Aerobic Oxidative Homo- and Cross-Coupling of Amines Catalyzed by

Article pubs.acs.org/joc

Cite This: J. Org. Chem. 2018, 83, 13481−13490

Aerobic Oxidative Homo- and Cross-Coupling of Amines Catalyzed by Phenazine Radical Cations Rok Brišar,†,§ Felix Unglaube,† Dirk Hollmann,‡ Haijun Jiao,† and Esteban Mejía*,† †

Leibniz Institute for Catalysis, Albert-Einstein-Str. 29a, 18059 Rostock, Germany Institute of Chemistry, University of Rostock, Albert-Einstein-Str. 3a, 18059 Rostock, Germany

‡

J. Org. Chem. 2018.83:13481-13490. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/02/18. For personal use only.

S Supporting Information *

ABSTRACT: Phenazine radical cations (PhRCs) were used for the first time as efficient metal-free catalysts for the oxidative homo- and cross-coupling of a variety of different amines. A series of functional PhRCs were prepared, characterized with X-ray diffraction, and their radical character was investigated with DFT calculations. They were tested as catalysts under neat conditions with low oxygen pressure to prepare homo- and crosscoupled aliphatic and aromatic imines in high yields. Although all synthesized phenazines were catalytically active, the highest reaction rates and the best selectivity were achieved using the 5,10-dihydro-5,10-dimethylphenazine radical cation. By means of fluorescence, UV−vis and EPR spectroscopy, a mechanism of the oxidative amine coupling, catalyzed by PhRCs, is proposed.

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azobis(isobutyronitrile)14 as well as Ogawa et al. using salicylic acid derivatives although the amount of catalyst renders this method relatively ineffective (Scheme 1B).15 The radicalmediated amine oxidation has been thoroughly investigated using heterogeneous carbocatalysts (Scheme 1C),16 including graphite oxide,17 porous graphene oxide,18 nanoporous carbons derived from MOFs,19 hierarchically porous carbon derived from POSS,20 carbon nitride,21 et cetera. We recently showed for the first time the potential of pyrazine radical cations as oxidation catalysts in amine homocoupling reactions (Scheme 1C).22 As an extension of this work, we exploited the redox chemistry of N,N′disubstituted-dihydrophenazines (DSPs), which, upon oxidation produce phenazine radical cations (PhRCs, Scheme 1D).23 These stable open-shell species are known as versatile one-electron reductants and oxidants in various biochemical processes.24 Very recently, PhRCs were introduced as excellent catalysts for atom transfer radical polymerization driven by visible light,25 as well as proton acceptors in the oxidative esterification of aldehydes.26 Here, we report the use of PhRCs for the first time as a highly effective and selective metal-free catalyst for the oxidative homo- and heterocoupling of amines under mild conditions. This homogeneous organocatalysts are chemically stable and readily available and achieve high reaction conversions and selectivity.

INTRODUCTION Imines are an important component of many relevant biologically active heterocycles,1 the latest anticancer drugs,2 and intermediates in organic transformations especially in asymmetric synthesis.3 Most imines are prepared by catalytic processes since these provide higher conversions and better reaction selectivity and significantly improve economic performance compared to noncatalytic systems. Among the available methods, the selective aerobic oxidation of amines is a promising approach, although it still remains a challenging task.4 Most of the effective systems are noble metal-based catalysts including Pd5 or Au.2,6 A considerable amount of effort has been put into replacing these expensive metals with cheaper compounds containing abundant metals like Cu,7 alas, not devoid of toxicity concerns. Satisfactory selectivity toward the cross-coupling products have so far been achieved for instance by highly active Fe8 and Pd5 complexes. Therefore, it is of high interest to replace metal-based systems by metal-free methodologies, particularly inspired by natural principles.4c,9 In a pioneering effort, Stahl et al. presented a biomimetic metalfree system using 4-tert-butyl-2-hydroxybenzoquinone (Scheme 1A), inspired by the copper amine oxidase cofactors (CuAOs).10 Later on, further bioinspired ortho-quinone catalysts were successfully applied in amine oxidations.11 Further examples of CuAOs mimicry for this reaction have been developed by the group of Largeron (Scheme 1A), all following an ionic transamination mechanism.12 In an elegant approach, Carbery and co-workers developed a flavin-organocatalyst cooperative system to mimic the activity of monoamine oxidase B, although it is only active for the homocoupling of benzylic amines (Scheme 1A).13 A radicalbased approach was described by Yan and co-workers, using © 2018 American Chemical Society

Received: September 11, 2018 Published: October 11, 2018 13481

DOI: 10.1021/acs.joc.8b02345 J. Org. Chem. 2018, 83, 13481−13490

Article

The Journal of Organic Chemistry

The DSPs (1) obtained by functionalization of 16-π-electron phenazine contain an antiaromatic 8-π-electron core. In order to obtain an active catalyst, 1 needs to be oxidized to its corresponding 15-π-electron radical cationic state (2). This was achieved by reaction with NOBF4 (Scheme 3). The formed NO is bubbled off with argon, while BF4− remains in the solution as the counterion to the obtained PhRC (2).

