Zum Mechanismus basekatalysierter Carbodesilylierungen von Aryl‐ und Heteroaryltrimethylsilanen

Abstract

Die Einfliisse verschiedener Basen auf die Spaltung von Silicium-Kohlenstoff-Bindungen in Aryl-und Heteroaryltrimethylsilanen unter Bildung von Aryl-bzw. Heteroaryl-Anionen in Gegenwart von Benzaldehyd als Abfangelektrophil werden untersucht. Aus den Umsetzungen mit 2-(Trimethylsilyl)benzothiazol(l) wird eine Reaktivitatsreihenfolge der verwendeten basischen Katalysatoren ermittelt. Die katalytische Aktivitat der Anionen nimmt mit ihrem Ionenpotential zu. Als erster Reaktionsschritt wird der Angriff der Base am Si-Atom unter Ausbildung einer pentakoordinierten Zwischenstufe postuliert, die im geschwindigkeitsbestimmenden Schritt unter Bildung des Carbanions dissoziiert, das rasch mit dem Benzaldehyd reagiert. Die Abhangigkeit der basisch katalysierten Carbodesilylierung von der ArylAnionen-Stabilitat wird an unterschiedlich substituierten Aryltrimethylsilanen 13, 1 3 und 1 3 ' untersucht. Die ermittelten Reaktionsgeschwindigkeiten korrelieren besser rnit denHammettschen o-Werten als mit den Aryl-Anionen-Stabilitaten. Mechanism of Base-Catalyzed Desilylations of Aryl-and Heteroaryltrimethylsilanes')The influence of different bases on the cleavage of silicium-carbon bonds in aryl-and heteroaryltrimethylsilanes is investigated in the presence of benzaldehyde as electrophilic scavenger for the aryl and heteroaryl anions formed in this process. A reactivity gradation of the various basic catalysts employed is determined from the reactions with 2-(trimethylsilyl)-bcnzothiazole (1). The increase of catalytic activity of the anions parallels that of their ion potential. Attack of the base at the Si atom is postulated as the first step in this reaction sequence, with subsequent dissociation of the pentacoordinated intermediate in the ratedetermining step. The carbanion thus liberated rapidly reacts with benzaldehyde. In the differently substituted aryltrimethylsilanes 13, 13, and 13' the dependency of aryl anion stability in the base-catalyzed carbodesilylation was investigated. The relative rates of reaction correlate with Hammett's o-constants rather than with the corresponding aryl anion stabilities.

Synthesis, Antibacterial and Antifungal Activities of Bifonazole Derivatives

“…The physico-chemical data of compounds 2b-c [13], 2e [14], 2f [15], 2h [14], 2i [16], 2j [17] are in agreement with the literature data.…”

Oxyanions catalyze group-transfer polymerization to give living polymers

“…However, it is unlikely that the equivalent of carboxylic acid that forms will survive in the presence of the silyl ketene acetal, the living polymer, or their corresponding activated complexes (analogous to 4 in Scheme III). Instead, the carboxylic acid will react with either the initiator (as shown in Scheme II), the living polymer, or their activated complexes (Scheme III) to give an inert methyl isobutyrate or dead (protonated) polymer and trimethylsilyl carboxylate (5) as byproducts. The silyl ester may then form a complex (6) with the monooxyanion via the equilibrium shown in Scheme III.…”

“…Figure5. GPC traces of PMMA prepared with various levels (expressed as mole percent of initiator) of tetrabutylammonium benzoate catalyst, using three sequential additions of MMA: (a) 2.2%, Mp = 5210 and 40 400; (b) 0.55%,MP = 4540, 20 100, and 108 000; (c) 0.14%, Mp = 12 000 (sh 4000) (Mp = peak molecular weight).Scheme II (RC02)2H"--RCOf + rco2h X…”

Synthesis of Highly Functionalizable Symmetrically and Unsymmetrically Substituted Triarylboranes from Bench‐Stable Boron Precursors

“…To obtain a triarylborane that can be orthogonally functionalized at all three aromatic rings, the para methyl group of mesitylene was exchanged for a trimethylsilyl group. This moiety permits many different functionalization reactions[ 90 , 91 , 92 , 93 , 94 ] and should be robust enough to tolerate the reaction conditions required for the triarylborane formation. We chose Ar H BF 3 K as the starting material and synthesized compound BAr H Ar Br Ar in a yield of 16 % over two steps in a one pot synthesis (Scheme 2 , Table 2 entry 5).…”