LongLarf

Fantastic twelve naphthochromenone photocatalysts (PCs) that absorb across the UV/Vis range and feature an extremely wide redox window (up to 3.22 eV) were synthesized on gram scale. Their excited‐state redox potentials, PC*/PC^.− (up to 1.65 V) and PC^.+/PC* (up to −1.77 V vs. SCE), are such that these novel PCs can engage in both oxidative and reductive quenching.

Abstract

Twelve naphthochromenone photocatalysts (PCs) were synthesized on gram scale. They absorb across the UV/Vis range and feature an extremely wide redox window (up to 3.22 eV) that is accessible using simple visible light irradiation sources (CFL or LED). Their excited‐state redox potentials, PC*/PC^.− (up to 1.65 V) and PC^.+/PC* (up to −1.77 V vs. SCE), are such that these novel PCs can engage in both oxidative and reductive quenching mechanisms with strong thermodynamic requirements. The potential of these bimodal PCs was benchmarked in synthetically relevant photocatalytic processes with extreme thermodynamic requirements. Their ability to efficiently catalyze mechanistically opposite oxidative/reductive photoreactions is a unique feature of these organic photocatalysts, thus representing a decisive advance towards generality, sustainability, and cost efficiency in photocatalysis.

The Front Cover shows an artist with benzene‐shaped hair painting polyfunctional (hetero)arenes on a canvas in dark blue with a big brush. The color symbolizes the active cobalt catalyst, which is used for several transformations described within this Minireview. This marvelous combination of a zinc brush together with the cobalt dye enables the creation of pure art. In their Minireview, F. H. Lutter et al. describe that, due to their relatively low toxicity and price, cobalt‐salts offer a useful alternative to other common metal catalysts (e. g. palladium complexes). This Minireview highlights the suitability of cobalt catalysts for cross‐coupling and electrophilic amination reactions with organozinc pivalates, a class of organozinc reagents with enhanced stability towards air and moisture. More information can be found in the Minireview by F. H. Lutter et al. on page 5188 in Issue 21, 2019 (DOI: https://doi.org/10.1002/cctc.20190007010.1002/cctc.201900070).

Photocatalysis: Reproducibility for photocatalytic CO₂ reduction has been evaluated on the example of [FeFe] hydrogenase mimics in combination with a heteroleptic Cu photosensitizer. Based on these results, we highlight the importance of testing reproducibility for new photocatalytic reaction protocols. Furthermore, we provide suggestions on how to ensure reproducibility of those transformations.

Abstract

Reproducibility of photocatalytic reactions, especially when conducted on small scale for improved turnover numbers with in situ formed catalysts can prove challenging. Herein, we showcase the problematic reproducibility on the example of attractive photocatalytic CO₂ reduction utilizing [FeFe] hydrogenase mimics. These Fe complexes, well‐known for their application in proton reduction reactions, were combined with a heteroleptic Cu photosensitizer and produced CO/H₂/HCO₂H mixtures of variable constitution. However, the reactions indicated a poor reproducibility, even when conducted with well‐defined complexes. Based on our experience, we make suggestions for scientists working in the field of photocatalysis on how to address and report the reproducibility of novel photocatalytic reaction protocols. In addition, we would like to highlight the importance of studying reproducibility of novel reaction protocols, especially in the fields of photocatalytic water splitting and CO₂ reduction, where TONs are widely used as the comparable measure for catalytic activity.

Publication date: 13 December 2019

Source: Tetrahedron, Volume 75, Issue 50

Author(s): Christopher J.A. Warner, Sian S. Berry, Simon Jones

Graphical abstract

Partner up: Photocatalysis offers a one‐step strategy to selectively functionalize the benzylic positions of electron‐rich arenes with alcohols. It merges the photoredox activation of arenes with copper(II)‐mediated oxidation of the resulting benzylic radicals, enabling the introduction of benzylic C−O bonds with high site selectivity, chemoselectivity, and functional‐group tolerance using only two equiv of the alcohol coupling partner.