Scheme 1. Exemplary Metal-Free Systems for the Oxidative Coupling of Amines

Scheme 3. Oxidation of the DSP (1) with BF4NO To Produce the Catalytically Active PhRC (2)

Once the DSPs were oxidized with NOBF4 the resulting PhRCs were characterized by electron paramagnetic resonance spectroscopy (EPR). These measurements showed the coupling of the unpaired electron to its molecular environment as the hyperfine signal splitting presented exemplarily in Figure 1. All EPR spectra are presented in Figure S9. The

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RESULTS AND DISCUSSION Catalysts Synthesis and Characterization. A series of DSPs (1) bearing different functionalities were prepared in two steps (Scheme 2).25a,27 First, phenazine was reduced with Scheme 2. Synthesis of DSPs Precatalysts (1a−f) Figure 1. EPR spectra of the PhRC compounds 2b and 2f after the oxidation with NOBF4 in toluene at 300 K and ambient pressure. Simulation using EPRSim3229 of 2f with g = 2.0043, 2xAN = 6.5 G, ΔB = 4 G; simulation of 2b with g = 2.0042, 2xAN = 6.5 G, 6xAH = 6.3 G, ΔB = 6 G.

measurements were performed in toluene after 1 was oxidized at 300 K. All EPR signals appear around g = 2.00, which is typical for organic radicals.28 In the case of 2a, no hyperfine coupling was observed (Figure S8), indicating a complete delocalization of the radical in the 7 π-electron system without visible coupling to a particular element or region of the catalyst. Compared to phenazine radical 2a, the spectrum of the dimethyl substituted phenazine (2b, Figure 1) shows a distinct hyperfine splitting (hfs) to the two nitrogen as well as six hydrogen nuclei of the methyl groups. If an aromatic ring is added, e.g. 2c−f (Figure 1), only the hyperfine splitting to the two nitrogen ring of the pyrazine ring was examined. The equality of the hfs indicates that the substitution on the para position of the phenyl ring with electron-donating or -withdrawing groups plays no major role in the radical delocalization. In order to gain a deeper

sodium dithionite to 5,10-dihydrophenazine (1a), which was then functionalized through a Buchwald−Hartwig coupling reaction to obtain the DSPs (1b−f) in high yields (see Supporting Information for full experimental details). 1c, 1d, and 1e were characterized by single crystal X-ray diffraction (see Figures S3, S4, and S5 and Table S1). 13482


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The Journal of Organic Chemistry understanding of the nature of the radical in 2, DFT calculations were performed. Using the B3PW9130 density functional level of theory with the all-electron TZVP basis set31 (using the Gaussian 16 program package), the neutral DSPs (1a, 1b, and 1f), and their corresponding radical states, PhRCs (2a, 2b, and 2f) were computed and optimized. Their geometry, ionization potential, and spin density of the highest occupied molecular orbitals (HOMO) were analyzed (see Figures S6, S6, and S8, Tables S2 and S3). Exemplarily, the optimized structure of PhRC 2b is shown in Figure 2. From the calculations, various interesting

Figure 3. Concentration profiles of the oxidative coupling of benzylamine (3a) catalyzed by a PhRC. Reaction conditions: neat, 1 mol % of 2b, 100 °C, O2 (1 bar).

several orders of magnitude lower than that of the amine substrate.32 With the optimized reaction conditions in hand, the effect of the phenazine substitution on the catalytic performance was investigated. The PhRCs, which were synthesized before, were tested as catalysts (2a−f). As shown in the Table 1, all the phenazines were catalytically active, although with marked differences in their activities.

Figure 2. Optimized structure (A) as well as the highest occupied molecular orbital (B) of 5,10-dimethyl-5,10-dihydrophenazine radical 2b.

Table 1. Effect of the Functionalization of PhRCs on Its Catalytic Performance

features were observed: (1) Upon formation of the radical, the resulting 7 π-electron antiaromatic ring system does not become planar (as it does not follow the Hückel’s rule); (2) the torsion angle between the planes containing the aryl rings is much smaller in the radical than the neutral state, indicating a better overlapping of the orbitals; (3) the spin density is always distributed via a delocalization over the phenanzine ring; and (4) an increase of the electron density in the substituents results in a decrease of the ionization energy, fostering the stability of the radicals. Catalytic Studies. In order to explore the activity of the PhRCs (2) as a catalysts in the oxidative coupling of amines, several different reaction conditions were screened using benzyl amine (3a). Interestingly, the temperature screening showed that, at temperatures below 70 °C, the conversion of the imine dimer is negligible, while when the temperature was increased to 100 °C, the reaction efficiency rose to almost quantitative conversion (Figure S2A). Even with a low catalyst loading of 0.5 mol %, 85% conversion was achieved (Figure S2B). If air (instead of pure oxygen as terminal oxidant) was applied, still 56% conversion was achieved after 16 h (with 1 mol % 2b). If the reaction is carried out in the absence of an oxidant, no imine product is detected. In order to investigate the reaction kinetics, a continuous sampling method by GC was chosen. The results show a zeroorder kinetic dependence on the amine substrate (Figure 3). Considering our reaction setup, it can be expected that the reaction rate is limited by diffusion of the oxygen into the reaction mixture, being that the concentration of O2 was

Entry

Catalyst

Conversion (mol %)a

Selectivity (mol %)b

1 2 3 4 5 6 7 8c

2a 2b 2c 2d 2e 2f 1b

32 99 99 84 71 56 55 54

96 81 97 81 90 96 98 83

a

Conversions were determined by GC. bCalculated by GC as 4a/(4a + 5a) * 100. cNonradical phenazine (1b) was used as a catalyst.

