Heterogeneous Organocatalysis for Photoredox Chemistry - ACS

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Heterogeneous Organocatalysis for Photoredox Chemistry Aleksandr Savateev, and Markus Antonietti ACS Catal., Just Accepted Manuscript • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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Heterogeneous Organocatalysis for Photoredox Chemistry Aleksandr Savateev and Markus Antonietti* Max-Planck Institute of Colloids and Interfaces, Department of Colloid Chemistry, Research Campus Golm, 14424 Potsdam, Germany. Corresponding author’s email: [email protected] Abstract Photoredox catalysis is a tool enabling a wide variety of chemical reactions with high selectivity under mild conditions of visible light. In this review we summarize recent experiments which use heterogeneous, covalent, metal free semiconductors with adjustable reactivity to drive such reactions. This class started with mesoporous graphitic carbon nitride, then continued with poly(heptazine imides), but is meanwhile extended to other polymers and solid state organics with conjugated electron system. Due to the high thermal and chemical stability, as well as adjustable conduction and valence band positions, the reaction space could be expanded to many organic reactions, such as photocatalytic synthesis of organosulfur compounds, or unconventional halogenation and cyanation reactions. Performance of carbon nitrides and homogeneous systems in certain reactions was compared in the present review.

Keywords: Metal free catalysis, visible light, photoredox catalysis, conjugated organic solids, carbon nitride Introduction Photoredox catalysis as a tool for chemical synthesis has experienced a blooming renaissance in the last 10 years. The most popular contributions are, however, based on homogeneous photoredox catalysts, such as ruthenium or iridium based transition metal complexes.1-3 This is complemented by organic dyes, for example, eosin Y,4 acridinium salts,5 perylene,6 Rhodamine 6G,7,8 the Fukuzumi-dye,9 or riboflavines10,11. These systems already point to the fact that appropriate conjugated organic systems, also in the shape of polymeric compounds or insoluble nanoparticles, will have appropriate activity.

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There is no need to explain to the catalytic community the complementary advantages of a heterogeneous catalyst: the ease of separation from the reaction mixtures, a higher photo and chemical stability due to restricted rotations and solid state effects, the ability to create immobilized catalytic beds and in general the "reusability". Heterogeneous catalysis is definitely less explored for photoconversions, but in the field of photosynthesis, i.e. uphill reactions to fix light energy in photochemical products, inorganic semiconductor nanostructures have a distinct history. It started with the observation that TiO2 can generate hydrogen from water by illumination under UVLight12 and in meanwhile expanded to a broad range of oxidic, sulfidic, and nitridic metal based semiconductors.13-15 Metal free boron containing catalysts also gained significant interest in recent years.16 For the interested reader, we point to some elaborated reviews on this subject,17-20 but will not further cover the subject as it is beyond our focus. Reaction medium pH applies certain restrictions on using inorganic systems in photocatalysis: (1) under extreme pH the surface is reconstructed up to amorphous layers and (2) pH affects the surface polarity that in turn controls the binding of the substrate to the surface of the photocatalyst, which could serve as a possible relevant catalytic descriptor. Covalent conjugated systems, graphitic carbons and carbon nitrides (CNs) are a meanwhile a popular choice for catalysis in general. Carbons are black, often semimetallic, and as such, up to some exceptions discussed later, in principle not able to perform photochemistry as such, as electron and hole have no sufficient energy gap and are also rather restricted in their energy position. Carbons as such, however, are also highly catalytically active and therefore constitute the field of “carbocatalysis”.21-23 Carbon nitride based semiconductors on the other hand are well known in material chemistry as photocatalysts,24-26 as a support to stabilize single metal atoms27,28 and as electrocatalysts.29-30 CNs comprise of only light elements, and possess suitable valence band maxima (VBM) and conduction band minima (CBM) for the controlled oxidation and reduction of substrates, see Figure 1.

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Figure 1. Introduction of chemical structures of the different system. (a) the chemical structure of C2N carbon-subnitride; (b) atomic-resolution scanning tunneling microscopy (STM) topography image of the C2N crystal (adapted from Nat. Commun. 2015, 6, 6486); (c), (d) and (j) the chemical structure of K-PHI, high resolution transmission electron microscopy (HR-TEM) image of K-PHI and powder X-Ray pattern of K-PHI; (e) the chemical structure of a covalent triazine framework CTF-Th; (f) transmission electron microscopy image of the CTF-Th@SBA-15 (adapted with permission from (ACS Catal., 2017, 7 (8), 5438). Copyright (2017) American Chemical Society; (g), (h) and (k) the chemical structure of the graphitic carbon nitride of the stoichiometry C3N4, its HR-TEM image and powder X-ray pattern; (i) an example of the conjugated polymer structure P3HT; (l) the band diagrams of the heterogeneous photocatalysts and conjugated polymers; (m) UV-vis absorption and photoluminescence spectra of K-PHI and g-C3N4. ACS Paragon Plus Environment

