The Cationic Mechanism

The computed Gibbs energy profile for the copper-free Sonogashira reaction with phenylacetylene (R = H) through a cationic mechanism is shown in Fig. 5.8. 

8 Gibbs energy profile in DCM (AGdcm. kcal-mol ) at 298 K for the cationic mechanism with R = H, and Base = pyrrolidine 

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Vinylethers are known to polymerize in a cationic mechanism in the conventional homopolymerization system (35). Williams et al. (36) and Naruse et al. (37) studied the radiation-induced polymerization of vinylethers and found that they polymerize in the cationic mechanism by ionizing radiations, too. The propagation rate constant of the polymerization of isobutylvinylether in bulk at 30° C was estimated to be 3 x 105 M l sec 1, much higher than that of the polymerization by... 

Ethene does not polymerize by the cationic mechanism because it does not have sufficiently effective electron-donating groups to permit easy formation of the intermediate growing-chain cation. 2-Methylpropene has electron-donating alkyl groups and polymerizes much more easily than ethene by this type of mechanism. The usual catalysts for cationic polymerization of 2-methylpropene are sulfuric acid, hydrogen fluoride, or a complex of boron... 

Polymerization by the cationic mechanism is most important for 2-methyl-propene (isobutylene), which does not polymerize well by other methods, and was discussed previously in considerable detail (Section 10-8B). 

A key feature of the cationic mechanism is that removal (or dissociation) of an anionic ligand from the palladium coordination sphere allows alkene complexation to occur while maintaining coordination of both phosphines of a bisphosphine ligand. That both phosphines can be accommodated in a square-planar four-coordinate intermediate during the insertion step has provided a simple rationalization for the higher enantioselectivities often observed for the cationic pathway. Concrete information on the enantioselective step of asymmetric Heck reactions proceeding by the cationic pathway has not been reported to date. It is likely to be either coordination of the alkene to generate 20.S or the insertion step (20.5 —> 20.6, Scheme 8G.20). 

The cationic mechanism of the polymerization is supported by the reference experiments in various solvents. The polymerization is suppressed in basic solvents such as acetonitrile and dimethylformamide. Alkyl halides and nitrobenzene are preferable for the formation of PPS in this polymerization. 

The anion is an activated species capable of further reaction. In the cationic mechanism, the epoxy group may be opened by active hydrogen to produce a new chemical bond and a hydroxyl group. This reaction may proceed in a number of different ways. 

Monomers listed above polymerize by the cationic mechanism. For some groups of monomers (lactones, carbonates) anionic or coordinate mechanism also operates and, from a synthetic point of view, this is the preferred method of converting cyclic esters into linear polyesters. The cationic polymerization of lactones, glycolide and it substituted analog, lactide, as well as spiroorthoesters and bicyclic orthoesters has been studied in some detail. 

C—C coupling than the cation mechanism, but separation of the newly generated car-boxy group from zirconium proved to be exothermic and did not require assistance by an acrylate molecule. 

The cation mechanism is perfectly reasonable as far as the diazonium salt is concerned but not do for the alkene. Conjugated esters are electrophilic and not nucleophihc alkenes. Eve were to attack the aryl cation, we should find the reverse regioselectivity. 

Hydroquinone at a concentration of 0.0050 M quenches photocrosslinking in the presence of TCPA completely. Possibly the basicity of hydroquinone is responsible for the inhibition, since the radical mechanism is disfavored compared to the cationic mechanism by several other pieces of evidence. The ideal quencher which was on order, is diphenylpicrylhydrazyl, which affects radical but not cationic processes. 

Anhydro sugars containing an oxirane ring as a side-chain to the sugar moiety have also been polymerized by the cationic mechanism 9> ... 

In polymerizations carried out to high conversions where the concentrations of propagating centers, monomer, and transfer agent as well as rate constants change, the polydispersity index increases considerably. Relatively broad molecular weight distributions are thus generally found in polymerizations by the cationic mechanism. 

