Acetal exchange mechanism

The most widely used approach for the separation of enantiomers by TLC is based on a ligand exchange mechanism using commercially available reversed-phase plates impregnated with a solution of copper acetate and (2S,4R,2 RS)-4-hydroxy-l-(2-hydroxydodecyl)proline in optimized amounts. Figure 7.9 (10,97,98,107-109). Enantiomers are separated based on the differences in the stability of the diastereomeric complexes formed between the sample, copper, and the proline selector. As a consequence, a prime requirement for separation is that the seumple must be able to form complexes with copper. Such compounds include... 

Ketones or aldehydes can undergo acetal exchange with orthoesters. The mechanism starts off as if the orthoester is going to hydrolyse but the alcohol released adds to the ketone and acetal formation begins. The water produced is taken out of the equilibrium by hydrolysis of the orthoester. 

B. Suggest a detailed mechanism for the acetal exchange used in this chapter to make an acetal of a ketone from an orthoester. 

The acetonide is cleaved under mild acidic conditions with catalytic amounts of / -toluene sulfonic acid, releasing hydroxyl diol 42. Performing the reaction in ethanol leads to an acetal exchange process, whose mechanism is analogous to the THP acetal cleavage presented earlier. 

Practice at drawing mechanisms for acetal exchange rather than straightforward acetal synthesis. 

Further results to which the exchange mechanism appears to be relevant have been reported by Cohen and Jankowski (1964) for the deamination of ira7isJro .s-2-decalylamine (20) in acetic acid-water mixtures. Alcohols and acetates of both retained (axial) and inverted (equatorial) configuration were found. The stereochemistry and... 

The first reaction is an acetal exchange controlled by entropy three molecules go in and four come out (the product, two molecules of methanol and one of water). We show just part of the mechanism. 

If the carbonyl oxygen of methyl and ethyl acetate exchanges with the solvent under the conditions of acid-catalyzed hydrolysis in the same way as ethyl benzoate, and if it is assumed that exchange and hydrolysis go by the same intermediate, then the mechanism is the slow bimolecular addition to the ester protonated on the carbonyl oxygen. Asknes and Prue (1959) have presented other evidence suggestive of carbonyl-oxygen protonation. 

Exchange of acetylacetone with Al(acac)3 takes place by a mechanism involving intermediates containing monodentate acac in non-aqueous as in aqueous solution. The kinetics of acetate exchange with trans-[Co(OAc)2 enal in acetic acid have been reported. ... 

Isomerisation studies of cobalt(iii) complexes cover a variety of compounds. The rate law for isomerisation of [Co(dien)(OH2)a] + indicates some isomerisation via the hydroxo-cation [Co(OH)dien(OHa)2] +. Rates and activation parameters for direct isomerisation of the tris-aquo-complex were determined. Further kinetic results for isomerisation of cis- and rra 5 -[Co(OH)2 en2]+ in strongly basic solution indicate an intramolecular mechanism. Isomerisation of cw-[CoCl2(diars)2] in methanol is also intramolecular, but isomerisation of cw-[Co(02C CH3)2en2]+ in acetic acid occupies an intermediate position between intra- and inter-molecu-larity, for the essential step is solvent-assisted acetate exchange within ion-pairs. This assignment of mechanism results from consideration of kinetics of acetate exchange as well as of isomerisation. ... 

It has now been shown by Knabe, Dyke, and their co-workers, that this interesting rearrangement occurs by a bimolecular exchange mechanism. Cycli-zation of an optically active ( ) sample of the amino acetal 31 provides optically active 30, most probably through transition state 32. ... 

As earlier mentioned, the resin does not hind thiocyanate hy an ion-exchange mechanism hut hy adsorption forces, prohahly related to the strong chaotropic effect of thiocyanate. This interpretation is supported hy the fact that thiocyanate is displaced from the resin hy other chaotropic ions such as perchlorate, trichloro-acetate and nitrate, hut not hy non-chaotropic ions such as chloride or sulfate (Table l). 

It was assumed that the role of the fight was simply to convert the trans isomer to the cis form and that the cyclization was an acid-catalyzed thermal reaction. The acceleration was realized because the cis isomer is in a conformation that is more favorable for cyclization. Later work by Dewar and Sutherland demonstrated that, in neutral ethanol, 60 underwent photocycfization to 61. They proposed a mechanism involving initial ESPT from the phenol to solvent, followed by attack of the phenolate on the carbonyl to give initially 62 and then 61 (thermal acetal exchange with solvent) ... 

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