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Non-modified CNs have a band gap of ca. 2.7 eV (λEx ~460 nm) that allows CNs to be used as visible light photoredox catalysts bypassing the use of UV light. CNs have excellent chemical and thermal stability up to 600°C31 . Although not commercialized now, the cost of synthesis of CNs is very low and in the few Euro/kg range, as both starting product (e.g. urea or melamin) as well as synthetic processes are simple.32 This can be compared to the well-known commercialized Ru- or Ir- based photocatalysts. Another general advantage of all conjugated covalent structures is the adjustment of substrate interactions which can be of van-der Waals type (“adjusted polarity”), charge transfer interactions (“electron rich/electron poor”), hydrogen bridges, or Coulombic in nature. In addition, covalent structures can be modified with different functional groups (see the synthetic examples below), which can bind substrates or stabilize reactive intermediates.33 A third, rapidly growing class of heterogeneous organic catalysts with similar properties and advantages are conjugated conducting polymers and all conjugated covalent organic frameworks (COFs), see Figure 1. Many of these systems are well explored for organic electronics and as organic conductors, and their potential use in catalysis is a recent accomplishment. The metal free character of all these systems simplifies synthesis and purification of medicinally important compounds (c.f., late stage functionalization reactions). Last but not the least, organophosphorus or organosulfur compounds that are considered as poisons34,35 in metal based catalytic methods could readily be used as reaction partners or additives for synthetic transformations. More generally, organocatalysts of the discussed type are usually tolerant against all type of chemical functionalities. Chemical Systems In the following, we will shortly introduce the character and synthesis of the different systems discussed, i.e. carbon nitrides, conducting polymers and frameworks, as well as some preliminary observations on “carbonaceous structures”. Photoredox catalysts based on more complex systems, such two system heterojunctions with other semiconductors,36 or nanocomposites with metal nanoparticles37 are just shortly discussed at the perspective part, but can be found elsewhere.38-40 a) Carbon nitrides Carbon nitrides represent a class of materials rather than a single chemical species. These compounds are built up from mainly carbon and nitrogen, typically around a molar C:N = 3:4 ratio, the conjugation is sp2 throughout, and the connectivity results in layers which pack in a graphitic fashion. The different systems differ, as typical for polymers, in repeat units, degree of condensation, and most importantly for organic semiconductors, motifs and symmetry of intermolecular packing. This is nicely seen by comparing thermally condensed polymeric carbon nitride with ionothermally ACS Paragon Plus Environment

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polymerized poly(heptazine imides), where better organization yields a material with stronger basicity, which forms stable potassium salt with bigger pores. The structural peculiarity of poly(heptazine imides) also lowers by 0.7 Volt valence band position (Figure 1). Heterogeneous catalysts rely in general on a high specific surface area, as the catalytic sites are located at the surface. This is why carbon nitrides were morphology-wise modified to mesoporous graphitic carbon nitride (mpg-CN) which possesses much larger surface area.41 Another option for two dimensional systems is delamination, and graphitic nanosheets of CN can be easily prepared by acid assisted delamination, ultrasound, or chemical etching techniques.42-45 Interestingly, contrary to graphene made from graphite, single nanolayers were rarely observed, CN seems to energetically prefer the few-layer thickness, which speaks for rather good electronic communication between the layers. The ability to form this 2Dobjects can however be seen as a general advantage to drive catalysis. g-C3N4 is prepared by thermolysis of urea, melamine or other nitrogen rich precursors at 550–600 °C.46 K-PHI is made by heat treatment of 5-aminotetrazole or triazole in LiCl/KCl eutectic salt melts also at 550-600 °C.47,48 Carbon nitrides have multiple possibilities for structural modification that in turn lead to the materials with altered properties. These methods, in general, can be divided into such using different precursors for the preparation of carbon nitrides and post treatment of the pristine carbon nitride material.38-39 Both g-C3N4 and K-PHI are built of tri-s-triazine rings49 that are considered to be thermodynamically the most stable at the synthesis temperature. It is worth mentioning here that similar to zeolites and ion exchange resins, the potassium cations in K-PHI can be easily and reversibly replaced with other metal cations.50 All structural details can be clearly observed in high resolution transmission electron microscopy (HRTEM) images.51 Both materials possess absorption spectra with an absorption edge at ca. 460 nm – typical for semiconductors, whereas K-PHI exhibits an additional, photoactive band, with slightly lower extinction coefficient, which covers the spectral range up to the red region. b) Conjugated organic polymers and frameworks for photocatalysis As dyes are long-known photocatalysts, many concepts from this area may be readily transferred to dye-like polymers, especially because there is a long track record to develop “Light-to electron converting polymers”, say for organic photovoltaics or organic light emitting diodes. In the context of catalysis, we have to differentiate between soluble, thereby “homogeneous” polymer catalysts, or colloidal or particulate insoluble species, thereby heterogeneous catalysts. Partition in both cases is simplified due to the molecular weight and polymer character of the catalyst, enabling very efficient separation processes.

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We also have to state that all modes of photocatalysis, i.e. photoredox chemistry, complete energy transfer, or only spin transfer have been described for polymer photocatalysts, and the following cases will illustrate these modes. A very illustrative case for the typical proceeding is given by the work of Vilela and Zhang.52,43 In this work, typical OPV donor-acceptor (simplifying exciton splitting) crosslinked polymer networks with high specific surface area were prepared and then applied as triplet sensitizer, i.e. for spin transfer to molecular oxygen. The resulting system showed outstanding activity for the oxygen addition to terpinene to create ascaridole, a model reaction for medical chemistry. On the same time, these catalysts were easily separated from the reaction mixture and recycled again. Later, Zhang et. al continued this work and improved similar polymers for water splitting,53 dehydrogenation reactions,54 bromination of aromatics,55 photooxidation of amines,56 photo-Suzuki couplings,57 oxidation of organic sulfides,58 as well as reductive dehalogenation reactions.59 Cooper et al. then followed the same tracks, but focused on using polymer framework and their photoredox chemistry.60-65 Their work included color management, dehydrogenation reactions, pore structure optimization, photocatalytic aza-Henry reactions, and photochemical water splitting for energy purposes Li, Shalom et al. used a rather orthogonal approach to generate novel organic frameworks useful as photocatalysts.66,67 They fabricated very stable supramolecular crystals of melamine with dihydroxybenzoquinone or related compounds and topotactically transformed those crystals into the corresponding fully condensed materials. Interestingly, the final product gave very well defined layers (2D material) or rods (1D material), as a result of the topotactic confinements in the crystal or surface layer (see Figure 2). The structures are indeed photochemically active up to the red region and can effectively liberate oxygen from water or decompose even stable dyes with rather high rates, that is they have a very positive valence band position.

