Carbenium transient formation

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 hydride transfer from a hydrosilane to the carbenium ion leads to the transient formation of a complex of tricoordinate silicon cation with solvent. Lambert et al. showed that if a stable counterion of very low nucleophilicity is used, the complex of silylenium ion with solvent is a persistent species [12]. Tetrakis(pentafluorophenyl)borate was successfiilly explored for this purpose, and this system was later used by Olah ef al. [1] for the generation of trisilyloxonium ions derived from D4... 

Biotransformation begins with the transient formation of a reactive intermediate, whose lifetime is long enough to allow an attack on cellular components. This occurs when a reactive intermediate (such as a radical or a carbenium ion) is formed and reacts rapidly with nucleophilic functions in cellular macromolecules (such as unsaturated lipids, proteins, nucleic acids), thus leading to their degradation and finally to cellular necrosis. 

Figure 26.4 Transient carbenium ion formation from an indolic framework.

In early 1993, Haw and co-workers (107) reported in situ studies of allyl alcohol-/-13C on HZSM-5 and CsHX. No persistent carbenium ions were observed, but 1,3 label exchange was observed for the alcohol on the weakly acidic zeolite. We interpreted this as support for a transient allyl cation. The low stability of this cation was invoked to explain the failure to observe this species as a persistent species. Downfield signals observed in that study were attributed to the formation of propanal. Later in 1993, Biaglow, Gorte, and White (BGW) (108) reported similar studies conducted at different loadings and assigned a downfield resonance (variously reported at 216 and 218 ppm by BGW) to the allyl cation in HZSM-5. 

This reaction has been the subject of many experimental, " and theoretical studies. The reaction, which initiates from a propylene physisorbed to the acidic proton, leads to the formation of a more stable chemisorbed propylene, or alkoxy species. Such reaction has been shown experimentally to occur readily at room temperature within an acidic zeolite. Whereas this reaction in principle can produce two different alkoxy species (viz. a primary and a secondary alkoxy species), experiment reports that only the secondary alkoxy species can be formed. This is explained by the fact that the formation of a transient primary carbenium ion is energetically more demanding than the formation of a secondary carbenium ion. " As already... 

As in the case of carbenium ions, one must distinguish between stable cation-radical salts which can be prepared and characterised without excessive precautions, and which owe their stability to considerable delocalisation of the positive charge and the unpaired electron, and transient, reactive cation-radicals (often without counterion) which undergo rapid secondary reactions immediately after their formation. While recently a few instances have been reported of initiation by the first type of species (see Sect. V-E), the presence of the less stable entities is characteristic in such initiation processes as the activation of charge-transfer complexes, the anodic oxidation of suitable anions... 

Althou the formation of cation-radicals is certainly the first step in the reaction sequences resulting from the use of the initiation techniques enumerated above, the subsequent steps are less well established. Various pathways are available to this first transient coupling to give dications, reaction with the monomer to give the carbenium ion or the dimer radical-cation, etc. Eventually, however the carbenium ions become the real active species, i.e., all these polymerisations involve a composite initiation mechanism where the cation-radicals are playing just a momentary role. 

It is well known that even modern Fourier-transform instruments require concentrations of at least 10 M to give a reasonably resolved NMR spectrum. Also, the minimum time necessary for a spectrum to be taken or for one given signal to be scanned is of the order of a minute. These constraints rule out the possibility of identifying short-lived transients or any compound present in low concentrations formed during a cationic polymerisation. Thus, while carbenium ions have been characterised by this technique under suitable conditions, their concentration in a polymerising solution is always too low to permit detection by NMR spectroscopy. On the other hand, one can easily follow the disappearance of the monomer and/or the formation of the polymer and of any other product (such as an ester) formed in appreciable quantities, an operation which is extremely convenient, not only from the point of view of the kinetics of pol)onerisation, Init also because it can lead to mechanistic information concerning the nature and the structure of various products. 

Kennedy and collaborators have studied the polymerisation of olefinic monomers induced by catalytic combinations consistir of mixtures of organic chlorides and aluminium alkyls. Although the authors claimed that these mixtures gave rise to carbenium salts capable of initiating viiq l polymermtion, we believe that the process leading to active species does not necessarily imply the formation of transient carbenium ions derived from the catalyst pair. For this reason, these systems are not discussed in this section but rather in Chapter IV together with other aluminium halide initiated polymerisations. 

A carbocation is an sp hybridized carbon bearing three substituents with an empty p orbital perpendicular to the plane of the other atoms (see 3), which has been termed a carbenium ion by Olah. It is a transient inter-mediate and reacts with reagents that can provide a pair of electrons to form a new bond to the electrophilic carbon. Formation of this cation is usually the slowest step (the rate determining step) in a... 

Interestingly, the product ratio depends upon neither the concentration of HCl nor the concentration of added chloride salts. If chloride and acetic acid (HOAc) compete for addition to a transient carbenium ion, one would expect increasing chloride concentrations to divert the intermediate to the formation of alkyl chloride products, but this is not seen. This... 

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]. 

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