The highest catalytic activity was observed for dihydro (2a) and dimethyl (2b) functional phenazines, with conversions reaching up to 99% (Table 1, entries 2 and 3). Although the phenazines functionalized with more sterically demanding aromatic substituents were catalytically active, these displayed only good to moderate conversions (2c−f, Table 1, entries 4 to 7). The ionization energy of 2c−f is lower than that of 2b, and the highest is found for 2a which results in an increased stability of the radical for the diaryl substituted PhRC. This can be correlated with the reactivity; lower radical stability causes higher catalytic activity. Substitution on the para position of 13483


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The Journal of Organic Chemistry the functional phenyl ring by electron-withdrawing (2e) or -donating groups (2d and 2f) seems to play only a minor role. These observations indicate that steric hindrance, hence, the accessibility of the radical as well as the stability of the radical, determines the catalyst’s performance. Interestingly, the formation of the amide side product (5a) could be observed; it results either from the oxidation of the benzylimine product (7)33 or, as Zhang et al. showed, as the result of oxidation of hemiaminal (see mechanistic proposal below).34 In terms of both activity and selectivity, catalyst 2b shows the best performance. Even in the case of the parent phenazine (1b), conversion of 54% was achieved (Table 1, entry 8), pointing out the necessity of the preactivation process with BF4NO (Scheme 3). To determine the versatility as tolerance toward functional groups, the scope of reaction was investigated. Having identified 2b as the best catalyst, a series of different amines bearing different functionalities were oxidized (Table 2). The

(−OCH3, −CH3, −H, −Cl, −F) was followed over time (Figures S10−S14). All kinetic constants are between 0.66 and 0.75 mol·L−1s−1, indicating that the reaction rate is nearly independent of the electronic nature of the substituent. This further confirms the assumption that the reaction rate might not be limited by the oxidation of the amine catalyzed by the phenazine radical cation, but by the diffusion of the oxygen in to the reaction mixture. Nevertheless, the amine oxidation is sensitive to sterically demanding substrates, as in the case of (2,6-dimethylphenyl)methanamine (Table 2, entries 3 and 4), for which the reaction time was extended to 24 h in order to obtain nearly quantitative conversions. Excellent conversions and selectivity achieved by homocoupling of benzylamines suggested that cross-coupling of different amines might be possible. The combination of benzyl amine with less readily oxidizable amines showed moderate selectivity toward the cross-coupling product (for cyclohexylamine, 51%; Table 3, entry 1). The low reaction rate might be

Table 2. Substrate Screening for the Oxidative HomoCoupling of Amines Catalyzed by 2b

Table 3. Substrate Screening for the Oxidative CrossCoupling of Amines Catalyzed by 2b

a

Conversions were determined by GC. bCalculated by GC as 7/(total products) * 100. c1 bar of O2.

a

Conversions were determined by GC. bCalculated by GC as 4/(4 + 5) * 100. cThe reaction time was extended to 24 h.

to blame for the poor reaction selectivity. Therefore, the oxygen pressure was increased 10-fold in order to increase the reaction rate and so shift the reaction selectivity toward the desired product. By doing so, the reaction selectivity increased up to 90% of the desired cross-coupling product with quantitative conversion (Table 3, entry 2). In all cases, the prevailing side product is the benzylamine homocoupling product which is clearly caused by the high nucleophilicity of benzylamine. Lower conversions were observed upon increasing bulkiness on the substrates, obtaining for adamantylamine

reaction of benzylamines with either electron-donating (Table 2, entries 2−5) or -withdrawing groups (Table 2, entries 6−9) afforded almost quantitative conversions and excellent selectivity in most cases. This indicates that the electronics of the benzyl ring does not have a major impact on reaction performance. This was verified by kinetic investigations, where the concentration of the para-substituted benzylamines 13484


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partial recovery of the nonradical phenazine 1c. Afterwards, 1c can be reoxidized (by oxygen) closing the catalytic cycle. To further confirm the nature of the phenazine redox ability to transfer the charge from the pyrazine core to benzylamine, the process was followed by UV−vis spectroscopy, similarly as presented above (Figure 5). 1a shows a strong absorption peak

and 1-naphylamine 65% and 51%, respectively (Table 3, entries 3 and 7). Long chain aliphatic amines, namely hexylamine and decylamine (entries 4 and 5), gave good selectivity, although the conversions were rather low, perhaps due to their low miscibility with benzylamine. Mechanistic Investigations. In order to explore the origin of the catalytic activity of PhRCs, we investigated the amine oxidative coupling reaction (Scheme 4) using UV−vis and fluorescence spectroscopy and EPR. Scheme 4. General Scheme of Phenazine (1) Oxidation with NOBF4 to Its Radical Cation Form (2) and Subsequent Single Electron Reduction with Benzylamine (3)

Figure 5. UV−vis spectrum of the phenazine 1a, 2a, and 2a after the addition of benzylamine 3a measured in acetonitrile.

in the range 250 to 400 nm, primarily due to π−π* transitions of the phenyl rings around the pyrazine core.38 After the addition of NOBF4 as the oxidizing agent, the main adsorption band broadens to the range 250 to 480 nm, while a new band appears in the range 500 to 750 nm (strong change of color from colorless to green). The appearance of a new band in the visible region can be attributed to the formation of the phenazine radical cation (2a) and its 7π-electron delocalized pyrazine core. Interestingly, the addition of 1 equiv of benzylamine is again able to completely reverse the process back to the initial state, which is shown by the complete disappearance of the absorption bands of 2a. This result confirms that rapid recovery of the initial catalyst state takes place, probably via single electron transfer (SET), accompanied by the formation of the charged bezylamine species (see proposed mechanism in Scheme 5). In order to understand the nature of the open-shell species formed upon SET, EPR experiments were performed (Figure 6). As described above, PhRCs give two different hyperfine splitting patterns, which depend on the substitution at the 5and 10-position. By the addition of benzylamine (3a) to the PhRCs (exemplary 2b and 2f) during the EPR measurement, the hyperfine splitting lost its intensity, while at the same time the number of lines increased to 13 or more. The complex hfs pattern could not be simulated; however, the pattern of hfs was the same, regardless of the phenazine substitution. Thus, it is reasonable to assume, that the same benzylamine radical species is formed. Fukuzumi et al. showed that the radical formed is of the benzyl type (PhCH•NH2) instead of being nitrogen-centered (PhCH2NH•).39 They also proved that the benzylamine radical may be detected by EPR spectroscopy, although it slowly decays due to its instability. Therefore, it can be assumed that the obtained hyperfine splitting belongs to the benzyl type radical. The hyperfine splitting presented in Figure 6 after the addition of benzylamine shows several shoulders, which indicates that more than one species is present in the solution. One of these species is most probably the phenazine

