Direct visible-light-induced synthesis of P-stereogenic phosphine oxides under air conditions

Over the past two decades, visible-light-induced transformations have been regarded as being among the most environmentally benign and powerful strategies for constructing complex molecules and diverse synthetic building blocks in organic synthesis. However, the development of efficient photochemical processes for assembling enantiomerically pure molecules remains a significant challenge. Herein, we describe a simple and efficient visible-light-induced C–P bond forming reaction for the synthesis of P-chiral heteroaryl phosphine oxides in moderate to high yields with excellent ee values (97–99% ee). Even in the absence of transition metal or photoredox catalysts, a variety of P-chiral heteroaryl phosphine oxides, including chiral diphosphine oxide 41, have been directly obtained under air conditions. Density functional theory (DFT) calculations have shown that the reaction involves intersystem crossing and single electron transfer to give a diradical intermediate under visible light irradiation.

We describe a simple and efficient visible-light-induced C–P bond forming reaction for the synthesis of P-chiral heteroaryl phosphine oxides in moderate to high yields with excellent ee values (97–99% ee).

Enantioselective [2 + 2] photocycloaddition of quinolone using a C1-symmetric chiral phosphoric acid as a visible-light photocatalyst

The enantioselective intra- and intermolecular [2 + 2] photocycloaddition of quinolone using a C₁-symmetric chiral phosphoric acid as a visible-light photocatalyst is developed. The thioxanthone chromophore on phosphoric acid plays an important role in both phototransformation and enantioselectivity.

Stereoselective, Ruthenium-Photocatalyzed Synthesis of 1,2-Diaminotruxinic Bis-amino Acids from 4-Arylidene-5(4H)-oxazolones

The irradiation of (Z)-2-phenyl-4-aryliden-5(4H)-oxazolones 1 in deoxygenated CH₂Cl₂ at 25 °C with blue light (465 nm) in the presence of [Ru(bpy)₃](BF₄)₂ (5% mole ratio) as a triplet photocatalyst promotes the [2+2] photocycloaddition of the C=C bonds of the 4-arylidene moiety, thus allowing the completely regio- and stereoselective formation of cyclobutane-bis(oxazolone)s 2 as single stereoisomers. Cyclobutanes 2 have been unambiguously characterized as the μ-isomers and contain two E-oxazolones coupled in an anti-head-to-head form. The use of continuous-flow techniques in microreactors allows the synthesis of cyclobutanes 2 in only 60 min, compared with the 24–48 h required in batch mode. Ring opening of the oxazolone heterocycle in 2 with a base affords the corresponding 1,2-diaminotruxinic bis-amino esters 3, which are also obtained selectively as μ-isomers. The ruthenium complex behaves as a triplet photocatalyst, generating the reactive excited state of the oxazolone via an energy-transfer process. This reactive excited state has been characterized as a triplet diradical ³(E/Z)-1* by laser flash photolysis (transient absorption spectroscopy). This technique also shows that this excited state is the same when starting from either (Z)- or (E)-oxazolones. Density functional theory calculations show that the first step of the [2+2] cycloaddition between ³(E/Z)-1* and (Z)-1 is formation of the C(H)–C(H) bond and that (Z) to (E) isomerization takes place at the 1,4-diradical thus formed.

Enantioselective synthesis enabled by visible light photocatalysis

Enantioselective photoreaction has been a synthetic challenge for decades. With the continuous development of modern visible light photocatalysis and asymmetric catalysis, remarkable advances have been achieved through the synergistic action of these catalytic reactions, allowing the construction of various enantiomerically enriched molecules that were once inaccessible using photocatalytic reactions. This review presents some of the contemporary developments in enantioselective visible-light photocatalysis reactions, covering the period from 2008 to March 2020, with the contents classified by catalysis type.

