Intramolecularly Sulfur-Stabilized Silicon Cations with Chiral Binaphthyl Backbones: Synthesis of Three Different Motifs and Their Application in Enantioselective Diels–Alder Reactions

Abstract

The formation and 29 Si NMR spectroscopic characterization of silicon cations that are intramolecularly stabilized by a dialkyl thioether are described. The chemical stability of the silicon−sulfur Lewis pair and, hence, the viability of the approach, were probed with a 2-[(alkylthio)methyl]phenylsubstituted hydrosilane as a proxy before three different motifs with chiral binaphthyl backbones were prepared in multistep sequences. The degree of shielding of the silicon atom in these cations was found to depend on the substitution pattern at the silicon atom and the ring size generated by the silicon−sulfur interaction. These sulfur-stabilized silicon cations are sufficiently reactive to promote Diels−Alder reactions of cyclohexa-1,3-diene with various dienophiles; the same set of reactions with cyclopentadiene is also reported. One of the three chiral Lewis acids induces low, but promising, enantioselectivity, and 24% ee is the highest value so far obtained with a cationic tetracoordinate silicon catalyst.

Planar Chiral, Ferrocene‐Stabilized Silicon Cations

“…The endo / exo ratios were determined by GLC analysis of the crude material, and the enantiomeric purity was verified by HPLC analysis . The analytical and spectroscopic data were in accordance with those reported previously.…”

“…The yields were moderate and inferior to those obtained with rac ‐ 2 (71 % for endo ‐ 23 and 50 % for endo ‐ 24 ), but endo / exo selectivities were excellent again. Unfortunately, cycloadducts endo ‐ 23 and endo ‐ 24 were isolated in the racemic form in all cases …”

“…After evaporation of solvents under reduced pressure, the resulting residue was purified by flash column chromatography on silica gel with mixtures of cyclohexane/ethyl acetate as the eluent to afford the analytically pure Diels–Alder adducts endo ‐tricyclo[5.2.1.0 2, 7 ]undec‐9‐en‐3‐one ( endo ‐ 23 ) and endo ‐3‐(bicyclo[2.2.2]oct‐5‐ene‐2‐carbonyl)oxazolidin‐2‐one ( endo ‐ 24 ), respectively. The endo / exo ratios were determined by GLC analysis of the crude material, and the enantiomeric purity was verified by HPLC analysis . The analytical and spectroscopic data were in accordance with those reported previously.…”

Refinement of the Catalyst Backbone of Chiral Intramolecular Silicon–Sulfur Lewis Pairs: Improved Enantioselectivity in the Diels–Alder Reaction of Cyclohexa‐1,3‐diene and Chalcone Derivatives

“…Our laboratory introduced sulfur‐stabilized silicon cations, as sufficiently strong Lewis acids for the promotion of challenging Diels–Alder reaction of cyclohexa‐1,3‐diene and unreactive dienophiles, particularly chalcones The goal of our efforts is the development of chiral catalysts to enable elusive enantioselective variants of these cycloadditions. [3b], [4e], After systematic examination of the generation and properties of these cationic silicon–sulfur Lewis pairs, we eventually arrived at catalyst ( S , S )‐ 1 that induced 34 % ee in the model reaction 2 + 3 → 4 and reached enantiomeric excesses in the region of 55 % for more hindered chalcones (Scheme , top). [3b] For us, the next steps have been understanding the influence of the different structural elements in ( S , S )‐ 1 on enantioinduction and, ultimately, the structural refinement of the catalyst backbone (Scheme , bottom).…”

“…[3b], [4e], After systematic examination of the generation and properties of these cationic silicon–sulfur Lewis pairs, we eventually arrived at catalyst ( S , S )‐ 1 that induced 34 % ee in the model reaction 2 + 3 → 4 and reached enantiomeric excesses in the region of 55 % for more hindered chalcones (Scheme , top). [3b] For us, the next steps have been understanding the influence of the different structural elements in ( S , S )‐ 1 on enantioinduction and, ultimately, the structural refinement of the catalyst backbone (Scheme , bottom). We report here new catalysts with modified silepine fragments, including replacement of the binaphthyl by a biphenyl unit.…”