Ein gezähmtes Silyliumion für Diels‐Alder‐Reaktionen bei niedrigen Temperaturen

Abstract

Manche mögen′s heiß: Eine neue Silicium‐Lewis‐Säure mit einem dreiwertigen, durch ein elektronenreiches Übergangsmetall stabilisierten Siliciumkation als „heißem“ Lewis‐saurem Zentrum katalysiert anspruchsvolle Diels‐Alder‐Reaktionen bei niedrigen Temperaturen mit ausgezeichneten Reaktionsgeschwindigkeiten und Selektivitäten (Beispiel siehe Schema). Der Silyliumion‐Katalysator ist in CH2Cl2 überraschend stabil.

Easy Access to Enantiomerically Pure Heterocyclic Silicon‐Chiral Phosphonium Cations and the Matched/Mismatched Case of Dihydrogen Release

“…In general, the spectroscopic data are quite the same for the cations in both compounds ( rac )‐ 2 a / b , thus indicating that the [HB(C 6 F 5 ) 3 ] − counterion in ( rac )‐ 2 a is not being coordinated via an Si⋅⋅⋅H⋅⋅⋅B interaction in solution [27] . An interaction with the solvent CD 2 Cl 2 can also be safely excluded [10b] . 29 Si NMR spectroscopy of compounds ( rac )‐ 2 a and b in CD 2 Cl 2 shows a signal at δ =13.7 and 13.6 ppm, respectively.…”

“Tamed” Silylium Ions: Versatile in Catalysis

“…As with hydrodefluorination, the strong bonding tendency of the Si À F bond (D(Me 3 Si À H) = 94.6 kcal mol À1 versus D(Me 3 Si À F) = 158.0 kcal mol À1 ) is exploited to defluorinate the substrate RÀF in the first step. [11] While the reactions carried out by Sawamura snd co-workers with Lamberts salt [Et 3 Si·toluene][B(C 6 F 5 ) 4 ] with the same substrates and a similar catalyst loading required "high" temperatures between 0 8C and room tem-perature, the [tBuFcMeSi][B(C 6 F 5 ) 4 ] salt (Fc = ferrocenyl; Scheme 6 and species E in Scheme 1) first described by Oestreich and co-workers is highly reactive even at low temperatures of down to À78 8C. The cationic s complex thus formed can then be deprotonated by reaction with a hydride source R 3 Si À H, which leads to the formation of the crosscoupling product, the release of hydrogen, and the regeneration of the silylium ion.…”

“…[9] Likewise, the Diels-Alder reactions described by the research groups of Sawamura and Oestreich can be ranked as C À C coupling reactions catalyzed by a silylium ion Lewis acid (Scheme 5). [11] While the reactions carried out by Sawamura snd co-workers with Lamberts salt [Et 3 Si·toluene][B(C 6 F 5 ) 4 ] with the same substrates and a similar catalyst loading required "high" temperatures between 0 8C and room tem-perature, the [tBuFcMeSi][B(C 6 F 5 ) 4 ] salt (Fc = ferrocenyl; Scheme 6 and species E in Scheme 1) first described by Oestreich and co-workers is highly reactive even at low temperatures of down to À78 8C. Both systems lead selectively to the endo diastereoisomers in almost quantitative yields.…”

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

“…The last 25 years have witnessedt he successfuld evelopment of silyl cations from laboratory curiosities to valuable catalysts in organic transformations. [1][2][3][4][5][6][7][8] By virtueo ft heir high Lewis acidity,s ilyl cations have provent ob ee xcellent catalysts in Diels-Alder reactions, [9][10][11][12] hydrodefluorination [13][14][15][16][17][18][19][20][21] and CO-activation reactions. [22,23] In these applications, it is mandatory that the high Lewis acidity of the tricoordinate silylium ion is pacified by an intramolecular or intermolecular donor to control the reactivity of the silyl Lewis acid.…”