Cations oxonium

One of the nonionic surfactants most used as an enhancer of chemiluminescent reactions is Brij-35. This surfactant increases the reaction of lucigenin with catecholamines by a factor of 2.6 compared with the CL intensity in an aqueous medium [42], This enhancement can be explained in the following way it is known that oxygen from the polyoxyethylene chains in Brij-35 can react with sodium ion to form an oxonium ion, by which means the polyoxyethylene chains act as an oxonium cation. In this way the increase in CL intensity due to Brij-35 can be attributed to the same effect described for the micelles of a cationic surfactant. 

Oxygen-containing solvents with a strong coordinating ability, such as diethyl ether, methyl /.so-butyl ketone and /.so-amyl acetate, form oxonium cations with protons under strongly acidic conditions, e.g. (R20) H. Metals which form anionic complexes in strong acid can be extracted as ion pairs into such solvents. For example, Fe(III) is extracted from 7 M hydrochloric acid into diethyl ether as the ion pair... 

Oxygen has two bonding electrons and two lone pairs. It can bond to two other atoms, and is usually divalent. It can also bond to one atom in a negatively charged form, or to three atoms in a positively-charged form. The oxonium cation produced still carries a lone pair, but these electrons will not participate... 

The amino alcohol intermediate is analogous to the hemiacetal, and both undergo protonation and loss of water, facilitated by the heteroatom. The iminium cation can then lose a proton, but the oxonium cation has no proton to lose instead, it is attacked by a nucleophile, namely a second molecule of alcohol. 

The reaction starts of with a protonation - use the catalyst. Resist the urge to protonate the 4-hydroxyl, but go for the one at position 1 that has the added functionality of the hemiacetal linkage. It is going to be the more reactive one. Protonation is followed by loss of water as leaving group. The intermediate oxonium cation shown is actually a resonance form of the simpler carbocation now you can see the role of the adjacent oxygen. The reaction is completed by attack of the nucleophile, the 4-hydroxyl of another molecule. This is not special, but is merely another version of the hemiacetal synthesis done in part (a). 

L. Hough and A. C. Richardson, Oxonium cation intermediates in the nucleophilic degradation of diethyl-sulphonylglycopyranosylmethane derivatives, Proc. Chem. Soc. (1959) 193-194. 

Since oxonium cations are sensitive to oxidation, C2H50H2+ could be implied in the formation of small amounts of methane... 

The facile synthesis of (24) from (22) has the oxonium cation (23) as an intermediate.62 Vinyl oxocarbenium ions are reported to take part in intermolecular... 

The acetal 14, activated by the iodotrimethylsilane 17, produces the oxonium cation 16 which can be intercepted by allylsilane 1 yielding homoallylic ether 15, one equivalent of methoxytrimethylsilane 18 and the catalyst 17. 

Simpkins et al. used an intramolecular variation of this allylsilane addition to oxonium cations for the synthesis of eight-membered rings [14]. Allylsilane 22, containing the oxonium ion precursor (acetal function) is transformed upon treatment with EtAlCl2 into the medium-sized ring 23 in moderate yield and stereoselectivity (Scheme 13.10). 

Mukaiyama rationalized this low selectivity by invoking the facile formation of benzylic oxonium cations which prefer to react via SN 1-type transition states with reduced chiral induction. His catalyst was prepared in situ by the addition of silver triflate to chlorodiphenylborane in a 1 1 ratio. 

The reaction is based upon the two components condensation between an aldehyde or ketone 6 (or their synthetic equivalents) and alcohol 95, which contains an allylsilane (or vinylsilane) moiety. The IMSC reaction is mediated by Lewis or Bronsted acids, which activate the carbonyl group of 6 towards nucleophilic attack. After addition of alcohol 95 on the activated carbonyl, the oxonium cation 96 is formed, which is intramolecularly captured by the pendant allylsilane function, leading to oxygen-containing rings 97 (Scheme 13.38). This process typically requires a stoichiometric (or more) amount of Lewis acid. 

This side reaction, which complicates the condensation of allylsilanes anti-126, was suppressed by using a-acetoxy acetals such as anti-131 as the oxonium cation precursor. Under these conditions, the desired cis-2,6-disubstituted dihydropyran 132 was isolated in moderate yields but high diastereoselectivity (dr = 94 6 Scheme 13.46). 

A plausible mechanism for the reaction is depicted in Scheme 13.73. Enol ether 200 reacts with the activated aldehyde to give the oxonium cation 204. This species is trapped intramolecularly by the allylsilane nucleophile and a new tetrahydro-pyran ring 202 is formed. 

The proposed mechanism for the reaction is shown in Scheme 13.75. In the first step, the oxonium cation 208, formed by TfOH-catalyzed condensation of an aldehyde with alcohol 206, undergoes an intramolecular cyclization to form the tertiary carbocation 209. In a subsequent step, cation 209 undergoes a pinacol rearrangement, leading to the observed tetrahydropyran 205. 

The stereoselective formation of spiroketals 242 can be explained in terms of the thermodynamic stability of the three possible products. Oxonium cation 245, formed by the condensation of ortholactone 244b and allylsilyl ether 106a, is in equilibrium with the starting materials. Spiroketal 242 also equilibrates under the reaction conditions with the other anomers. The thermodynamically more stable product 242b, stabilized by a double anomeric effect, is obtained as the only product of the reaction (Scheme 13.89) as the substituents attempt to occupy equatorial positions in the newly generated tetrahydropyran ring. 

The proposed mechanism for these reactions is shown in Scheme 13.96 [106], The initial formation of hemiketal 262 is followed by loss of water and generation of oxonium cation 263. Subsequent intramolecular addition of the azide function onto the cation produces intermediate 264. Elimination of a proton and of N2 directly affords the heterocyclic products. An alternative mechanism involving a... 

Like gitonic 1,2-dicationic species, analogous superelectrophiles having two oxonium cationic centers in a 1,3-dicationic structure are so far virtually unknown. There have been no reports of persistent bis-oxonium... 

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