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Accepted Article Title: Visible light induced radical arylations of arenes and heteroarenes with aryldiazonium salts do not require a photocatalyst Authors: Michael C. D. Fürst, Eva Gans, Michael J. Böck, and Markus Rolf Heinrich This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Chem. Eur. J. 10.1002/chem.201703954 Link to VoR: http://dx.doi.org/10.1002/chem.201703954
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10.1002/chem.201703954
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COMMUNICATION Visible light induced radical arylations of arenes and heteroarenes with aryldiazonium salts do not require a photocatalyst Michael C. D. Fürst, Eva Gans, Michael J. Böck and Markus R. Heinrich*
Abstract: In the absence of a photocatalyst and other additives, the radical arylation of diverse arenes and heteroarenes has been achieved with aryldiazonium salts under visible light irradiation from a blue LED. While the course of some reactions can be rationalized by the formation of strongly light-absorbing CT complexes between the diazonium ion and the aromatic substrate, several further examples indicate that the simple presence of an aromatic substrate – showing only weak interactions to the diazonium ion – is fully sufficient to enable product formation.
Biaryls represent an ubiquitous structural motif in many fields of chemistry.[1] Synthetic methods for the construction of aryl-aryl bonds are mainly based on transition metal catalysis, including Suzuki,[2,3] Stille[4] and Ullmann couplings,[5] and biaryls may as well be obtained through C-H activation reactions,[6] oxidative couplings[7] and reactions via arynes.[8] Radical-based transformations, which are a further alternative for biaryl synthesis,[9,10] have recently received attention through new developments in the field of catalysis,[11] particularly photocatalysis.[12] Such photocatalyzed reactions can be conducted with a number of aryl radical precursors including diazonium[13] and iodonium salts,[14] diazoanhydrides,[15] carboxylic acids,[16] azo sulfones,[17] and iodo-, bromo- and chloroarenes.[18] Typically employed photocatalysts[19] are complexes of ruthenium[13a,13e,14] or iridium,[16,18g] eosin Y,[13b,18c] titanium dioxide[13c,d] or strong electron-donors,[18f] and double excitation of perylene bisimide recently allowed the conversion of aryl chlorides.[18a] A generalized mechanism for typical photocatalyzed aryl-aryl coupling reactions is depicted in Scheme 1a. [12] Activation of the photocatalyst (PC) by irradiation provides the initiator PC * (step 1), which triggers the reductive formation of the aryl radical from ArX (step 2). After addition to the substrate (step 3), the electron resulting from rearomatization (step 4) is taken up by the oxidized photocatalyst PC+. The key role of the photocatalyst thereby is to close the radical chain, as only few combinations of Ar-X and substrate are known (e.g. electron-poor diazonium salts with electron-rich benzenes),[20,21] for which the electron resulting from step 4 can directly induce the formation of a new aryl radical from Ar-X.[22] Alternatively, the chain transfer in radical arylation reactions can be promoted by strong bases, which is known as base-promoted homolytic aromatic substitution (BHAS).[23-25]
Additional irradiation allows to conduct BHAS reactions under milder conditions.[18g-18i]
Scheme 1. Radical aryl-aryl coupling in the presence and absence of a photocatalyst. ET: electron transfer.
In this overall context it was a challenging question whether a photocatalyst is really required to promote radical aryl-aryl coupling reactions that are unable to maintain a chain mechanism by themselves. Aryl radical generation from diazonium ions[26] under UV irradiation is well known,[27] and particular effects, such as the formation of charge-transfer (CT) complexes between diazonium ions and the aromatic substrates, might be exploited as well.[20,28] Herein, we now report that a broad range of radical aryl-aryl couplings with diazonium salts can be induced through visible light irradiation, not requiring a photocatalyst or other additive (Scheme 1b). Hydroquinone (1), 1,4-dimethoxybenzene (2), 3-hydroxypyridine (3) and furfurylamine (4) were chosen as substrates for the initial photoinduced arylation experiments, whereat acidic solvent systems were used to protect the diazonium ions from ionic sidereactions.[26],[29] Analysis of the UV-Vis difference spectra[28] revealed that CT complexes are formed when the diazonium salt 5a is combined with the electron-rich substrates 1, 2 and 4, but only a very weak absorption was measured for 5a and the 3hydroxypyridinium ion 3 at wavelength over 420 nm (Figure 1). absorption
scaled by ×10 nm
Dipl. Pharm., Apotheker M. C. D. Fürst, E. Gans, M. J. Böck, Prof. Dr. Markus R. Heinrich Department of Chemistry and Pharmacy, Pharmaceutical Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg Schuhstraße 19, 91052 Erlangen, Germany, Email: [email protected] Supporting information for this article is given via a link at the end of the document.
