Chiral phosphoric acid catalyzed asymmetric transfer hydrogenation of bulky aryl ketones with ammonia borane

Zhou, Qiwen; Meng, Wei; Feng, Xiujuan; Du, Haifeng; Yang, Jing

doi:10.1016/j.tetlet.2019.151394

“…Further advancement of the asymmetric TH of imines and ketones with H 3 N•BH 3 was described by Du and co-workers when they used enantioenriched phosphoric acid (Scheme 19 a,b). [54] The chiral ammonia-borane complex 21 was isolated but, while it was found to be off-cycle for ketone reduction (Scheme 19 b), it was proven to be an active intermediate in the TH of imines by stoichiometric studies and DFT calculations (Scheme 19 a). Interestingly for both processes, DFT calculations supported the formation of a sixmembered-ring TS which accounted for substrate activation by amine-borane chiral complex 21 for imine reduction, while it involves substrate activation by phosphoric acid to form species 22, followed by TH from H 3 N•BH 3 for ketone reduction.…”

Section: Metal-free Thmentioning

confidence: 99%

Amine–Boranes as Transfer Hydrogenation and Hydrogenation Reagents: A Mechanistic Perspective

Lau

¹

,

Gasperini

²

,

Webster

³

2021

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Transfer hydrogenation (TH) has historically been dominated by Meerwein–Ponndorf–Verley (MPV) reactions. However, with growing interest in amine–boranes, not least ammonia–borane (H3N⋅BH3), as potential hydrogen storage materials, these compounds have also started to emerge as an alternative reagent in TH reactions. In this Review we discuss TH chemistry using H3N⋅BH3 and their analogues (amine–boranes and metal amidoboranes) as sacrificial hydrogen donors. Three distinct pathways were considered: 1) classical TH, 2) nonclassical TH, and 3) hydrogenation. Simple experimental mechanistic probes can be employed to distinguish which pathway is operating and computational analysis can corroborate or discount mechanisms. We find that the pathway in operation can be perturbed by changing the temperature, solvent, amine–borane, or even the substrate used in the system, and subsequently assignment of the mechanism can become nontrivial.

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“…According to DFT studies, the phosphoric acid promotes the double hydrogen transfer between the ammonia borane and the ketone, as the transfer of the hydridic and protic hydrogens take place simultaneously through a pericyclic six-membered transition state as a source of asymmetric induction. [28] Scheme 5. Asymmetric hydroarylation of electron rich stryrenes.…”

Section: Reduction Of C = C and C = O Double Bondsmentioning

confidence: 99%

Chiral Phosphoric Acids as Versatile Tools for Organocatalytic Asymmetric Transfer Hydrogenations

Pálvölgyi

¹

,

Scharinger

²

,

Schnürch

³

et al. 2021

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Herein, recent developments in the field of organocatalytic asymmetric transfer hydrogenation (ATH) of C=N, C=O and C=C double bonds using chiral phosphoric acid catalysis are reviewed. This still rapidly growing area of asymmetric catalysis relies on metal-free catalysts in combination with biomimetic hydrogen sources. Chiral phosphoric acids have proven to be extremely versatile tools in this area, providing highly active and enantioselective alternatives for the asymmetric reduction of α,β-unsaturated carbonyl compounds, imines and various heterocycles. Eventually, such transformations are more and more often used in multicomponent/cascade reactions, which undoubtedly shows their great synthetic potential and the bright future of organocatalytic asymmetric transfer hydrogenations.

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“…Further advancement of the asymmetric TH of imines and ketones with H 3 N⋅BH 3 was described by Du and co‐workers when they used enantioenriched phosphoric acid (Scheme 19 a,b). [54] The chiral ammonia–borane complex 21 was isolated but, while it was found to be off‐cycle for ketone reduction (Scheme 19 b), it was proven to be an active intermediate in the TH of imines by stoichiometric studies and DFT calculations (Scheme 19 a). Interestingly for both processes, DFT calculations supported the formation of a six‐membered‐ring TS which accounted for substrate activation by amine–borane chiral complex 21 for imine reduction, while it involves substrate activation by phosphoric acid to form species 22 , followed by TH from H 3 N⋅BH 3 for ketone reduction.…”

Section: Catalyzed Classical Th Reactionsmentioning

confidence: 99%

Amine–Boranes as Transfer Hydrogenation and Hydrogenation Reagents: A Mechanistic Perspective

Lau

¹

,

Gasperini

²

,

Webster

³

2021

View full text Add to dashboard Cite

Transfer hydrogenation (TH) has historically been dominated by Meerwein–Ponndorf–Verley (MPV) reactions. However, with growing interest in amine–boranes, not least ammonia–borane (H3N⋅BH3), as potential hydrogen storage materials, these compounds have also started to emerge as an alternative reagent in TH reactions. In this Review we discuss TH chemistry using H3N⋅BH3 and their analogues (amine–boranes and metal amidoboranes) as sacrificial hydrogen donors. Three distinct pathways were considered: 1) classical TH, 2) nonclassical TH, and 3) hydrogenation. Simple experimental mechanistic probes can be employed to distinguish which pathway is operating and computational analysis can corroborate or discount mechanisms. We find that the pathway in operation can be perturbed by changing the temperature, solvent, amine–borane, or even the substrate used in the system, and subsequently assignment of the mechanism can become nontrivial.

show abstract

Chiral phosphoric acid catalyzed asymmetric transfer hydrogenation of bulky aryl ketones with ammonia borane

Cited by 10 publications

References 56 publications

Amine–Boranes as Transfer Hydrogenation and Hydrogenation Reagents: A Mechanistic Perspective

Amine–Boranes as Transfer Hydrogenation and Hydrogenation Reagents: A Mechanistic Perspective

Chiral Phosphoric Acids as Versatile Tools for Organocatalytic Asymmetric Transfer Hydrogenations

Amine–Boranes as Transfer Hydrogenation and Hydrogenation Reagents: A Mechanistic Perspective

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