Carbocationic species

Carboxoninm ions, for example, do not react with alkanes. However, in superacid solution acetaldehyde (or acetone), for example, readily reacts with isobutane involving diprotonated, highly reactive carbocationic species. 

In the following sections, we detail another functionality which is of major value in the area of carbocationic domino processes, namely the epoxides. On the basis of their high tendency to be opened in the presence of Lewis or Bronsted acids, thereby furnishing carbocationic species, several challenging domino procedures have been elaborated relatively recently. 

The first mechanism was proposed by Baird et al. [189]. The carbocationic species is schematically shown in Scheme IX. The attack of monomer, well known on the carbocationic center of a metal-ion-activated olefin, proceeded in the normal manner for carbocationic polymerization. This mechanism is based on the following two evidences. Alcoholysis of the polymerization system, TiMe3Cp /B(C6F5)3, resulted in the presence of an alkoxy group at an end group, and vinyl ethers and iV-vinylcarbazole were polymerized by using the same system. 

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. 

Alkyl cations like the tert-butyl cation (2) and 2-propyl cation (89) are significantly stabilized by hyperconjugative C-H and C-C a -back donation into the empty carbocationic p-orbitals. Protosolvation involving a -bonds can diminish this hyperconjugative stabilization and thus lead to super-electrophilic carbocationic species. 

Suggest a mechanism for the formation of compound 10 and for the demethylation of the intermediate carbocationic species 9. 

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]. 

The initiating systems based on Lewis acids with covalent esters and halides offer some advantages. First, the number of chains can be easily controlled by the concentration of the ester or halide used as an initiator. Second, the polymerization rate and the proportion of carbocationic species may be easily adjusted by the strength and concentration of the Lewis... 

These equilibria also strongly affect copolymerization. Monomer reactivity ratios in controlled/living systems should be identical to those in conventional cationic copolymerizations, if the comonomers react exclusively with carbocationic species. The equilibrium between active and... 

Termination by Reactions of More Reactive Species Existing in Equilibrium with Stable Onium Species As already discussed, in the systems, in which unimolecular ring-opening of cyclic onium ion leads to highly stabilized carbocationic species, a concentration of the latter species in equilibrium with onium ions may be significant. This is, for example, the case of cationic polymerization of cyclic acetals, where carboxonium ions exist in equilibrium with their oxonium counterpart ... 

Summary of the v(C = C) Vibrational Mode (Observed by IR Spectroscopy) and of the k->k Electronic Transition (Observed by UV-Vis Spectroscopy) for Carbocationic Species Derived from Acetylene, Methylacetylene, and Ethylacetylene Oligomerization on HZSM-5... 

Carbocationic species are also considered to be intermediates in reactions between iodobenzene difluoride and alkenes [265, 266]. Tetrafluorination of alkynes is possible using nitrosonium tetrafluoroborate and pyridine-(HF) [267] (Figure 3.60). 

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). 

Oxiranes of terminal monosubstituted and internal disubstituted olefins do not undergo the isomerization under standard conditions, but give aldehydes at elevated temperature. For the special substrates described in Sch. 35, different modes of reaction originated from intermediary carbocationic species, involve neighboring functional group participation, oxidation, etc. An improvement employing other silicon Lewis acids, for example Mc3SiI and McsSiBr, was developed by Kraus, Detty, and Sakurai [17,19f,62]. 

Yanagisawa, M, Mukaiyama, T, Catalytic and stereoselective glycosylation with glycosyl fluoride using active carbocationic species paired with tetrakis(pentafluorophenyl)borate or trifluorometha-nesulfonate, Chem. Lett., 224-225, 2001. 

A Lewis acid catalyst can interact with the reagent containing a functional group having a donor atom with nonbonded pairs of electrons. This gives rise to a positively polarized complex or a carbocationic species, which then reacts with the -rr-donor substrate (aromatic, alkenic or alkynic hydrocarbons). Though this process can occur under strictly anhydrous conditions, this generally is not the case as impurity, moisture, or other cocatalysts are usually present. In the case of reactions of alkenes and alkynes... 

Frequently occurring side-reactions are elimination and rearrangements of the carbon skeleton [127], because of the intermediate formation of carbocationic species. 

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