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Article

Transition-Metal-Catalyzed Diarylation of Isocyanides with Triarylbismuthines for the Selective Synthesis of Imine Derivatives

by
Shintaro Kodama
1,
Yuki Yamamoto
1,
Yohsuke Kobiki
1,
Hitomi Matsubara
1,
Cong Chi Tran
1,
Shin-ichi Kawaguchi
2,*,
Akihiro Nomoto
1 and
Akiya Ogawa
1,*
1
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan
2
Center for Education and Research in Agricultural Innovation, Faculty of Agriculture, Saga University, 152-1 Shonan-cho, Karatsu, Saga 847-0021, Japan
*
Authors to whom correspondence should be addressed.
Materials 2021, 14(15), 4271; https://0-doi-org.brum.beds.ac.uk/10.3390/ma14154271
Submission received: 1 July 2021 / Revised: 22 July 2021 / Accepted: 28 July 2021 / Published: 30 July 2021
(This article belongs to the Special Issue Research of Organic Chemical Molecules and Materials for Biological Applications)
Browse Figures
Versions Notes

Abstract

:
The transition-metal-catalyzed diarylation of isocyanides with triarylbismuthines was investigated in detail, and rhodium catalysts such as [RhCl(nbd)]2 were found to selectively afford N-alkyl diaryl ketimines. On the other hand, palladium-catalyzed diarylation proceeded with the incorporation of two molecules of isocyanide, preferentially yielding N,N’-dialkyl or N,N’-diaryl α-diimines. In addition, a cascade synthesis of 2,3-diarylquinoxalines starting from the palladium-catalyzed diarylation of isocyanides with triarylbismuthines was successfully achieved.

1. Introduction

As the chemistry of heteroatom-containing compounds has significantly grown in recent decades, the properties and reactivities of high-period elements have gradually attracted more attention [1,2,3,4,5,6,7,8,9,10,11,12]. Bismuth is the heaviest of the group 15 elements, and its organic and inorganic compounds are regarded to be nontoxic [13]. However, organobismuth compounds are generally unstable due to the weakness of the carbon–bismuth bond. An exception is triarylbismuthines (BiAr3, 1), which are stable and some of them are commercially available. Therefore, synthetic applications of triarylbismuthines [14,15,16] have been investigated by many organic chemists [17,18,19,20,21,22,23,24,25] and N-, O-, S- and C-arylation reactions have been developed using triarylbismuthines as arylating reagents.
Recently, we developed novel palladium-catalyzed diarylation reactions of isocyanides 2 [26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41] using triarylbismuthines 1 and tetraarylleads 4 (Scheme 1). The use of BiAr3 1 result in α-diimines 3 being selectively obtained (Scheme 1a) [42]. Using PbAr4 4 instead of 1 led to the formation of α-diimines 3 and/or ketimines 5 (Scheme 1b) [43]. With aliphatic isocyanides 2 (R = alkyl), N-alkyl diaryl ketimines 5 were preferentially obtained, whereas N,N’-diaryl α-diimines 3 were formed when electron-rich aromatic isocyanides 2 (R = electron-rich Ar) were used.
However, it is unclear what factors would affect the product selectivity of α-diimines 3 and/or ketimines 5 using BiAr3. Hence, we investigated the transition-metal-catalyzed diarylation of isocyanides with BiAr3 1 under several reaction conditions for the selective synthesis of imine derivatives (3 or 5) (Scheme 1c).