Introduction of a 7-aza-6-MeO-indoline auxiliary in Lewis-acid/photoredox cooperative catalysis: highly enantioselective aminomethylation of α,β-unsaturated amides

An efficient cooperative chiral Lewis acid/photoredox catalytic system for engaging highly reactive radicals in highly enantioselective conjugate addition to α,β-unsaturated carbonyls is highly desirable. Direct photoexcitation of unbound substrates typically induces undesired background pathways for racemic products and remains a formidable challenge to be addressed in the area of enantioselective photocatalysis. Herein, we report a cooperative catalytic system comprising a chiral Cu(i) complex and an Ir(iii) photocatalyst fueled by visible-light irradiation that allows for seamless integration of the catalytic formation of α-amino alkyl radicals and subsequent enantioselective addition to α,β-unsaturated amides. A 7-aza-6-MeO-indoline attachment on the amide substrates plays a pivotal role in suppressing the undesired pathways, resulting in excellent enantioselectivity and enabling expedited access to valuable γ-aminobutyramides. The indoline amide was readily diversified with full recovery of the azaindoline attachment, highlighting the synthetic utility of this cooperative catalytic system.

An efficient cooperative chiral Lewis acid and photoredox catalytic system towards the highly enantioselective radical conjugate addition of α-amino radicals to α,β-unsaturated amides is developed with the implementation of unique auxiliaries.

Visible light-induced aerobic oxidative cross-coupling reaction: preparation of α-indolyl glycine derivatives

Rose Bengal, an organic dye, has been used as a photocatalyst in the cross-coupling reaction between glycine derivatives and indoles.

Stereoselective Organic Reactions in Heterogeneous Semiconductor Photocatalysis

The most significant feature of heterogeneous semiconductor photocatalysis is that both oxidation and reduction occur in a one-pot process. Thus, photocatalysis leads to unique redox organic reactions that cannot be achieved by conventional techniques using oxidants or reductants. Semiconductor photocatalysis is expected to be a new method for fine chemical syntheses of highly valuable molecules such as chiral medicines. However, the use of semiconductor photocatalysts in stereoselective reactions has been limited so far. This mini-review highlights recent progress in stereoselective organic reactions using semiconductor photocatalysts, briefly summarizing the enantio- and diastereoselective reactions based on the currently available literature.

Unified Mechanism of Oxygen Atom Transfer and Hydrogen Atom Transfer Reactions with a Triflic Acid-Bound Nonheme Manganese(IV)-Oxo Complex via Outer-Sphere Electron Transfer

Outer-sphere electron transfer from styrene, thioanisole, and toluene derivatives to a triflic acid (HOTf)-bound nonheme Mn(IV)-oxo complex, [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂ (N4Py = N, N-bis(2-pyridylmethyl)- N-bis(2-pyridyl)methylamine), has been shown to be the rate-determining step of different types of redox reactions such as epoxidation, sulfoxidation, and hydroxylation of styrene, thioanisole, and toluene derivatives, respectively, by [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂. The rate constants of HOTf-promoted epoxidation of all styrene derivatives with [(N4Py)Mn^IV(O)]²⁺ and electron transfer from electron donors to [(N4Py)Mn^V(O)]²⁺ exhibit a remarkably unified correlation with the driving force of outer-sphere electron transfer in light of the Marcus theory of electron transfer. The same electron-transfer driving force dependence is observed in the oxygen atom transfer from [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂ to thioanisole derivatives as well as in the hydrogen atom transfer from toluene derivatives to [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂. Thus, mechanisms of oxygen atom transfer (epoxidation and sulfoxidation) reactions of styrene and thioanisole derivatives and hydrogen atom transfer (hydroxylation) reactions of toluene derivatives by [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂ have been unified for the first time as the same reaction pathway via outer-sphere electron transfer, followed by the fast bond-forming step, which exhibits the singly unified electron-transfer driving force dependence of the rate constants as outer-sphere electron-transfer reactions. In the case of the epoxidation of cis-stilbene by [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂, the isomerization of cis-stilbene radical cation to trans-stilbene radical cation occurs after outer-sphere electron transfer from cis-stilbene to [(N4Py)Mn^IV(O)]²⁺-(HOTf)₂ to yield trans-stilbene oxide selectively, which is also taken as evidence for the occurrence of electron transfer in the acid-catalyzed epoxidation.