Figure 2. Organic semiconductors by topotactic transfer of DA-monomer crystals. (a,c) Co-crystal systems. The corresponding spontaneous nanostructures after condensation: ACS Paragon Plus Environment

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(b) nanosheets. Adapted with permission from ref 66. Copyright 2016 Wiley-VCH. (d) nanotubes. Adapted with permission from ref 67. Copyright 2017 The Royal Society of Chemistry. (e) Scheme of the topotactic transformation process. Reproduced with permission from ref 67. Copyright 2017 The Royal Society of Chemistry. c) Photocatalysis with Carbon nanostructures There is a number of papers pointing out that carbon nanostructures can also serve as active photocatalysts for organosynthesis, and a recently appeared mini review lists oxidation of alcohols, epoxidation of alkenes, hydroxylation of phenols, and photoreduction of CO2 as documented cases.68 Indeed, absorbing materials, such as black carbons, can act as photosensitizers, a local photothermal heat source, and also as an active, non-innocent support for other photocatalysts, e.g. organic dyes or titania. Another rather open field is the so called Carbon quantum dots (CQDs).69 Many of them show pronounced blue or white fluorescence,70-75 i.e. they are able to generate a high energy electron-hole pair. It means that CQDs can also serve as photoredox catalyst. This of course is in apparent contradiction that sp2-conjugated graphitic carbons are usually semi-metals, with a Fermi-level close to the standard hydrogen potential. There is therefore a good chance that these CQDs are chemically very different to the expectations of pure carbon structures, and indeed, many of them contain only about 60 wt. % of carbon, i.e. they are rather conjugated polymer resins than carbons. Another option to explain the found optimal behaviour and thereby also the assumed chemical reactivity is a core-shell heterojunction structure, with the fluorescence coming from the recombination of charges located at the functional surface and carbon core structures. We however think that such structures can be very powerful photocatalysts and also run enzyme-like reactions, just that their branding as “CQD” is a little unfortunate as it suggests to carbon experts a potentially wrong structure and wrong physical behaviour.

Photoredox properties The photocatalysis of dye molecules, metal based complexes and semiconductors are easily quantified: electrons are lifted from the VB to CB upon visible light photoexcitation, leaving a positive hole behind in the CB. For catalysts in the visible region the photon energy is greater than 2 eV. This is about the maximal energy the activation barrier in a chemical reaction can be lowered, as long as we can split up the reaction in at least two half reactions with an electron transfer from the reagents to the catalyst. This energy input is translated into 200 kJ·mol-1, i.e. a remarkable acceleration of the process. Then in most of the cases the process occurs practically spontaneous. It is clear that such an involved energy also enables then endergonic, “uphill” processes, which is in these special cases not only photocatalysis, but photosynthesis, as the process would never occur without light. Concerning the required acceleration of reaction, photoluminescence lifetime of the photocatalysts is a good measure. This is based on the simplification that charge ACS Paragon Plus Environment

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carriers, which recombine are similar to those who enter the chemical reaction. Notably, especially in semiconductors, a major fraction of photogenerated charge carriers do not recombine, but they are trapped in surface states and can participate in photocatalysis even on longer timescales. Nevertheless, substrate oxidation at the VB site, and substrate reduction at the CB site, have to take place in comparable times scales, as else the catalyst spectrochemically “waits” for the lower reaction to close the catalytic cycle, i.e. it is always the slower reaction which controls the overall effectivity. Typical lifetimes for the diverse photocatalysts are in the range 2 ns – 50 ns, i.e. this is the typical time to couple chemistry to the excited state. In other words, if a photocatalyst was always in the active state, it would run the reaction with a TOF larger than 2 x 107 s1, an impressive illustration what indeed 200 kJ·mol-1 lowering of the activation barrier can be accomplished. To trick a little around this, otherwise strict balancing of oxidative and reductive processes, two measures can be applied. The one is that the catalyst as such can indeed store or buffer a number of charges of the slower-to-react type, and carbon nitride for instance is very effective to store electrons at up to 16 % of the repeating units,76 which can be only explained when it is a bulk effect. Another trick for reaction acceleration is the use of a reduction or hole mediator, which quickly reacts with the slower charge species and transports it from the catalyst into the reaction medium, when it then builds up a substrate concentration until both reactions are kinetically balanced again. Typical mediators are NAD+/NADH77 or methylviologen,78 while it has to be noted that the mediator consumes some of the light energy for the added transport itself, the amount depending on its own redox potentials. Application of the heterogeneous carbon nitride materials in organic photoredox catalysis. Among the variety of heterogeneous organocatalysts available in the literature, a large fraction of them belongs to the family of carbon nitrides. Even though, these materials are built mostly from carbon and nitrogen, they have different chemical structures. Therefore different carbon nitrides exhibit different activity in the specific catalytic process. In the present paragraph we are focused on the application of carbon nitrides in the catalysis rather than the structure of these materials. We encourage the readers to refer to the original works to find out more about the specific structure of the carbon nitride photocatalyst (CNP) used in the reactions discussed below. Depending on the absorption spectrum of carbon nitrides, they can be excited with a range of wavelength. The restriction here is that the photon energy should be shorter or equal to the band gap. However, one has to keep in mind, even though UV light can be used for CNP excitation, it might also cause undesirable photolysis of the organic molecules present in the reaction mixture. The spectrum of light sources to be used in photocatalysis is very broad.79 The majority of light sources used in the present review are represented by common household light bulbs, light emitting diodes (LEDs) and xenon solar simulator. Household light bulbs are ACS Paragon Plus Environment

