Synthesis and Structural Aspects of <i>N</i>‐Triflylphosphoramides and Their Calcium Salts—Highly Acidic and Effective Brønsted Acids

Rueping, Magnus; Nachtsheim, Boris J.; Koenigs, René M.; Ieawsuwan, Winai

doi:10.1002/chem.201001438

Cited by 97 publications

(39 citation statements)

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“…Hierzu wurden kürzlich von unserer Gruppe systematische Untersuchungen durchgeführt, die im folgenden Absatz kurz erläutert werden sollen. [12] Zunächst [12,13] Allerdings können die Calciumsalze ebenfalls in der asymmetrischen Katalyse eingesetzt werden.…”

Section: Metall-liganden Oder Brønsted-säurenunclassified

“…Hierzu wurden kürzlich von unserer Gruppe systematische Untersuchungen durchgeführt, die im folgenden Absatz kurz erläutert werden sollen. [12] Zunächst wurden von uns Kristallisationsexperimente durchgeführt, um die räumliche Struktur des Katalysators besser verstehen zu können. Allerdings lieferten diese ersten Kristallisationsexperimente nicht die freie Brønsted-Säure 18 c, sondern das entsprechende Calcium(II)-Salz (Abbildung 5 a).…”

Section: Metall-liganden Oder Brønsted-säurenunclassified

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Modulation der Acidität – hoch acide Brønsted‐Säuren in der asymmetrischen Katalyse

et al. 2011

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Chirale, hoch acide Brønsted‐Säuren haben sich als metallfreie Katalysatoren in der organischen Synthese etabliert und konnten mittlerweile in einer Vielzahl enantioselektiver C‐C‐ und C‐X‐Bindungsknüpfungen eingesetzt werden. Die hohe Acidität dieser neuen Katalysatoren macht sie zu nützlichen Hilfsmitteln für die Aktivierung verschiedener Imine, Carbonyle und weiterer schwach basischer Substrate. Durch die Bildung chiraler Kontaktionenpaare ergeben sich völlig neue Aktivierungsmöglichkeiten für die asymmetrische Katalyse. Dieser Kurzaufsatz gibt einen Überblick über das rationale Design dieser neuen Organokatalysatoren und beschreibt verschiedenste, kürzlich erschienene Anwendungen.

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Section: Metall-liganden Oder Brønsted-säurenunclassified

Modulation der Acidität – hoch acide Brønsted‐Säuren in der asymmetrischen Katalyse

et al. 2011

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“…Changing the bulky ortho substituent on the binol core from 2,4,6-triisopropylphenyl to 9-anthracene further improved the selectivity, thus providing 3,6-dihydro-2H-pyran 8 with ee values as high as 90 %. [13] By using a reliable asymmetric synthetic approach [14] we have unambiguously determined that when the S enantiomers of B-G are used the absolute stereochemistry of the major enantiomer of 8 is of the R configuration (as shown). [15] Although longer reaction times and higher reaction temperatures are needed when these chiral Bronsted acids are used to catalyze the vinyl oxetane ring expansion, [16] the ee values are superior and very encouraging for this challenging system.…”

Section: Methodsmentioning

confidence: 99%

Catalytic Ring Expansion of Vinyl Oxetanes: Asymmetric Synthesis of Dihydropyrans Using Chiral Counterion Catalysis

2012

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As a part of our program aimed at developing new ringexpansion reactions from strained heterocyclic precursors [1] we are exploring vinyl oxetanes as potential candidates. We were particularly eager to learn if our success with the ring expansion of vinyl oxiranes to 2,5-dihydrofurans using [Cu-(hfacac) 2 ] could be translated into an equivalent transformation for vinyl oxetanes (Scheme 1). Given the allylic nature of the system and the favorable release of strain energy, vinyl oxetanes seemed like perfect ring-expansion candidates. Furthermore, we predicted that a broader range of catalysts would be competent for this new ring-expansion reaction because the competing 1,2-hydride shift pathway we needed to overcome for the vinyl oxirane ring expansion is less likely to occur for the vinyl oxetanes.In general, vinyl oxetanes have received limited attention, where the few published studies have focused on nucleophilic ring openings [2] of vinyl oxetanes and insertions of heteroatoms [3] into the oxetane. [4] There is only one report in the literature of a related lactonic vinyl oxetane ring expansion (Scheme 1). In 2000, when trying to expand the scope of their cationic-palladium-mediated b-lactone synthesis to include conjugated aldehydes Hattori et al. realized that in many cases d-lactones were formed. [5] The scope and yield of their cascade reaction, which was developed using classic palladium chemistry and relies on a superior leaving group to that in our ether ring expansion, was reasonable but limited to a few enals; enones were found to be poor substrates.We selected vinyl oxetane 1 as our model substrate. It is readily accessible by a nucleophilic addition to a commercially available b-chloro carbonyl precursor [6] and it has a phenyl group that we postulated would aid oxetane bond breaking and stabilization of a cationic intermediate. When 1 was subjected to the standard catalytic [Cu(hfacac) 2 ] conditions we used for the vinyl oxiranes, [1a] ring expansion to 3,6dihydro-2H-pyran 2 did indeed occur after prolonged heating at 150 8C, albeit in poor yield (Table 1, entry 1). The detrimental hydride shift pathway is presumably a far less likely competing path for vinyl oxetanes than vinyl oxiranes, therefore we decided to evaluate other stable readily available catalysts. Metal(II) triflates were the most effective catalysts and dichloromethane the best solvent. Copper(II) triflate (Table 1, entries 5-7) proved to be a remarkably wellsuited catalyst for the ring expansion, thus affording the desired product (2) both rapidly and quantitatively using only 1 mol % of catalyst at À78 8C. These results prompted us to Scheme 1. Proposed catalytic ring expansion of vinyl oxetanes. Table 1: Catalyst screening for vinyl oxetane ring expansion. Entry Catalyst mol % Solvent T [8C] t [h] % Conv.

