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Hydrolysis iminium ion

Step 1 Enamine alkylation followed by iminium ion hydrolysis. [Pg.111]

Remarkably, the presence of water showed beneficial effects on both reaction rates and selectivities, while facilitating the iminium ion hydrolysis step in the catalytic cycle. Computational studies suggest an asynchronous mechanism for the reaction [23-25], where the attack of the diene to the / -carbon atom of the iminium ion is the rate-limiting step [23], and the 7r,7r-interaction between the phenyl ring of the catalyst s benzyl group and the olefinic 7r-system of the iminium ion accounts for the selectivity of the reaction [6, 24]. [Pg.99]

Michael additions [63] and is now established as a general strategy for the asymmetric conjugate addition of nucleophiles at the (3-position of a,(3-unsaturated carbonyl compounds. The standard catalytic cycle for a chiral pyrrolidine-catalyzed (3-functionalization of an a,(3-unsaturated carbonyl compound is shown in Scheme 2.12, and it begins with the acid-promoted condensation of the carbonyl with the amine to form an unsaturated iminium ion, more electrophilic than the starting unsaturated carbonyl. This reactive intermediate suffers then the addition of the nucleophile at the (3-position, leading to a (3-functionalized enamine in tautomeric equilibrium with an iminium ion. Hydrolysis of this intermediate releases both the product and the chiral ammonium salt, which can reenter the catalytic cycle. [Pg.31]

Acid catalysed iminium ion hydrolysis (compare hydrolysis of an acetal) then occurs ester hydrolysis does not occur under these mild conditions. [Pg.106]

BF4] or [bmim][PF6] (Scheme 22.8). Bis-amide 19-catalyzed aldol reactions performed in [bmimllBFJ required a much lower excess of donor ketone 21 (3 equiv. instead of 30 equiv. in proline-catalyzed reactions) and allowed a synthesis of chiral compounds 22 bearing heterocyclic, prenyl, or metallocene units [43], The improved catalytic performance of prolinamide derivatives in ionic liquids might be due to a stabihzation of the iminium intermediate formed from the ketone and the catalyst or because of the enhanced nucleophilicity of the enamine [42]. Notably, IL dilution with water (1 1 by volume) accelerated the enamine/iminium ion hydrolysis and raised reaction rates and product yields, with the enantioselec-tivity being retained or even becoming somewhat higher than under water-free conditions [45], Furthermore, the catalyst/lL/water system could be easily recycled five times without aldol yield, dr, and ee losses. [Pg.623]

The tetrasubstituted isomer of the morpholine enamine of 2-methyl-cyclohexanone (20) because cf the diminished electronic overlap should be expected to exhibit lower degree of enamine-type reactivity toward electrophilic agents than the trisubstituted isomer. This was demonstrated to be the case when the treatment of the enamine with dilute acetic acid at room temperature resulted in the completely selective hydrolysis of the trisubstituted isomer within 5 min. The tetrasubstituted isomer was rather slow to react and was 96% hydrolyzed after 22 hr (77). The slowness might also be due to the intermediacy of quaternary iminium ion 23, which suffers from a severe. 4< strain 7,7a) between the equatorial C-2 methyl group and the methylene group adjacent to the nitrogen atom, 23 being formed by the stereoelectronically controlled axial protonation of 20. [Pg.9]

In all the reactions described so far a chiral Lewis acid has been employed to promote the Diels-Alder reaction, but recently a completely different methodology for the asymmetric Diels-Alder reaction has been published. MacMillan and coworkers reported that the chiral secondary amine 40 catalyzes the Diels-Alder reaction between a,/ -unsaturated aldehydes and a variety of dienes [59]. The reaction mechanism is shown in Scheme 1.73. An a,/ -unsaturated aldehyde reacts with the chiral amine 40 to give an iminium ion that is sufficiently activated to engage a diene reaction partner. Diels-Alder reaction leads to a new iminium ion, which upon hydrolysis af-... [Pg.46]

Iminium ions (2) would be expected to undergo hydrolysis quite readily, since there is a contributing form with a positive charge on the carbon. Indeed, they react with water at room temperature." Acid-catalyzed hydrolysis of enamines (the last step of the Stork reaction, 12-18) involves conversion to iminium ions " ... [Pg.1178]

The reactivity of dipyrrolylmethane 236 has similarly been exploited in its reaction with 4-dimethylamino-l,l,l-trifluoro-3-buten-2-one 239 and trifluoromethanesulfonic anhydride to generate the iminium ion 240 followed by hydrolysis to give the fluorinated product 241 in 56% yield (Scheme 16) <2003CHE776>. [Pg.739]

A basic group removes a proton from the P-carbon of the iminium and forms the enamine. This enamine then reacts as a nucleophile towards the aldehyde group of glyceraldehyde 3-phosphate in a simple addition reaction, and the proton necessary for neutralizing the charge is obtained from an appropriately placed amino acid residue. Finally, the iminium ion loses a proton and hydrolysis releases the product from the enzyme. [Pg.369]

One simple example was the hydrolysis of imines hack to carbonyl compoimds via nucleophilic attack of water. The Mannich reaction is only a special case of nucleophilic addition to iminium ions,... [Pg.369]

Most enamines, unfortunately, are sensitive to hydrolysis. The parent enamine, iV,iV-dimethylvinylamine, has in fact been prepared [3], but appears to be unstable. Enamines of cyclic ketones and many aldehydes can readily be isolated, however [4-7]. The instability of enamines might at first appear to diminish the utility of enamines as nucleophiles, but actually this property can be viewed as an added benefit enamines can be readily and rapidly generated catalytically by using a suitable amine and a carbonyl compound. The condensation of aldehydes or ketones with amines initially affords an imine or iminium ion, which then rapidly loses a proton to afford the corresponding enamine (Scheme 1). [Pg.30]

Ogilvie monitored the Diels-Alder reaction between cinnamaldehyde and cyclopentadiene by H NMR using his hydrazide catalyst 18 and was able to conclude that under the reaction conditions adopted (18-TfOH 100 mol% CDjNOj/D O (19 1) 0.1 M) cycloaddition was the rate limiting step of the catalytic cycle, iminium ion formation and hydrolysis being rapid [48]. In addition, it was also shown that under these reaction conditions the key cycloaddition step was reversible. Although this unexpected reversibility was slow, the possibility of exploiting this in a dynamic resolution procedure appears tempting. [Pg.336]


See other pages where Hydrolysis iminium ion is mentioned: [Pg.136]    [Pg.872]    [Pg.284]    [Pg.311]    [Pg.320]    [Pg.872]    [Pg.872]    [Pg.707]    [Pg.708]    [Pg.707]    [Pg.708]    [Pg.136]    [Pg.872]    [Pg.284]    [Pg.311]    [Pg.320]    [Pg.872]    [Pg.872]    [Pg.707]    [Pg.708]    [Pg.707]    [Pg.708]    [Pg.461]    [Pg.303]    [Pg.307]    [Pg.826]    [Pg.417]    [Pg.1655]    [Pg.70]    [Pg.82]    [Pg.91]    [Pg.266]    [Pg.108]    [Pg.111]    [Pg.48]    [Pg.161]    [Pg.824]    [Pg.111]    [Pg.39]    [Pg.285]    [Pg.286]    [Pg.289]    [Pg.315]    [Pg.316]    [Pg.337]   
See also in sourсe #XX -- [ Pg.885 ]




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Iminium ion

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