Figure 1. Difference spectra obtained for substrates 1-4 in combination with 4chlorophenyldiazonium chloride (5a).
Additional UV measurements confirmed that under the acidic conditions, none of the substrates 1-4 is protonated at a hydroxy
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Chemistry - A European Journal
COMMUNICATION or methoxy group, or at the furan ring system, which could lead to a particular type of activation. In the next step, the mixtures of 4-chlorophenyldiazonium chloride (5a) with each substrate 1-4 were irradiated with three different light sources (Scheme 2).
Scheme 2. Arylation under irradiation with three different light sources: UV, UVVis and visible light.
From these experiments all desired biaryls 6a-9a were obtained with the highest yields under visible light irradiation from a blue LED, which provided a total emission range from 425-530 nm, whereat the core region (> 50% relative spectral intensity) stretches from 450-475 nm.[29] In contrast to that, the reactions performed under UV or UV-Vis irradiation led to numerous sideproducts in trace amounts. Firstly, these results demonstrate that CT-based arylations are feasible for typical diazonium ions, such as the 4-chlorophenyldiazonium ion. Particular conditions and highly activating substitution patterns (F5C6N2+, Cl3F2C6N2+, (O2N)2C6H3N2+), which lead to efficient underlying chain reactions, are not required. [20] Moreover, and even more interesting was the observation made with 3-hydroxypyridine (3) under visible light, as this experiment suggests that a strong CT complex is not necessary for a successful arylation. Due to constant cooling through a stream of air or a water bath, thermal initiation could be ruled out for all irradiation experiments. Further insight was gained by monitoring the reaction course by 1 H-NMR spectroscopy in the presence and absence of the radical acceptor (Figure 2).
When 5a was irradiated with light from the blue LED (450-475 nm), only partial decomposition was observed with 78% or 67% of 5a remaining after 24 hours, respectively. In the presence of each of the substrates 1-4, in contrast, product formation was largely completed after 6 hours showing the strongly accelerating effect of the aromatic compounds on the conversion of 5a. The faster reactions of hydroquinone (1) and 1,4-dimethoxybenzene (2) with 5a to give 6a and 7a thereby correlate with the lower ionization potentials of 1 and 2 (7.9 and 7.8 eV), compared to the heterocyclic substrates 3 and 4 (9.5 and 8.4 eV).[30] Although it is difficult to determine a reaction order from the curves depicted in Figure 2, their progression nevertheless indicates that an autocatalytic (self-accelerating) effect of the formed biaryl compounds 6a-9a is not of importance. A participation of a chain transfer mechanism could be ruled out for 5a in combination with substrates 1, 2 and 4 through control experiments initiated by catalytic amounts of iodide.[20],[29] For 3-hydroxypyridine (3), this control revealed a weak chain transfer, that is most likely supported by the excellent aptitude of 3-hydroxypyridinium as an aryl radical acceptor.[31] The photoinduced arylations, which had so far been conducted in NMR tubes (0.1 mmol), were next transferred to a larger reaction scale (0.5 to 1.0 mmol). The related optimization showed that only minor differences in yield are observed when the reactions are conducted under argon or air atmosphere. [29] As the presence of dioxygen is known to facilitate rearomatization (Scheme 1b, step 3),[32] the lifetime of the preceding cyclohexadienyl radical cannot be of decisive importance for the reaction course, which further supports the assumption of a non-chain mechanism.[20] Participation of the initally formed substrate radical cation (step 1) in the rearomatization step (step 3) can currently not be ruled out. The results obtained under optimized conditions, and under variation of the substitution pattern on the diazonium salt, are summarized in Scheme 3.
%
Scheme 3. Increased reaction scale and variation of the substituent on the aryldiazonium salt 5. Yields determined after purification by column chromatography. [a] Yield determined by 1H-NMR spectroscopy.
h
Figure 2. Time course experiments under irradiation with a blue LED (450-475 nm).