2. Results and Discussion

In our previous paper [42], we reported that Pd(OAc)2-catalyzed diarylation of isocyanides 2 with BiAr3 1 selectively afforded α-diimines 3 (a representative result is shown in Table 1, entry 1). When the catalyst was changed to Pd(PPh3)4, a typical zero-valent palladium complex, the yield of 3aa decreased significantly (entry 2). On the other hand, other divalent palladium complexes such as PdCl2 and Pd(PPh3)2Cl2 selectively afforded 3aa with good yields (entries 3 and 4). Addition of PPh3 to Pd(OAc)2 resulted in lower yield of 3aa (entry 5). The zero-valent Pd complex, Pd2(dba)3, gave 3aa in moderate yield (entry 6). In the absence of a catalyst, barely any diarylation occurred (entry 7).
The reaction conditions for the Pd(OAc)2-catalyzed diarylation of 2a with 1a were also investigated in more detail (entries 8–18). In all cases, α-diimine 3aa was obtained as the major product, mostly along with very small amounts of ketimine 5aa. Reducing the loading of 1a resulted in a decrease in the yield of 3aa (entry 8). The presence of air did not inhibit the formation of 3aa (entry 9). When the reaction was conducted at room temperature, the yield of 3aa decreased (entry 10). Decreasing the amount of Pd(OAc)2 resulted in a gradual decrease in the yield of 3aa (entries 11 and 12). Among the solvents examined (entries 13–16), acetonitrile gave the best result, obtaining 3aa in 84% yield (entry 15). The present diarylation also proceeded even in a shorter time (4 h), affording 3aa in 81% yield (entry 17). A similar result was also obtained under Ar atmosphere as under air (entry 18 vs. 9).
Surprisingly, changing the catalyst to rhodium complexes selectively afforded ketimine 5aa without formation of 3aa (Table 2, entries 1–4). For example, the diarylation using [RhCl(nbd)]2 exclusively afforded 5aa in 50% yield (entry 1). Reducing the loading of this catalyst resulted in a lower yield of 5aa (entry 2). In addition, use of excess 1a improved the yield of 5aa with this catalyst (entry 3). RhH(CO)(PPh3)3, which is an active catalyst for hydroformylation, was ineffective when used for the present diarylation of isocyanide 2a (entry 4).
Since several rhodium complexes exhibited good ketimine selectivity, we next investigated the rhodium-catalyzed diarylation of tert-butyl isocyanide 2a with triphenylbismuthine 1a. The Rh-catalyzed diarylation was performed on a 0.4 mmol scale (1a and 2a) using [RhCl(nbd)]2, and the desired ketimine 5aa was obtained in 51% yield by adding (p-MeO-C6H4)3P as the ligand (entry 5). The diarylation using 1.0 mL of C6H6 improved the yield of 5aa to 71% yield (entry 7). Use of 0.6 mmol of 1a resulted in a slightly lower yield of 5aa (entry 9 vs. 7). Some other rhodium catalysts, such as RhCl(PPh3)3, RhH(PPh3)3, RhBr(PPh3)3, [Rh(dppp)(cod)]+BF4−, RhCl3, and [Rh(OAc)2]2, were ineffective for the desired diarylation (entries 10, 11, 12, and 14–16). In addition, the diarylation failed when using other transition-metal catalysts, such as [Ru(NH3)5Cl]Cl2, RuCl3•nH2O, [Ir(cod)Cl]2, Ir(CO)Cl[n-C10F21]PPh2]2, Ir(CO)Cl(PPh3)2, CuI, CuCl2, and CoCl(PPh3)3, with no reaction taking place in most cases (these data are not shown in Table 2). Among the catalysts examined, trans-RhCl(CO)(PPh3)3 exhibited a moderate catalytic activity for the diarylation to give 5aa (entry 13). Overall, when mononuclear Rh complexes were used as catalysts, many byproducts were formed via polymerization of tert-butyl isocyanide. In contrast, this polymerization was suppressed by using [RhCl(nbd)]2. These results suggest that the choice of catalysts is very important for the selective reaction between isocyanides and BiAr3.
With the optimal reaction conditions in hand (Table 2, entry 7), the scope of the Rh-catalyzed diarylation of aliphatic isocyanides 2 with BiAr3 1 was examined (Scheme 2). The reaction of t-BuNC 2a with BiPh3 1a afforded 5aa in 71% yield, whereas cyclohexyl isocyanide 2c underwent the diarylation to provide 5ac in 40% yield. The reactions of t-BuNC 2a with p-Me, p-CF3, and p-Cl-substituted triarylbismuthines 1 also proceeded to give the corresponding N-tert-butyl diaryl ketimines (5ba, 5da, and 5fa) in moderate to good yields (see Materials and Methods).
