1- Phenethyl cation

Carbocations are intermediates in several kinds of reactions. The more stable ones have been prepared in solution and in some cases even as solid salts, and X-ray crystallographic structures have been obtained in some cases. An isolable dioxa-stabilized pentadienylium ion was isolated and its structure was determined by h, C NMR, mass spectrometry (MS), and IR. A P-fluoro substituted 4-methoxy-phenethyl cation has been observed directly by laser flash photolysis. In solution, the carbocation may be free (this is more likely in polar solvents, in which it is solvated) or it may exist as an ion pair, which means that it is closely associated with a negative ion, called a counterion or gegenion. Ion pairs are more likely in nonpolar solvents. 

The 9-fluorenyl cation also reacts at the diffusion limit with substituted styrenes, as indicated by the observation that the rate constants are 3 x 10 M s independent of the styrene substituent. The reaction of styrene with the phenethyl cation (71), the first step in the polymerization of styrene (Sty) catalyzed by Brpnsted acids, has also been directly observed (Scheme 1.8). Photoprotonation of styrene... 

Since a bridgehead carbonium ion would be expected to be shorter-lived than an a-phenethyl cation, the additional equilibration occurs at the stage of the correspondingly longer-lived diazonium ion pair. This work also provides the clearest demonstration that nitrogen may be lost from within an ion pair without causing its disruption. 

Equation 16 illustrates a celebrated example where ring closure competes with vicinal hydride shift (a common form of atom transfer in cations, which does not take place in free radicals or anions). The gas phase reaction was explored by preparing the dimethylfluoronium ion, (CH3)2F" , by y-radiolysis of fluoromethane. Exothermic methylation of a sample of C-yd-phenethyl chloride (where the asterisk in equation 16 symbolizes the labeled position) in the gas mixture gives a vibrationally excited ion that loses chloromethane to form two isomeric ions, a-phenethyl cation and spirooctadienyl cation (sometimes called ethylenebenzenium). Nucleophilic attack by methanol in the reaction mixture yields PhCH(CH3)OCH3, whose isotopic label remains almost entirely at the methyl group. The recovered PhCH2CH20CH3 contains equally distributed between the two methylene positions. The spirooctadienyl ion does not isomerize to a-phenethyl cation, even though DFT calculations predict the latter to be 55 kJ/mol more stable. 

From the standard Gibbs energy change for the formal hydride exchange between a-phenethyl and tert-butyl cations as determined by FT ICR in Ref. 51. Entropies corrected using the experimental values for iio-butene and styrene (82.48 cal moF K ) and our computed value for the phenethyl cation. Scaled FlF/6-31G(d) value from this work. 

Olah and co-workers used NMR spectroscopy to study the relative stability of phenonium ions, benzylic ions, and phenethyl cations as a function of substituents on the benzene ring. For the imsubstituted benzene ring, the phenonium ion was more stable than the other two jjossible structures. With a p-methoxy substituent, however, the benzylic ion was most stable, while the phenethyl cation was most stable for a p-trifluoromethyl substituent. Olah, G. A. Comisarow, M. B. Kim, C.].J.Am. Chem. Soc. 1969,91,1458. Also see Olah, G. A. Head, N. J. Rasul, G. Prakash, G. K. S. /. Am. Chem. Soc. 1995,117, 875. 

Solutions were 2 x 10-3 mol dm-3 in compound, and potentials were determined with reference to the SCE, b Three-electron reversible oxidation process. Two-electron reversible oxidation process. Separation between anodic and cathodic peak potentials values for ferrocene under identical conditions ranged from 80 to 90 mV. Shift in respective ferrocenyl oxidation potential produced by presence of guest cation (2 equiv) added as their thiocyanate salts for potassium and ammonium, and their picrate salts for methylammonium and phenethyl-ammonium. 

It can be synthesized by reaction of a 1 1 molar ratio of ethyl vinyl ether and phenethyl alcohol in the presence of cation exchange resin [141]. It imparts fresh, floral, green notes and is used in fine fragrances as well as in soap, cosmetics and detergents. 

Richard and Jencks combined the above method with use of the azide clock to determine values of pA R for a-phenethyl carbocations bearing electron-donating substituents in the benzene ring and for the cumyl cation for a wider range of substituents.22,89 They inferred values for the parent... 

