Styryl cations

In contrast to the thermal solvolysis, a rearranged enol ether 45 (and also the hydrolysis product, acetophenone) is formed in addition to the unrearranged product 44. The rearrangement is more apparent in less nucleophilic TFE. The results are best accounted for by heterolysis to give the open primary styryl cation 46 (Scheme 8). This cation gives products of substitution 44 and elimination 30 by reaction with the solvent. Alternatively, 46 can rearrange to the a-phenyl vinyl cation 47 via 1,2-hydride shift, which gives rise to 45 and 30. 

A side-effect was the spectroscopic hunt for the elusive styryl cation which resulted in the identification of the principal cations formed by the action of acids on styrene the subsequent quest for a non-spectroscopic method to identify organic cations led to the development of the polarography of carbenium and oxonium ions. 

We also obtained NMR spectra of the phenylindanyl cation 13 in the large-pore zeolite HY, and a small amount of cation 14 was formed on HZSM-5 by dimerization of a-methylstyrene. The dimethylphenyl carbenium ion 15 was not persistent on any zeolite we examined. This is not surprising if one reads the solution acid literature. 15 cannot be observed in 100% H2S04 stabilizing this cation requires 30% oleum (S03/H2S04) or other superacids (115). HZSM-5 is not a superacid. The observation of the much less stable styryl cation 11 was hailed as a triumph of superacid solution chemistry (116). If the styryl cation, with the phenyl group provid-... 

A comparison of Figs. 5 and 6 shows that between the anionic polymerization of styrene and the transition metal polymerization of styrene there is a region of poor catalysts which produce either atactic or no polymers. It has been shown by the substitution effects of the reaction that both the styryl anion and the styryl cation propagate on the... 

Using the isodesmic equations 9a-c Tsuno and coworkers calculated from the experimental gas phase basicities that the a-trimethylsilyl group stabilizes the styryl cation by 4.5 kcalmol-1 (equation 9a). Equations 9b and 9c which compare the effect of the a-trimethylsilyl group with those of the t-butyl- and the methyl substituent, respectively, are nearly thermoneutral, indicating the order Alkyl = SiMe3 > H of styryl cation stabilization ability18. 

Monomeric styryl cations were observed recently at 315 and 325 nm during flash photolysis of styrene and a-methylstyrene, respectively, in trifluoroethanol [20]. Absorbance of these model compounds at wavelengths lower than those of the propagating species may be due to the... 

The dissociation constants of trityl and benzhydryl salts are KD 10 4 mol/L in CH2C12 at 20° C, which corresponds to 50% dissociation at 2-10-4 mol/L total concentration of carbocationic species (cf. Table 7) [34]. The dissociation constants are several orders of magnitude higher than those in analogous anionic systems, which are typically KD 10-7 mol/L [12]. As discussed in Section IV.C.l, this may be ascribed to the large size of counterions in cationic systems (e.g., ionic radius of SbCL- = 3.0 A) compared with those in anionic systems (e.g., ionic radius of Li+ 0.68 A), and to the stronger solvation of cations versus anions. However, the dissociation constants estimated by the common ion effect in cationic polymerizations of styrene with perchlorate and triflate anions are similar to those in anionic systems (Kd 10-7 mol/L) [16,17]. This may be because styryl cations are secondary rather than tertiary ions. For example, the dissociation constants of secondary ammonium ions are 100 times smaller than those of quaternary ammonium ions [39]. 

For example, styryl cations react with perchlorate to form covalent esters that are relatively stable at low temperatures [81], whereas the more basic triflate anion tends to abstract j3-protons from carbenium ions in a transfer reaction [56]. The basicity of the counteranion determines the contribution of /3-proton elimination relative to propagation, and therefore the limiting molecular weight in a polymerization. 

Rate constants were calculated by assuming the extinction coefficient of growing macromolecular styryl cation to be c - 10,000 mol lL/cm l. Values of kp might be three times larger, if t 30,000 mol -L-cm 1 b Relative reactivities of ions and ion pairs estimated from common ion effect. 

