Heteroleptic Cu-Based Sensitizers in Photoredox Catalysis - American

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Heteroleptic Cu-Based Sensitizers in Photoredox Catalysis Published as part of the Accounts of Chemical Research special issue “Photoredox Catalysis in Organic Chemistry”. Augusto C. Hernandez-Perez and Shawn K. Collins* Département de Chimie and Centre for Green Chemistry and Catalysis, Université de Montréal, CP 6128 Station Downtown, Montréal, Québec H3C 3J7, Canada CONSPECTUS: 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 UVlight 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.

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at shorter, more energetic wavelengths.4 The use of sensitizers and visible light activation can be used to promote single electron transfers between organic molecules and has ushered in a new age of radical chemistry, causing organic chemists to revisit traditional bond formations and imagine alternative synthetic routes based upon new mechanistic pathways. As a result, novel preparative methods are appearing that are ever more chemoselective, functional group tolerant, and efficient. It is important to note that photocatalysis is not without its own drawbacks. The majority of the chemistry developed exploits

INTRODUCTION Photochemistry is a powerful synthetic technique that can promote various functional group transformations through the use of light as a green1 and traceless reagent.2 Despite the apparent advantages, photochemical activation has not experienced the same usage in synthesis as thermal activation. The failure to fully grasp the potential of photochemistry could perhaps be traced to the inconveniences of working with ultraviolet (UV) light, which include specialized and expensive glassware, health and safety concerns, and challenges associated with developing reactions on larger scales.3 Photocatalysis offers the possibility to employ light in the visible spectrum for synthetic chemistry without the challenges associated with light © XXXX American Chemical Society

Received: May 24, 2016

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DOI: 10.1021/acs.accounts.6b00250 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research the long-lived excited states associated with Ru-based polypyridyl-type complexes.5 When different excited state oxidation or reduction potentials are needed, the use of Irbased complexes is often explored.6 While significant efforts have been reported to improve catalyst stability and lower catalyst loadings,7 the development of alternative sensitizers based upon more abundant metals has emerged.8 The use of copper is appealing, and a number of photoactive complexes have appeared in the literature as competent catalysts for photocatalysis.9 Synthetic photochemistry is generally influenced by issues of light penetration and homogeneous irradiation. One could argue that photochemical routes, regardless of application on small or process-level scales, could be enhanced by the shorter reaction times and often improved yields afforded by continuous flow. The efficient mixing observed in flow is also beneficial to photocatalytic synthesis.10 Consequently, the exploitation of continuous flow chemistry has grown increasingly important in modern synthetic photochemistry, regardless of the wavelength of the light source.11 While the interplay of photocatalysis employing novel sensitizers and continuous flow techniques can present certain alternatives to UV-light mediated synthetic photochemistry, there is significant potential to develop both visible and UV-light methods in parallel, exploiting differences in their mechanistic pathways to discover new complementary synthetic tools.

Scheme 1. Synthesis of [5]Helicene via a UV-Mediated Photocyclization

Scheme 2. Synthesis of [5]Helicene via a Visible-Light Mediated Photoredox Transformation