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cheap and available in every store. They might be a choice when high quality of light is not needed. Even though single colour LEDs are not monochromatic, their low price and narrow distribution of the photons by energy in the emission spectrum, make them a reasonable replacement of expensive lasers. They are an excellent choice to study the dependence of the activity of the catalysts on the wavelength of the incident light. Besides, single colour LEDs in combination with photon flux meter can be used to estimate the quantum yield of the reaction. Finally, solar simulators are indispensable in the applications aiming to use natural Sun light as energy input to run certain photocatalytic reaction, i.e. photosynthesis. The reaction parameters can be easily adjusted to the outdoor conditions, when the setup is transferred from the lab bench outside.

1. Controlled oxidation reactions Controlled oxidation of the organic molecules, which includes, for instance, introduction of hydroxyl or carbonyl group and dehydration, is an indispensable tool of the synthetic organic chemistry toolbox.80 This reaction, quite often, is accomplished via a proton coupled electron transfer (PCET) in a concerted fashion.81 Given that the heterogeneous CNP are very efficient in handling electrons and protons – water reduction, for instance, was accomplished with 57%-60% apparent quantum yield at 420 nm,82,83 a large number of articles focuses on using CNP for controlled oxidation of organic compounds.84 Selective oxidation of alcohols to aldehydes is not only a model benchmark reaction to assess the efficacy of the novel photocatalyst, it is also an important industrial process.85 Scheme 1 briefly summarizes the variety of conditions used in this reaction. Thus, Su et. al successfully oxidized primary and secondary alcohols to aldehydes and ketones at 100°C and O2 pressure 8 bar (Scheme 1a).86 In case of primary alcohols, the selectivity was lower due to the over oxidation of the aldehydes to the corresponding acids. Generally, two possible paths for alcohol oxidation are considered: (1) H-radical abstraction from the alkoxide anion by the HO2· radical (standard electrode potential of the half-reaction O2(g) + H+ + e- = HO2· (aq) is -0.13 V) followed by the organic radical anion oxidation by the photocatalyst (Scheme 1a, left); (2) oxidation of the alkoxide anion by the photocatalyst followed by the H-radical abstraction from the O-centered radical (Scheme 1a, right).86 The aforementioned problem of benzyl alcohol over oxidation while keeping the conversion close to 100% was solved by Savateev and Antonietti, who performed the reaction at +50°C and used S8 as a selective electron acceptor (Scheme 1b).51 In this reaction elemental sulfur is gradually converted into H2S (standard electrode potential of the half-reaction S8(s) + 2H+ + 2e- = H2S(g) is +0.14 V), being inert towards benzaldehyde.

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Chen et. al have circumvented the problem of benzaldehyde over oxidation by applying milder conditions (O2 pressure 1 bar, T = 50°C) and keeping the conversion of the alcohol below 100% (Scheme 1c).87 Reisner et. al have used a combination of NiP, a hydrogenase-inspired molecular catalyst, and cyanamide surface-functionalized carbon nitride for selective oxidation of 4-methylbenzyl alcohol to the aldehyde coupled with H2 production with TOF of NiP 1082 h-1 depending on the substrate (Scheme 1d).88 Herein, the reaction also proceeds via a reductive quenching of the excited state of the photocatalyst.89 Zhang et. al have used the model reaction of benzyl alcohol oxidation by fixing the reaction conditions, in order to evaluate the photocatalytic performance of a series of mesoporous carbon nitride materials (Scheme 1e).90 Shiraishi et. al used the photocatalytic reduction of O2 at the expense of ethanol to produce H2O2 (Scheme 1f).91 Zheng and Zhou have applied heterogeneous carbon nitride photocatalyst to convert a series of α-hydroxy ketones to 1,2-diketones (Scheme 1g).92 Despite oxidation of the alcohols is typically performed in organic solvents such as acetonitrile or trifluorotoluene, Long et. al have shown that water is also a suitable green solvent for this application (Scheme 1h).93 In addition to the reported CNPs, various alcohols can be also splitted into hydrogen and the corresponding carbonyl compound using Ni-modified CdS nanoparticles.94 Summarizing the variety of methods for selective oxidation of alcohols to the corresponding carbonyl compounds known from the literature and presented on the Scheme 1, we can point out the following approaches that chemists use in order to achieve high conversion of the alcohol along with high selectivity toward carbonyl compounds. (1) Even though O2 is the most common reagent for this reaction, to bind two hydrogen atoms, using specific H2 evolution co-catalysts or mild oxidants always pays off in high conversion along with high selectivity. (2) The structure of the photocatalyst and its surface properties apparently have greater influence onto the selectivity of alcohols oxidation rather than oxygen pressure, temperature and reaction time. (3) These parameters, however, can be freely adjusted, in order to accelerate the oxidation of secondary alcohols as the product, the ketone, is much more robust against further oxidation compared to the alcohols.

