By Benjamin List
As nucleophiles, uncomplicated alkenes are usually so unreactive that in simple terms hugely energetic electrophiles, equivalent to carbocations, peroxides, and halogens will react with them. For the iteration of carbon-carbon bonds, milder tools will usually be required. thankfully, it's attainable to extend the reactivity of alkene-type p-nucleophiles through introducing electron-donating substituents. Substitution of 1 H with an OH or OR offers an enol or a vinyl ether, that are already far better nucleophiles. utilizing nitrogen rather than oxygen, one obtains even higher nucleophiles, enamines. Enamines are one of the such a lot reactive impartial carbon nucleophiles, showing premiums which are even akin to a few charged nucleophiles, equivalent to enolates [1, 2]. such a lot enamines, regrettably, are delicate to hydrolysis. The mother or father enamine, N, N-dimethylvinylamine, has in reality been ready , yet seems to be uns- ble. Enamines of cyclic ketones and plenty of aldehydes can conveniently be remoted, in spite of the fact that [4-7]. The instability of enamines may perhaps in the beginning seem to lower the software of enamines as nucleophiles, yet truly this estate could be seen as an additional benefit: enamines will be without problems and swiftly generated catalytically through the use of an appropriate amine and a carbonyl compound. The condensation of aldehydes or ketones with amines firstly provides an imine or iminium ion, which then swiftly loses a proton to manage to pay for the corresponding enamine (Scheme 1).
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Additional resources for Asymmetric Organocatalysis
Bode and coworkers reported the catalytic, diastereoselective synthesis of b-hydroxy esters from a,b-epoxy aldehydes using thiazolium pre-catalyst173 Eq. 16a . MeOH, EtOH, and BnOH are effective nucleophiles providing upwards of >10:1 diastereoselectivity. Aziridinylaldehyde 174 has also been shown to provide the desired N-tosyl-b-aminoester 175 in 53% yield Eq. 16b. 110 J. L. Moore, T. Rovis Me Me O Cl N Bn 10 mol% 173 8 mol% DIPEA CH2Cl2, 30 C S O BnOH H Ph Me 171 Ts O N Ph 174 OBn a Me 172 89% >10:1 dr (16) TsNH O 10 mol% 173 8 mol% DIPEA CH2Cl2, 30 oC EtOH H Ph OH O Ph OEt 175 53% b The proposed catalytic cycle for this reaction begins with the initial attack of the in situ generated thiazolylidene carbene on the epoxyaldehyde followed by intramolecular proton transfer (Scheme 28, XXXII–XXXIII).
N Cl N Ph 20 mol% 177 O Ph O N NuH H Br 176 O Ph OEt Ph 178 78% O Ph N N Ph XXXVII Et3N, PhMe O R N Ph Me O NHPh Nu O Ph 179 91% 180 65% Scheme 29 Rovis and co-workers acylation reaction via activated carboxylate XXXVII The reaction conditions are mild and generally tolerant of epimerizable stereocenters. For instance, the use of (S)-ethyl lactate 181 under the reaction conditions produces desired ester 182 in 94% ee Eq. 17a. The subjection of racemic ethyl lactate 181 to standard reaction conditions with chiral pre-catalyst 183 provides ester 182 in 32% ee Eq.
Diastereoselective protonation provides activated carboxylate intermediate XXXVI. Nucleophilic attack of the activated carboxylate regenerates the catalyst and provides the desired b-hydroxy ester. O O O H S R O R' O Bn H R N XXXII R' Me S Me OH S R R' Me N Bn Me XXXIII Me O Me OH S R N Bn R' Bn Me Me XXXIV OH O R N OR'' OH O OH O R' S R R' Bn N Me XXXVI Me S R R' R''OH Bn Me N XXXV Me Scheme 28 Proposed mechanism for the formation of b-hydroxy esters Concurrently with Bode’s work, Rovis and co-workers reported an internal redox reaction of a-haloaldehydes to provide a variety of esters in good yields .