Dedicated to Professor Gerhard Quinkert on the occasion of his 80th birthdayThe application of chiral Brønsted acids in metal-free enantioselective catalysis is increasing rapidly.[1] Within the last two years the first highly enantioselective transformations have been developed in which chiral Brønsted acids function as biomimetic catalysts. The central role performed by the Brønsted acids in such reactions is the activation of the electrophile by catalytic protonation, thereby enabling them to react with a nucleophile. In this manner it has been possible to realise enantioselective transformations with aldimines and ketoimines using chiral Brønsted acids such as binol phosphates [Eq. (1)]. [2][3][4][5] In these transformations a proton is initially transferred from the Brønsted acid to an aldimine or ketoimine to form an intermediary chiral ion pair which subsequently reacts with a nucleophile to form the corresponding amine and the regenerated Brønsted acid. Within this field of chiral ion pair catalysis, only aldimines and ketoimines have been activated to date. More recently, however, we have been successful in the activation of both the electrophile and the nucleophile in a new double Brønsted acid catalyzed reaction.[5] In these reactions the simultaneous and co-operative activation of the aldimine by the chiral binol phosphate *BH [Eq. (1)], and the carbonyl nucleophile by an achiral Brønsted acid [Eq. (2)], result in the desired products.[5]The enantioselective Brønsted acid catalyzed activation of a "pure" carbonyl compound using a chiral binol phosphate has not previously been described. Here we report for the first time the development of such a reaction: a Brønsted acid catalyzed enantioselective Nazarov cyclization. The Nazarov reaction belongs to the group of electrocyclic reactions and is one of the most versatile methods for the synthesis of fivemembered rings, which are the key structural elements of numerous natural products.[6] In general, the Nazarov cyclization can be catalyzed by Brønsted or Lewis acids. However, only a few asymmetric variations have been described, of which most require the use of large amounts of chiral metal complexes. [7] Building on our previous results, [3][4][5] we decided to examine a metal-free Nazarov reaction catalyzed by a binol phosphate. This would not only be the first example of a Brønsted acid catalyzed, enantioselective, electrocyclic reaction but would additionally provide a simple and direct route to optically pure cyclopentenones.We assumed that the catalytic protonation of a divinylketone A by the binol phosphate (*BH) would result in the formation of an adduct B, which consisted of a cyclopentadienyl cation and a phosphate anion (Scheme 1). Subsequent conrotatory 4p electrocyclization would lead to oxyallyl cation C which, through the elimination of a proton, would form enolate D. Successive protonation of this enolate should then result in the formation of cyclopentenone E and the regenerated Brønsted acid catalyst *BH.At the outset of our experiment...

Recently, chiral highly acidic Brønsted acids have emerged as powerful catalysts for enantioselective C-C and C-X bond-forming reactions. Their strong acidity renders them valuable tools for the activation of imines, carbonyl compounds, and other weakly basic substrates. As a result, new perspectives are opened and highly stereoselective transformations based on the concept of chiral contact-ion-pair catalysis can be realized. This Minireview gives an overview of the design and application of these new organocatalysts and presents recent results in this rapidly growing field.

A highly efficient BiA C H T U N G T R E N N U N G (OTf) 3 -catalyzed benzylation of arenes and heteroarenes has been developed. The mild reaction conditions, high yields, operational simplicity and practicability, broad scope, and remarkably low catalyst loading render this environment friendly process an attractiv approach to diarylmethane derivatives. The extension to an intramolecular variant of this procedure provides a valuable route to substituted fluorenes.

Professor Gerhard Quinkert zum 80. Geburtstag gewidmet Die Anwendungen chiraler Brønsted-Säuren in der metallfreien enantioselektiven Katalyse nehmen rasant zu.[ Eine enantioselektive Brønsted-Säure-katalysierte Aktivierung von "reinen" Carbonylverbindungen mit chiralen Binolphosphaten ist bisher jedoch nicht bekannt. Wir berichten hier erstmals über die Entwicklung einer solchen Reaktion, der Brønsted-Säure-katalysierten enantioselektiven Nazarov-Cyclisierung. Die Nazarov-Reaktion gehört zu den elektrocyclischen Reaktionen und ist eine der vielseitigsten Methoden zum Aufbau von fünfgliedrigen Ringen, die das Grundgerüst zahlreicher Naturstoffe bilden.[6] Gewöhn-lich wird die Nazarov-Cyclisierung durch Zusatz von Brøn-sted-oder Lewis-Säuren katalysiert, aber nur wenige Beispiele asymmetrischer Varianten sind beschrieben, wobei in den meisten Fällen hohe Mengen an chiralen Metallkomplexen verwendet werden müssen. [7] Daher beschlossen wir, auch aufbauend auf unseren frü-heren Ergebnissen, [3][4][5] eine metallfreie, durch Binolphosphat katalysierte Nazarov-Reaktion zu untersuchen. Eine solche Reaktion wäre nicht nur das erste Beispiel einer Brønsted-Säure-katalysierten enantioselektiven elektrocyclischen Reaktion, sondern würde zudem einen einfachen und direkten Zugang zu enantiomerenreinen Cyclopentenonen liefern.Wir nahmen an, dass eine katalytische Protonierung des Divinylketons A durch das Binolphosphat *BH ein Addukt B bestehend aus einem Cyclopentadienylkation und einem Phosphatanion ergibt (Schema 1). Eine direkt anschließende konrotatorische 4p-Elektrocyclisierung sollte dann zum Oxyallylkation C führen, das unter Abspaltung eines Protons das Enolat D ergibt. Nachfolgende Protonierung des Enolats sollte dann das Cyclopentenon E und die regenerierte Brønsted-Säure *BH ergeben.Zu Beginn unserer experimentellen Untersuchungen begannen wir daher mit der Suche nach einem geeigneten Brønsted-Säure-Katalysator für die enantioselektive Elektrocyclisierung des Dienons 2 (Tabelle 1).[8] Bereits die ersten Reaktionen, die mit verschiedenen Binolphosphaten 1 a-e in Toluol bei 60 8C durchgeführt wurden, ergaben die Cyclo-

Recently, 1,1'-bi-2-naphthol (BINOL)-based N-triflylphosphoramides emerged as a new class of potent Brønsted acid catalysts. In this paper we describe the efficient synthesis of various BINOL-based N-triflylphosphoramides and their calcium salts. Furthermore, X-ray crystal structure analysis combined with energy-dispersive X-ray spectroscopy (EDX) measurements confirmed that the synthesised chiral N-triflylphosphoramides are highly acidic metal-free catalysts.