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Scheme 1. Oxidation of primary and secondary alcohols to the corresponding carbonyl compounds. Conversion of the alcohols and selectivity (in parentheses) of carbonyl compound formation are shown. Isolated yields are marked with asterisks. Oxidation of the secondary amines toward the synthetically useful imines was enabled by the CNP and O2 as the electron acceptor.95

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Scheme 2. Oxidation of the secondary amines. Conversion of the amine and the selectivity (in the parentheses) of the imine are shown. Hexagonal boron carbon nitride as a metal free visible light photocatalyst was also used in dehydrogenation of N-containing heterocycles.96 A selective oxidation of sulfides to sulfoxides, rather than sulfates, was accomplished by such heterogeneous carbon nitride materials, in the presence of isobutyraldehyde additive (Scheme 3).97 In addition, Wang et. al used “oxidized CNP” to enable oxidation of sulfides to sulfoxides without extra additives.98

Scheme 3. Aerobic oxidation of sulfides to sulfoxides by carbon nitride photocatalyst. Conversion of the sulfide and selectivity (in parentheses) of the sulfoxide are presented. Zhang et. al used a combination of CNP, catalytic amount of NHPI and H2O2 under visible light irradiation to oxidize selectively a benzylic position in alkylarenes to the carbonyl compounds (Scheme 4a).99 Verma et. al expanded the scope of the substrates to 12 hydrocarbons using CNP modified with vanadium (II) oxide (Scheme 4b).100

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Scheme 4. C-H oxidation of hydrocarbons. Conversion of the hydrocarbons and selectivity (in parentheses) of the carbonyl compounds are shown. Isolated yields marked with asterisks. Industrially valuable oxidation of benzene to phenol with 11.9% conversion was implemented using CNP loaded with iron and H2O2 as a green oxidant.101 Similarly phenol was synthesized from benzene with 98% yield using CNP modified with vanadium (II) oxide.100 2. C-C and C-heteroatom bond forming reactions Cross coupling reactions are indispensable tools for assembling complex molecules from small fragments.102 In terms of photocatalysis, this approach is largely represented by homogeneous transition metal redox complexes, e.g. [Ru(bpy)3]2+;1 as well as organic dyes – 9,10-dicyanoanthracene,103 thioxanthone-based systems,104 and using various techniques of energy transfer, e.g. photon upconversion triplet-triplet anihilation.105 In this paragraph we focus on cross-coupling reactions mediated by heterogeneous photocatalysts. Tetrahydroisoquinoline (THIQ) moiety occurs widely in natural biologically active compounds.106 Therefore a lot of efforts were put in order to functionalize THIQ core, including photocatalytic techniques.107 Oxidation potential of the THIQ determined by cyclic voltammetry is +1.25 V vs. Ag/AgCl.108 Thus, THIQs were coupled with Cnucleophiles, such as nitroalkanes or dimethylmalonates (Scheme 5).109 In most of the cases CNP demonstrates comparable actvivity to the reported earlier homogeneous systems represented by eosyn Y110 and Ir(ppy)2(dtbbpy)PF6.111 However, in the more challenging case, e.g. Aza-Henry reaction between N-phenylpyrrolidine and nitromethane, higher yield, 36%, of the product was obtained when CNP was used as a photocatalyst (Scheme 5). The mechanism includes the formation of Naryltetrahydroisoquinolinium cation that is subsequently attacked by the corresponding nucleophile.

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Scheme 5. Oxidative Mannich reaction. Isolated yields are presented. Furthermore, the dual oxidative Mannich reaction was put into practice using a combination of the heterogeneous carbon nitride photocatalysis and proline organocatalysis (Scheme 6).109 The yields of certain compounds synthesized by mean of Ru(bpy)3(PF6)2 are shown in parentheses for comparison in Scheme 6.112

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Scheme 6. Dual photo/organocatalytic oxidative Mannich reaction. Isolated yields are shown. The spectrum of the coupling agents was further expanded by Möhlmann and Blechert, who conducted the Sakurai reaction with weak nucleophiles such as allylstannanes (Scheme 7).113 Thus, 12 compounds were prepared with 69-96% yield. Moreover, tributylallenylstannane was used as a nucleophile giving the corresponding homopropargylic amine, as a highly reactive compound, in 89% yield.

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R1 R2

N

R1

[CNP], O2 (1 bar)

or

+ Ar

N

R1

SnBu3 R1

R2

60W light bulb r.t., 6-26 h, MeOH

SnBu3 C

or R1 N

R1

N

N

N

Ph

Ar

Ar

N

OMe 83%

93%

79%

70%

MeO N

MeO

N

N

Ph

Ph

N tBu

OMe 93%

65%

71%

83% MeO

MeO N

N

MeO

N

Ph

N

MeO

Ph

Br 96%

89%

89%

75%

Photocatalytic mechanism O2

h

e CNP

O2

CNP*

N

h

N

Ar

HO2

H

Ar

Nu H

H2O2 N

Ar N

Ar

Nu

Scheme 7. Photo-Sakurai reaction between THIQs and allylstannanes. Isolated yields are presented. The possibility to use allylsilanes as the coupling partners with THIQ was investigated.113 However, CuI is required in order to obtain the product. In this reaction only trimethyl(2-methylallyl)silane gave the coupling product, while in the presence of trimethylallylsilane, due to its low nulceophilicity, only methanol adduct that was eventually converted into the lactam, was detected (Scheme 8).