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PHOTOCHEMICAL SYNTHESIS OF HELICENES Helical carbon-based materials, specifically helicenes, are of interest due to their inherently chiral and conjugated frameworks that present numerous applications, most notably in materials science.12 In the past two decades, new synthetic methods have focused on non-photochemical routes to provide facile access to the twisted and strained aromatic skeletons, because the traditional photochemical route employing UV light was considered impractical for scale up and limited in scope to certain polyaromatic systems. The UV light-mediated photocyclodehydrogenative cyclization of stilbenes is a reversible 6π-electrocyclization, which results in the formation of a carbon−carbon bond between two arenes.13 In situ oxidation re-establishes aromaticity and affords the desired polycyclic aromatic product. A representative example that highlights the challenges associated with the UV-light method is the synthesis of [5]helicene, 2. The irradiation of stilbenyl precursor 1 with a high intensity mercury lamp, using molecular iodine as oxidant, was reported as affording 25% yield of [5]helicene, 37% yield of the over oxidized polycyclic aromatic 3, and 38% yield of the regioisomeric product dibenzo[b,g]phenanthrene 4, whereby cyclization has occurred with a different carbon atom (indicated by the solid blue circle, Scheme 1).14 An alternative photochemical synthesis of [5]helicene via photoredox catalysis would involve treating stilbenyl precursor 1 under irradiation with visible light in the presence of a sensitizer, molecular iodine as oxidant, and propylene oxide as a HI trap (Scheme 2).15 For the cyclization of a stilbenoid 1 under visible irradiation (using a 20 W household light bulb and molecular iodine as oxidant), two catalysts were selected that had been popularized for photoredox catalysis at the time, Ru(bpy)2(PF6)2 (5) and Ir(ppy)2(dtbbpy)PF6 (6); however each only afforded the desired [5]helicene in 90% yield and generally readily crystallizable. Recognizing the need for straightforward syntheses of sensitizers, a protocol for the in situ formation of the heteroleptic complexes was devised (Scheme 3). Because other common sensitizers based upon Ru Scheme 3. In Situ Preparation of Cu-Based Sensitizers

and Ir require synthesis of the discrete complex, an in situ protocol for sensitizer synthesis would allow a more rapid screening of sensitizer structures for a given photoredox transformation. The developed method for the synthesis of heteroleptic complexes involved sequential addition of the diamine and bisphosphine ligands to a solution of a commercially available copper source, [Cu(MeCN)4]BF4. Exploiting the in situ synthesis of heteroleptic complexes afforded the Cu(Xantphos)(neo)BF4 complex 9 as optimal for the synthesis of [5]helicene, providing it in 40−47% yield on small scale (50 mg) (Scheme 4). While the synthesis of [5]helicene using the Cu(XantPhos)(neo)BF4 sensitizer represented the first use of heteroleptic Cu-complexes in photoredox catalysis, it should be noted that several reports of C


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Accounts of Chemical Research Scheme 5. Continuous Flow Synthesis of [5]Helicene Using an in Situ Formed, Heteroleptic Cu Complex 9 as Sensitizer

Scheme 6. Continuous Flow Synthesis of Pyrene/Helicene Hybrids Using an in Situ Formed, Heteroleptic Cu Complex 9 as Sensitizer

(neo)BF4 catalyst 9 formed in situ (25 mol %) the synthesis of various pyrene/helicene hybrids was undertaken (Scheme 6). The targets included a hybrid representing a merger of [5]helicene and a pyrene unit at its 2-position (10), a hybrid merging an electron rich trimethoxyarene with a pyrene at its 2position (11), and a hybrid fusing [5]helicene and a pyrene at its 4-position (12). Batch experiments were all stopped after 5 days, regardless of conversion, and the resulting helicenes 10, 11, and 12 were isolated in 12−23% yield. To improve the synthesis of the pyrene−helicene hybrids, a new flow protocol was devised that exploited a commercially available pumping module whose respective 10 mL reactor volume coils (PFA tubing) were placed around a standard household lightbulb. Again, the flow synthesis using the in situ generated heteroleptic Cu-based sensitizer 9 was considerably improved under flow conditions, where the yields increased to 41−46% and reaction times were again decreased to approximately 18 h. The improved reaction conditions allowed for isolation of enough material for characterization of the electronic and solid state properties of the helicenes and comparison with [5]helicene analogs.

