Hydroxy carbenium ion

Electrolysis of /J-hydroxy carboxylic acid (90) gave ketone (91) as the main product, which was converted to dl-muscone (Scheme 34) [113]. The migratory aptitude of alkyl substituents at the fi-position in the intermediate -hydroxy carbenium ion was scrutinized to show that vinyl, cyclopropyl, and benzyl groups rearrange predominantly. 

Of interest is a comparison of a-methoxy with a-hydroxy substituents. The a-hydroxy carbenium ions correspond to protonated ketones and their pAR values may be derived from a combination of a hydration equilibrium constant and a pfor protonation of the ketone, as illustrated by the thermodynamic cycle based on acetophenone in Scheme 15.I37,I3X Corresponding data are available for benzaldehyde138,139 and acetone70,140,141 and lead to the values of... 

The OH-group signals in the PMR spectra of hydroxybenzeniiun ions are usually located in a much higher field (by 4-6 ppm) than those in the spectra of the hydroxy-carbenium ions of. the aliphatic series (cf. this is due to a more effective delocalization of the positive charge in the ions of the former type. 

In the initial step one hydroxy group is protonated, and thus converted into a good leaving group—i.e. water. Subsequent loss of water from the molecule proceeds in such a way that the more stable carbenium ion species 2 is formed. The next step is a 1,2-shift of a group R to the tertiary carbenium center to give a hydroxycarbenium ion species 4 ... 

The reaction is strictly intramolecular the migrating group R is never completely released from the substrate. The driving force is the formation of the more stable rearranged carbenium ion 4, that is stabilized by the hydroxy substituent. The... 

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. 

As described in Section III, practically all known conversions of 2-benzopyrylium salts are the result of their reactions with nucleophiles, and 2-benzopyrylium, similar to its monocyclic analog [82AHC(Suppl)], behaves chemically more like a carbenium ion, and not like an oxonium salt. This fact can be easily understood if one takes into account resonance formulas a-f, where an oxonium form f is added for the presence of an alkoxy (hydroxy) substituent in position 6. 

A carbenium ion with a /3-hydroxy group, E stabilizes itself by way of a carbenium ion —> carboxonium ion rearrangement. Such a rearrangement occurs in the third step... 

The isomerization of acyloxycarbocations 170 into tertiary carbenium ions 171 is due to the almost equal energies of both intermediates110. Therefore, in comparison with the related hydroxy- and alkoxycarbocations 105, the acyloxycarbenium ions 163 have far lower stability and can be regarded as destabilized carbocations111. 

Starting with the knowledge of carbenium ion stability and the understanding that it is necessary to decrease this stability enabling the reaction of these ions with nitriles102 (Section III.B.l), it should be assumed that acetals 209, oc-haloethers 211, enols 217 and vinyl ethers 218 are ineffective precursors since they are the sources of the highly stable hydroxy- (223) and alkoxycarbenium ions 224. In contrast, the carbonyl compound derivatives, which can produce destabilized ions (in comparison to ions 223 and 224) are the most interesting for reactions with nitriles. This includes acyloxycarbocations 225 (see Section III.B.3), halocarbocations 226, A-acyliminium ions 228 and vinyl cations 229. 

The electrolysis products of different carboxylates have been compared with the ionization potentials of the intermediate radicals. From this it appeared that alkyl radicals with gas-phase ionization potentials smaller than 8 eV mainly lead to carbenium ions. Accordingly, a-substituents such as carboxy, cyano or hydrogen support the radical pathway, whilst alkyl, cycloalkyl, chloro, bromo, amino, alkoxy, hydroxy, acyloxy or aryl more or less favor the route to carbenium ions. Besides electronic effects, the oxidation seems also to be influenced by steric factors. Bulky substituents diminish the extent of coupling. The main experimental factors that affect the yield in the Kolbe electrolysis are the current density, the pH of the electrolyte, ionic additives, the solvent and the anode material. 

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