Carbenium ions as intermediates

This section is devoted to displacement at the silicon atom involving transient formation of a tricoordinate silicon cation, e.g., according to Eq. (1). Generation of carbenium ions as intermediates is a common... 

The possibility of NMR to follow the intramolecular migration of the selective label in adsorbed hydrocarbons gave the researches a chance to provide an evidence for the formation of alkyl carbenium ions as intermediates in reactions on solid acid catalysts, including zeolites. Carbenium ions in superacids exhibit a unique property to scramble the selective... 

The hydrocarbon catalytic cracking is also a chain reaction. It involves adsorbed carbonium and carbenium ions as active intermediates. Three elementary steps can describe the mechanism initiation, propagation and termination [6]. The catalytic cracking under supercritical conditions is relatively unknown. Nevertheless, Dardas et al. [7] studied the n-heptane cracking with a commercial acid catalyst. They observed a diminution of the catalyst deactivation (by coking) compared to the one obtained under sub-critical conditions. This result is explained by the extraction of the coke precursors by the supercritical hydrocarbon. 

Fig. 14. 90.4-MHz 13C MAS spectra of styrene- -l3C reacting on zeolite HZSM-5. The methylindanyl cation 12 (251 ppm), formed through the cracking of the cyclic dimer (cf. Fig. 6) followed by intramolecular hydride transfer, was converted to naphthalene at 523 K. This is the clearest example of a free carbenium ion as a reaction intermediate on a zeolite.

Undoubtedly, such scrambling phenomena are possible only with the participation of carbenium ions as reaction intermediates or transition states. Molecular rearrangements that rationalize the scrambling of deuterium atoms are shown in Scheme 5. 

We would expect peculiarities (1) and (2) to be also inherent in carbenium-ion-type intermediates of other reactions (such as the cracking, alkylation, and isomerization of hydrocarbons) occurring in the pores of solid acid catalysts. 

The occurrence of carbenium ions as reaction intermediates is strongly supported by the observation that the isotopic exchange can be totally suppressed in the presence of carbon monoxide.68 Furthermore, trapping of the intermediate carbenium ions by CO and water has been observed by insitu NMR spectroscopy when isobutane, water, and CO reacted on HZSM-5 zeolite to form pivalic acid.69,70 Regarding the small conversion, only a limited number of acid sites are suggested to be strong enough for the initial protolytic activation to take place. 

It was previously noted that superelectrophilic carboxonium ions may be generated from suitable precursors, including amino-ketones, /V-heteroaromatic ketones and aldehydes, amino-acetals, and other substrates.45 In their superacid-promoted condensation reactions, these compounds often produce ammonium-carbenium superelectrophiles as intermediates in the reactions. As an example, the amino-acetal (229) reacts with arenes in the presence of superacid to give the arylated product (231, eq 72).43... 

Carbenium ions become intermediates in two-step additions to alkenes when they are more stable than the corresponding cychc onium intermediates. With many electrophiles (e. g., Br2 and Cl2), this occasionally is the case. With others, such as protons and carbenium ions, it is always the case (Figure 3.51). 

In their protonated forms, zeolites are widely employed in the oil and petrochemical industries, in processes such as the conversion of alcohols to gasoline, catalytic cracking, isomerization and alkylations of hydrocarbons [3]. These chemical reactions most probably involve proton transfer from the acidic site of the zeolite to the organic substrate. In the case of hydrocarbons, this transfer gives rise to carbenium (I) or carbonium (II) ions as intermediates or transition states. 

Lockwood, R. F., Nicholas, K. M. Transition metal-stabilized carbenium ions as synthetic intermediates. I. a-[(Alkynyl)dicobalt hexacarbonyl] carbenium ions as propargylating agents. Tetrahedron Lett. 1977, 4163-4166. 

The non-classical version of the intermediate carbenium ion (also referred to as a carbonium ion) resulting upon dissociation of the pyrophosphate anion from famesylpyrophosphate explains the cyclization to several cyclic carbenium ions as demonstrated for some sesquiterpenes (Fig. 3). Additional diversity arises from 1,2-hydride and 1,2-alkyl shifts (Wagner-Meerweest rearrangements) sbA sigmatropic reactions (COPE rearrangements) on the one hand, and on the other hand from the formation of diastereomers and enantiomers provided that the cyclizations generate new asymmetric carbon atoms (Fig. 3)... 

The 1,3-dicarbonyl unit has been shown to be an excellent leaving group in 5 1 reactions catalysed by FeCl3. °" The catalysis is thought to occur when the iron coordinates with the two carbonyl oxygens before the ionization step. These reactions proceed via either a free carbenium ion or a carbenium ion-pair intermediate, which is consumed in a second reaction with a nucleophile such as C(3) of 5-bromoindole. Yields of these alkylation reaetions range from 73 to 99%. 

It is speculated that the mechanism concerning pentacoordinated carbonium ion intermediates (eg. Ri-CH3 -CH2-R2, Ri-CH2 =CH-R2, CcHy" ) occurs at temperatures above 500°C with the intermediates undergoing ft-scission to smaller paraffins and carbenium ions. As well, the carbonium ions are converted to carbenium ions through the loss of hydrogen, present as molecular hydrogen in the cracking products. This mechanism is also favoured by low conversion, low hydrocarbon partial pressure and high constraint indexed zeolites (Scherzer, 1989). 

Under mild conditions, the label scrambling was also observed in oct-l-ene on H-ZSM-5 [131] and in propane on H-ZSM-5 at523-573K [132]. Becausethe scrambling of the label between the 1- and 2-positions is impossible without the formation of protonated methylcyclopropane, which is in the equilibrium with the butyl cation [127] (Scheme 1), one can conclude that carbenium ions as transient intermediates are formed on solid acid catalysts. Recent theoretical estimates of the stability of some cations in zeolites proved this possibility [133-135]. 

S-Substituted thiiranium ions react with water and alcohols to give trans ring opening (Scheme 72). A report that oxygen nucleophiles attack sulfur as well as carbon has been shown to be incorrect (79ACR282). The intermediate thiiranium ion (57) in the presence of lithium perchlorate readily yields the carbenium ion which undergoes a transannular hydride... 

The mechanism was then reexamined 25 years later in 1997 by Kappe. Kappe used H and C spectroscopy to support the argument that the key intermediate in the Biginelli reaction was iminium species 16. In the event, 5 reacted with 3a to form an intermediate hemiaminal 17 which subsequently dehydrated to deliver 16. Iminium cation 16 then reacted with 6 to give 14, which underwent facile cyclodehydration to give 15. Kappe also noted that in the absence of 6, bisureide 8 was afforded as a consequence of nueleophilic attack of 16 by urea (3a). This discovery confirmed the conclusion of Folkers and Johnson in 1933. As far as the proposal from 25 years earlier by Sweet and Fissekis, Kappe saw no evidenee by H and NMR spectroscopy that a carbenium ion was a required species in the Biginelli reaetion. When benzaldehyde (5) and ethyl... 

The reaction mechanism has been confirmed by trapping of intermediates 13, 14 and 15. Because of the fact that neither a carbene nor a carbenium ion species is involved, generally good yields of non-rearranged alkenes 2 are obtained. Together with the easy preparation and use of tosylhydrazones, this explains well the importance of the Shapiro reaction as a synthetic method. 

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