Isolated carbenium ions

Fluoboric acid is also an efficacious promoter of cyclic oxo-carbenium ions (Scheme 4.24) bearing an activated double bond which, in the presence of open-chain and cyclic dienes, rapidly undergo a Diels-Alder reaction [91]. Chiral a, -unsaturated ketones bearing a -hydroxy substituents, protected as acetals, react with various dienes in the presence of HBF4, affording Diels-Alder adducts that were isolated as alcohols by hydrolysis of the acetal group by TsOH. Some examples of reactions with isoprene are reported in Table 4.23. The enantios-electivity of the reaction is dependent on the size of the substituent R on the of-carbon high levels of asymmetric induction were observed with R = z-Pr (90 1) and R = t-Bu (150 1) and low levels with R = Me (2.7 1) and R = Ph (3.0 1). Scheme 4.24 shows the postulated reaction mechanism. 

Explanation The increase of both Y and DP with time implies a growing species of long life, which is characteristic of esters but not of carbenium ions, and an absence of transfer reactions. The suppression of all kinds of transfer reactions seems to require some very special features in the anionoid moiety of the ester which are not yet understood fully but which, by hypothesis, cannot affect the reaction pattern of an isolated, unpaired cation. The fact that living polymerisations can occur in toluene is a convincing demonstration that these reactions cannot involve carbenium ions, because growing cations alkylate toluene [33-37], a process which produces low DPs, independent of Y. 

X-ray photoelectron (ESCA) spectra of carbenium ions have also been obtained in frozen superacid solutions, generally in SbF5-S02 solution or as isolated salts. Sulfur dioxide was subsequently removed by the usual freeze-thaw procedure. A thin layer of the viscous SbF5 solution was deposited on the precooled sample holder, in a dry nitrogen... 

When R—X is enantiomerically enriched exo-2-norbornyl 4-bromobenzenesulfonate (brosylate), 1 in Fig. 4.5, and SOH is aqueous ethanol, for example, solvolysis is accompanied by racemisation. However, the rate of racemisation is faster than the rate of product formation, and starting material isolated before completion of the solvolysis is partially racemised [14]. The most economical interpretation of these results (and much other evidence, see Chapter 7) is that R+ X- includes the achiral nonclassical carbenium ion (3 in Fig. 4.5), and the ion pair undergoes internal return faster than nucleophilic capture. In other words, k- > k2 in Scheme 4.4, the precise value of the ratio k- lk2 depending upon the particular solvent, and the greater this ratio, the closer the initial reversible process approaches a pre-equilibrium. 

Dication 56 has been isolated and studied by crystallography, revealing a separation of the carbenium ion centers by 3.66 A. 

This cation can be drawn either as an oxonium ion or as a primary carbenium ion. The oxonium ion structure is the more realistic. Primary carbenium ions are not known in solution, let alone as isolable intermediates, and the proton NMR spectrum of the cation compared with that of the isopropyl cation (this is the best comparison we can make) shows that the protons on the CH2 group resonate at 9.9 p.p.m. instead of at the 13.0 p.p.m. of the true carbenium ion. 

Let us discuss in more detail all these factors. First of all, it should be stressed that onium ions (e.g., trialkyloxonium, tetraalkylammonium, trialkylsulfonium) are inherently stable different onium salts may be isolated and stored in a pure state, some of them are commercially available. This is in contrast to cationic vinyl polymerization, where carbenium ion species are not inherently stable due to the possibility of proton expulsion ... 

Some organic cation salts can be isolated as crystalline solids. Stability of carbenium ions is enhanced if the electron-deficient carbon is con jugated with olefin or aromatic groups or with atoms with unshared electron pairs (O, N, S). The positive charge is diflused over a larger region as a consequence. Examples of such initiators are hexachloroantimonate (SbCl ) salts of triphenyl methyl ((Cf,H5)3C ) and cyclohepirienyl (C7H ) carbenium ions. 

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