Hochreaktive Trialkylsilylierungsreagentien aus Bis(trifluormethansulfonyl)imid - Silylierung von funktionellen Gruppen, alkinen und reaktiven Aromaten

Attempts to Synthesize a Thiirane, Selenirane, and Thiirene by Dealkylation of Chalcogeniranium and Thiirenium Salts

Thiiranium salts [Ad2 SR]+ X- (5, 8, 9, 11, 12; X- =Tf2 N- (Tf=CF3 SO2 ), SbCl6 - ) and seleniranium salts [Ad2 SeR]+ X- (14, 16, 17, 23-25; X- =Tf2 N- , BF4 - , CHB11 Cl11 - , SbCl6 - ) are synthesized from strained alkene bis(adamantylidene) (1). The disulfides and the diselenides (Me3 SiCH2 CH2 E)2 (4, 13), (tBuMe2 SiCH2 CH2 E)2 (7, 22), and (NCCH2 CH2 E)2 (10, 15; E=S, Se) have been used. The thiirenium salts [tBu2 C2 SR]+ X- (34) and [Ad2 C2 SR]+ X- (35, 36) are prepared from the bis-tert-butylacetylene (2) and bis-adamantyl-acetylene (3) with R=Me3 SiCH2 CH2 and tBuMe2 SiCH2 CH2 . Attempts to cleave off the groups Me3 SiCH2 CH2 , tBuMe2 SiCH2 CH2 , and NCCH2 CH2 resulted in thiiranes 27, 30. No selenirane Ad2 Se (33) is formed from seleniranium salts, instead cleavage to the alkene (1) and diselenide (13, 15) occurs. The thiirenium salt [Ad2 C2 SCH2 CH2 SiMe3 ]+ Tf2 N- (35) does not yield the thiirene Ad2 C2 S (37), the three-membered ring is cleaved, forming the alkyne (3) and disulfide (4). All compounds are characterized by ESI mass spectra, NMR spectra, and by quantum chemical calculations. Crystal structures of the salts 8, 12, 25, 17, 26, 36 and the thiiranes 27, 30 are presented.

Catalytic stereospecific O-glycosylation

Glycosylation using Tf₂NH or Tf₂NTMS as the catalysts and a trichloroacetimidate donor gives glycosides with inverted anomeric stereochemistry.

Glycosylation intermediates studied using low temperature1H- and19F-DOSY NMR: new insight into the activation of trichloroacetimidates

Low temperature¹H- and¹⁹F-DOSY have been used for analyzing reactive intermediates in glycosylation reactions, where a glycosyl trichloroacetimidate donor has been activated using different catalysts.

Electrophilic C-H Borylation and Related Reactions of B-H Boron Cations

Catalytic procedures are described for aminedirected borylation of aliphatic and aromatic tertiary amine boranes. Sequential double borylation is observed in cases where two or more C-H bonds are available that allow 5-center or 6-center intramolecular borylation. The HNTf₂ catalyzed borylation of benzylamine boranes provides a practical means for the synthesis of ortho-substituted arylboronic acid derivatives, suitable for Suzuki-Miyaura cross-coupling applications.

Lewis acid organocatalysts

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 degrees C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of their cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis.