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

TiCl4-mediated deoxygenative reduction of aromatic ketones to alkylarenes with ammonia borane

A rapid and mild protocol for the exhaustive deoxygenation of various aromatic ketones to corresponding alkanes was described, which was mediated by TiCl4 and used ammonia borane (AB) as the reductant. This reduction protocol applies to a wide range of substrates in moderate to excellent yields at room temperature. The gram-scale reaction and syntheses of some key building blocks for SGLT2 inhibitors demonstrated the practicability of this methodology. Preliminary mechanistic studies revealed that the ketone is first converted into an alcohol, which then undergoes a carbocation to give the alkane via hydrogenolysis.

Recent Advances in Organocatalyzed Asymmetric Reduction of Prochiral Ketones: An Update

Chiral alcohols are important synthetic intermediates and building blocks for the synthesis of drugs, agrochemicals, and natural products. Asymmetric reduction of prochiral ketones has been the most investigated method for accessing chiral alcohols. In this regard, organocatalyzed asymmetric reduction, as a complementary method to transition-metal- and enzyme-catalyzed reactions, has attracted tremendous interest in the past decades due to the reactions with such catalysts being metal-free and easy to operate, and principally, the ease of recovery and the ability to reuse the catalysts. Following up on a comprehensive overview on organocatalyzed asymmetric reductions of prochiral ketones in early 2018, this short review is intended to summarize the recent progress in this area from the beginning of 2018 until the end of August 2021.

1 Introduction

2 Boron-Based Chiral Organocatalysts

2.1 Boron-Containing Chiral Schiff Base Catalysts

2.2 Chiral Alpine-Borane Catalysts

2.3 Boron-Containing Chiral Frustrated Lewis Pair Catalysts

2.4 Chiral Borate Ester–Amine Complex Catalysts

3 Phosphorus-Based Chiral Organocatalysts

3.1 Chiral Phosphoric Acid Organocatalysts

3.2 Chiral Phosphinamide and Phosphoramide Organocatalysts

4 Chiral Ionic Liquid Organocatalysts

5 Chiral-Oxazoline-Based Organocatalysts

6 Conclusion and Outlook

Chiral Phosphoric Acids as Versatile Tools for Organocatalytic Asymmetric Transfer Hydrogenations

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.

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

Transfer hydrogenation (TH) has historically been dominated by Meerwein-Ponndorf-Verley (MPV) reactions. However, with growing interest in amine-boranes, not least ammonia-borane (H3 N⋅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 H3 N⋅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.

Zirconium-hydride-catalyzed transfer hydrogenation of quinolines and indoles with ammonia borane

Herein, by applying zirconium-hydride complex as the catalyst, the transfer hydrogenation of quinoline and indole derivatives with ammonia borane as a proton and hydride source is achieved.

Ammonia borane as a reducing agent in organic synthesis

Ammonia borane is gaining increasing attention as a sustainable and atom-economical winning reagent for the reduction of several substrates.