N-Arylphenothiazines as strong donors for photoredox catalysis - pushing the frontiers of nucleophilic addition of alcohols to alkenes

A new range of N-phenylphenothiazine derivatives was synthesized as potential photoredox catalysts to broaden the substrate scope for the nucleophilic addition of methanol to styrenes through photoredox catalysis. These N-phenylphenothiazines differ by their electron-donating and electron-withdrawing substituents at the phenyl group, covering both, σ and π-type groups, in order to modulate their absorbance and electrochemical characteristics. Among the synthesized compounds, alkylaminylated N-phenylphenothiazines were identified to be highly suitable for photoredox catalysis. The dialkylamino substituents of these N-phenylphenothiazines shift the estimated excited state reduction potential up to -3.0 V (vs SCE). These highly reducing properties allow the addition of methanol to α-methylstyrene as less-activated substrate for this type of reaction. Without the help of an additive, the reaction conditions were optimized to achieve a quantitative yield for the Markovnivkov-type addition product after 20 h of irradiation.

Advances in asymmetric visible-light photocatalysis, 2015–2019

Asymmetric visible-light photocatalysis has recently drawn considerable attention of the scientific community owing to its unique activation modes and significance for the enantioselective green synthesis.

A chiral nickel DBFOX complex as a bifunctional catalyst for visible-light-promoted asymmetric photoredox reactions

The enantioselective photoredox reaction of α,β-unsaturated carbonyl compounds and tertiary/secondary α-silylamines was enabled by a readily available single NiII-DBFOX catalyst (DBFOX = 4,6-bis((R)-4-phenyl-4,5-dihydrooxazol-2-yl)dibenzo[b,d]furan) under visible light conditions. The non-precious chiral catalyst is involved in the photochemical process to initiate single electron transfer and at the same time provides a well-organized chiral environment for the subsequent radical transformations. Good to excellent enantioselectivities (80-99% ee) were obtained for the formation of chiral γ-amino carboxylic acid derivatives and γ-lactams.

First use of a divalent lanthanide for visible-light-promoted photoredox catalysis

We report the first catalytic use of a divalent lanthanide in visible-light-promoted bond-forming reactions. Our new precatalyst uses europium in the +2 oxidation state and is active in the presence of blue light from light-emitting diodes. The use of low-energy visible light reduces the occurrence of potential side reactions that might be induced by higher-energy UV light. The system described here uses zinc metal as a sacrificial reductant and is tolerant to wet, protic solvents. The catalyst can be made in situ from relatively inexpensive and air-stable EuCl3·6H2O, and the ligand can be synthesized in large quantities in two steps. With 0.5% loading of precatalyst, an average of 120 turnovers was observed in six hours for reductive coupling of benzyl chloride. We expect that the results will initiate the study of visible-light-promoted photoredox catalysis using divalent europium in a variety of reactions.

A visible-light-activated rhodium complex in enantioselective conjugate addition of α-amino radicals with Michael acceptors

An efficient enantioselective conjugate addition of photogenerated α-amino radicals to Michael acceptors catalyzed by a single and newly prepared chiral-at-metal rhodium complex was established.

Visible-Light Photocatalytic Aerobic Annulation for the Green Synthesis of Pyrazoles

A selective and high yielding synthesis of polysubstituted pyrazoles through a VLPC (visible light photoredox catalysis)-promoted reaction of hydrazine with Michael acceptors is reported. The method employs very mild reaction conditions and uses air as the terminal oxidant, which makes the process environmentally benign. Different types of Michael acceptors with various substituents can undergo the reaction to afford corresponding pyrazoles in good to excellent yields. The reaction is proposed to go through VLPC-promoted oxidation of hydrazine to diazene followed by its addition to Michael acceptors, other than the conventional condensation of hydrazine with a carbonyl.

