and the ethan-olysis rate constants of the corresponding diarylmethyl chlorides [39] allows the chloride affinity scale of the diarylmethyl cations to extend to the less stabilized systems (Scheme 7, bottom left). In the AG° scale shown in Scheme 7, 10 kJ mol-1 corresponds to K = 374. 

Scheme 7 Relative chloride affinities of diarylcarbenium ions and of metal chlorides in CH2C12 (-70° C). (From Ref. 40.)...

The benzhydryl chlorides and BC13 react with formation of ion pairs (ionization constant, Ki) which dissociate to give the free ions (dissociation constant, KD). Because paired and free diarylcarbenium ions show only slightly different UV-visible spectra, [41], spectrophotometric measurements allow the determination of the total carbocation concentration. On the other hand, only free ions are detected by conductometric analysis, and a combination of both methods allows the determination of Ki and Kd using the theory of binary ionogenic equilibria [42,43]. 

Figure 3 Work station for determining reactivities of diarylcarbenium ions. (Reprinted with permission from Ref. 58.)...

Figure 10 Reactivities of terminal alkenes toward diarylcarbenium ions (-70° C, CH2CI2, reference reaction Aryl2CH+ + 2-methyl-l-pentene). (Reprinted with permission from Ref. 128. Copyright 1990 American Chemical Society.)...

The relationship between structure and reactivity of diarylcarbenium ions has already been mentioned in the discussion of Figures 10 and 11 (Section III.D.4.b). Some numbers on which these figures are based, are given in Table 6. They show that the ditolylcarbenium ion (TohCH+) reacts 105 times faster with 2-methyl-1-pentene and 106 times faster with 2-methyl-2-butene than the better stabilized dianisylcarbenium ion (An2CH+). It... 

Table 6 Rate Constants and Activation Parameters for the Additions of Diarylcarbenium Ions to 2-Methyl-l-pentene and 2-Methyl-2-butene in Dichloromethane...

Figure 14 Correlation between AG for the reactions of diarylcarbenium ions with alkenes and A G,° for the ionization of the corresponding diarylmethyl chlorides with BCI3. (From Ref. 69, reprinted with permission of VCH Verlags-gesellschaft.)...

The reactions of diarylcarbenium ions, Ar2CH+, with allylstannanes are kinetically first order in each reagent and involve irreversible addition to give the cation which is stabilised by the P-stannyl substituent, followed by loss of R3Sn+ (or sometimes attack of a nucleophile at C2).40... 

The accumulation of diarylcarbenium ion can be confirmed by NMR spectroscopy. For example, a solution of bis(p-flurorophenyl)carbenium ion exhibits a signal at 192.6 ppm because of the carbenium carbon. This... 

Scheme 5.23 Generation of diarylcarbenium ion pools by C—H bond dissociation...

As exemplified in Scheme 5.24, the dicarylcarbenium ion pool reacts with various nucleophiles including allylsilanes and ketene silyl acetals very quickly. Friedel-Crafts-type reactions with aromatic and heteroato-matic compounds also take place smoothly. Therefore, diarylcarbenium ion pools serve as powerful reagents for flash chemistry. 

Okajima M, Soga K, Nokami T et al (2006) Oxidative generation of diarylcarbenium ion pools. Org Lett 8 5005-5007... 

Okajima M, Soga K, Watanabe T et al (2009) Generation of diarylcarbenium ion pools via electrochemical C-H bond dissociation. Bull Chem Soc Jpn 82 594-599... 

Microsystem-controlled cationic polymerization technology requires extremely reactive initiators and cation pools serve as effective initiators for this technology. Usually carbocations are generated by a reversible process from their precursor. Yoshida et al. developed the cation pool method [35], in which carbocations are generated irreversibly by low-temperature electrolysis and are accumulated in relatively high concentration in the absence of nucleophiles. N-Acyliminium ions, alkoxycarbenium ions [36-40] and diarylcarbenium ions [41] have been generated by this method. Such cation pools are expected to serve as extremely reactive initiators for cationic polymerization. 

NuciGOphiliCity of Monomers. Figure 1 shows cationically polymerizable monomers in order of increasing nucleophilicity. The numbers listed are nu-cleophilicity parameters (N) developed by Mayr (24,25). N values were derived by measuring the rate of reaction between diarylcarbenium ions and various monomers using UV-vis spectroscopy and are discussed more later in this article. 

Various other initiator systems for the controlled/living polymerization of isobutylene have been reported (222). The capping reaction of living polsrisobuty-lene with 1,1-diphenylene ethylene (DPE) results in the formation of a stable and fully ionized diarylcarbenium ion, which can be used for chain-end functionalization by quenching with appropriate nucleophiles as shown in reactions 82 and 83 (258-261). 

The use of NHC-boranes as hydride donors has also been investigated by Curran and coworkers [91] through the measurement of nucleophilicity parameters based on diarylcarbenium ions as reference electrophiles. This method is based on a study by Mayr and Ofial [92]. The authors were also able to demonstrate the potential of NHC-boranes as hydride donors by using them to reduce iminium ions evidence for hydride transfer of two and three hydrides from IMe-borane has also been provided as this particular complex was able to produce />-chloroben2ylaniline in an 86% yield even when 0.33 equivalents of the NHC-borane were used. 

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