Crucial Role of the Conjugate Base for Silyl Lewis Acid Induced Mukaiyama Aldol Reactions

Abstract

The silyl Lewis acid induced Mukaiyama aldol reaction proceeds through each catalytic cycle under the influence of their conjugate bases; there is an especially significant difference between the low nucleophilic conjugate bases, –NTf2 and –CTf3, and the relatively high nucleophilic –OTf. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Supersilyl Protective Groups in Aldol Reactions

“…The excellent diastereomeric purity of the product (dr = 95 : 5) indicates that the second aldol step proceeds with nearly complete selectivity (Table 4.1, entry 16). Alternatively, acetone SEE (18) could be used as the second nucleophile, giving methyl ketone (30) with similar results. Use of Lewis acid Me 2 AlNTf 2 (0.5 mol%) was found to promote the second addition with greater reproducibility [69].…”

“…The addition product is obtained in high yield, with transfer of the super silyl group to the β-hydroxyl. The active catalytic species is believed to be the silylium Lewis acid species TMS 3 Si + · − NTf 2 , generated by protodesilylation of the SEE, rather than the Brønsted acid HNTf 2 (see below) [15][16][17][18][19]. The unique reactivity of the super silyl group is highlighted by comparing to the TBS SEE, which when reacted with aldehydes under identical conditions low yields are obtained (Eq.…”

Chemistry of Fluorinated Carbon Acids: Synthesis, Physicochemical Properties, and Catalysis

“…33,34) This species activates the carbonyl substrates via the O-silylation reaction, and then nucleophilic attack of unreacted ketene silyl acetals on the resulting silyl carboxonium B forms the C-C bond (formation of C). Furthermore, the silyl transfer reaction from intermediate C to unreacted carbonyl substrates regenerates intermediate B along with the formation of aldol (or Michael) product D. 35) If highly Lewis acidic species A is inactivated by a trace amount of water existing in the reaction system, a selfrepairing pathway through the protonation of ketene silyl acetals by Tf 2 CHCH 2 CHTf 2 3 will bring about the regeneration of catalytically active A. 36) Although our efforts to observe intermediate A failed, some experimental results supported this plausible mechanism.…”

Asymmetrische Gegenanionen‐vermittelte Katalyse: Konzept, Definition und Anwendungen

“…Yamamoto et al haben Binol-abgeleitete N-Triflylphosphoramide 87 entwickelt, die in der Lage sind, Ketone für einen nucleophilen Angriff zu aktivieren. [70] Yamamoto und Mitarbeiter schlugen vor, dass das tatsächliche Gegenanion ein Addukt aus silyliertem Triflimid und einem Produktmolekül sein kçnnte, das in unserem System 92 a oder 92 b entsprechen würde (Schema 18). Als Ergebnis unserer Versuche zur Aktivierung dieser anspruchsvollen Substratklassen berichteten wir und die Gruppe um Ishihara von der Synthese der hochaciden, Binaphthyl-abgeleiteten Disulfonsäure 88 und deren Anwendung in der Brønsted-Säurekatalyse.…”