Carbocations. Carbocationic species

Zeolites are the main catalyst in the petrochemical industry. The importance of these aluminosilicates is due to their capacity to promote many important reactions. By analogy with superacid media (1), carbocations are believed to be key intermediates in these reactions. However, simple carbocationic species are seldom observed on the zeolite surface as persistent intermediates within the time-scale of spectroscopic techniques. Indeed, only some conjugated cyclic carbocations were observed as long living species, but covalent intermediates, namely alkyl-aluminumsilyl oxonium ions (2) (scheme 1), where the organic moiety is bonded to the zeolite structure, are usually thermodynamically more stable than the free carbocations (3,4). 

A carbocationic species in which there is at least one pentavalent carbon atom (e.g., CHs ). 2. Traditional name for chemical species that are now referred to as carbenium ions. Considerable confusion exists in the literature with this term for carbocations. The -onium suffix usually refers to a higher covalency when compared to the neutral atom thus, CH5+ would be a true carbonium ion (in terms of the first definition). Additional ambiguity results when the term ethyl carbonium ion is used to describe both CH3CH2 and to R—CH2CH2. For these reasons, the terms carbocation or carbenium ion are now preferred. 

Superelectrophilic onium dications have been the subject of extensive studies and their chemistry is discussed in chapters 4-7. Other multiply charged carbocationic species are shown in Table 2. These include Hogeveen s bridging, nonclassical dication (14)26 the pagodane dication (15)27 Schleyer s l,3-dehydro-5,7-adamantane dication (16)28 the bis(fluroenyl) dication (18)29 dications (17 and 19) 19a trications (20-21)19a,3° and tetracations (22-23).31 Despite the highly electrophilic character of these carbocations, they have been characterized as persistent ions in superacids. 

The now-classic technique pioneered by Nobel Laureate George Olah and co-workers [52, 53] for preparing relatively stable long-lived carbocations, and their direct observation in solution by NMR, has been applied to the study of a number of classes of fluorinated carbocationic species [52-55], including alkyl, aryl, allyl and tropylium cations (Table 4.9). 

Potentially, there are greater numbers of monomers that are suitable for cationic polymerization than for anionic, but the cationic method is less successful in block copolymer synthesis because, in many systems, the existence of a living carbocationic species is doubtful. Consequently, the involvement of carbocations in block copolymer synthesis tends to be limited to mixed reactions, e.g., the couphng of poly(tetrahydro-furan) cations with polystyryl anions to give an (A - B) diblock (Equation 5.19). 

When a counteranion is not so nucleophilic as the iodide anion, the propagating carbocation may be stabilized instead by adding an base (Z) so that living polymerization proceeds (Eq. 9 see also section 1.1) (4). This method is particularly effective for the polymerization initiated with ethylaluminum dichloride (EtAlCl2) (13) and typically, the bases may be 1,4-dioxane and related ethers that form a "base-stabilized" carbocationic species like where Z is an ether oxygen. We have recently synthesized end-functionalized polymers via these base-stabilized living species (14). 

Carbocations derived from the alcohol are probably the reactive species, but data concerning by-products expected with carbocationic intermediates are lacking. Rearrangement of 2-alkylaminothiazoles to 2-amino-5-alkylthiazoles may also explain the observed products 2-aminothiazole with benzyl chloride yields first 2-benz Iaminothiazole (206). which then rearranges to 2-amino-5-benzvlthiazole (207) (Scheme 130) (163. 165. 198). 

We have recently shown that metal-exchanged zeolites give rise to carbocationic reactions, through the interactions with alkylhalides (metal cation acts as Lewis acid sites, coordinating with the alkylhalide to form a metal-halide species and an alkyl-aluminumsilyl oxonium ion bonded to the zeolite structure, which acts as an adsorbed carbocation (scheme 2). We were able to show that they can catalyze Friedel-Crafts reactions (9) and isobutane/2-butene alkylation (70), with a superior performance than a protic zeolite catalyst. 

After discussing the dehydration of methanol and formation of DME, we are able to illustrate a number of key theoretical concepts. The first is that carbocation fragments are found in transition states, rather than as stable intermediates. Furthermore, the nature of these species is different from what is predicted from gas-phase studies, experimental or theoretical. The cluster, i.e., the zeolite, controls the stabilization of this carbocationic fragment. Second, we see that each different reaction requires a different transition state, rather than formation of a transition state that can be converted in a number of possible reactions. (This latter view received some support as a result of different processes possessing very similar activation barriers.)... 

An overview of the reactions over zeolites and related materials employed in the fields of refining, petrochemistry, and commodity chemicals reviewed the role of carbocations in these reactions.15 An overview appeared of the discovery of reactive intermediates, including carbocations, and associated concepts in physical organic chemistry.16 The mechanisms of action of two families of carcinogens of botanical origin were reviewed.17 The flavanoids are converted to DNA-reactive species via an o-quinone, with subsequent isomerization to a quinone methide. Alkenylbenzenes such as safrole are activated to a-sulfatoxy esters, whose SnI ionization produces benzylic cations that alkylate DNA. A number of substrates (trifluoroacetates, mesylates, and triflates) known to undergo the SnI reaction in typical solvolysis solvents were studied in ionic liquids several lines of evidence indicate that they also react here via ionization to give carbocationic intermediates.18... 

Most carbocationic and cationic ring-opening polymerizations are chain processes proceeding with carbocations and/or onium ions as the active species. Nonchain processes which occur via cationic and electrophilic intermediates will be discussed in Chapter 7. 

It is possible to work at either lower cation concentrations—but this could lead to stronger effects of impurities (adventitious moisture)—or to use more specialized flow reactors. The dynamic equilibration between active and dormant species offers another solution to this problem. In this case, the sensitivity to impurities is low due to the high total number of chains, but the momentary concentration of propagating carbocations is tremendously reduced. This approach is always used in new controlled/ living carbocationic polymerizations, as we will discuss in detail in this chapter. 

The mechanistic details and roles of all constituents of the multicomponent initiating systems for new controlled/living carbocationic polymerization are also discussed in Section VI. At this stage it suffices to say that in both the new systems and conventional carbocationic polymerization, monomer is consumed by the repetitive electrophilic addition of growing carbocations whether or not in dynamic equilibrium with either covalent species or onium ions. 

Trapping agents, such as malonate anions, naphthoxides, and phosphines have been used to determine the concentration of chain carriers in controlled/living and other carbocationic systems [85,249,250]. These strong nucleophiles react with all sufficiently electrophilic species, including not only carbocations but also onium ions and covalent esters. Thus, the discussed measurements can provide only the total concentration of active and dormant end groups. In principle, the kinetics of formation of the product in the trapping experiments could resolve more and less active species but only if they are present at comparable concentrations. As discussed before, carbocations are present in ppm quantities in comparison with dormant species. 

Controlled polymerization requires that the initiation rate is at least comparable to that of propagation. Initiation in controlled/living carbocationic systems is usually carried out using models of growing species in their dormant state (e.g., the adducts of a monomer with protonic acids). This enables a similar set of equilibria to be established between carbocations and dormant species for initiation and for propagation. For example, 1-phenylethyl halides have similar reactivity as the macromolecular dormant species in styrene polymerizations, and I-alkoxyethyl derivatives are as reactive as the macromolecular species in the polymerization of vinyl ethers [Eq. (38)] ... 

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