Structure–Reactivity Relationships in Lithiated Evans Enolates: Influence of Aggregation and Solvation on the Stereochemistry and Mechanism of Aldol Additions
Abstract:Aldol additions to isobutyraldehyde and cyclohexanone with lithium enolates derived from acylated oxazolidinones (Evans enolates) are described. Previously characterized trisolvated dimeric enolates undergo rapid addition to isobutyraldehyde to give a 12:1 syn:syn selectivity in high yield along with small amounts of one anti isomer. The efficacy of the addition depends critically on aging effects and the reaction quench. Unsolvated tetrameric enolates that form on warming the solutions are unreactive toward i… Show more
“…8 Rate and computational studies implicated monomer-based transition structure 6 . We would be remiss if we did not also mention that computational studies suggested that the H–H interaction highlighted in 6 causes facial selectivity rather than an explicit interaction of the aldehyde with the benzyl moiety.…”
Building on structural and mechanistic studies of lithiated enolates derived from acylated oxazolidinones (Evans enolates) and chiral lithiated amino alkoxides, we found that amino alkoxides amplify the enantioselectivity of aldol additions. The pairing of enantiomeric series affords matched and mismatched stereoselectivities. The structures of mixed tetramers showing 2:2 and 3:1 (alkoxide-rich) stoichiometries are determined spectroscopically. Rate and computational studies provide a viable mechanistic and stereochemical model based on the direct reaction of the 3:1 mixed tetramers, but they raise unanswered questions for the 2:2 mixed aggregates.
“…8 Rate and computational studies implicated monomer-based transition structure 6 . We would be remiss if we did not also mention that computational studies suggested that the H–H interaction highlighted in 6 causes facial selectivity rather than an explicit interaction of the aldehyde with the benzyl moiety.…”
Building on structural and mechanistic studies of lithiated enolates derived from acylated oxazolidinones (Evans enolates) and chiral lithiated amino alkoxides, we found that amino alkoxides amplify the enantioselectivity of aldol additions. The pairing of enantiomeric series affords matched and mismatched stereoselectivities. The structures of mixed tetramers showing 2:2 and 3:1 (alkoxide-rich) stoichiometries are determined spectroscopically. Rate and computational studies provide a viable mechanistic and stereochemical model based on the direct reaction of the 3:1 mixed tetramers, but they raise unanswered questions for the 2:2 mixed aggregates.
“…[61] Über die faszinierende Chemie der Aggregate ist noch viel zu lernen. Vielleicht ist diese Rückbesinnung auf die Grundlagen sinnvoll, mit einem Fokus auf einem Alkalimetallenolat in einer chiralen Umgebung.D ie Stereoselektivitätd er Enolisierung sollte lçsbar sein, das vielleicht grçßte Problem ist aber die Kontrolle der Aggregatzustände.…”
Section: Zusammenfassung Und Ausblickunclassified
“…Vielleicht ist diese Rückbesinnung auf die Grundlagen sinnvoll, mit einem Fokus auf einem Alkalimetallenolat in einer chiralen Umgebung.D ie Stereoselektivitätd er Enolisierung sollte lçsbar sein, das vielleicht grçßte Problem ist aber die Kontrolle der Aggregatzustände. [61] Über die faszinierende Chemie der Aggregate ist noch viel zu lernen. Das im Zusammenhang mit Lithium und anderen Alkalioder Erdalkalimetallen entstehende Bild lässt interessanterweise darauf schließen, dass sich das chemische Verhalten am besten im Bereich der Koordinationschemie betrachten lässt, und nicht als vereinfachendes Problem von Aciditätu nd Basizität.…”
Die Alkylierung von Ketonen ist Teil des Grundstudiums der Chemie. In vielen Fällen führt diese Umwandlung zu einem neuen stereogenen Zentrum. Allerdings täuscht die scheinbare Einfachheit der Umwandlung über eine Reihe von Schwierigkeiten hinweg, die so groß sind, dass eine allgemeine Methode für die direkte asymmetrische Alkylierung von Ketonen bisher unerreicht ist. Trotz der Weiterentwicklung der Organo‐ und Übergangsmetallkatalyse liefert bisher keine von beiden eine adäquate Lösung. Sogar die Anwendung eines effizienten und allgemein verwendbaren stöchiometrischen Reagens muss erst noch beschrieben werden. Dieser Kurzaufsatz beschreibt den aktuellen Stand in Bezug auf direkte Alkylierungsreaktionen einiger Carbonylgruppen und bespricht den begrenzten Fortschritt bei Ketonen sowie mögliche Wege, die letztlich zu einer breit anwendbaren Methode für die asymmetrische Alkylierung von Ketonen führen können.
“…9,10 By contrast, few experimental probes of mechanism have been carried out with either simple boron enolates 11 or oxazolidinone-based variants. 6,12–15 On the heels of investigations of lithium-based aldol additions of Evans enolates, 12 we undertook structural and mechanistic studies of the boron variant in Scheme 1. 16 …”
The soft enolization of an acylated oxazolidinone using di-n-butylboron triflate (n-Bu2BOTf) and trialkylamines and subsequent aldol addition was probed structurally and mechanistically using a combination of IR and NMR spectroscopies. None of the species along the reaction coordinate shows a penchant for aggregating. Complexation of the acylated oxazolidinone by n-Bu2BOTf was too rapid to monitor, as was the subsequent enolization with Et3N (triethylamine). The pre-formed n-Bu2BOTf·Et3N complex displaying muted Lewis acidity and affiliated tractable rates reveals a rate-limiting complexation via a transition structure with a complicated counterion. n-Bu2BOTf·i-Bu3N bearing a hindered amine shifts the rate-limiting step to proton transfer. Rate studies show that the aldol addition with isobutyraldehyde occurs as proffered by others.
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