Sulfur–Sulfur‐Based Ligands Derived from D‐Sugars: Synthesis of Pd^II Complexes, Application in Palladium‐Catalyzed Allylic Alkylation for the Synthesis of Both Members of Enantiomer Pairs, and Structural Studies

Abstract

A divergent synthetic approach for the synthesis of optically pure bis(thioglycosides) of type I is reported. A common chiral intermediate with up to eight free hydroxy groups is obtained in only two steps, and from this common intermediate a large number of ligands can be synthesized. A strategy resembling positional scanning enabled the rapid discovery of an efficient catalyst for the palladium-catalyzed asymmetric allylation of malonate. Both enantiomers of the allylated product could be obtained in up to 90 % ee values through the use of natural D-sugars as catalyst precursors, thanks to the structural similarity of α-D-arabinose and β-L-galactose.

Carbohydrates as enantioinduction components in stereoselective catalysis

“…In a related study, Khiar and co-workers glycosylated dithiols with peracylated glycosyl donors to give chelating C 2 -symmetric carbohydrate-bis(thioether) ligands, which were evaluated in Pd AAA. 38 Additionally, Pregosin and co-workers have reported carbohydrate derived S-oxazoline ligands. These were accessed by alkylating anomeric thiols with oxazoline-based alkyl chlorides.…”

Carbohydrates as Synthetic Tools in Organic Chemistry

“…Examples for structurally very simple, C 2 ‐symmetrical S,S‐donor ligands were described by Khiar et al. who prepared bis(thioglucoside) 70 from ethanedithiol,26 and its pseudo‐enantiomer ps ent ‐ 70 derived from D ‐arabinose26b,c (Figure 4). Upon coordination to a metal, the sulphur of these ligands becomes stereogenic, therefore, even an enantiomerically pure ligand can give rise to mixtures of diastereomeric complexes.…”

“…The cyclic substrates 61 gave good to excellent ee values with ribo ligand 69 c. The results achieved for unsymmetrical and cyclic substrates are among the best reported to date, [25] with d-ribo-configured 3,5-diphosporoamidites the results could even be improved for cyclic substrates. [25b] Examples for structurally very simple, C 2 -symmetrical S,Sdonor ligands were described by Khiar et al who prepared bis(thioglucoside) 70 from ethanedithiol, [26] and its pseudoenantiomer ps ent-70 derived from d-arabinose [26b,c] (Figure 4). Upon coordination to a metal, the sulphur of these ligands becomes stereogenic, therefore, even an enantiomerically pure ligand can give rise to mixtures of diastereomeric complexes.…”

Recent Progress in Asymmetric Catalysis Using Chiral Carbohydrate‐Based Ligands

“…In the search for both enantiomers of the alkylation product, the authors successfully prepared pseudo‐enantiomers 52 and 53 derived from D ‐galactose and D ‐arabinose, respectively (Figure 14). 38b…”