The highly electron-rich phenazine derivatives with their πconjugated pyrazine core are known to have distinctive optical properties: many phenazine derivatives are used as fluorescent tracers or dyes in medicinal35 and electronic applications.36 A strong fluorescence was detected in the case of diphenyl substituted phenazine (1c). The emission spectrum of 1c in acetonitrile showed a strong band in the range 450−600 nm, which disappeared almost completely upon oxidation with NOBF4 (Figure 4). This fluorescence quenching was reported before by Sokołowska et al.37 However, after the addition of 1 equiv of benzylamine to the mixture, reappearance of the fluorescence band resulted in exactly the same position as before the oxidation. The recovery of the fluorescence indicates that a single electron transfer process from 2c to benzylamine takes place which results in

Figure 4. Fluorescence spectrum of the phenazine 1c, 2c, and 2c after the addition of benzylamine measured in acetonitrile at standard conditions. 13485


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slow, as indicated by the kinetic investigations. Hence, the use of preoxidized phenazine (by NOBF4) results in the improvement in reaction performance (Table 2 entry 3 compared to entry 8). In the second step of the catalytic cycle, a single electron transfer with the amine substrate 3 takes place, regenerating the nonradical phenazine 1. The overall process is assisted by the superoxide anion which enables the proton abstraction from 3 to produce H2O2 and benzylimine (4) as proposed by Loh and others.10,15,18,39 The formed hydrogen peroxide is also known to promote the imine formation to form amine derivatives.40 The imine intermediate 4, which was detected by GC/MS analysis,11b can then undertake two transformation pathways. The primary pathway involves the condensation with amine 6 to form the main product 7 and ammonia. The secondary reaction is the hydrolysis of 4 to form benzaldehyde.41 The coupling of benzaldehyde and another equivalent of 6 results in the formation of hemiaminal (8), which can be either dehydrated to yield the imine (7) or oxidized to form the amide side product (5).34 The latter (5) is usually detected in only 1−5 mol % of the overall product.

Scheme 5. Proposed Reaction Mechanism for Phenazine Catalyzed Coupling of Benzylamines to Imine Dimers in the Presence of Atmospheric Oxygen

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CONCLUSION In summary, we presented for the first time the use of phenazine radical cations (PhRCs) as metal-free catalysts for the aerobic oxidative homo- and cross-coupling of amines. The results show that the catalyst is not sensitive to the electronic nature of the benzylamine substrate; therefore, high reaction conversions and selectivity could be obtained in every case. In addition to the formation of symmetric imines, we showed that selective amine cross-coupling is also accessible. In that case it was necessary to increase the pressure of oxygen to 10 bar. Mechanistic investigations using spectroscopic techniques revealed that initially a single electron transfer from the PhRC to the amine substrate takes place, followed by the reoxidation of the phenazine by oxygen. The novel catalytic system presented in this work follows the trend of replacement of metal-based catalysts both in academia and industry, as the need exists for the introduction of environmentally friendlier catalysts for chemical transformations. Because no solvents, harmful reagents, or metals are used for the oxidative coupling of amines, this process employing PhRC as a catalyst could potentially replace commonly known metal-based processes in the future. Efforts toward increasing the availability of oxygen in the reaction mixture, including safer use of this (potentially dangerous) gas, are currently the subject of active research in our laboratories.

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Figure 6. EPR spectra of 2f and 2b before (red) and after (blue) the addition of benzylamine (3a) in toluene at 300 K.

EXPERIMENTAL SECTION

General Information. All reactions were performed under Schlenk, air-/moisture-free conditions. All solvents were dried, degassed, and stored in septum-sealed flasks over molecular sieves under an argon atmosphere. Solvents for catalyst preparation were distilled over Na, CaH2, or molecular sieves following standardized procedures. All chemicals were purchased from commercial sources (unless stated otherwise). Most of the chemicals were used without further purification. The purity of purchased products was confirmed by either NMR spectroscopy or GC/MS analysis. All NMR spectra were recorded in CDCl3 or toluene-D8 using a Bruker Avance 300 spectrometer with a QNP probe head (1H: 300 MHz, 13C: 75 MHz) or a Bruker Avance 400 spectrometer (1H, 400 MHz; 13C, 100 MHz). Calibrations of the spectra were carried out referencing residual solvent shifts and were reported as parts per million (ppm) relative to SiMe4. Multiplicities are abbreviated as follows: singlet (s), doublet (d), triplet (t), quartet (q), doublet−doublet (dd), doublet-triplet

radical cation, which was not completely reduced to its closedshell form 1b or 1c. Unfortunately, attempts to simulate these spectra were unsuccessful; hence, the composition of this radical mixture remains unclear. Based on the presented fluorescence, UV−vis and EPR measurements of phenazine catalyzed amine oxidation, and several control experiments, a general reaction pathway is proposed (Scheme 5). In the initial stage of the catalytic cycle, the phenazine 1 is oxidized, by molecular oxygen dissolved on the reaction mixture, producing the corresponding PhRC 2 and a superoxide anion (O2•−). The concentration of oxygen in the reaction mixture is low due to the high temperature (100 °C) and low pressure (1 atm); therefore, the reoxidation is 13486