Among all experiments, limitations were only found for the 4methoxyphenyldiazonium ion 5c, which apparently not possess enough electron affinity to act as oxidant under visible light irradiation, so that the related biaryls 6c-9c are formed in low yields. Control reactions with 5c in the absence of an aromatic substrate confirmed this assumption, as 5c turned out as fully
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COMMUNICATION stable under LED irradiation.[29] Acceptor substitution on the diazonium ion, as exemplified by the cyano, nitro and ester derivatives 5d-5f did, on the other hand, not generally lead to increased yields compared to the more or less electron-neutral diazonium salts 5a and 5b. Only with the electron-rich substrates hydroquinone (1) and 1,4-dimethoxybenzene (2), above average yields were obtained for 5d and 5e, which could point to some underlying radical chain transfer for these particular transformations.[21,22] The photochemical reactivity of the diazonium ions is in agreement with the half-wave potentials determined by polarographic reduction, where the 4-methoxy derivative 5c shows a significantly lower potential (E1/2 = 0.14 V vs. SCE) than all other diazonium ions incorporated in this study (0.35 V < E 1/2 < 0.45 V vs. SCE).[33] Control reactions under identical conditions, but in the dark, gave the biaryls 6a-9a only in low yields of 15%, 420 nm so that the reaction to biaryl 11a can be explained by excitation of an intermediate CT complex. For thiophene, 1,4-benzoquinone and benzene, no or at best only very weak CT absorptions were measured above 420 nm in combination with 5a/5a’. Along with the previously discussed observations for 3-hydroxypyridine (3), these results clearly demonstrate that a strong CT complex is not required for a successful arylation reaction induced by visible light. Having found that radical arylations with arenediazonium salts can be conducted under such simple reaction conditions, we finally evaluated whether the addition of the typical photocatalyst [Ru(bpy)3]Cl2 (2 mol%)[29, 31c] could further improve the reaction outcome. From eight comparative experiments conducted with 4chlorophenyldiazonium chloride (or tetrafluoroborate) 5a,a’ and the arenes 1-4, thiophene, N-Boc pyrrole, 1,4-benzoquinone and benzene, only the two reactions to the thiophene- and benzoquinone-derived biaryls 10a and 12a gave yields increased by 8% and 11%, respectively, when the photocatalyst was present. The slow arylation of benzene could be shortened to 24 h through the addition of [Ru(bpy)3]Cl2, whereat the yield of 13a (79%) however remained more or less unchanged (c.f. Figure 3).[37] In summary, these results demonstrate that a broad range of radical aryl-aryl coupling reactions with arenediazonium salts can be conducted under simple visible light irradiation, not requiring a particular catalyst, photocatalyst and other additive such as a base. A key finding regarding the reaction scope was that photoinduced transformations are able to proceed independently from the formation of a strong charge-transfer complex between diazonium salt and aromatic substrate. The only true limitation was found to be an insufficient oxidation potential of the diazonium ion, as it is caused by strong donor substitution. Further investigations on the photochemical activation and on the underlying mechanism are currently underway in our laboratory.
Acknowledgements The authors are grateful for the financial support by the Studienstiftung des Deutschen Volkes (M. C. D. F.). We would further like to thank Johannes Köckenberger for experimental assistance and Dominik Sauerbeck (Josef Hofmann Modell- und Leuchtentechnik) for recording the emission spectra of the LED.
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COMMUNICATION Keywords: Radical reactions • Arylation • Biaryl • Photochemistry • Diazonium
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COMMUNICATION Michael C. D. Fürst, Eva Gans, Michael J. Böck and Markus R. Heinrich* Page No. – Page No.
The radical arylation of diverse arenes and heteroarenes has been achieved with arenediazonium salts under visible light irradiation, and in the absence of a photocatalyst or other additives, such as a bases. The simple presence of an aromatic substrate in the reaction mixture – showing only weak interactions to the diazonium ion – thus appears to be fully sufficient to enable product formation.
