Phosphonic acid catalyzed synthesis of pyrazolidines

Davis, Lindsey O.; Daniel, William F. M.; Tobey, Suzanne L.

doi:10.1016/j.tetlet.2011.11.083

Cited by 8 publications

(4 citation statements)

References 15 publications

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“…Subsequently, complex C releases the pyrazolidine product, regenerating the complex A with another molecule of hydrazone. Although monopolar (3 + + 2) cycloadditions with hydrazonium cations have been documented since the 1970s, the synthetic applications and especially the catalytic reactions are rare . Does the N -triflylphosphoramide really protonate hydrazones and enable the (3 + + 2) cycloadditions with alkenes?…”

Section: Introductionmentioning

confidence: 99%

Mechanism and Selectivity of N-Triflylphosphoramide Catalyzed (3⁺ + 2) Cycloaddition between Hydrazones and Alkenes

et al. 2014

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Brønsted acid catalyzed (3(+) + 2) cycloadditions between hydrazones and alkenes provide a general approach to pyrazolidines. The acidity of the Brønsted acid is crucial for the catalytic efficiency: the less acidic phosphoric acids are ineffective, while highly acidic chiral N-triflylphosphoramides are very efficient and can promote highly enantioselective cycloadditions. The mechanism and origins of catalytic efficiencies and selectivities of these reactions have been explored with density functional theory (M06-2X) calculations. Protonation of hydrazones by N-triflylphosphoramide produces hydrazonium-phosphoramide anion complexes. These ion-pair complexes are very reactive in (3(+) + 2) cycloadditions with alkenes, producing pyrazolidine products. Alternative 1,3-dipolar (3 + 2) cycloadditions with the analogous azomethine imines are much less favorable due to the endergonic isomerization of hydrazone to azomethine imine. With N-triflylphosphoramide catalyst, only a small distortion of the ion-pair complex is required to achieve its geometry in the (3(+) + 2) cycloaddition transition state. In contrast, the weak phosphoric acid does not protonate the hydrazone, and only a hydrogen-bonded complex is formed. Larger distortion energy is required for the hydrogen-bonded complex to achieve the "ion-pair" geometry in the cycloaddition transition state, and a significant barrier is found. On the basis of this mechanism, we have explained the origins of enantioselectivities when a chiral N-triflylphosphoramide catalyst is employed. We also report the experimental studies that extend the substrate scope of alkenes to ethyl vinyl ethers and thioethers.

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Section: Introductionmentioning

confidence: 99%

Mechanism and Selectivity of N-Triflylphosphoramide Catalyzed (3⁺ + 2) Cycloaddition between Hydrazones and Alkenes

et al. 2014

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“…Simi-larly, in the 13 C NMR spectrum, whilst the two allylic methyl groups and one of the methoxy groups appeared as very sharp res- tertiary carbenium ion 6 featured in our pyrrolidine synthesis (Scheme 1), subsequent trapping of which by the newly generated hydrazine leads to the observed products 7. Of course, as pointed out, 4 alternative mechanisms could well be in operation, but at least a compatibility between such masked hydrazines and an acid-catalysed reaction looked likely. 5 We relied on the Mitsunobu reaction 6 to obtain suitable sub-strates for our investigation of the idea shown in Scheme 2, as out-lined in Scheme 4.…”

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confidence: 95%

“…2 Hence, often it is preferable to assemble such structures using a stepwise approach. 3,4 The inspiration for the present methodology was derived from a possible extension of our Þnding that unsatu-rated sulfonamides 1 are readily converted into the corresponding ⇑ Corresponding author.…”

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confidence: 99%

“…In this study, the discovery of novel, twostep cyclisations was described in which the acylhydrazones 5 having a distal prenyl alkene under-went conversion into the annulated pyrazolidines 7 (Scheme 3). 4 A likely mechanism involves imine protonation followed by cyclisa-tion to form a cyclohexane which generates exactly the type of symmetrically protected hydrazine 13 by a Ôhalf-MitsunobuÕ wherein no additional nucleophile is added; the hydrazine by-pro-duct plays this role.…”

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A new method for the synthesis of pyrazolidines

Chaudhry

Kariuki

Knight

2016

Tetrahedron Letters

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Asymmetrische Brønsted‐Säure‐katalysierte Cycloadditionen – effiziente enantioselektive Synthese von Pyrazolidinen, Pyrazolinen und 1,3‐Diaminen aus N‐Acylhydrazonen und Alkenen

et al. 2012

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Eine allgemeine, metallfreie, hoch enantioselektive Brønsted‐Säure‐katalysierte [3+2]‐Cycloaddition von Hydrazonen mit Alkenen wurde entwickelt, die zu nützlichen Pyrazolidin‐Derivaten führt (siehe Schema). Die erhaltenen optisch aktiven Pyrazolidine können auf vielfältige Weise chemisch modifiziert werden, was unter anderem die enantioselektive Synthese wichtiger 1,3‐Diamine ermöglicht.

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Phosphonic acid catalyzed synthesis of pyrazolidines

Cited by 8 publications

References 15 publications

Mechanism and Selectivity of N-Triflylphosphoramide Catalyzed (3⁺ + 2) Cycloaddition between Hydrazones and Alkenes

Mechanism and Selectivity of N-Triflylphosphoramide Catalyzed (3⁺ + 2) Cycloaddition between Hydrazones and Alkenes

A new method for the synthesis of pyrazolidines

Asymmetrische Brønsted‐Säure‐katalysierte Cycloadditionen – effiziente enantioselektive Synthese von Pyrazolidinen, Pyrazolinen und 1,3‐Diaminen aus N‐Acylhydrazonen und Alkenen

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Phosphonic acid catalyzed synthesis of pyrazolidines

Cited by 8 publications

References 15 publications

Mechanism and Selectivity of N-Triflylphosphoramide Catalyzed (3+ + 2) Cycloaddition between Hydrazones and Alkenes

Mechanism and Selectivity of N-Triflylphosphoramide Catalyzed (3+ + 2) Cycloaddition between Hydrazones and Alkenes

A new method for the synthesis of pyrazolidines

Asymmetrische Brønsted‐Säure‐katalysierte Cycloadditionen – effiziente enantioselektive Synthese von Pyrazolidinen, Pyrazolinen und 1,3‐Diaminen aus N‐Acylhydrazonen und Alkenen

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Mechanism and Selectivity of N-Triflylphosphoramide Catalyzed (3⁺ + 2) Cycloaddition between Hydrazones and Alkenes

Mechanism and Selectivity of N-Triflylphosphoramide Catalyzed (3⁺ + 2) Cycloaddition between Hydrazones and Alkenes