Carbocation long-lived, observation

My work on long-lived (persistent) carbocations dates back to the late 1950s at Dow and resulted in the first direct observation of alkyl cations. Subsequently, a wide spectrum of carbocations as long-lived species was studied using antimony pentafluoride as an extremely strong Lewis acid and later using other highly acidic (superacidic) systems. 

Silyl effects in carbocations which were predicted computationally and have been observed in the gas phase and in solvolysis reactions have been proven by NMR spectroscopic investigation of long-lived silyl-substftuted carbocations in... 

Cyclobutenyl carbocations have been under study in our laboratory for some years (11). In (34), unusually strong temperature dependence of l3C chemical shifts (up to 0.077 ppm/K) was observed for long-lived cations 17a-c. The authors suggested that this resulted from a very low inversion barrier for the four-membered ring (Scheme 13). 

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). 

P-Fluonne or fluorine further removed from the cation center always inductively destabilizes carbocations [115, 116] No simple P-fluoroalkyl cations have been observed in either the gas phase or solution, and unlike the cases of the other halogens, there is no evidence for formation of alkyl fluoronium ions (5) in solution [117, 118], although (CH3)2F+ is long-lived in the gas phase [7791 The only P-fluonnated cations observed m solution are those that benefit from additional conjugativc stabilization, such as a-trifluoromethylbenzyl cations [772] and per-fluonnated allyl [729], cyclopropenium [772], and tropyliiim [727] ions... 

Under the long-lived conditions of carbocations in superacid (Section 5.3, p. 235), 1,2-shifts interconverting ions of like stability also occur and are very rapid. For example, at — 180°C the five methyl groups of 2,3,3-trimethylbutyl cation have only one peak in the nmr. This observation implies that the methyl shift in Equation 6.6 occurs at the rate of 75 x 103 sec -1 with an activation barrier of <5 kcal mole-1.8... 

This chapter begins with a short historic retrospect about the development of the carbocation concepts and covers the techniques used for their generation, observation, and characterization under superacidic long-lived conditions. This is followed by an extensive coverage of the multitude of trivalent (classical) and equilibrating (degenerate) and higher (five or six) coordinate (nonclassical) carbocations. 

On the basis of the study of carbocations by direct observation of long-lived species, it became increasingly apparent that the carbocation concept is much wider than previously realized and necessitated a general definition.21 Therefore, such a definition was offered based on the realization that two distinct, limiting classes of carbocations exist (Figure 3.1). 

The first stable, long-lived carbocation observed was the triphenylmethyl (trityl) cation 135.6 y... 

Pentafluorophenyl has an excellent ability to enchance the stability of allylic linear and cyclic carbocations. Species 26 and 27 were observed as long-lived species in the reaction of SbF5 with corresponding olefins [69] ... 

Until the early 1960s information about carbocations has been obtained almost exclusively from indirect evidence. A major change occurred in 1962 when Olah reported the generation and direct observation of carbocations as long-lived species in solvents of low nucleophilicity (superacidic conditions) [11,12]. Many types of carbocations have since... 

The ions [315] are long-lived compared with other p-aryl-substituted carbonium ions such as 2,3-dimethyl-3-aryl-2-butyl cations [93] and [95] and are therefore suitable for observation of intramolecular donor-acceptor com-plexation between an aromatic donor at C and a carbocation centre. Borodkin et al. (1973) studied the UV-spectra of ions [315 X = CH3, H, Cl, F or CF3] and found charge-transfer bands as a result of the donor-acceptor interaction. There is a linear relationship between the frequency of these... 

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). 

Resonance-stabilised long-lived carbocations such as 4.26A and 4.26B have been generated from precursor alcohols in superacidic solution [72] but, if conjugative stabilisation is absent as in 4.26C, then only protonated alcohols are observed. Similarly, ketones with up to three a-fluorine atoms can be protonated giving, for example, 4.26D, whilst hexafluoroacetone is not protonated in a superacidic medium [73] (Figure 4.26). 

Using the GIAO-MP2 method (tzp/dz basis set), Rasul et aU " calculated the B NMR chemical shifts of hypercoordinate boron atoms in selected boron compounds and NMR chemical shifts of the corresponding isoelectronic and isostructural carbocations for systems not yet observed under long-lived superacidic conditions B NMR and NMR chemical shift values for the methonium ion CH5+ (5) and the corresponding neutral boron analog BHs (15) were shown to reproduce well the experimental values. Calculations were also performed for the six-coordinate carbocation (diprotonated methane, 18) seven-coordinate carbocation (triprotonated methane, 27) the... 

Since it was considered less difficult to suppress side reaction in the pMOS polymerization due to the stabilization of the growing carbocation, cationic polymerization of pMOS was investigated in detail using iodine. The MWDs of product polymers became unimodal in the polymerization in CCI4 at 0 °C. A nearly linear relationship was observed between the peak molecular weight of the product polymers and monomer conversion, indicative of polymerization mediated by long-lived active species. At -15 °C, the of product polymers increased in almost direct proportion to monomer conversion after the second feed of pMOS. Moreover, block copolymers with IBVE were obtained under similar conditions. 

The variations in allyl cation geometries are sensitive to the hyperconjugative interactions of alkyl substituents. Alkylated allyl cations were experimentally characterized with H-NMR, UV, and IR spectra. Recent developments of a matrix isolation technique for generating carbocations enabled the direct observation of small allyl cations with only a few stabilizing alkyl substituents. The parent allyl cation, the smallest long-lived carbocation observed in a solid SbFs matrix, could be characterized by IR but not by NMR spectroscopy. 

In principle, although not always in practice, reaction intermediates may be isolated. They exist at minima on the energy curve and have a definite lifetime, which may be long or short. For example, in the reaction of an excess of bromoethane with methylamine to give the ammonium salt [Et3NMe]+Br (8.6), the intermediates ethylmethylamine and diethylmethylamine are stable compounds, and if the reaction was stopped at an appropriate time, they could be isolated. Other intermediates, such as the carbocation, 8.2, are short lived and unstable and cannot be isolated, although they can sometimes be observed spectroscopically (Figure 8.7). 

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