The equilibrium constants Ka, K, and h2o are conveniently summarized in Scheme 10 in the form of a cycle similar to that shown above for the a-phenethyl and t-butyl cations (Schemes 1 and 4). It is worth noting that P h2o measures the stability of the double bond relative to the alcohol (hydrate). If p fR was converted to HIA, p h2o in the cycle would be replaced by the energy of hydrogenation. The latter provides the conventional measure of double bond stability, save that here free energy in aqueous solution is measured rather than the more usual heat of hydrogenation in the gas phase. 

On the other hand, when the nucleophile reacts with the carbenium ion after it has separated from the leaving group, the reaction takes place with complete racemization. This is the case with more stable and consequently longer-hved carbenium ions. For example, the a-methyl benzyl cation, which is produced in the rate-determining step of the solvolysis of R-phenethyl bromide in a water/ethanol mixture, is such a cation (Figure 2.14). As in the solvolysis of Figure 2.13, the nucleophile is a water/ethanol mixture. 

The idea is to add a latent Friedel-Crafts catalyst to the polymerization feed that will be inert during the polymerization. Then at >200 °C it becomes activated and converts the residual styrene monomer to phenethyl carbocations. These carbocations can either initiate cationic polymerization of the styrene or can alkylate the benzene rings of PS to become attached. With this approach PS itself is the scavenger for residual styrene monomer in PS (Scheme 4.3). 

Isocyanates and isothiocyanates have been used as precursors to formamidyl and thioformamidyl cation equivalents. The reaction shown in equation (25), catalyzed by boron trifluoride etherate, was used in a synthesis of the alkaloid lycoricidine, and proceeded in 89% yield. This type of reaction also works efficiently using other -phenethyl isocyanates. 

We present in Fig. 35 the structure (C symmetry) optimized at the MP2(full)/6-31G(d) level (this work). Recently, a large computational study has been published on benzyl and larger carbocations. It uses both the optimized force field MMP2 extended to carbocations and MP4sdq/6-31G(d)//MP2(full)/6-31G(d) calculations. It provides, inter alia, valuable estimates of standard heats of formation. In this case, the difference in stability between phenethyl and 4-methylbenzyl cations (1.7 by ab initio computations and 2.9 kcal mol by MMP2) is in good agreement with the experimental results in Ref. 51. Our own G2(MP2) results are presented in Table 9. 

Figure 4. Retention of anionic and cationic compounds with I-phenethyl-2-picolinium (PEP) in the mobile phase [18]. Stationary phase /iBondapak Phenyl (10 fim). Mobile phase l-phenethyl-2-picolinium (PEP) in 0.1 M acetic acid. Reproduced from M. Denkert dal., ]. Chromatogr., 218. 37 (1981) by permission of Elwvier Science Publishers.

This catalytic cycle is shown in Scheme 16.17 for the reactions of cyclohexadiene catalyzed by paUadium-phosphine complexes. By this mechanism, the combination of a diene and either a Ni(0) or Pd(0) complex and acid or a cationic nickel(ll) or palladium(ll) hydride generates a cationic ir-allyl complex. Similarly, the combination of a vinylarene and one of these combinations of species generates a cationic phenethylpalladium complex. These allyl and phenethyl complexes have been isolated using this procedure. - The amine then attacks the ir-allyl or ir-phenethyl complex to form tfie organic product and regenerate Ni(0) or Pd(0). This nucleophilic attack on ir-allyl and ir-benzyl complexes was discussed in detail in Chapter 11. 

Another polymerization technique is cationic polymerization. For example, styrene can be polymerized by treatment with a small amount of 1-phenethyl chloride, tin tetrachloride, and tetra-n-butylammonium chloride [2]. Presumably, the mechanism is as follows. 

This type of the poljnner may act as an effective complexing agent with cation by the polymer back-bone, and may exert a strong stereoregulation by the isotactic structure of the polymer and by the asymmetric environment produced by chiral amine functions. Preliminary results of the enantiomer selection in the reaction of potassium acetate and a-phenethyl bromide by the optically active isotactic polyepichlorohydrin functionalized by L-2-dimethylamino-3-methylbutylamine gave 5-7% of the stereoselection. Further efforts are needed to achieve high enantioselection. 

Perrot, A.L., de Lijser, H.J.P., and Arnold, D.R., The importance of conformational effects on the carbon-carbon bond cleavage of P-phenethyl ether radical cations. Can. J. Chem., 75, 384, 1997. 

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