Visible light induced radical arylations of arenes and heteroarenes with aryldiazonium salts do not require a photocatalyst
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Accepted Article Title: Visible light induced radical arylations of arenes and heteroarenes with aryldiazonium salts do not require a photocatalyst Authors: Michael C. D. Fürst, Eva Gans, Michael J. Böck, and Markus Rolf Heinrich This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Chem. Eur. J. 10.1002/chem.201703954 Link to VoR: http://dx.doi.org/10.1002/chem.201703954
Supported by
10.1002/chem.201703954
Chemistry - A European Journal
COMMUNICATION Visible light induced radical arylations of arenes and heteroarenes with aryldiazonium salts do not require a photocatalyst Michael C. D. Fürst, Eva Gans, Michael J. Böck and Markus R. Heinrich*
Abstract: In the absence of a photocatalyst and other additives, the radical arylation of diverse arenes and heteroarenes has been achieved with aryldiazonium salts under visible light irradiation from a blue LED. While the course of some reactions can be rationalized by the formation of strongly light-absorbing CT complexes between the diazonium ion and the aromatic substrate, several further examples indicate that the simple presence of an aromatic substrate – showing only weak interactions to the diazonium ion – is fully sufficient to enable product formation.
Biaryls represent an ubiquitous structural motif in many fields of chemistry.[1] Synthetic methods for the construction of aryl-aryl bonds are mainly based on transition metal catalysis, including Suzuki,[2,3] Stille[4] and Ullmann couplings,[5] and biaryls may as well be obtained through C-H activation reactions,[6] oxidative couplings[7] and reactions via arynes.[8] Radical-based transformations, which are a further alternative for biaryl synthesis,[9,10] have recently received attention through new developments in the field of catalysis,[11] particularly photocatalysis.[12] Such photocatalyzed reactions can be conducted with a number of aryl radical precursors including diazonium[13] and iodonium salts,[14] diazoanhydrides,[15] carboxylic acids,[16] azo sulfones,[17] and iodo-, bromo- and chloroarenes.[18] Typically employed photocatalysts[19] are complexes of ruthenium[13a,13e,14] or iridium,[16,18g] eosin Y,[13b,18c] titanium dioxide[13c,d] or strong electron-donors,[18f] and double excitation of perylene bisimide recently allowed the conversion of aryl chlorides.[18a] A generalized mechanism for typical photocatalyzed aryl-aryl coupling reactions is depicted in Scheme 1a. [12] Activation of the photocatalyst (PC) by irradiation provides the initiator PC * (step 1), which triggers the reductive formation of the aryl radical from ArX (step 2). After addition to the substrate (step 3), the electron resulting from rearomatization (step 4) is taken up by the oxidized photocatalyst PC+. The key role of the photocatalyst thereby is to close the radical chain, as only few combinations of Ar-X and substrate are known (e.g. electron-poor diazonium salts with electron-rich benzenes),[20,21] for which the electron resulting from step 4 can directly induce the formation of a new aryl radical from Ar-X.[22] Alternatively, the chain transfer in radical arylation reactions can be promoted by strong bases, which is known as base-promoted homolytic aromatic substitution (BHAS).[23-25]
Additional irradiation allows to conduct BHAS reactions under milder conditions.[18g-18i]
Scheme 1. Radical aryl-aryl coupling in the presence and absence of a photocatalyst. ET: electron transfer.
In this overall context it was a challenging question whether a photocatalyst is really required to promote radical aryl-aryl coupling reactions that are unable to maintain a chain mechanism by themselves. Aryl radical generation from diazonium ions[26] under UV irradiation is well known,[27] and particular effects, such as the formation of charge-transfer (CT) complexes between diazonium ions and the aromatic substrates, might be exploited as well.[20,28] Herein, we now report that a broad range of radical aryl-aryl couplings with diazonium salts can be induced through visible light irradiation, not requiring a photocatalyst or other additive (Scheme 1b). Hydroquinone (1), 1,4-dimethoxybenzene (2), 3-hydroxypyridine (3) and furfurylamine (4) were chosen as substrates for the initial photoinduced arylation experiments, whereat acidic solvent systems were used to protect the diazonium ions from ionic sidereactions.[26],[29] Analysis of the UV-Vis difference spectra[28] revealed that CT complexes are formed when the diazonium salt 5a is combined with the electron-rich substrates 1, 2 and 4, but only a very weak absorption was measured for 5a and the 3hydroxypyridinium ion 3 at wavelength over 420 nm (Figure 1). absorption
scaled by ×10 nm
Dipl. Pharm., Apotheker M. C. D. Fürst, E. Gans, M. J. Böck, Prof. Dr. Markus R. Heinrich Department of Chemistry and Pharmacy, Pharmaceutical Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg Schuhstraße 19, 91052 Erlangen, Germany, Email: [email protected] Supporting information for this article is given via a link at the end of the document.
