A

(S,S)-19 (20% mol) Star*(S)-12 (20%mol) Star*-p-TSA (20 %mol) H-bonding catalyst (1 eqv)

Phosphate buffer pH 7.5

H'V^N^bTMs

R1 H

Concentrated organic phase

R1= n-hexyl, n-octyl, n-decyl vno2

Scheme 4.12 Site-isolation techniques in multicatalyst reactions effect of sulphonic acid [85] (Scheme 4.12a). In a very interesting and recent report of Frechet et al. [86] two aldehydes with the same chemical reactivity underwent distinct reactions by polarity-driven confinement into two different liquid phases, that enabled the selective formation of cross-cascade products instead of a statistically mixture of four possible products. Indeed, carrying out the process in aqueous phosphate buffer, l-proline, nitromethane and a hydrophilic aldehyde were preferentially dissolved in water phase whereas more hydrophobic prolinol 12 and long-chain aliphatic aldehydes formed a concentrated organic phase. After addition of nitromethane to the first aldehyde had occurred in water, the hydrophobic nitroalkene intermediate diffused into the organic phase wherein reacted with the second aldehyde to give the expected cross-product in excellent enantio- and diastereomeric purity (Scheme 4.12b).

The multicatalyst strategy in multicomponent tandem or cascade reactions is still in its infancy and enormous potentiality could be expected from coupling of organocatalysts and transition metal complexes, up to now mainly limited to counterion/gold (or palladium) catalysis yet discussed in the text, as an exciting research field in which these two complementary branches of asymmetric catalysis could merge opening the way to new concepts and synthetic tools.

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