Oxonium ions phenols

Donation of a proton to the reactant often forms a carbenium ion or an oxonium ion, which then reacts ia the catalytic cycle. For example, a catalytic cycle suggested for the conversion of phenol and acetone iato bisphenol A, which is an important monomer used to manufacture epoxy resias and polycarbonates, ia an aqueous mineral acid solution is shown ia Figure 1 (10). 

Another similarity with water is that alcohols and phenols are both weakly basic and weaJkly acidic. As weak bases, they are reversibly protonated by strong acids to yield oxonium ions, TOFty. ... 

Oxonium ions are excellent alkylating agents, and ethers can be conveniently prepared by treating them with alcohols or phenols. Quaternary ammonium salts can sometimes also be used. ... 

One must treat a reaction mixture with the sodium salt RO-Na+ of an alcohol or phenol, and search for compounds of the type H-[0(CH2)20CH2] -0R, which would be formed from homologues of (VII) (Mainz theory), but not from secondary oxonium ions such as (V) or (VI) (Y = H) (Keele theory), which would form ROH. 

The test (b) we carried out typically as follows but we have also used many variations of this procedure [18]. We used an assembly of connected reaction tubes attached to the vacuum line. In one tube we polymerised (I) by perchloric acid in methylene dichloride. Reaction was stopped by adding sodium phenate, and any phenol formed from secondary oxonium ions was neutralised with sodium hydride. The volatile compounds were distilled into a second tube where the same experiment was repeated. This technique is based on that of Saegusa and Matsumoto [19] phenol and phenyl ethers can be estimated separately by their UV spectra. 

A related dienediol-phenol rearrangement which can occur by different pathways was reported as a new method for synthesis of the oxepine system180. Protonation of the starting diol 344 produces a cation 345 which can follow normal dienone-phenol rearrangement (path a) when the substituents R2 = Me, Ph and R1 = t-Bu are eliminated in the step 346 — 347. However, when R1 = t-Bu and R2 is a substituted phenyl which decreases the nucleophility, the cationoid intermediate 345 cyclizes to the oxonium ion 348 (path b) which then undergoes deprotonation to give the oxepine 349 (equation 124)180. 

Bisphenol A [(bis-4-hydroxyphenol)dimethylmethane], used for the production of epoxy resins and polycarbonates, is obtained by the acidic condensation of phenol and acetone. Here, the carbonium ion produced by the protonation of acetone attacks the phenol molecule at the para position producing a quinoidal oxonium ion that loses water and rearranges to a p-isopropylphenol carbonium ion. The water attacks another phenol molecule, also in the para position, giving another quinoidal structure that rearranges to bisphenol A. It has been found that bisphenol A may be involved in one of the endocrine systems. The consequences of this are still being determined. 

There has been some controversy about the mechanisms of the Simonis and Pechmann reactions, which still remain in doubt. It has been suggested (50BSF1132) that the condensations proceed through a common oxonium ion (467). Dehydration to the phenoxyacrylic ester (468) is followed by cyclization to the chromone whilst a rearrangement to the substituted phenol (469) subsequently affords the coumarin (Scheme 171). 

The addition of alcohols and phenols to double bonds is catalyzed by acids or bases. When the reactions are acid catalyzed, the mechanism is electrophilic, with H" " as the species attacked by the 7i-bond. The resulting carbocation combines with a molecule of alcohol to give an oxonium ion, 38. 

The removal of all methyl ethers in 22 is performed by using the Lewis acid BBrs. This trivalent boron compound is very electrophilic and attacks the oxygen atom in aryl methyl ether 40 to give intermediate 41. Then, the resulting oxonium ion 42 is attacked by Br in an Sn2 reaction providing the free phenol 44 after aqueous workup. 

The hemiacetal gives a locally planar oxonium ion that can add the phenol from the top or bottom face. The bottom face is preferred in this instance as axial C-O bonds are more stable in acetals because of the anomeric effect (p. 1130) and acetal formation is under thermodynamic control. 

Like the amino group, the phenolic group powerfully activates aromatic rings toward electrophilic substitution, and in essentially the same way. The intermediates are hardly carbonium ions at all, but rather oxonium Ions (like I and II), in which every atom (except hydrogen) has a complete octet of electrons ... 

This observation is in line with the preferential protonation at the ring, not at the oxygen atom, of phenol or anisole (cf. Section IV.A). Distonic dehydro-oxonium ions 50 are therefore not generated in these chemical ionization experiments, in line with the fact that they are more than 200 kJ mol less stable than ions 49 (Chart 7). A major fragmentation of ions 50 should be a loss of HOH or ROH with the production of benzyne ions (m/z 76), but the relative intensity of this peak is not increased, thus confirming that ions 50 are not produced to a significant extent in the protonation-debromination sequence. 