Abstract

Methods that enable the direct C−H alkoxylation of complex organic molecules are significantly underdeveloped, particularly in comparison to analogous strategies for C−N and C−C bond formation. In particular, almost all methods for the incorporation of alcohols by C−H oxidation require the use of the alcohol component as a solvent or co‐solvent. This condition limits the practical scope of these reactions to simple, inexpensive alcohols. Reported here is a photocatalytic protocol for the functionalization of benzylic C−H bonds with a wide range of oxygen nucleophiles. This strategy merges the photoredox activation of arenes with copper(II)‐mediated oxidation of the resulting benzylic radicals, which enables the introduction of benzylic C−O bonds with high site selectivity, chemoselectivity, and functional‐group tolerance using only two equivalents of the alcohol coupling partner. This method enables the late‐stage introduction of complex alkoxy groups into bioactive molecules, providing a practical new tool with potential applications in synthesis and medicinal chemistry.

Abstract

An efficient additive‐free manganese‐catalyzed hydrogenation of nitriles to primary amines with molecular hydrogen is described. The pre‐catalyst, a well‐defined bench‐stable alkyl bisphosphine Mn(I) complex fac‐[Mn(dpre)(CO)₃(CH₃)] (dpre=1,2‐bis(di‐n‐propylphosphino)ethane), undergoes CO migratory insertion into the manganese‐alkyl bond to form acyl complexes which upon hydrogenolysis yields the active coordinatively unsaturated Mn(I) hydride catalyst [Mn(dpre)(CO)₂(H)]. A range of aromatic and aliphatic nitriles were efficiently and selectively converted into primary amines in good to excellent yields. The hydrogenation of nitriles proceeds at 100 °C with a catalyst loading of 2 mol % and a hydrogen pressure of 50 bar. Mechanistic insights are provided by means of DFT calculations.

A solvent‐free, efficient atom‐economical N‐alkylation and dehydrogenative coupling of amines catalyzed by a ruthenium–pincer is reported. Sodium alkoxide generated in situ from alcohols using sodium serve a dual role as alkyl precursor and as a base. The water formed in the reaction promotes the catalytic cycle by regenerating alcohol from sodium alkoxide.

We report the use of ruthenium–NNN‐pincer complexes of the type (^R2NNN)RuCl₂(PPh₃) (R = tBu, iPr, Cy and Ph) for the catalytic N‐alkylation of primary amines under solvent‐free conditions. For the first time, the base that is required to promote these reactions is generated in situ from the alcohol by the use of sodium. The resulting sodium alkoxide regenerates the alcohol substrate while acting as the water scavenger thus mitigating the need of an additional base. Among the catalysts screened, ( ^tBu2NNN)RuCl₂(PPh₃) (0.02 mol‐%) gives very high turnovers and good yields at 140 °C. The ( ^tBu2NNN)RuCl₂(PPh₃) catalyzed N‐alkylation tolerates a variety of amine and alcohol substrates. While excellent turnover (29000) was obtained for the ( ^tBu2NNN)RuCl₂(PPh₃) (0.002 mol‐%) catalyzed alkylation of aniline with cyclohexyl methanol, the turnovers obtained in the corresponding catalytic methylation of p‐anisidine was also very high (12000). The ( ^tBu2NNN)RuCl₂(PPh₃) catalyzed reactions have also been accomplished under open‐vessel conditions resulting in a net dehydrogenative coupling reaction. This protocol has been used to transform benzene‐1,2‐diamines to benzimidazoles with high productivity (12000 turnovers). DFT studies indicate that while β‐hydride elimination is rate‐determining (RDTS: 24.31 kcal/mol) for the alcohol dehydrogenation segment which is endothermic, insertion of the imine is rate‐determining (RDTS: 11.26 kcal/mol) for its hydrogenation that is exothermic.

Putting garbage to good use: Organocatalysis has allowed many issues to be addressed in the development of sophisticated, but less polluting, chemical processes. However, minimizing waste also means efficient utilization of raw and renewable materials. This review discusses the use of hetero‐ and homogeneous organocatalysts derived from waste biopolymers, renewable platform molecules, terpenes and rosin, and natural proteins.