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The Journal of Organic Chemistry (dt), quintet (quint), sextet (sext), septet (sept), multiplet (m), and broad (br). Samples were generally measured at room temperature (297 K). Deuterated solvents were degassed and distilled if necessary. ESI-TOF spectra were obtained on Waters Q-TOF micro mass spectrometer. The samples were dissolved in a methanol−water (9:1) mixture with 0.1% HCOOH to achieve sufficient ionization. GC analysis was performed on a Hewlett-Packard 6890 Series. Samples were dissolved in tetrahydrofuran (THF) and sampled into the GC column. GC/MS analysis was performed on an Agilent 6890/5973 GC-MS or Agilent 7890/5977 GC-MS. Samples were dissolved in tetrahydrofuran (THF). The 5,10-dihydrophenazine (1a) was prepared following the procedure reported by Zheng et al.27c The aromatic functionalized phenazines (1c−f) were prepared via Buchwald−Hartwig coupling following the procedure reported by Theriot et al.25a The active radical cation catalysts (2) were obtained by oxidation via addition of a stoichiometric amount of nitrosonium tetrafluoroborate in acetonitrile at room temperature. Catalyst Preparation. 5,10-Dihydrophenazine (1a). Phenazine (1g, 5.5 mmol) was dissolved in 20 mL of boiling ethanol in a 500 mL three-neck flask. During 1 h, 12.3 g (70.6 mmol) of sodium dithionite dissolved in 100 mL of water were added with a dropping funnel to the phenanzine solution. Once the sodium dithionite was completely added, the mixture was boiled for 2 h and stirred at room temperature for a further 15 h overnight. The resulting light green precipitate was filtered off and washed three times with 200 mL of water. Afterward, the light green solid was dried for 24 h under vacuum at room temperature. The title product was obtained as a light green solid (Yield: 0.89 g, 5.3 mmol, 96%). 1H NMR (300 MHz, DMSO-d6) δ 7.2 (s, 2H), 6.3−6.2 (m, 4H), 6.0−5.9 (m, 4H); 13C{1H} NMR (75 MHz, DMSO-d6) δ 134.2, 120.7, 111.8 5,10-Diphenyl-5,10-dihydrophenazine (1c). In a 250 mL Schlenk flask, 1 g (5.4 mmol) of 5,10-dihydrophenazine (1a) was mixed with 2.11 g (21.9 mmol) of NaOtBu, 103 mg (0.2 mmol) of RuPhos, and 178 mg (0.2 mmol) of RuPhos palladacycle precatalyst dissolved in 8 mL of 1,4-dioxane. Upon complete dissolution, 4.5 g (22 mmol) of 4iodobenzene were added. After stirring for 12 h at 110 °C, the mixture was cooled down to room temperature and 200 mL of CH2Cl2 were added. The mixture was washed three times with 200 mL of H2O and dried over MgSO4. After filtration the mixture was adsorbed on silica and purified by column flash chromatography with a solvent mixture of n-hexane and ethyl acetate (10:1). The title product was obtained as a deep green crystalline solid (Yield: 0,46 g, 1.3 mmol, 25%). Single crystals of X-ray quality were grown from a mixture of CH2Cl2 and nhexene (1:2) and after 4 days at room temperature. 1H NMR (300 MHz, Benzene-d6) δ 7.0 (m, J = 4.2 Hz, 2H), 6.3 (m, J = 6.4 Hz, 4H), 5.9−5.7 (m, 4H); 13C{1H} NMR (75 MHz, C6D6) δ 140.8, 137.2, 131.4, 128.4, 128.1, 127.7, 113.1, 30.2. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C24H18N2 334.1465; found 334.1461. 5,10-Di(4-toluoyl)-5,10-dihydrophenazine (1d). Following the same procedure as that for 1c, using 0.5 g (2.7 mmol) of 5,10dihydrophenazine (1a), 1.08 g (11.2 mmol) of NaOtBu, 48 mg (0.1 mmol) of RuPhos, 93 mg (0.1 mmol) of RuPhos palladacycle precatalyst, and 2.4 g (11 mmol) of 4-iodotoluene. The title compound was obtained as a green crystalline solid (Yield: 0.46 g, 1.3 mmol, 45%). Single crystals of X-ray quality were obtained as follows: to a supersaturated solution of purified 1d in benzene, diethyl ether was added until the solid was completely dissolved. The solvents were allowed to evaporate through a thin cannula until crystals appeared. 1H NMR (300 MHz, Benzene-d6): δ 7.1 (m, 4H), 7.0 (m, 4H), 6.3 (m, 4H), 5.9 (m, 4H), 2.1 (s, 1H); 13C{1H} NMR (75 MHz, Benzene-d6): δ 138.2, 137.8, 137.4, 132.2, 131.4, 121.4, 113.1, 21.1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C26H21N2 362.1778; found 362.1778. 5,10-Di(4-trifluoromethylphenyl)-5,10-dihydrophenazine (1e). Following the same procedure as that for 1c, using 0.5 g (2.7 mmol) of 5,10-dihydrophenazine (1a), 1.14 g (11.9 mmol) of NaOtBu, 50 mg (0.1 mmol) of RuPhos, 87 mg (0.1 mmol) of RuPhos palladacycle precatalyst, and 2.5 g (11.1 mmol) of 4-bromobenzotrifluoride. The title compound was obtained as a yellow-green