Figure 1. Difference spectra obtained for substrates 1-4 in combination with 4chlorophenyldiazonium chloride (5a).
Additional UV measurements confirmed that under the acidic conditions, none of the substrates 1-4 is protonated at a hydroxy
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10.1002/chem.201703954
Chemistry - A European Journal
COMMUNICATION or methoxy group, or at the furan ring system, which could lead to a particular type of activation. In the next step, the mixtures of 4-chlorophenyldiazonium chloride (5a) with each substrate 1-4 were irradiated with three different light sources (Scheme 2).
Scheme 2. Arylation under irradiation with three different light sources: UV, UVVis and visible light.
From these experiments all desired biaryls 6a-9a were obtained with the highest yields under visible light irradiation from a blue LED, which provided a total emission range from 425-530 nm, whereat the core region (> 50% relative spectral intensity) stretches from 450-475 nm.[29] In contrast to that, the reactions performed under UV or UV-Vis irradiation led to numerous sideproducts in trace amounts. Firstly, these results demonstrate that CT-based arylations are feasible for typical diazonium ions, such as the 4-chlorophenyldiazonium ion. Particular conditions and highly activating substitution patterns (F5C6N2+, Cl3F2C6N2+, (O2N)2C6H3N2+), which lead to efficient underlying chain reactions, are not required. [20] Moreover, and even more interesting was the observation made with 3-hydroxypyridine (3) under visible light, as this experiment suggests that a strong CT complex is not necessary for a successful arylation. Due to constant cooling through a stream of air or a water bath, thermal initiation could be ruled out for all irradiation experiments. Further insight was gained by monitoring the reaction course by 1 H-NMR spectroscopy in the presence and absence of the radical acceptor (Figure 2).
When 5a was irradiated with light from the blue LED (450-475 nm), only partial decomposition was observed with 78% or 67% of 5a remaining after 24 hours, respectively. In the presence of each of the substrates 1-4, in contrast, product formation was largely completed after 6 hours showing the strongly accelerating effect of the aromatic compounds on the conversion of 5a. The faster reactions of hydroquinone (1) and 1,4-dimethoxybenzene (2) with 5a to give 6a and 7a thereby correlate with the lower ionization potentials of 1 and 2 (7.9 and 7.8 eV), compared to the heterocyclic substrates 3 and 4 (9.5 and 8.4 eV).[30] Although it is difficult to determine a reaction order from the curves depicted in Figure 2, their progression nevertheless indicates that an autocatalytic (self-accelerating) effect of the formed biaryl compounds 6a-9a is not of importance. A participation of a chain transfer mechanism could be ruled out for 5a in combination with substrates 1, 2 and 4 through control experiments initiated by catalytic amounts of iodide.[20],[29] For 3-hydroxypyridine (3), this control revealed a weak chain transfer, that is most likely supported by the excellent aptitude of 3-hydroxypyridinium as an aryl radical acceptor.[31] The photoinduced arylations, which had so far been conducted in NMR tubes (0.1 mmol), were next transferred to a larger reaction scale (0.5 to 1.0 mmol). The related optimization showed that only minor differences in yield are observed when the reactions are conducted under argon or air atmosphere. [29] As the presence of dioxygen is known to facilitate rearomatization (Scheme 1b, step 3),[32] the lifetime of the preceding cyclohexadienyl radical cannot be of decisive importance for the reaction course, which further supports the assumption of a non-chain mechanism.[20] Participation of the initally formed substrate radical cation (step 1) in the rearomatization step (step 3) can currently not be ruled out. The results obtained under optimized conditions, and under variation of the substitution pattern on the diazonium salt, are summarized in Scheme 3.
%
Scheme 3. Increased reaction scale and variation of the substituent on the aryldiazonium salt 5. Yields determined after purification by column chromatography. [a] Yield determined by 1H-NMR spectroscopy.
h
Figure 2. Time course experiments under irradiation with a blue LED (450-475 nm).
