Oxonium ions, nucleophilic attack

C(6) to give an oxonium ion. Nucleophilic attack, probably by acdtic acid, on the benzylic carbon gives a O-acetyl sugar derivative and benzyl acetate. Glycosides are also acetolysed under these reaction conditions. 

The difference in reactivity between alkylated and acylated pentenyl glycosides can be rationalised as follows the elctrophilic iodonium ion will add to the double bond of the pentenyl moiety to give a cyclic iodonium ion. Nucleophilic attack by the oxygen will lead to an oxonium ion intermediate which then forms an oxocarbenium ion and an iodo-tetrahydrofuran derivative. The aglycone oxygen will be of low... 

Jones and Plesch have reported on decomposition of trietl rloxonium salts with BF, PFe, and SbFg in CH2CI2 solution. At 60° rate constants of decomposition were as follows (in s ) 3.3 lO" (BF ), 7. lO CPFe), 1.3 lO- (SbFe). Thus SbFs anion is at these conditions approx. 50 times nrore stable than PFe anion. Decomposition rate is influenced by additions of nucleophiles into reaction s) tem (e.g. (C2Hs)20, 1,3-dioxolane). Solvation shell, formed by additives, retards oxonium ion against attack of the anion, because overaU enthalpy of activation increases upon addition of ether (AAH = 7 kcal mole ). Entropy of activation increases even to higher extent (AAS < 30 e.u.). This is connected with release of solvating ether molecules in the course of reaction. 

Step 2 Nucleophilic attack by water on carbon of the oxonium ion The carbon-oxygen bond of the ring is broken in this step and the ring opens... 

Protonation of the carbonyl oxygen as emphasized earlier makes the carbonyl group more susceptible to nucleophilic attack A water molecule adds to the carbonyl group of the protonated ester m step 2 Loss of a proton from the resulting oxonium ion gives the neutral form of the tetrahedral intermediate m step 3 and completes the first stage of the mechanism... 

Propa.ga.tlon, The tertiary THF oxonium ion undergoes propagation by an S. mechanism as a result of a bimolecular colHsion with THF monomer. Only colHsions at the ring a-carbon atoms of the oxonium ion result in chain growth. Depropagation results from an intramolecular nucleophilic attack of the penultimate chain oxygen atom at the exocycHc a-carbon atom of the oxonium ion, followed by expulsion of a monomer molecule. 

These give the products expected from electrophilic attack on oxygen by the electrophilic reagent atom, followed by nucleophilic opening of the cyclic oxonium ion (e.g. 42 Scheme 29) <64HC(19-1)436). 

The use of iodotrimethylsilane for this purpose provides an effective alternative to known methods. Thus the reaction of primary and secondary methyl ethers with iodotrimethylsilane in chloroform or acetonitrile at 25—60° for 2—64 hours affords the corresponding trimethylsilyl ethers in high yield. The alcohols may be liberated from the trimethylsilyl ethers by methanolysis. The mechanism of the ether cleavage is presumed to involve initial formation of a trimethylsilyl oxonium ion which is converted to the silyl ether by nucleophilic attack of iodide at the methyl group. tert-Butyl, trityl, and benzyl ethers of primary and secondary alcohols are rapidly converted to trimethylsilyl ethers by the action of iodotrimethylsilane, probably via heterolysis of silyl oxonium ion intermediates. The cleavage of aryl methyl ethers to aryl trimethylsilyl ethers may also be effected more slowly by reaction with iodotrimethylsilane at 25—50° in chloroform or sulfolane for 12-125 hours, with iodotrimethylsilane at 100—110° in the absence of solvent, " and with iodotrimethylsilane generated in situ from iodine and trimcthylphenylsilane at 100°. ... 

The boron trifluoride-alkyl thiol reagent combination also operates on the basis of nucleophilic attack on an oxonium ion generated by reaction of the ether with boron trifluoride.90... 

The oxonium ion intermediate of the type 34 normally collapses by the attack of a nucleophile OH or OAc to yield a masked aldehyde or hemi-acetal-acetate group. However, in this particular case the approach of... 

The dehydration of 2,4,4-trimethylchroman-2-ol to the chromene is of interest because of the source of the chromanol. 1,1,3-Trimethylindane hydroperoxide (178) undergoes an acid-catalyzed rearrangement to a mixture of the chromanol and chromene. The intermediacy of an oxonium ion can be envisaged, with nucleophilic attack at C-2 leading to the chromanol and proton loss giving the chromene (Scheme 29) (56JCS4785). 

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