crystalline solid (Yield: 0.95 g, 2 mmol, 73%). Single crystals of X-ray quality were obtained as follows: to a supersaturated solution of purified 1e in benzene, diethyl ether was added until the solid was completely dissolved. The solvents were allowed to evaporate through a thin cannula until crystals appeared. 1H NMR: (300 MHz, Benzened6): δ 7.3 (q, J = 7.1 Hz, 4H), 7.0 (s, 4H), 6.3 (s, 4H), 5.7 (s, 4H); 19F NMR (282 MHz, Benzene-d6): δ −62.2. 5,10-Di(4-methoxyphenyl)-5,10-dihydrophenazine (1f). Following the same procedure as that for 1c, using 0.5 g (2.7 mmol) of 5,10dihydrophenazine (1a), 0.92 g (8.2 mmol) of KOtBu, 24 mg (0.1 mmol) of Pd(OAc)2, 17 mg (0.084 mmol) of P(tBu)3, and 1.21 g (5.2 mmol) of 4-iodoanisole, product was purified by flash chromatography column with a solvent mixture of n-hexane and ethyl acetate (20:1). The title compound was obtained as a yellow crystalline solid (Yield: 0.89 g, 2.5 mmol, 82%). 1H NMR (300 MHz, Chloroform-d): δ 7.9−7.7 (m, 3H), 7.4−7.3 (m, 4H), 7.1−7.0 (m, 4H), 6.2−6.1 (m, 4H), 3.3 (s, 6H); 13C{1H} NMR (75 MHz, Chloroform-d): δ 143.7, 138.3, 130.7, 129.8, 127.9, 116.5, 114.4, 55.5; HRMS (ESI-TOF) m/ z: [M + H]+ calcd for C26H22N2O2 394.1676; found 394.1675. Oxidative Coupling of Amines. General Procedure. In a Schlenk tube, 5 mmol of substrate were added to 0.05 mmol of catalyst (1 mol %) with a syringe. The solution was flushed tree times with oxygen. A balloon with oxygen was attached to the Schlenk valve. The reaction mixture was stirred for 15 h at 100 °C in an oil bath. The experiments at a 10 bar of O2 were carried out in a 12 mL glass vial placed inside a Parr stainless steel autoclave. This autoclave was flushed tree times with oxygen with a pressure of 10 bar. The reaction mixture was stirred for 15 h at 100 °C in an oil bath. An optimization of the reaction parameters (temperature and catalyst concentration) was carried out, and the optimal values (Figure S1) were used in all the experiments, unless otherwise noted. Gas Chromatography. GC analysis was performed on a HewlettPackard 6890 Series. Samples were dissolved in tetrahydrofuran (THF) and sampled into the GC column. The chromatogram peak area ratios of the product and internal standard anisole (Ap/As) were used for calibration. The GC yields are calculated with the calibration curve with anisole as an internal standard. (E)-N-Benzyl-1-phenylmethanimine (4a).42 Following the general procedure, using 5,10-dimethyl-5,10-dihydrophenazine (10.5 mg, 0.005 mmol) and phenylmethanamine (500 mg, 4.7 mmol). The pure product was obtained by flash column chromatography on silica gel using a mixture of pentane and ethyl acetate (10:1). The desired product was a yellowish oil (444 mg; 2.2 mmol) 97%. 1H NMR (300 MHz, Chloroform-d) δ 8.3 (s, 1H), 7.8−7.6 (m, 4H), 7.4−7.0 (m, 16H), 4.7 (d, J = 1.5 Hz, 4H); 13C{1H} NMR (75 MHz, CDCl3) δ 162.2, 139.5, 136.3, 131.0, 128.8, 128.7, 128.5, 128.2, 127.2, 65.2. (E)-N-(4-Methylbenzyl)-1-(p-tolyl)methanimine (4b).43 Following the general procedure, using 5,10-dimethyl-5,10-dihydrophenazine (10.5 mg, 0.005 mmol) and p-tolylmethanamine (600 mg, 4.9 mmol). The pure product was obtained by flash column chromatography on silica gel using a mixture of pentane and ethyl acetate (10:1). The desired product was a yellowish oil (519 mg; 2.3 mmol) 95%. 1H NMR (300 MHz, Chloroform-d) δ 8.5 (s, 1H), 7.8 (d, J = 8.1 Hz, 2H), 7.4−7.3 (m, 8H), 4.9 (s, 1H), 2.5 (s, 4H), 2.5 (s, 4H); 13C{1H} NMR (75 MHz, CDCl3) δ 161.8, 141.1, 136.5, 133.8, 129.5, 128.4, 64.9, 21.7, 21.3. (E)-N-(2,6-Dimethylbenzyl)-1-(2,6-dimethylphenyl)methanimine (4c).22 Following the general procedure, using 5,10-dimethyl-5,10dihydrophenazine (10.5 mg, 0.005 mmol) and (2,6-dimethylphenyl)methanamine (700 mg, 5.1 mmol). The pure product was obtained by flash column chromatography on silica gel using a mixture of pentane and ethyl acetate (10:1). The desired product was a yellowish oil (307 mg; 1.2 mmol) 48%. 1H NMR (300 MHz, Chloroform-d) δ 8.4 (d, J = 1.8 Hz, 0H), 7.1−6.9 (m, 1H), 4.8 (d, J = 1.8 Hz, 0H), 2.3 (s, 1H), 2.3 (s, 1H); 13C{1H} NMR (75 MHz, CDCl3) δ 160.9, 137.5, 137.1, 134.8, 134.5, 128.7, 128.4, 128.3, 127.4, 58.9, 20.5, 19.9. (E)-N-(4-Methoxybenzyl)-1-(4-methoxyphenyl)methanimine (4d).44 Following the general procedure, using 5,10-dimethyl-5,10dihydrophenazine (10.5 mg, 0.005 mmol) and (4-methoxyphenyl)methanamine (700 mg, 5.1 mmol). The pure product was obtained by 13487