photocyclization to form carbazoles involve natural product synthesis (glycozoline)30 and the synthesis of chloro-31 and azacarbolines.32 In exploring a photoredox variant toward carbazoles, triphenylamine 13 was employed as a model compound. A series of batch experiments were performed to survey a variety of Cu-based sensitizers.33 The in situ preparation was exploited to vary the nature of the bisphosphine and amine ligand. In all cases, control reactions were performed with the preformed complex, and yields were similar within experimental error. The Cu(XantPhos)(neo)BF4 complex 9 was identified as optimal (5 mol %), providing a 56% yield of carbazole 14 after 120 h (Scheme 7). Under the reactions conditions, an organic sensitizer eosin Y34 did not improve yield, and other oxidants such as O2 or MV(PF6)2 were not superior to molecular iodine. An analogous photoredox experiment for the conversion of 13 using Ru(bpy)3(PF6)2 (5 mol %) provided 27% yield of carbazole 14. When each transition metal-based catalyst was compared under continuous flow conditions (residence times of 10 h), the Cu-based catalyst provided carbazole 14 in 75% yield, while the Ru-based catalyst provided 14 in 53% yield. In exploring the scope of the photoredox synthesis of carbazoles (Scheme 8), it was demonstrated that electron-rich triarylamines were good substrates for cyclization, because the trimethoxy-derivative 15 was isolated in good yield (55% yield), and the mesityl-substituted carbazole 16 (95% yield), N-tolylsubstituted carbazole 19 (70% yield), and N-anisyl-substituted

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PHOTOCHEMICAL SYNTHESIS OF CARBAZOLES Given the success observed with the heteroleptic Cu complexes in the synthesis of helicenes, it was reasoned that similar photoredox processes could promote a Mallory-like cyclization of other compounds that typically undergo 6π-electrocyclization under UV-light conditions. Investigating the synthesis of carbazoles from diaryl- or triaryl-substituted amines was particularly attractive because the heterocyclic nucleus has found extensive application in medicinal chemistry and materials science.28 The photocyclization of di- and triaryl amines using UV-light photochemistry has been known since the 1970s, where it was mainly investigated for mechanistic understanding,29 but the technology was never fully developed into a robust synthetic method. Some of the few reports of D


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Accounts of Chemical Research Scheme 7. Photoredox Synthesis of N-Phenyl Carbazole 14 Using Transition Metal-Based Sensitizers

65% yield, and both N-ethyl and N-isopropyl carbazoles were also isolated in good yields (79% and 65% yield).36 Other Nalkylated carbazoles that were formed include the polycyclic carbazole derivative 24 (64% yield), the 3,9-dimethyl carbazole 27 (60% yield), 3-methoxy-9-methylcarbazole 28 (63% yield), and the heterocyclic derivatives N-methyl α-carboline derivative 25 (51% yield) and pyrimido[5,4-b]indole derivative 26 (53% yield). Following the development of the photoredox synthesis of carbazoles, it became apparent that advantages presented by continuous flow technology (increased light penetration and subsequent decrease in reaction time) would be equally viable in a UV-light mediated synthesis of carbazoles. As such, the characteristics of each photochemical method (yields, reaction times, functional group compatibilities, and regioselectivities) could be compared. The UV-light experimental setup consisted of PFA reactors coils (10 mL), placed inside a photoreactor equipped with 350 nm lamps (Scheme 9).37 Reaction times for Scheme 9. UV-Mediated Photocyclization of Aryl Amines To Afford Carbazoles

Scheme 8. Photoredox Synthesis of Carbazoles Using a Heteroleptic Cu-Based Sensitizer

the cyclization of triphenylamine 13 to the corresponding 9phenylcarbazole were much faster than those observed for the photoredox protocol. At 350 nm irradiation, a 71% yield of the carbazole 14 was isolated following chromatography after a residence time of only 15 min. Further improvement in reaction time and yield was observed using a commercially available UV-reactor, where a 91% yield of 14 was obtained in 10 min. During the investigation of the scope of the photoredox synthesis of carbazoles, the cyclization of triaryl amines bearing substituted phenyl rings resulted in a mixture of products, termed “exo” and “endo” depending on whether the substituted ring was found as the N-substituent or within the carbazole nucleus, respectively. In general, substituents located in the para-position of the aryl rings of the starting material tend to