Heteroleptic Cu-Based Sensitizers in Photoredox Catalysis

Photochemistry is an important tool in organic synthesis that has largely been underdeveloped in comparison to thermal activation. Recent advances in technology have ushered in a new era in synthetic photochemistry. The emergence of photocatalysis, which exploits sensitizers for the absorption of visible light, has provided organic chemists with a new route to the generation of radical intermediates for synthesis. Of particular interest is the development of Cu-based complexes for photocatalysis, which possess variable photophysical properties and can display complementary reactivity with common photocatalysts based on heavier transition metals such as Ru or Ir. Heteroleptic Cu-based sensitizers incorporating the presence of both a bisphosphine and diamine ligand bound to the copper center are a promising class of photocatalysts. Their synthesis is a single step, often involving only precipitation for purification. In addition, it was shown that the sensitizers could be formed in situ in the reaction mixture, simplifying the experimental setup. The heteroleptic nature of the Cu-complexes also affords opportunities to fine-tune properties. For example, structurally rigidified bisphosphines reinforce geometries about the metal center to extend the excited state lifetime. Variation of the diamine ligand can influence the excited state oxidation/reduction potentials and optical absorbances. The heteroleptic complex Cu(XantPhos)(neo)BF4 has demonstrated utility in the synthesis of helical polyaromatic carbocycles. The synthesis of [5]helicene, a relatively simple member of the helicene family, was improved from the existing UV-light mediated method by eliminating the formation of unwanted byproducts. In addition, the Cu-based sensitizers also promoted the formation of novel pyrene/helicene hybrids for materials science applications. The synthetic methods that were developed were augmented when combined with continuous flow technology. The irradiation of reaction mixtures as they are pumped through small diameter tubing provides a more homogeneous and increased photon flux compared with irradiation in round-bottom flasks or other batch reactors. The value of continuous flow methods is also evident when examining UV-light photochemistry, where the simple and safe experimental set-ups allow for further exploration of high energy light for synthetic purposes. The synthesis of functionalized complex carbazoles was also studied using both a visible light method exploiting a heteroleptic copper-based sensitizer and a UV-light mediated method. It was demonstrated that both the photocatalysis methods and UV light photochemistries were rendered more user-friendly, safe, and reproducible when using continuous flow methods. Interestingly, the two photochemical methods often afford contrasting selectivities as a result of their inherently different mechanisms. It can be expected that the complementarity of the various photochemical methods will be an asset to synthetic chemists as the field continues to evolve.

Highly Fluorinated Ir(III)-2,2':6',2″-Terpyridine-Phenylpyridine-X Complexes via Selective C-F Activation: Robust Photocatalysts for Solar Fuel Generation and Photoredox Catalysis

A series of fluorinated Ir(III)-terpyridine-phenylpyridine-X (X = anionic monodentate ligand) complexes were synthesized by selective C-F activation, whereby perfluorinated phenylpyridines were readily complexed. The combination of fluorinated phenylpyridine ligands with an electron-rich tri-tert-butyl terpyridine ligand generates a "push-pull" force on the electrons upon excitation, imparting significant enhancements to the stability, electrochemical, and photophysical properties of the complexes. Application of the complexes as photosensitizers for photocatalytic generation of hydrogen from water and as redox photocatalysts for decarboxylative fluorination of several carboxylic acids showcases the performance of the complexes in highly coordinating solvents, in some cases exceeding that of the leading photosensitizers. Changes in the photophysical properties and the nature of the excited states are observed as the compounds increase in fluorination as well as upon exchange of the ancillary chloride ligand to a cyanide. These changes in the excited states have been corroborated using density functional theory modeling.

Enantioselective rhodium/ruthenium photoredox catalysis en route to chiral 1,2-aminoalcohols

A visible-light-induced enantioselective redox coupling of α-silylalkylamines with 2-acyl imidazoles is catalyzed by a combination of a chiral Rh Lewis acid and a Ru photoredox sensitizer.

Radical–radical cross coupling reactions of photo-excited fluorenones

Radical–radical cross coupling reactions of photoexcited 9-fluorenones have been accomplished for the first time, leading to the synthesis of 9-alkyl, pyrollidinyl and spiro-THF derivatives of 9-fluorenones.