The acidic hydrogen of phenol may participate in the formation of a phenolacetyl nitrate-silica complex, in which the nitro group is well positioned in a six-membered transition state, for the ortlio-attack. In other words, the initially formed oxonium ion is... 

The reaction sites in alcohols, phenols, and ethers are the polar bonds (carbon-oxygen and oxygen-hydrogen) and the lone pairs of electrons on the oxygen. The unshared electron-pairs on alcohols and ethers make these compounds Lewis bases. Oxoniums ions, in which the oxygen has three bonds and is positive, result from the protonation of alcohols and ethers. Most reactions of alcohols involve the O-H bond, C-0 bond, or both. 

Chantal et al. (8,10) have studied the effect of methanol cofeeding on product distribution from phenolic compounds. They proposed an oxonium mechanism to explain alkylation where oxonium ions are generated from diphenylether and anisole, intermediates that were formed from phenolic starting materials. They used low levels of methanol in these experiments 90/10 (phenol/methanol) vs 1/1 (wood pyrolzate/methanol) in the results reported in this paper and by Chen et al. (12). Under conditions of high methanol concentrations, and in the presence of carbohydrate-derived material, the formation of an oxonium ion from methanol (CH30H2 ) as proposed by Aranson et al. (24) is also possible. Direct reaction with either wood-derived reactants or various products from zeolite catalysis could explain the observed synergistis effect. 

It is believed that methyl aromatic ether is protonated from the thermally stable pyridine hydrochloride to form an oxonium ion, which undergoes nucleophilic substitution with chloride to evolve methyl chloride and yield phenol derivatives, as exemplified here by the reaction of anisole. 

With acid-catalyzed curing, the phenol alcohol is protonated at the more basic methylol group. The oxonium ion formed eliminates water to form a benzyl carbonium ion and then reacts with a compound containing at least two nucleophilic groups, HY, to produce cross-links. Here, Y can be O-alkyl,... 

The stability of the intermediate oxonium ions V and VII determine which of the two possible routes leading either to the phenolic species IV, or to the aromatic vinyl ether species VIII is preferred. In the case of an aliphatic group R, the oxonium ion V, stabilized by an aromatic system, is preferred over the aliphatic oxonium ion VII. This favors the cleavage to the phenolic compound IV but not the formation of the vinyl ether VIII. This vinyl ether reacts further to the symmetrical species III as depicted in Scheme 1. High catalyst concentration or high temperature causes the formation of the aromatic vinylether VIII to compete with the reaction leading to IV (42). In this case an intermolecular crosslinking between two polymer chains occurs. 

Rearrangement of oxonium ions. In the acid-catalysed cleavage of cumene hydroperoxide (to phenol and acetone), an important step is aryl transfer from carbon to oxygen in the intermediate oxonium ion ... 

Bakelite A can be acid- or base-cured or cured without catalysts. In the acid-curing, the more basic alcoholic hydroxyl group of the phenol alcohol is protonated. A benzyl carbonium ion, which is formed by the elimination of water from the oxonium ion, then reacts with a compound containing at least two nucleophilic groups HY— ... 

Alcohols (and phenols) function not only as weak acids but also as weak bases. They have unshared electron pairs on the oxygen and are therefore Lewis bases. They can be protonated by strong acids. The product, analogous to the oxonium ion, H30, is an alkyloxonium ion. 

Formation of the oxonium ion is accompanied by formation of the nucleophilic bromide ion. Bromide ion attacks the sp hybridized and less hindered methyl group in an Sn2 reaction rather than the sp carbon of the benzene ring, which requires significantly higher energy to achieve the requisite transition state. Therefore, the product is bromomethane and phenol. 

Notably, optimization studies exposed the critical influence of phenol on the reactivity and enantioselectivity within this manifold, suggesting a two-step pathway as illustrated in Figure 9. Initially, enantioselective protonation takes place from the chiral Brdnsted acid 57 or oxonium ion pair 60, generated by rapid proton transfer between 57 and phenol, to silyl enol ether 61 to form chiral ion pair 62. This is followed by desilylation with phenol to form the corresponding ketone 63, silylated phenol, and catalyst 57 for further turnover. 

Because the oxonium ion resonance form has an octet of electrons around each of the atoms, it is the most important contributor. The resultant stabilization of the ion compared to a typical carbocation is part of the reason for the rearrangement reaction. (We recall that the cyclohexadienyl cation intermediates formed in aromatic substitution reactions are also stabilized by the resonance donation of electrons from the oxygen atom of phenols and anisoles.) Loss of the hydrogen atom bonded to oxygen from the resonance-stabilized cation yields the final product, 3,3-dimethyl-2-butanone, pinacolone. 

---