carbazole 20 (70% yield) were all isolated in good yields. The N-tolyl- and N-anisyl-substituted carbazoles were isolated as 7:1 and 9:1 ratios of isomers, respectively, in which the major isomer had the substituted aryl group “exo” to the carbazole skeleton (as the N-substituent). Carbazoles whose triaryl amine precursors contained electron poor heterocyclic aromatics afforded the pyridine-containing carbazole 18 and the pyrimidine-containing carbazole 17 in good isolated yields of 55% and 60%, respectively. In addition, the synthesis of N-alkyl carbazoles was also possible, despite other reports that photoredox catalysis can promote amine demethylation.35 The cyclization to form N-methyl carbazole 21 occurred in E


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Accounts of Chemical Research exert a large effect on the regioselectivity.38 For example, when the fluorinated derivative 29 was cyclized under visible light photoredox conditions using the Cu(Xantphos)(neo)BF4 catalyst, a 95% total yield was obtained; however, the products were a 10:90 ratio of carbazoles, in which the major isomer was “endo” product having the fluorinated aromatic included in the carbazole skeleton (Scheme 10). Interestingly, when the

Scheme 11. Regioselectivity Observed When Comparing the Cyclization of Halogenated Triaryl Amines under Photoredox or UV-Light Mediated Photocyclization

Scheme 10. Regioselectivity Observed When Comparing the Cyclization of Triaryl Amines under Photoredox or UVLight Mediated Photocyclization

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CHALLENGES AND OPPORTUNITIES The UV absorption window of photoactive Cu catalysts differs greatly based on the ligands used. For the homoleptic complex Cu(dap)2Cl 7, an absorption band in the 500 nm range allows for excitation using blue or green LEDs. However, the heteroleptic complex Cu(DPEPhos)(neo)BF4 8 lacks the strong absorbance for the homoleptic analog 7 and is more easily excited from bands in 350−400 nm range. Developing new heteroleptic Cu complexes with extended absorption windows would enhance the utility of the complexes in photoredox catalysis. Another challenge associated with heteroleptic Cu-complexes involves taming their ligand disassociation behaviors.41 Reiser and co-workers have reported the synthesis and characterization of a series of heteroleptic Cu-complexes in which the bisphosphine has been replaced with a bisisonitrile ligand (Scheme 12).42 The change in reactivity observed with the resulting complexes was proposed to derive from a change in the ligand dynamics of the heteroleptic complex. Bulky bisphosphines were proposed to encourage disassociation and, hence, the formation of complex mixtures including homoleptic complexes. The less sterically demanding nature of the bisisonitrile ligand was believed to stabilize the heteroleptic structure (Scheme 12a). The complex Cu(binc*)(dpp)BF4, 39, was observed to have a longer excited-state lifetime compared with the known homoleptic catalyst Cu(dpp)2Cl, 40, and demonstrated activity at low catalyst loading (0.5 mol %) in the visible light-mediated atom transfer radical addition reaction of a bromomalonate (Scheme 12b). Finally, Cu-based photocatalysts could be explored for their potential in asymmetric synthesis. Recently, Reiser and coworkers demonstrated that the heteroleptic Cu-complex Cu(binc*)(dpp)BF4, 39, promotes the formation of a different product in trifluoromethylchlorosulfonylation reactions (Scheme 13).43 The preference for Cu(binc*)(dpp)BF4 (39) to form the sulfonylated compound 41 was proposed to be due to the possibility of an inner sphere mechanism. If accurate, the possibility of tuning the ligand structure would provide a new avenue to enable asymmetric copper-catalyzed photoredox transformations.44