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Scheme 8. Cross-coupling between N-phenyl-THIQ and allylsilane. Apparently allylborane compounds as allyl transferring reagents are more suitable in the reaction with N-substituted THIQs compared to allylsilanes as the products of coupling were isolated with higher, 70-87% yields, (Scheme 9).113

Scheme 9. Photo-allyboration of THIQs. Isolated yields are shown. Carbon nitrides were demonstrated to be useful photocatalyst to convert aldehydes and ketones to acetals using methyl viologen (MV2+) as a very rapid electron shuttle (Scheme 10).114 MV2+/MV·+ couple has a redox potential -0.445 vs NHE that enables quick removal of the electron from the conduction band of the excited photocatalyst.115

Scheme 10. Conversions and TONs of methyl viologen (in parentheses) are shown. The scope of the alcohols used in synthesis of the corresponding acetals could be expanded even further by employing O2 as a sacrificial electron acceptor (Scheme 11).116

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Scheme 11. Conversions of the carbonyl compound and selectivity (in parentheses) of the acetals are shown. Photocatalytic decarboxylative fluorination over heterogeneous carbon nitride photocatalyst was successfully implemented in flow photoreactor using Serial Microbatch Reactors (Scheme 12).117 The oxidation potential of phenoxyacetic acid is +1.809 V vs. Ag/AgCl, while selectfluor used as a fluorination reagent in this reaction has the reduction potential of -0.284 V vs. Ag/AgCl. In general, the isolated yields are similar to those obtained using Ru(bpy)3Cl2 photocatalyst in batch.118

Scheme 12. Decarboxylative fluorination of the phenoxycarboxylic acids. Retention time and isolated yield are shown. Image of the polytetrafluoroethylene tubing as an efficient continuous reaction site is reproduced with permission from Angew. Chem. Int. Ed. 2018, 57, 1.117 Oxidative coupling of two benzyl amines to the corresponding imine molecule was realized photocatalytically over the heterogeneous CNP (Scheme 13).95 ACS Paragon Plus Environment

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Scheme 13. Oxidative coupling of the benzylamines. Conversion of the benzylamines and selectivity of the imine (in parentheses) are shown. Isolated yield is marked with asterisk. Potassium poly(heptazine imide) (Figure 1) was employed in the assembly of thioamides from benzylamines and elemental sulfur under mild conditions (Scheme 14).119 The method offers 14 thioamides, in good to excellent yields, from primary or secondary aromatic, heterocyclic and aliphatic amines. Moreover, unsymmetrically substituted thioamides were prepared by mixing of two different amines. The mechanism of this reaction includes two sequential photocatalytic oxidation reactions that imply (1) formation of the intermediary imine, similarly to the reaction on Scheme 13, that attaches H2S furnishing aminothiol 1 and (2) oxidation of the aminothiol 1 to the thioamide.

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S

NH2

R or N H S

R

[CNP]

+ S8

R

=461 nm, 70oC, 20 h, Ar, dioxane

N H

R

R S

S N H

N H

N H

Me Me

90%

Me

Me

92%

90%

S

S

S

N H

N

N H

MeO

OMe H2N

NH2

91%

72%

76% S

S

S

S

n

N H

N

N H

N

N

S

O

N H

n

n

C5H11

Ph

N

76%

83%

H2S S

NH R

e

h

h

1st PC Cycle

N H

h

NH2

CNP*

Sn-1

NH2

e CNP*

CNP

CNP

R

Sn-1

Ph

e

S8

Pr

72%

H2S

Photocatalytic mechanism

N H

S

N

C5H11

78%

85%

CNP

N H

Pr

N

68% S

89% S

O

N

N

88%

n

N H

2nd PC Cycle

S8

Ph

h e

1

1

CNP

Sn

Sn

Sn

Sn Ph

NH + Ph

Ph

N

NH2

Ph

N

Ph + NH3

SH Ph + H2S

Ph

N H 1

Ph

Scheme 14. The scope of the thioamides prepared by the heterogeneous carbon nitride photocatalyst. Lock and Su reported light-tuned selective synthesis of azo- and azoxy-compounds by the CNP (Scheme 15).32 Using this method, 24 compounds were prepared in excellent yields. The procedure can be scaled up using 80 L batch reactor.

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Scheme 15. CNP assisted synthesis of azoxy- and azo-compounds. Conversion of the nitroarene and selectivity (in parentheses) of the azoxy- or azo-compound is shown.

3. Oxidative coupling One of the advantages of the photocatalysis is that it allows running the reaction without preactivation of the coupling partners by introduction of the functional groups as it is done in the transition metal catalysis. For instance, the pairs of aryl halide and R-B(OH)2 (Suzuki reaction), aryl halide and Grignard reagents (Kumada reaction) can be named.120-121 The radical cation generated upon one-electron oxidation of the substrate is more acidic than the neutral molecule.122-124 Upon deprotonation, reactive radical species are generated and can be coupled with the substrate of interest. Rational design of the heterogeneous photocatalytic system enabled for a variety of oxidative C-C, C-N, C-S and C-O coupling reactions, which we have summarized in this section. ACS Paragon Plus Environment

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Earlier, C-H thiolation of aromatic compounds was implemented by Wu and Lei using a combination of homogeneous cobalt catalyst and Ru(bpy)3(PF6)2.125 Nevertheless, C-H methyl bonds in methylarenes were successfully activated by only one heterogeneous photocatalyst – potassium poly(heptazine imide) (Figure 1) to generate the reactive benzylradicals that eventually were intercepted by S8 (Scheme 16).76 This reaction affords 6 dibenzyldisulfanes, including BOC-protected one, from the corresponding methylarenes in one step, under mild, transition metal-free conditions. It was shown that the reaction proceeds via a long-lived negatively charged radical of the CNP reported earlier.89, 126 Oxidation potential of toluene is 2.26±0.02 V vs SCE.127

