Achieving Enantioselectivity in Difficult Cyclohexa-1,3-diene Diels–Alder Reactions with Sulfur-Stabilized Silicon Cations as Lewis Acid Catalysts

Abstract

A novel cationic silicon−sulfur Lewis pair with a chiral H 8 -binaphthyl backbone is reported. It catalyzes otherwise sluggish Diels−Alder reactions of cyclohexa-1,3-diene and chalcone derivatives in good yields and decent enantioselectivities (up to 81% ee). The enantioinduction is highest with a [1,1′-biphenyl]-4-yl substituent at the carbonyl carbon atom. This moiety can be later converted into a carboxyl group by Baeyer−Villiger oxidation. The same oxidant also epoxidizes the double bond in the cycloadduct, and the epoxide engages in a lactonization with the carboxylic acid. Synthetically interesting hexahydro-3,6-methanobenzofuran-2(3H)-one skeletons are obtained in one pot.

Chiral Chalcogenyl‐Substituted Naphthyl‐ and Acenaphthyl‐Silanes and Their Cations

“…[25,26] In as eries of investigations, Oestreich and co-workers used different families of intramolecularly stabilized silyl cations to introduce chirality on these silyl Lewis acids. [27][28][29][30][31][32][33] Particularly interesting in the context of our study are silylsulfoniumi ons I-IV ( Figure 1). [28,[30][31][32][33] In thesee xamplest he neighboring group effecto ft he sulfur donor creates new centers of chirality that are controlled by the present stereo element (centered chirality at the silicona tom or axial chirality).…”

“…Later, Ghosez and co‐workers used non‐ionic chiral silyltriflimides derived from substituents from the chiral pool [25, 26] . In a series of investigations, Oestreich and co‐workers used different families of intramolecularly stabilized silyl cations to introduce chirality on these silyl Lewis acids [27–33] . Particularly interesting in the context of our study are silylsulfonium ions I – IV (Figure 1).…”

“…Chiral silyl cations synthesized by the groups of Oestreich and Landais. [27,28,32,36,37] [a] Dr.S. Künzler catalyzes challenging Diels-Alder reactions at low temperatures in good yields and decent enantioselectivities (up to 81 % ee).…”

“…Künzler catalyzes challenging Diels-Alder reactions at low temperatures in good yields and decent enantioselectivities (up to 81 % ee). [32] This approach based on chiral silyl Lewisa cids competes with the asymmetricc ounter-anion-directed silylium catalysis popularized by the List group. [34,35] The question whether the stereochemical information at the silicon atom is retained by the neighboring group effect of an electron-donating group during the typicali onization/nucleophilic addition sequenceo fs ilylium ion catalysis was recently addressed by Landais, Roberta nd co-workers.…”

“…Particularly interesting in the context of our study are silylsulfonium ions I – IV (Figure 1). [28, 30–33] In these examples the neighboring group effect of the sulfur donor creates new centers of chirality that are controlled by the present stereo element (centered chirality at the silicon atom or axial chirality). Consequent structure modifications finally led to the silylsulfonium species IV , which catalyzes challenging Diels–Alder reactions at low temperatures in good yields and decent enantioselectivities (up to 81 % ee ) [32] .…”