Abstract

One of the greatest challenges facing our society is to reconcile our need to develop efficient and sophisticated chemical processes with the limited resources of our planet and its restricted ability to adsorb pollution. Organocatalysis has allowed many issues to be addressed in the development of sophisticated, but less polluting, processes. However, minimizing waste also means an efficient utilization of raw and renewable materials. Waste biomass represents an alternative to conventional petroleum‐based chemical manufacturing and is a highly attractive renewable resource for the production of chemicals and high‐value‐added organocatalysts. Recent achievements in the use of renewable biomass feedstocks for the synthesis of organocatalysts are presented. Their application in synthetic methodologies, including multicomponent reactions, which are performed under solvent‐free conditions or in eco‐friendly reaction media, as well as recycling and reusing the organocatalysts, is illustrated. A few pioneering examples that demonstrate the potential of these promoters in asymmetric synthesis have also been documented. In particular, this review covers examples on the use of hetero‐ and homogeneous organocatalysts derived from 1) waste biopolymers, such as chitosan, alginic acid, and cellulose; ii) renewable platform molecules, such as levoglucosenone, isosorbide, mannose, d‐glucosamine, and lecithin; 3) terpenes and rosin, such as pinane, isosteviol, and abietic acid; and iv) natural proteins (gelatin, bovine tendons, silk fibroin proteins).

It's hard to stand just on one leg. Proper bidentate coordination of a Ph₂PN( ⁱ Pr)PPh₂ ligand via two P atoms to a Cr complex catalyst is essential for high performance in homogeneous ethylene tetramerization. This is only accomplished by a methylaluminoxane activator (MMAO), while other activators enable only monodentate (AlEt₃, AlⁱBu₃, AlOct₃) or even no coordination of PNP to Cr (AlMe₃), which lowers activity.

Abstract

The effect of different AlR₃ activators (R=methyl, ethyl, isobutyl, n‐octyl) has been studied in comparison to modified methylaluminoxane (MMAO) by operando EPR as well as by in situ UV‐vis, ATR‐IR and XANES/EXAFS spectroscopy during oligomerization of ethylene at 20 bar and 40 °C with a homogeneous Cr complex catalyst formed in situ upon mixing a Cr(acac)₃ precursor, a Ph₂PN( ⁱ Pr)PPh₂ ligand (PNP) and the activator. Coordination of PNP to Cr(acac)₃ is initiated only in the presence of an activator. Highest 1‐octene productivity (detected during operando EPR measurements) was obtained with MMAO which promotes bidentate coordination of the ligand to form an active (PNP)Cr^II(CH₃)₂ chelate complex. Rising bulkiness of R in AlR₃ leads to only monodentate coordination of PNP to the Cr center by one P atom and increasing reduction to Cr^I to a maximum extend of around 30 % for AlOct₃. This lowers the catalytic performance, which is mainly governed by the mode of PNP coordination rather than by the Cr^I content.

Deracemization is an attractive strategy for asymmetric synthesis, but intrinsic energetic challenges have limited its development. Here, we report a deracemization method in which amine derivatives undergo spontaneous optical enrichment upon exposure to visible light in the presence of three distinct molecular catalysts. Initiated by an excited-state iridium chromophore, this reaction proceeds through a sequence of favorable electron, proton, and hydrogen-atom transfer steps that serve to break and reform a stereogenic C–H bond. The enantioselectivity in these reactions is jointly determined by two independent stereoselective steps that occur in sequence within the catalytic cycle, giving rise to a composite selectivity that is higher than that of either step individually. These reactions represent a distinct approach to creating out-of-equilibrium product distributions between substrate enantiomers using excited-state redox events.

A stereodivergent hydrogenation of α,β‐unsaturated phosphonates is disclosed using a single enantiomer of the catalyst. This enables generation of the R‐ or S‐configured β‐chiral phosphonate with equal and opposite selectivity. Enantiodivergence is regulated at the substrate level through the development of a facile E → Z isomerisation. This has been enabled for the first time by selective energy transfer catalysis using anthracene as an inexpensive organic photosensitiser.