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The Journal of Organic Chemistry flash column chromatography on silica gel using a mixture of pentane and ethyl acetate (10:1). The desired product was a yellowish oil (638 mg; 2.5 mmol) 98%. 1H NMR (300 MHz, Chloroform-d) δ 8.3 (s,1H), 7.7 (d, 2H, J = 8.3 Hz), 7.2 (d, 2H, J = 8.2 Hz), 6.8−7.1 (m, 4H), 4.7 (s, 2H) 3.8 (s, 3H), 3.8 (s, 2H); 13C{1H} NMR (75 MHz, CDCl3) δ 161.9, 161.1, 158.4, 131.1 129.3, 129.1, 114.1, 113.9, 64.3, 55.1, 54. 1 (E)-N-(4-Chlorobenzyl)-1-(4-chlorophenyl)methanimine (4e).44 Following the general procedure, using 5,10-dimethyl-5,10-dihydrophenazine (10.5 mg, 0.005 mmol) and (4-chlorophenyl)methanamine (700 mg, 4.9 mmol). The pure product was obtained by flash column chromatography on silica gel using a mixture from pentane and ethyl acetate (10:1). The desired product was yellowish oil (625 mg; 2:4 mmol) 97%. 1H NMR (300 MHz, Chloroform-d) δ 8.1 (s, 1H), 7.5 (d, J = 8.5 Hz, 3H), 7.2 (d, J = 8.5 Hz, 3H), 7.1 (q, J = 8.6 Hz, 6H), 4.6 (s, 0H); 13C{1H} NMR (75 MHz, CDCl3) δ 160.9, 137.7, 136.9, 134.5, 132.8, 129.6, 129.4, 129.0, 128.7, 64.1. (E)-N-(4-Fluorobenzyl)-1-(4-fluorophenyl)methanimine (4f).44 Following the general procedure, using 5,10-dimethyl-5,10-dihydrophenazine (10.5 mg, 0.005 mmol) and (4-fluorophenyl)methanamine (600 mg, 4.7 mmol). The pure product was obtained by flash column chromatography on silica gel using a mixture of pentane and ethyl acetate (10:1). The desired product was a yellowish oil (500 mg; 2.1 mmol) 92%. 1H NMR (300 MHz, Chloroform-d) δ 8.4 (s, 1H), 7.8 (m, 2H), 7.3 (m, 2H), 7.1 (d, 2H), 4.8 (s, 1H); 13C{1H} NMR (75 MHz, CDCl3) δ 160.6, 130.3, 130.2, 129.6, 129.5, 115.9, 115.6, 115. 5, 115.2, 64.1. (E)-1-(Pyridin-3-yl)-N-(pyridin-3-ylmethyl)methanimine (4g).22 Following the general procedure, using 5,10-dimethyl-5,10-dihydrophenazine (10.5 mg, 0.005 mmol) and pyridin-3-ylmethanamine (550 mg, 5 mmol). The pure product was obtained by flash column chromatography on silica gel using a mixture of pentane and ethyl acetate (10:1). The desired product was a yellowish oil (483 mg; 2.5 mmol) 98%. 1H NMR (300 MHz, Chloroform-d) δ 8.8 (s, 1H), 8.5− 8.4 (m, 1H), 8.4 (m, 1H), 8.3 (s, 0H), 8.0 (dt, J = 7.9, 2.0 Hz, 1H), 7.5 (dt, J = 7.8, 1.9 Hz, 0H), 7.2 (m, 1H), 4.7 (s, 1H); 13C{1H} NMR (75 MHz, CDCl3) δ 159.7, 151.5, 150.0, 149.0, 148.3, 135.5, 134.6, 134.4, 131.2, 123.6, 123.4, 62.2. (E)-1-(Thiophen-2-yl)-N-(thiophen-2-ylmethyl)methanimine (4h).44 Following the general procedure, using 5,10-dimethyl-5,10dihydrophenazine (10.5 mg, 0.005 mmol) and thiophen-2-ylmethanamine (600 mg, 5.3 mmol). The pure product was obtained by flash column chromatography on silica gel using a mixture of pentane and ethyl acetate (10:1). The desired product was a yellowish oil (538 mg; 2.6 mmol) 98%. 1H NMR (300 MHz, Chloroform-d) δ 8.2 (d, J = 1.2 Hz, 1H), 7.2 (m, 1H), 7.1 (dd, J = 3.7, 1.2 Hz, 1H), 7.0 (m, 1H), 7.0 (m, 1H), 6.9 (m, 1H), 6.8 (m, 1H), 4.7 (s, 1H); 13C{1H} NMR (75 MHz, CDCl3) δ 155.2, 151.0, 147.6, 141.8, 141.4, 148.3, 135.0, 131.0, 127.2, 126.5, 126.4, 124.6, 123.4, 58.3. (E)-N-Cyclohexyl-1-phenylmethanimine (7a).45 Following the general procedure, using 5,10-dimethyl-5,10-dihydrophenazine (5.25 mg, 0.0025 mmol), phenylmethanamine (270 mg, 2.5 mmol), and cyclohexanamine (240 mg, 2.5 mmol). The pure product was obtained by flash column chromatography on silica gel using a mixture of pentane and ethyl acetate (10:1). The desired product was a yellowish oil (926 mg; 49 mmol) 99%. The GC/MS (electronic ionization) spectra match the desired product (see Supporting Information). (E)-N-(Adamantan-1-yl)-1-phenylmethanimine (7b).46 Following the general procedure, using 5,10-dimethyl-5,10-dihydrophenazine (5.25 mg, 0.0025 mmol), phenylmethanamine (270 mg, 2.5 mmol), and adamantan-1-amine (380 mg, 2.5 mmol). The pure product was obtained by flash column chromatography on silica gel using a mixture of pentane and ethyl acetate (10:1). The desired product was a yellowish oil (29 mg; 3.3 mmol) 65%. The GC/MS (electronic ionization) spectra match the desired product (see Supporting Information). (E)-N-Hexyl-1-phenylmethanimine (7c).45 Following the general procedure, using 5,10-dimethyl-5,10-dihydrophenazine (5.25 mg, 0.0025 mmol), hexan-1-amine (250 mg, 2.5 mmol), and cyclohexan-

