Photoredox-catalyzed Direct Reductive Amination of Aldehydes without an External Hydrogen/Hydride Source

Visible-Light-Promoted Dual Photoredox/Nickel-Catalyzed Chemoselective Reduction of Secondary and Tertiary Amides with Hydrosilanes in the Presence of an Ester

We report a one-step procedure to selectively reduce secondary and tert-amides to their corresponding amine derivatives in the presence of an ester. This was achieved via the synergistic combination of a photoredox, a nickel catalytic system, and phenyl silane as a reductant in the presence of blue light-emitting diode light (455 nm) at room temperature. Further, this mild light-promoted dual metallaphotoredox catalytic system was also successful in selectively reducing a lactam to the cyclic amines, without affecting the ester moiety present in the molecules.

Photocatalyst-free visible-light triggered amination of benzo[c][1,2,5]thiadiazole: direct C-N bond formation from C(sp2)-H bond

The photoredox amination of arene protocols mostly comprises photocatalyst-mediated transformations. Herein, we presented the photocatalyst-free, visible-light promoted, direct conversion of C(sp²)-H to C(sp²)-N method. Multipurpose benzothiadiazoles are used as model synthons and secondary amines as aminating agents. Mechanistic study reveals that the radical reaction mechanism proceeds through nitrogen-centered radical generation, followed by the addition of arenes, which was demonstrated for the present amination protocol of benzothiadiazole with secondary amines in an atom economical fashion.

Recent advances in visible light-induced C(sp3)–N bond formation

Synthetic chemists have long focused on selective C(sp ³)-N bond-forming approaches in response to the high value of this motif in natural products, pharmaceutical agents and functional materials. In recent years, visible light-induced protocols have become an important synthetic platform to promote this transformation under mild reaction conditions. These photo-driven methods rely on converting visible light into chemical energy to generate reactive but controllable radical species. This Review highlights recent advances in this area, mostly after 2014, with an emphasis placed on C(sp ³)-H bond activations, including amination of olefins and carbonyl compounds, and cross-coupling reactions.

Organic Photoredox Catalysts Exhibiting Long Excited-State Lifetimes

Organic photoredox catalysts with a long excited-state lifetime have emerged as promising alternatives to transition-metal-complex photocatalysts. This paper explains the effectiveness of using long-lifetime photoredox catalysts for organic transformations, focusing on the structures and photophysics that enable long excited-state lifetimes. The electrochemical potentials of the reported organic, long-lifetime photocatalysts are compiled and compared with those of the representative Ir(III)- and Ru(II)-based catalysts. This paper closes by providing recent demonstrations of the synthetic utility of the organic catalysts.

1 Introduction

2 Molecular Structure and Photophysics

3 Photoredox Catalysis Performance

4 Catalysis Mediated by Long-Lifetime Organic Photocatalysts

4.1 Photoredox Catalytic Generation of a Radical Species and its Addition to Alkenes

4.2 Photoredox Catalytic Generation of a Radical Species and its Addition to Arenes

4.3 Photoredox Catalytic Generation of a Radical Species and its Addition to Imines

4.4 Photoredox Catalytic Generation of a Radical Species and its Addition to Substrates Having C≡X Bonds (X=C, N)

4.5 Photoredox Catalytic Generation of a Radical Species and its Bond Formation with Transition Metals

4.6 Miscellaneous Reactions of Radical Species Generated by Photoredox Catalysis

5 Conclusions

Ru(dppbsa)-catalyzed hydrodeoxygenation and reductive etherification of ketones and aldehydes

Ru(dppbsa)-catalyzed hydrodeoxygenation and reductive etherification of ketones and aldehydes were developed. The carbonyl substrates without β-CH functionality follow the hydrogenation-hydrogenolysis path, wherein the hydrogenolysis of the alkanol intermediates presents as...

Mechanochemistry Activated Covalent Conjugation Reactions in Soft Hydrogels Induced by Interfacial Failure

This work reports the development of a mechanochemistry activated covalent conjugation (MACC) reaction that shows areas of interfacial failure in soft hydrogels. Hydrogels are prone to delamination from rigid substrates due to the competition between swelling and adhesion, which can lead to bonding failure in a mechanism similar to crack propagation in harder materials. In this work, reductive amination was shown to occur when a ketone-bearing fluorescein derivative was bonded to an amine-functionalized hydrogel, as both of these moieties were found to be necessary for covalent conjugation into the gel network. For thin, circular polyacrylamide hydrogels, wrinkle patterns and regions of subsequent delamination at the edge of the gel were found to be selectively tagged by the dye. This reaction was then used to explore the effect of gel properties on patterns of interfacial failure. As cross-linker loading increased, the propagation of the delamination front and the area fraction of delamination were both found to increase, as shown by fluorescence images of gels. Increasing the thickness of the gel increased the fraction of delaminated area but did not change its propagation toward the center of the gel. This MACC reaction shows how mechanochemical reactions can be used for fluorescence tagging without incorporating mechanophores into the polymer gel matrix.

Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis

Organic compounds that show Thermally Activated Delayed Fluorescence (TADF) have become wildly popular as next-generation emitters in organic light emitting diodes (OLEDs). Since 2016, a subset of these have found increasing use as photocatalysts. This review comprehensively highlights their potential by documenting the diversity of the reactions where an organic TADF photocatalyst can be used in lieu of a noble metal complex photocatalyst. Beyond the small number of TADF photocatalysts that have been used to date, the analysis conducted within this review reveals the wider potential of organic donor-acceptor TADF compounds as photocatalysts. A discussion of the benefits of compounds showing TADF for photocatalysis is presented, which paints a picture of a very promising future for organic photocatalyst development.

Effects of Propylene Oxide End Capping on Amination of Polyalkylene Glycols

Polyalkylene glycols with two different end-capping groups of ethylene oxide (EO) and propylene oxide (PO) were used for amination to produce polyetheramine (PEA) on cobalt-based catalysts. Although it is known that the amination of secondary alcohol is more difficult than that of primary alcohol, PO end-capped block copolymers showed remarkably enhanced activity toward PEA and selectivity toward the primary amine compared to EO end-capped block copolymers.

Visible-Light-Promoted Photoredox Dehydrogenative Coupling of Phosphines and Thiophenols

Herein, by applying visible-light photoredox catalysis, we have now achieved the first example of catalytic dehydrogenative coupling of phosphines and thiophenols that proceeds at room temperature. Key to our success is the use of benzaldehyde as a soft oxidant, which avoids the issue of phosphine oxidation. Furthermore, we observed the unexpected dealkylative coupling of secondary and tertiary alkylphosphine with thiophenols.

Understanding the role of thermodynamics in catalytic imine reductions

The past decade has witnessed a booming growth of research activity in catalytic imine reduction, due to the ongoing motivation to find more efficient conversions of the rather unreactive C[double bond, length as m-dash]N bonds to the desired C-N segments found in many pharmaceuticals. While several timely reviews have well documented various C[double bond, length as m-dash]N reduction methodologies with respect to the type of catalyst (acid, base, or transition metal), a detailed discussion of the core role of thermodynamic driving forces in governing these catalyses is still lacking, however. Hence, this tutorial review describes some of the most practical considerations for adjusting reduction thermodynamics by choosing appropriate catalytic strategies, in order to make the target reduction energetically feasible. The combined use of relevant thermodynamic parameters of the substrate imines, hydrogen donors, and catalysts in realizing such a goal is demonstrated on the basis of the energetics of the possible elementary paths. Experimental observations from the literature that are in line with the present energetics-based analyses are exemplified.

Proton-coupled multi-electron transfer and its relevance for artificial photosynthesis and photoredox catalysis

Photoinduced PCET meets catalysis, and the accumulation of multiple redox equivalents is of key importance.

Visible-Light-Driven External-Reductant-Free Cross-Electrophile Couplings of Tetraalkyl Ammonium Salts

Cross-electrophile couplings between two electrophiles are powerful and economic methods to generate C-C bonds in the presence of stoichiometric external reductants. Herein, we report a novel strategy to realize the first external-reductant-free cross-electrophile coupling via visible-light photoredox catalysis. A variety of tetraalkyl ammonium salts, bearing primary, secondary, and tertiary C-N bonds, undergo selective couplings with aldehydes/ketone and CO₂. Notably, the in situ generated byproduct, trimethylamine, is efficiently utilized as the electron donor. Moreover, this protocol exhibits mild reaction conditions, low catalyst loading, broad substrate scope, good functional group tolerance, and facile scalability. Mechanistic studies indicate that benzyl radicals and anions might be generated as the key intermediates via photocatalysis, providing a new direction for cross-electrophile couplings.

Enantioselective synthesis of amines by combining photoredox and enzymatic catalysis in a cyclic reaction network

Visible light-driven reduction of imines to enantioenriched amines in aqueous solution is demonstrated for the first time. Excitation of a new water-soluble variant of the widely used [Ir(ppy)3] (ppy = 2-phenylpyridine) photosensitizer in the presence of a cyclic imine affords a highly reactive α-amino alkyl radical that is intercepted by hydrogen atom transfer (HAT) from ascorbate or thiol donors to afford the corresponding amine. The enzyme monoamine oxidase (MAO-N-9) selectively catalyzes the oxidation of one of the enantiomers to the corresponding imine. Upon combining the photoredox and biocatalytic processes under continuous photo-irradiation, enantioenriched amines are obtained in excellent yields. To the best of our knowledge, this is the first demonstration of a concurrent photoredox- and enzymatic catalysis leading to a light-driven asymmetric synthesis of amines.

Visible light catalyzed synthesis of quinolines from (aza)-Morita–Baylis–Hillman adducts

A mild and efficient protocol for the synthesis of quinoline scaffolds from (aza)-MBH adducts under visible light catalysis has been established.