Polymerization of 7V-vinylcarbazole catalyzed by dimethylglyoxime complexes of different metals immobilized on PVC follows the cationic mechanism. Lewis acids immobilized in a volume of swollen polymer gel catalyze cationic polymerization and oligomerization of vinyl ethers, etc. Cationic complexes of Pd(II) bound to modified PS initiate alternative copolymerization of fluorinated olefins (C F2 +i)(CH2)mCH=CH2 with carbon monoxide [112,113]. The product thus obtained was polyspiroketal rather than polyketone. 

Various reaction schemes were proposed to explain the cationic mechanism. They tend to resemble the schemes suggested for polymerizations of cyclic ethers. The initiation step involves an equilibrium that is followed by a ring-opening reaction ... 

Thus, we have a closed system of Equations 3.4-3.10 which allow the MWD and average MW to be calculated for a random distribution of polymer yields in each zone (AM,). For determining the relationship between the polymer yield in a separate zone and the amount of catalyst loaded, knowledge of polymerisation kinetic mechanisms is necessary. In particular, using the cationic mechanism of isobutylene polymerisation, the following expressions may be derived [1] ... 

Since a carbonium ion or closely related species is the intermediate, orientation would be expected to follow the Saytzeff rule favoring the most stable olefin. The carbonium ion mechanism also explains the general trend in relative reactivity exhibited in this reaction. Tertiary alcohols are the most reactive, and reactivity decreases on going to secondary and primary alcohols. Also in accord with the cationic mechanism is the fact that rearranged products are found in cases where a carbonium ion intermediate could be expected to rearrange. ... 

Much of the recent hteiature on the mechanism of the enantioselective intramolecular Mizoroki-Heck reaction has focused on the anionic mechanism, o-iodoanilide substrates, pathways involving neutral pentacoordinate palladium intermediates and the influence of additives. The new examples and mechanistic findings indicate that the potential may exist to control the stereoselectivity of the intramolecular Mizoroki-Heck reaction through pathways other than the cationic mechanism. However, further research is needed to obtain the level of effectiveness of the traditional cationic pathway. 

Pretreatment involving the partial reduction of the oxide with hydrogen can similarly produce significant effects that vary with the metal oxide used. The effect that prereduction has on supported chromia and molybdenum oxides is widely different. On a chromia catalyst, the reduction step leads to the formation of Br0nsted sites, which then catalyze the isomerization reaction via a cationic intermediate. On a reduced molybdena catalyst, metathesis-type mechanisms dominate, with the cationic mechanism proceeding only on a fully oxidized molybdena surface. 

The arenium cation mechanistic pathway was first proposed in 1935 by Baddeley and was later reinforced by Balaban and Nenitzescu. Computational calculations performed by King and co-workers have produced several important conclusions. First, it was determined that the arenium cation mechanistic pathway was thermodynamically favored in studies under both vacuum and solvated conditions due to lower energy transition states than those found in the radical cation mechanistic pathway. Second, due to the increasingly exergonic nature of the reaction and the observed nonaccumulation of intermediates, C-C bond formation, in the case of hexaphenylbenzene, was found to occur slowest for the first bond and fastest for the last bond. The arenium cation mechanistic pathway is further supported by evidence that the Scholl reaction can proceed in acidie solutions that do not promote radical formation, such as anhydrous HF. The cationic mechanism has been shown to predominate in strongly protic conditions. ESR and EPR studies have concluded that the radieal cations previously observed during the Scholl reaction are not part of the actual reaction, but... 

When the system Et4N" BF4 /styrene/nitrobenzene was electrolyzed with 0.35 mA for 60 min, some polystyrene was observed around the anode. Another field in which cations are assumed to be the intermediate is styrene polymerization by y-rays [94]. Careful work showed that the rate of polymerization gradually changes from 0.5 order, indicating a radical process, to a first-order process, suggesting an unimolecular termination characteristic for the cationic mechanism [95 99]. 

Many other acidic catalysts possessing a Lewis acid as a component are used in propylene oligomerization. The most popular are BF3/H3PO4 [648-651], BF3/ MeOH [652], BF3/BUOH [653], AICI3 [654], and ZnClj [655,656], and they are listed later in Table 3. The cationic mechanism of propylene oligomerization in the presence of Lewis acids is represented in the following reaction series ... 

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