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“…Whereas in metal-catalyzed reactions the activation of the carbonyl or imine group is achieved through coordination to Lewis acids, the basis of organocatalytic methodologies lies either in protonation through chiral Brønsted acids or in hydrogen-bond activation by chiral small-molecule H-bond donor catalysts. [83] Chiral thioureas [84] thus induce enantioselectivity in the nucleophilic addition to the cationic electrophile through hydrogen-bond formation, whereas optically active phosphoric acids or dicarboxylic acids, [85] as well as N-triflylphosphoramides, [86] act as proton donors. Although there are a good number of reports on asymmetric organocatalytic reactions involving intermediary iminium ions (in both inter-and intramolecular processes) [87] and N-acyliminium ions in intermolecular processes, [88] intramolecular α-amidoalkylation reactions with π-electrophiles, and specifically with aromatic and heteroaromatic rings, are still limited to a few examples that take advantage of both activation methods, using chiral Brønsted acids, such as BINOL-derived phosphoric acids or trifylphosphoramides, together with thioureas.…”

Section: Enantioselective α-Amidoalkylation Reactionsmentioning

confidence: 99%

Strategies Based on Aryllithium and N‐Acyliminium Ion Cyclizations for the Stereocontrolled Synthesis of Alkaloids and Related Systems

et al. 2011

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The intramolecular α-amidoalkylation reactions of aromatic and heteroaromatic ring systems constitute a versatile approach for the synthesis of nitrogen heterocyles in a diastereoselective or enantioselective fashion. On the other hand, the intramolecular reactions of aryllithium compounds have also been extensively used in the synthesis of carbocycles and heterocycles. The use of imides as internal electrophiles is particularly attractive because of the potential to introduce diverse functionality into the cyclized products by subjecting the resulting α-hydroxylactams to intermolecular IntroductionAryllithium compounds and N-acyliminium ions are extremely versatile intermediates for the formation of carboncarbon bonds in organic synthesis. Cyclization of aryllithium compounds generated by halogen/lithium exchange with internal electrophiles (Parham cyclization) has become a valuable protocol for the stereoselective construction of carbocyclic and heterocyclic systems.[1] Many electrophiles remain inert during halogen/metal exchange reaction at low temperature, but are reactive enough to participate in a subsequent cyclization reaction. Halides, epoxides, or alkenes [2] are thus among the different types of internal electrophiles used in the Parham cyclization. When the internal electrophile is a carboxylate derivative, this anionic cyclization could be considered an anionic Friedel-Crafts equivalent, with the advantage that it lacks the electronic requirements of the classical reaction. Although it is possible to use carboxylic acids [3] or esters, [4] carbamates have proven to be much more effective internal electrophiles in Parham cycliacylations.[5] Amides are also useful electrophiles in Parham cyclizations, and it has been reported that in some cases there is an influence of the natures of the substituents at the nitrogen atom on the course of the cyclization reaction. [6,7] [a] Departamento In connection with our interest in aromatic lithiation, we have developed an anionic cyclization approach directed towards the construction of the pyrrolo[1,2-b]isoquinolone core based on N-(o-halobenzyl)pyrrole-2-carboxamides, showing that Weinreb amides and morpholine amides function as excellent internal electrophiles.[8] This observation could be explained by assuming that halogen/lithium exchange could be favored by a complex-induced proximity effect (CIPE).[9] This concept has been invoked to explain other metal/metal, hydrogen/metal, or halogen/metal [10] exchange reactions. Lithium/halogen exchange would thus be favored first by coordination of the inducing organolithium compound with amide or carbamate groups, and then by stabilization of the resulting aryllithium compound. The better behavior of Weinreb and morpholine amides relative to N,N-diethyl amides could be attributed to the extra stabilization of the intermediate generated after cyclization through the formation of an internal chelate. The scope of these Parham-type cyclizations has been extended to the formation of seven-and eight-membered rings [8b] ...

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Synthesis and Structural Aspects of N‐Triflylphosphoramides and Their Calcium Salts—Highly Acidic and Effective Brønsted Acids

Cited by 97 publications

References 58 publications

Modulation der Acidität – hoch acide Brønsted‐Säuren in der asymmetrischen Katalyse

Modulation der Acidität – hoch acide Brønsted‐Säuren in der asymmetrischen Katalyse

Catalytic Ring Expansion of Vinyl Oxetanes: Asymmetric Synthesis of Dihydropyrans Using Chiral Counterion Catalysis

Strategies Based on Aryllithium and N‐Acyliminium Ion Cyclizations for the Stereocontrolled Synthesis of Alkaloids and Related Systems

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