amine (240 mg, 2.5 mmol). The pure product was obtained by flash column chromatography on silica gel using a mixture of pentane and ethyl acetate (10:1). The desired product was a yellowish oil (671 mg; 3.5 mmol) 71%. The GC/MS (electronic ionization) spectra match the desired product (see Supporting Information). (E)-N-Decyl-1-phenylmethanimine (7d): 47 Following the general procedure, using 5,10-dimethyl-5,10-dihydrophenazine (5.25 mg, 0.0025 mmol), phenylmethanamine (270 mg, 2.5 mmol), and decan-1-amine (390 mg, 2.5 mmol). The pure product was obtained by flash column chromatography on silica gel using a mixture of pentane and ethyl acetate (10:1). The desired product was a yellowish oil (613 mg; 2.5 mmol) 50%. The GC/MS (electronic ionization) spectra match the desired product (see Supporting Information). (E)-N,1-Diphenylmethanimine (7e)..47,48 Following the general procedure, using 5,10-dimethyl-5,10-dihydrophenazine (5.25 mg, 0.0025 mmol), phenylmethanamine (270 mg, 2.5 mmol). and aniline (230 mg, 2.5 mmol). The pure product was obtained by flash column chromatography on silica gel using a mixture of pentane and ethyl acetate (10:1). The desired product was a yellowish oil (688 mg; 3.8 mmol) 76%. The obtained spectra match those previously reported. 1 H NMR (300 MHz, Chloroform-d) δ 8.47 (s, 1H), 8.0−7.9 (m, 2H), 7.51−7.47 (m, 3H), 7.4−7.4 (m, 2H), 7.3−7.2 (m, 4H); 13C{1H} NMR (75 MHz, CDCl3) δ 160.6, 152.2, 136.35, 131.5, 129.3, 129.0, 128.9, 126.1, 123.0, 121.0. (E)-N-(Naphthalen-1-yl)-1-phenylmethanimine (7f)..47,49 Following the general procedure, using 5,10-dimethyl-5,10-dihydrophenazine (5.25 mg, 0.0025 mmol), phenylmethanamine (270 mg, 2.5 mmol), and naphthalen-1-amine (360 mg, 2.5 mmol). The pure product was obtained by flash column chromatography on silica gel using a mixture of pentane and ethyl acetate (10:1). The desired product was a yellowish oil (577 mg; 2.5 mmol) 51%. Obtained spectra match those previously reported. 1H NMR (300 MHz, Chloroform-d) δ 8.8 (s, 1H), 8.7−8.6 (m, 1H), 8.31−8.24 (m, 2H), 8.14−8.09 (m, 1H), 7.98 (dtd, J = 8.3, 1.0, 0.4 Hz, 1H), 7.80−7.7 (m, 6H), 7.31 (dd, J = 7.3, 1.1 Hz, 1H); 13C{1H} NMR (75 MHz, CDCl3) δ 160.4, 149.4, 136.5, 134.0, 131.5, 129.1, 128.9, 127.7, 126.5, 126.2, 125.9, 125.8, 124.1, 112.8.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b02345. Crystallographic data for 1c (CIF) Crystallographic data for 1d (CIF) Crystallographic data for 1e (CIF) Complete description of the experimental procedures of catalysts preparation and catalysis; characterization data of the catalysts and reaction products including 1H and 13 C NMR spectra, GC-MS analysis, single crystal X-ray diffraction, computational (DFT) details; mechanistic investigations including descriptions of the kinetic experiments, UV−vis and fluorescence spectra, and EPR measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (E.M.). ORCID

Haijun Jiao: 0000-0002-2947-5937 Esteban Mejía: 0000-0002-4774-6884 Present Address §

National Institute of Chemistry, Hajdrijova 19, 1001 Ljubljana, Slovenia.

Notes

The authors declare no competing financial interest. 13488


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ACKNOWLEDGMENTS We would like to thank Dr. Anke Spannenberg for the X-ray diffraction measurements. The financial support from Henkel AG (R.B.) is gratefully acknowledged.

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REFERENCES

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