Thus, how should block copolymers between styrene and a vinyl ether be prepared Starting with styrene or with a vinyl ether In the former system, the propagating styryl cation is intrinsically more reactive but present at much lower concentration. A rough estimate of the ratio of cation reactivities is = 103 but the ratio of carbocations concentrations is = I0 S. Thus, the ratio of apparent rate constants of addition is 10-2. Macromolecular species derived from styrene should add to a standard alkene one hundred times slower than those derived from vinyl ethers. Thus, one cross-over reaction St - VE will be accompanied by =100 homopropagation steps VE - VE. Therefore, in addition to a small amount of block copolymer, a mixture of two homopolymers will be formed. Blocking efficiency should be very low, accordingly. 

This compares with fep for free cationic growth of 3.5 x 10 1 mole sec at 15°C and act 0- It is worthwhile to consider why the styryl cation reacts about 10 times faster with its monomer than does the styryl radical. A similar comparison is possible in the case of JV-vinylcarbazole for which the reported [106] free radical propagation coefficient is 6.0 Imole sec ... 

With suitable substituent groups (which also prevent transalkylations) secondary styryl cations were found as stable, long-lived ions116,1,7 ... 

In methanol, the main enol ether products are the unrearranged 38 in TFE by far the major product is the rearranged enol ether 80. This difference is due to the difference in lifetime of the primary styryl cation (82) in the two solvents. Its... 

In the thermolysis the E isomers of fluoro- and chlorostyrene ( -84-F and -84-Cl) are formed exclusively, which indicates the intermediacy of the vinylenebenze-nium ion 29 (see Scheme 29) as the species abstracting a fluoride from the counterion BF4, or a chloride from the solvent. Photochemically both E- and Z-84 are formed, indicating the intermediacy of the primary styryl cation 82 (see Scheme 52). Preferentially the Z-isomer of 84-F is formed. The fluoride-abstracting species must not be 82 itself, but a tight ion-molecule pair of 82 and iodobenzene (see Scheme 53), which hampers the approach of the fluoride donor to the side of 82 where this group leaves. 

The initiation rate was found to be proportional to the initiator concentration and roughly first order with respect to the monomer concentration, although the amount of the styryl cation formed was only 1-4% of the total acid concentration. 

First, a rise in transient absorption of styryl cation due to protonation was observed (Scheme 39). Between the rise and decay (ocmax = 340 nm), there is a short period of stationary state (or smooth maximum), which is longer at low temperature and shorter at higher temperature. 

An interesting reaction related to the selective dimerization of styrene may be the oxo-acid catalyzed addition of styrene to aromatic hydrocarbons to yield 1,1 -diphenyl-ethane derivatives (37) (Eq. (26)). This process differs from the cationic dimerization of styrene in that the unsaturated bond to be attacked by the styryl cation is aromatic, not vinylic as in the latter. Mechanistically, however, these reactions are quite similar, particularly in that both require rapid and efficient deprotonation of the carbo-cationic intermediate formed by the addition of the styryl cation. The high proton-... 

Diphenylethanes (57) are used in industry as heat media and condenser oil, and can be prepared by Friedel-Crafts reaction between a 1-chloro-l-arylethane and an aromatic hydrocarbon in the presence of an MX catalyst (Eq. (26a)). An alternative route is the addition of the styryl cation to an aromatic hydrocarbon (Eq. (26b)). This reaction may be advantageous over the above Friedel-Crafts process in view of the better availability of the starting material (styrene) a disadvantage is that styrene oligomers are usually obtained as by-products. 

The photochemistry of alkenyl(phenyl)iodonium tetrafluoroborates in methanol as well as 2,2,2-trifluoroethanol, dichloromethane, and toluene has been investigated (Equation 110.2). Homolysis as well as heterolysis of both bonds, the vinyhc C-I bond and the aromatic C-I bond, occur. The predominant formation of styryl cation accounts for the product derived from nucleophilic substitution, elimination, and rearrangement, as shown in Scheme 5. 

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