Among all experiments, limitations were only found for the 4methoxyphenyldiazonium ion 5c, which apparently not possess enough electron affinity to act as oxidant under visible light irradiation, so that the related biaryls 6c-9c are formed in low yields. Control reactions with 5c in the absence of an aromatic substrate confirmed this assumption, as 5c turned out as fully
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10.1002/chem.201703954
Chemistry - A European Journal
COMMUNICATION stable under LED irradiation.[29] Acceptor substitution on the diazonium ion, as exemplified by the cyano, nitro and ester derivatives 5d-5f did, on the other hand, not generally lead to increased yields compared to the more or less electron-neutral diazonium salts 5a and 5b. Only with the electron-rich substrates hydroquinone (1) and 1,4-dimethoxybenzene (2), above average yields were obtained for 5d and 5e, which could point to some underlying radical chain transfer for these particular transformations.[21,22] The photochemical reactivity of the diazonium ions is in agreement with the half-wave potentials determined by polarographic reduction, where the 4-methoxy derivative 5c shows a significantly lower potential (E1/2 = 0.14 V vs. SCE) than all other diazonium ions incorporated in this study (0.35 V < E 1/2 < 0.45 V vs. SCE).[33] Control reactions under identical conditions, but in the dark, gave the biaryls 6a-9a only in low yields of 15%, 420 nm so that the reaction to biaryl 11a can be explained by excitation of an intermediate CT complex. For thiophene, 1,4-benzoquinone and benzene, no or at best only very weak CT absorptions were measured above 420 nm in combination with 5a/5a’. Along with the previously discussed observations for 3-hydroxypyridine (3), these results clearly demonstrate that a strong CT complex is not required for a successful arylation reaction induced by visible light. Having found that radical arylations with arenediazonium salts can be conducted under such simple reaction conditions, we finally evaluated whether the addition of the typical photocatalyst [Ru(bpy)3]Cl2 (2 mol%)[29, 31c] could further improve the reaction outcome. From eight comparative experiments conducted with 4chlorophenyldiazonium chloride (or tetrafluoroborate) 5a,a’ and the arenes 1-4, thiophene, N-Boc pyrrole, 1,4-benzoquinone and benzene, only the two reactions to the thiophene- and benzoquinone-derived biaryls 10a and 12a gave yields increased by 8% and 11%, respectively, when the photocatalyst was present. The slow arylation of benzene could be shortened to 24 h through the addition of [Ru(bpy)3]Cl2, whereat the yield of 13a (79%) however remained more or less unchanged (c.f. Figure 3).[37] In summary, these results demonstrate that a broad range of radical aryl-aryl coupling reactions with arenediazonium salts can be conducted under simple visible light irradiation, not requiring a particular catalyst, photocatalyst and other additive such as a base. A key finding regarding the reaction scope was that photoinduced transformations are able to proceed independently from the formation of a strong charge-transfer complex between diazonium salt and aromatic substrate. The only true limitation was found to be an insufficient oxidation potential of the diazonium ion, as it is caused by strong donor substitution. Further investigations on the photochemical activation and on the underlying mechanism are currently underway in our laboratory.
Acknowledgements The authors are grateful for the financial support by the Studienstiftung des Deutschen Volkes (M. C. D. F.). We would further like to thank Johannes Köckenberger for experimental assistance and Dominik Sauerbeck (Josef Hofmann Modell- und Leuchtentechnik) for recording the emission spectra of the LED.
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10.1002/chem.201703954
Chemistry - A European Journal
COMMUNICATION Keywords: Radical reactions • Arylation • Biaryl • Photochemistry • Diazonium
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COMMUNICATION Michael C. D. Fürst, Eva Gans, Michael J. Böck and Markus R. Heinrich* Page No. – Page No.
The radical arylation of diverse arenes and heteroarenes has been achieved with arenediazonium salts under visible light irradiation, and in the absence of a photocatalyst or other additives, such as a bases. The simple presence of an aromatic substrate in the reaction mixture – showing only weak interactions to the diazonium ion – thus appears to be fully sufficient to enable product formation.
Visible light induced radical arylations of arenes and heteroarenes with aryldiazonium salts do not require a photocatalyst
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