cyclization of 29 was performed under UV light irradiation (300 nm), a similar yield was obtained (87%) but the regioselectivity had inverted, where the exo product 30 in which the 4-fluorophenyl group became the N-substituent, was preferred in a 87:13 ratio. Another example of inverted selectivities between the visible and UV-light mediated methods occurs with an electron donating methoxy group in the paraposition of the aromatic substituent of the precursor 31 (Scheme 10). When the methoxy derivative 31 was cyclized under visible light photoredox conditions using the Cu(Xantphos)(neo)BF4 catalyst, a 70% total yield was obtained with a ratio of 90:10 favoring the exo product 32. Conversely, when the cyclization of 31 was performed under UV light irradiation (300 nm), an 81% yield was obtained where the endo product 32 was preferred (10:90 exo/endo). Complementarity of the UV-light and Cu-based photoredox methods was also observed in the cyclization to form halogenated carbazoles. While cases of inversed regioselectivity in the cyclization of fluorinated derivatives was mentioned above, functional group tolerance of other halogens was markedly different between the two photochemical methods (Scheme 11). The heteroleptic Cu-based catalyst 9 was able to promote cyclization to form chloro-, bromo-, and iodinated carbazoles 34, 36, and 38, respectively, in yields ranging from 19% to 95%. In contrast, high energy UV light conditions (300 nm) promoted carbon−halogen bond homolysis (Scheme 11).39 In some instances, 9-phenylcarbazole 14 could be isolated from the complex reaction mixtures, but no halogenated carbazoles or uncyclized amines were isolated. While in most of the cyclizations to form carbazoles, the UV light method provided higher general yields and faster reaction times, the tolerance to sensitive functionality such as halogens stands out as one of the advantages of the photoredox method.40 F


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Accounts of Chemical Research Scheme 12. (a) Ligand Disassociation Behavior of Heteroleptic Complexes and (b) Use of Some Heteroleptic Complexes in Atom Transfer Reactions

Université de Montréal and pursued his graduate studies (Ph.D.) at the Université de Montréal under the supervision of Prof. Shawn K. Collins. He is currently a Research Scientist at IntelliSynRD in Montréal, Québec. Shawn K. Collins obtained a B.Sc. degree from Concordia University in 1996 and his Ph.D. in 2001 at the University of Ottawa (Prof. A. G. Fallis). After an NSERC postdoctoral fellowship with Professor L. E. Overman (University of California, Irvine), he joined the faculty at Université de Montréal in September 2003 and was promoted to Full Professor in 2015.

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ACKNOWLEDGMENTS S.K.C. thanks the students who have contributed to the research described in this Account: Augusto Hernandez-Perez, Anna Vlassova, Anne-Catherine Bédard, Antoine Caron, Shawn Parisien-Collette, and Clémentine Minozzi. The authors also acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC), Université de Montreal, the Centre for Green Chemistry and Catalysis (CGCC), and the NSERC CREATE program in Continuous Flow Science for generous funding. The Canadian Foundation for Innovation (CFI) is acknowledged for generous funding of the flow chemistry infrastructure.

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Scheme 13. Influence of Ru- and Cu-Based Catalysts on the Trifluoromethylchlorosulfonylation of Alkenes

In summary, Cu-based complexes are potent catalysts for the formation of carbon−carbon bonds through processes that are analogous to 6π-electrocyclizations. Given the variety of such processes in the literature, further work into the development of photoredox variants for classical UV-mediated electrocyclization is warranted. The usefulness of continuous flow chemistry to photoredox and UV-light mediated transformations cannot be overstated: in addition to decreasing reactions times and increasing yields, the flow technology renders scale-up more facile and also enables the safe use of UV-light for preparative chemistry. The synthesis of substituted carbazoles via both photoredox catalysis and UV-light mediated photocyclizations demonstrates the potential complementarity of the different photochemical methods. If harnessed, the ability to select combinations of irradiation wavelengths and catalysts would allow chemists to access different products from the same starting material, expanding the photochemical toolbox in organic chemistry.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Augusto C. Hernandez-Perez was born in Montréal, Québec, Canada, in 1987. In 2009, he received his B.Sc. in Chemistry from the G


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Accounts of Chemical Research

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Heteroleptic Cu-Based Sensitizers in Photoredox Catalysis - American

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