Scheme 16. Oxidative thiolation of the substituted toluenes. Isolated yields are shown. The photos depict the suspension of the CNP in the ground state (orange) and the in-situ created, long lived radical-anion (green). CNP was used to generate CF3· radical upon single electron reduction of trifluoromethanesulfonylchloride (Scheme 17).128 A reduction potential of TfCl is +0.4 V vs. Ag/AgCl.129 A broad scope of heterocycles, 15 compounds, was investigated, while the coupling products were obtained in moderate to excellent yield and in most cases good selectivity. In photocatalytic trifluoromethylation reaction CNP showed somewhat lower selectivity compared to the homogeneous Ru- and Ir-based photoredox systems.129 The possibility to introduce perfluorobutyl substituents into the pyrrole ring was demonstrated.

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Scheme 17. Photocatalytic trifluoromethylation of the aromatic substrates. Yields were determined by 1H, 19F NMR or GC-MS. Isolated yields are marked with asterisk. Oxidative coupling between styrenes or dihydronaphthalenes was successfully accomplished using heterogeneous CNP and nitrobenzene as the electron scavenger under blue (455 nm) light irradiation (Scheme 18).130 The mechanism was proposed taking into account the oxidation potential of benzene sulfonate (Eox = 0.833 V vs. SCE at pH ≈ 6) and the reduction potential of nitrobenzene (Ered = -0.713 V vs. SCE at pH ≈6). In general, the yields of sulfones are comparable to those obtained by using eosin Y as a homogeneous photocatalyst.131-132

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Scheme 18. Oxidative coupling between sodium sulfinates and styrenes or 1,2dihydronaphthalenes. Isolated yields are shown. Similarly, coupling of styrene and sodium benzenesulfinate can be performed under green (520 nm) light irradiation using carbon nitride photocatalyst with improved absorption in the visible range of the light spectrum.133 Yang and Wang have used CNPs prepared from the biotic molecules, i.e. adenine, guanine, etc., in a series of reaction discussed above: oxidative coupling of benzylamine to imine, synthesis of benzophenone from diphenylmethane, oxidation of benzyl alcohol to benzaldehyde and C-C couplings.134 4. Heterocyclizations The importance of the heterocyclic compounds for medicinal chemistry, agriculture and material chemistry cannot be overestimated.135 Typically aromatic heterocycles are prepared from the saturated or partially saturated precursors using oxidative cyclization approach, which relies on the combination of C-C or C-heteroatom coupling and a controlled oxidation reaction.136 A plethora of heterocyclization methods was developed using different and quite often expensive reagents in stoichiometric amounts. Given that atom efficiency of the common oxidizers, e.g. the Dess-Martin reagent,137 is much below 100%, the use of these chemical oxidants leads to the formation of solid waste in the quantity comparable to that of the target heterocycle. In this regard, photocatalysis allows for using cheap, abundant and atom efficient oxidants such as elemental sulfur or oxygen. Thus, Kurpil et al. has prepared 2,5-disubstitued-1,3,4-oxadiazoles from the corresponding hydrazones in 18-82% yields using K-PHI (Figure 1) as a heterogeneous ACS Paragon Plus Environment

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catalyst and S8 as an electron acceptor and the terminal acceptor of the protons (Scheme 19).138 Similarly to discussed above (Scheme 16), the reaction proceeds via a long lived radical-anion of the CNP.

Scheme 19. Photocatalytic oxidative cyclization of hydrazides to 1,3,4-oxadizoles. By means of the CNP a series of dihydrobenzofuranes was prepared directly by a regioselective coupling of resveratrol or its deriatives using carbon nitride photocatalyst and O2 as an electron mediator (Scheme 20).139 Furthermore, this method was applied for the synthesis of dioxins when 3,4-dihydroxy-trans-stilbenes were used as coupling partners. The method is tolerant to different substituents and 15 compounds were obtained with 40-88% yield.

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Scheme 20. Synthesis of benzofuranes and benzodioxines via coupling of the radicals derived from resveratrol or its analogues. Isolated yields are shown. Condensation of benzylamines with ortho-substituted anilines photocatalyzed by carbon nitrides under O2 atmosphere gave benzimidazoles, benzoxazoles and benzothiazoles, depending on the heteroelement in the ortho-position to the amine functionality (Scheme 21).95 In this reaction, on the first step benzylamines are oxidatively coupled with anilines giving the intermediary imines, as was discussed earlier (Scheme 13). On the second step the corresponding imines underwent intramolecular cycloaddition and subsequent oxidation furnishing the products.

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Scheme 21. Photocatalytic synthesis of benzimidazoles, benzoxazoles and benzothiazoles. Conversion and selectivities (in parentheses) based on benzylamines. The photo-Hantzsch synthesis of 1,4-dihydropyridines using CNP and S8 as a mild electron acceptor was reported (Scheme 22).51 In this reaction acetaldehyde is generated from ethanol right before consumption.