Abstract

Enantiodivergent, catalytic reduction of activated alkenes relays stereochemical information encoded in the antipodal chiral catalysts to the pro‐chiral substrate. Although powerful, the strategy remains vulnerable to costs and availability of sourcing both catalyst enantiomers. Herein, a stereodivergent hydrogenation of α,β‐unsaturated phosphonates is disclosed using a single enantiomer of the catalyst. This enables generation of the R‐ or S‐configured β‐chiral phosphonate with equal and opposite selectivity. Enantiodivergence is regulated at the substrate level through the development of a facile E → Z isomerisation. This has been enabled for the first time by selective energy transfer catalysis using anthracene as an inexpensive organic photosensitiser. Synthetically valuable in its own right, this process enables subsequent Rh^I‐mediated stereospecific hydrogenation to generate both enantiomers of the product using only the S‐catalyst (up to 99:1 and 3:97 e.r.). This strategy out‐competes the selectivities observed with the E‐substrate and the R‐catalyst.

The Cs₂CO₃‐promoted synthesis of α‐acyloxy‐α′‐hydroxy ketones is reported, in which tertiary bromopropargylic alcohols undergo hydration/acylation with carboxylic acids via in situ formation of cyclic carbonates.

A facile approach towards α‐acyloxy‐α′‐hydroxy ketones by reaction of readily available tertiary bromopropargylic alcohols and carboxylic acids in system Cs₂CO₃/H₂O/DMF (50–55 °C, 4 h) was developed. Key intermediates of this synthesis are cyclic carbonates generated in situ from bromopropargylic alcohols and Cs₂CO₃ which have been utilized as both reagent and base promoter.

Visible light irradiation of 2‐iodophenyl‐thionocarbonates enabled Barton–McCombie‐type radical deoxygenations to proceed in the absence of a photocatalyst, a radical initiator, or tin or silicon hydrides. Applying this strategy to deoxygenative borylation reactions allowed a wide range of structurally diverse aliphatic alcohols to be transformed into synthetically valuable boronic esters.

Abstract

A photochemical method for converting aliphatic alcohols into boronic esters is described. Preactivation of the alcohol as a 2‐iodophenyl‐thionocarbonate enables a novel Barton–McCombie‐type radical deoxygenation that proceeds efficiently with visible light irradiation and without the requirement for a photocatalyst, a radical initiator, or tin or silicon hydrides. The resultant alkyl radical is intercepted by bis(catecholato)diboron, furnishing boronic esters from a diverse range of structurally complex alcohols.

A metal‐free method for reduction of cyclic N‐sulfonyl ketimines catalyzed by B(C₆F₅)₃, using commercially available methylphenylsilane as a reducing reagent under mild conditions has been developed. This reductive method was effective, not only providing the expected cyclic N‐Sulfonamides in good to excellent yields, but also showing good functional‐group tolerance.

A metal‐free method for reduction of cyclic N‐sulfonyl ketimines catalyzed by B(C₆F₅)₃, using commercially available methylphenylsilane as a reducing reagent under mild conditions has been developed. This reductive protocol was effective, not only providing the expected cyclic N‐sulfonamides in good to excellent yields, but also showing good functional‐group tolerance.

Hydrogen peroxide (H₂O₂) synthesis generally requires substantial postreaction purification. Here, we report a direct electrosynthesis strategy that delivers separate hydrogen (H₂) and oxygen (O₂) streams to an anode and cathode separated by a porous solid electrolyte, wherein the electrochemically generated H⁺ and HO₂^– recombine to form pure aqueous H₂O₂ solutions. By optimizing a functionalized carbon black catalyst for two-electron oxygen reduction, we achieved >90% selectivity for pure H₂O₂ at current densities up to 200 milliamperes per square centimeter, which represents an H₂O₂ productivity of 3.4 millimoles per square centimeter per hour (3660 moles per kilogram of catalyst per hour). A wide range of concentrations of pure H₂O₂ solutions up to 20 weight % could be obtained by tuning the water flow rate through the solid electrolyte, and the catalyst retained activity and selectivity for 100 hours.