Intramolecularly Sulfur-Stabilized Silicon Cations as Lewis Acid Catalysts

Abstract

The synthesis and spectroscopic characterization of previously unprecedented sulfur-stabilized silicon cations are reported. Several 1,3-dithiolan-2-yl- and 1,3-dithian-2-yl-substituted silanes were prepared and successfully transformed into the corresponding silicon cations by hydride abstraction. The silicon–sulfur interaction creates three consecutive stereocenters at three different elements. It is remarkable that the present stereocenter at the silicon atom determines the stereochemical outcome at the formerly prochiral sulfur and carbon atoms with excellent diastereoselectivity. All sulfur-stabilized silicon cations are shown to be potent catalysts in a challenging Diels–Alder reaction. Moreover, structurally related oxazoline-stabilized silicon cations were generated and characterized but found to be unreactive.

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

“…8, 9 As part of our ongoing research, we recently reported the synthesis of intramolecularly sulfur-stabilized silicon cation 6 with a planar chiral ferrocenyl substituent (Figure 2, left). 10 Its ability to act as a Lewis acid catalyst was demonstrated in the difficult Diels−Alder reaction of cyclohexa-1,3-diene and chalcone, but no asymmetric induction was seen.…”

“…Inspired by cation 5 (Figure 1, bottom right), we also targeted the axially chiral dihydrosilepine (S)-10 with a 2-[(alkylthio)methyl]phenyl group installed at the silicon atom (Figure 3, bottom). Different from our previous work, 10 we aimed to accomplish stabilization by acyclic rather than cyclic dialkyl thioethers (cf. 1,3-dithiolan-2-yl and 1,3-dithian-2-yl substituents in 6 and 7a− 7d, respectively, Figure 2).…”

“…10 That difference is likely the result of the larger ring size of (S)-8, i.e., seven vs five. These findings are in good agreement with a recent study by Muller and coworkers, where it was shown that the silicon nucleus of silicon cations stabilized in six-membered rings is considerably more shielded than in its five-membered counterparts.…”

Synthesis, Characterization, and Electrochemistry of Diferrocenyl β-Diketones, -Diketonates, and Pyrazoles

“…The synthetic methodologies to prepare the ferrocenyl-functionalized β-diketones 5 – 8 is shown in Scheme 1 . The therefore necessary starting compounds 1 – 3 were synthesized accordingly to references [ 79 , 80 , 81 ]. Ketone 4 was prepared by a Friedel-Crafts acylation of ferrocene with 2-[2-(2-methyoxyethoxy)ethoxy]acetyl chloride, which is accessible by refluxing the appropriate carboxylic acid with thionyl chloride in analogy to references [ 82 , 83 ], with ferrocene in presence of the Lewis acid catalyst AlCl 3 ( Section 4.6 ).…”

Facile Access to Stable Silylium Ions Stabilized by N-Heterocyclic Imines

“…To date, most isolable silylium ions are typically limited to alkyl and aryl substituents and heteroatom-substituted silylium derivatives have remained extremely scarce. However, introduction of a heteroatom such as N, O, or S to the silylium center [20][21][22] produced silylium ions with good catalytic activity [23].…”