Scheme 22. Photo-Hantzsch 1,4-dihydropyridine synthesis. GC-MS yield is shown. The abovementioned 1,4-dihydropyridines can be further oxidized to the aromatic pyridines using the same carbon nitride photocatalyst, but exchanging ethanol to more stable against oxidation solvent – acetonitrile, in order to promote oxidation reaction at the 1,4-dihydropyridine core (Scheme 23).51 Oxidation potential of diethyl 2,4,6trimethyl-1,4-dihydropyridine-3,5-dicarboxylate shown in Scheme 23 is +0.44 V vs Ag/AgCl measured at pH 10.6.140

Scheme 23. Photocatalytic oxidation of 1,4-dihydropyridines to pyridines. Isolated yields are shown. Diels-Alder reaction is a powerful technique to assemble cyclic compounds. Usually it is applied in a non-catalysed fashion, but it can also be highly simplified/enabled by heterogeneous CNP (Scheme 24).141 Similar to the reported earlier homogeneous version of this reaction mediated by Ru(bpz)3(BArF)2, CNP gave comparable yields of the Diels-Alder adducts for most of the substartes.142 On the other hand, 9-vinyl-9Hcarbazole gave the adduct in 91% yield only after 1 h or irradiation, while Ru(bpz)3(BArF)2 produced the same product in 65% yield after 24 h of irradiation. ACS Paragon Plus Environment

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In this reaction, catalytic amounts of O2 participated and played the role of the electron mediator via single electron transfer (SET). It was also confirmed that one of the intermediates in this reaction is 4-membered cycle 4 as shown on the Scheme 27. In the absence of diene, the dimer 5 of the starting alkene 1 was detected.

Scheme 24. Carbon nitride catalyzed Diels-Alder reaction and a proposed photocatalytic mechanism. A series of bromomalonates was converted to the corresponding cyclopentanes by heterogeneous CNP via an intermolecular C-centered radical addition to the double bond (Scheme 25).143 The reaction mixture was pumped through the fluorinated ethylene propylene (FEP) tubing as a continuous reactor filled with CNP dispersion. The same reaction of cyclopentanes synthesis was also enabled by homogeneous Ru(bpy)3Cl2 photocatalyst.144 For comparison purposes, the yields of the products obtained using homogeneous Ru(bpy)3Cl2 photocatalyst are shown on Scheme 25 as well.

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Scheme 25. Photocatalytic synthesis of cyclopentanes. GC-FID yields and isolated yields are shown in parentheses.

Other options and perspectives It is clear that the modern heterogeneous organocatalysts have grown in a way that practically all the reactions previously described with the homogeneous catalysts can be also accomplished with these heterogeneous systems, with the additional advantages of being easy to separate and to recycle. In addition to the given cases, rather classical tools of copolymerization and donor-acceptor structures essentially coming from the closely related field of organic photovoltaics (OPV) already know, allow to address different spectral ranges to be used and different reactions to be addressed in a partly highly specific way. However, it is our opinion that we are just at the beginning to see even more powerful tools where typical semiconductor physics is entering the field of organic reactions. Heterojunction design, i.e. the combination of two semiconductors in one joint system where the two phases are separated in nanometer dimensions is one obvious approach. Heterojunction solar cells were shown to improve charge separation in OPV devices,145 and we simply may expect that heterojunction photocatalysts will resolve some of the dynamic problems of redox photocatalysis, as spontaneous charge recombination is not the restricting dynamic step anymore, as well as that oxidation and reduction halfreactions occur well separated at two different reaction sites. The so called “Z-scheme” photocatalysis, as exerted in biological photosynthesis,146 is a special case of such heterojunction design. Here the two semiconductors do not couple by direct electron transfer from HOMO to HOMO or LUMO to LUMO, but appropriately placed electron ACS Paragon Plus Environment

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mediator (EM) is coupling the lower LUMO with the higher HOMO see Figure 3. That way, the photons in absorbed in the 2 systems can add up to high energy activation, e.g. two red or infrared photons can be summed up to drive a higher energy reaction. Indeed, there are already some selected cases indicative for successful Z-scheme photocatalysis.147-149

Figure 3. “Z-scheme” of two semiconductors separated by the electron mediator (EM). Another rather open field is the so called Carbon quantum dots (CQDs) and Carbonnitrogen quantum dots. Many of them show pronounced blue or white fluorescence,70-75 i.e. they are able to generate a high energy electron-hole pair. That means that also photocatalysis is possible, as already mentioned above. This of course is in apparent contradiction that graphitic carbons are usually semi-metals, with a Fermi-level close to the standard hydrogen potential. There is therefore a good chance that these CQDs are chemically very different to the expectations. Another option is a core-shell heterojunction structure, with the fluorescence coming from the recombination of functional surface and carbon core structures CONCLUSIONS AND OUTLOOK In the previous parts, we gave broad evidence that heterogeneous, conjugated, organic solids are very effective photocatalysts for a broad range of applications. Contrary to dyes, metal complexes, or also inorganic semiconductors, the redox properties of the resulting structures can be controlled by choice of monomers, copolymerizaton, as well as intermolecular structure control towards the 3-dimensional interacting electronic nano entity, thus allowed often a fine adjustment of reactivity and selectivity to give products in highest yields under otherwise rather mild conditions. The reactions are essentially metal-free, the catalysts are therefore to a wide extent function-tolerant, and most systems are so stable that they can be reused for many operations. In special cases, the systems even exceed the best homogeneous photocatalysts in their performance. The reaction portfolio also contained previously unknown synthetic photoredox transformations, such as the synthesis of disulfanes and thioamides. In spite of the wide variety of possible organic semiconductors and the ability to functionalize them, we assume that we are just at the beginning of the discovery of a new field of catalysts. ACS Paragon Plus Environment

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Acknowledgements The authors are grateful to the Max Planck Society and Forschungsgemeinschaft for the financial support (DFG-An 156 13-1).

the

Deutsche

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