Thiol and disulfide derivatives of ephedra alkaloids 2: A mechanistic study of their effect on the addition of diethyl zinc to benzaldehyde

Abstract

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Nonlinear Effects in Asymmetric Catalysis

“…Subsequently, Noyori and co-workers [22,24] Bolm et al, [25] and Kellogg and co-workers [26] reported the presence of an NLE in this class of reactions. The formation of stable and catalytically inactive heterochiral dimers 2 was suggested as a possible explanation for the occurrence of the NLE.…”

Synthesis and application of chiral β‐amino disulfides as ligands for the enantioselective addition of diethylzinc to aldehydes

“…1,75,76 To elucidate the mechanistic aspects, we assumed that the disulfide linkage is cleaved by nucleophilic attack of diethylzinc, resulting in the thiolate 5 and the thioether 6 (Scheme 2), according to the results reported by Kellogg 35 and Wirth. 30 Aiming to identify the catalytically active species, we prepared the compound 6 via 2a ring-opening reaction by attack of EtSSEt/NaBH 4 77 as shown in Scheme 3.…”

“…25 In the last few years, a significant improvement in enantioselectivity has been observed by the use of amino sulfur or selenium moieties as catalysts for the enantioselective addition of diethylzinc to aldehydes. [26][27][28][29][30][31][32] Sulfur containing ligands 33 like b-amino thiols and b-amino disulfides derivatives from ephedrine, [34][35][36] norephedrine, [37][38][39] (S)-proline, [40][41][42] (S)-phenylglycine, 43 (S)-valine, 44,45 (R)-cysteine, 5,46-50 (R)-cystine, 51 and others [52][53][54][55][56][57][58] have gained more and more interest as catalysts in this stereoselective reaction. In a similar way, chiral amino diselenides have been efficiently used as catalysts for this addition, [28][29][30][31][32] but due to the most difficult accessibility of these compounds, only a few number of reports can be found in literature.…”

Enantioselective Synthesis Mediated by Catalytic Chiral Organoselenium Compounds

“…We assumed that the diselenide linkage is cleaved by nucleophilic attack of diethylzinc, resulting in zinc selenolate 33a and selenoether 33b, according to the results reported by Kellogg [27] and Wirth [15]. We believe that the active catalyst of the reaction is selenolate 33a, since in an additional experiment selenoether 33b did not catalyze the alkylation of benzaldehyde.…”