As mentioned above, the Pd and Rh catalysts were found to afford α-diimines 3 and ketimines 5, respectively, with excellent product selectivity. Conceivably, α-diimines 3 might be more important than ketimines 5 as synthetic intermediates. Hence, we examined the scope and limitations of this catalytic diarylation using the reaction conditions found in entry 1 of Table 1 (Scheme 3). In the cases of aliphatic isocyanides 2a–2d, the corresponding N,N’-dialkyl α-diimines 3aa–3ad were successfully obtained in good yields. The diarylation of electron-rich aromatic isocyanide 2e also afforded the corresponding N,N’-diaryl α-diimine 3ae in good yield, whereas aromatic isocyanides with electron-withdrawing groups such as p-nitro and p-cyano groups resulted in the formation of a complex mixture.
In addition, the scope and limitations of the triarylbismuthines were investigated (Scheme 4). The diarylation of t-BuNC 2a, with BiAr3 1b–1d was conducted, and the corresponding N,N’-di-tert-butyl α-diimines 3ba–3da were formed in moderate yields. When the p-methoxyphenyl isocyanide 2e was used for the arylation, similar results were observed.
The Pd(OAc)2-catalyzed diarylation of t-BuNC 2a with Bi(C6H4-F-p)3 1c was carried out under several different conditions to explore the reason for the lower yield of 3ca (Table 3).
An equimolar reaction of 2a with 1c was found to afford ketimine 5ca in parallel to α-diimine 3ca (entry 1), while decreasing the amount of 1c resulted in the selective formation of 3ca (entry 3). Therefore, the molar ratio of 1c to 2a was an important factor for the selective synthesis of α-diimine 3ca.
The impact of reducing the catalyst loading was then examined to allow for the easy isolation of α-diimines 3 (Table 4). The catalytic diarylation of isocyanide 2a was conducted using 5 mol% Pd(OAc)2 and one equivalent of triphenylbismuthine 1a to 2a under an atmosphere of N2, and α-diimine 3aa was obtained in low yield (entry 1). Interestingly, under an atmosphere of air, the yield of 3aa was dramatically improved (entry 2). Using molecular oxygen instead of air was also effective (entry 3). However, the combination of divalent copper salts was ineffective for the diarylation (entries 4 and 5). Moreover, the effect of reducing the amount of BiPh3 1a was examined (entries 6–8). Even when using 1/3 equivalent of 1a, α-diimine 3aa was obtained in satisfactory yield (entry 6). This clearly indicates that all three phenyl groups on 1a could be used for the formation of 3aa. When the loading of Pd(OAc)2 was reduced to 1 mol%, the yield of 3aa slightly decreased (entry 7). However, the use of 2 mol% of Pd(OAc)2 led to the formation of 3aa in a satisfactory yield (81%). As can be seen from the data in Table 4, the use of a combination of Pd(OAc)2 and air reduced the loading of both catalyst and triarylbismuthine. Further examination of reaction conditions for the Pd(OAc)2-catalyzed diarylation in air revealed that acetonitrile was the best for the present diarylation [42].
In our previous paper, we proposed a possible pathway for the Pd(OAc)2-catalyzed diarylation of isocyanide 2 with triarylbismuthine 1 to afford α-diimine 3, the essence of which is shown in Scheme 5.
Transmetalation between Pd(OAc)2 and BiAr3 1 might generate arylpalladium species I, into which isocyanide 2 inserts to form the imidoylpalladium species II. The subsequent ligand-exchange reaction of II with itself then leads to the palladium complexes III and IV. Reductive elimination from III affords the α-diimines 3 along with the Pd(0) species. Since the Pd(OAc)2-catalyzed diarylation proceeds smoothly in the presence of oxidizing agents such as air, the Pd(0) species might be oxidized to the Pd(II) species by air, or by the bismuth compounds present in the reaction system.
In the case of rhodium catalysts such as [RhCl(nbd)]2, oxidative addition of BiAr3 1, followed by insertion of isocyanide 2 into the Rh–Ar bond results in the formation of an imidoylrhodium species. Presumably, the ligand-exchange reaction is less important for the imidoylrhodium species compared with the Pd(OAc)2-based system. Accordingly, transmetalation of imidoylrhodium species with BiAr3 1 might generate aryl imidoylrhodium species of the type “ArC(=NR)–RhLn–Ar,” and the subsequent reductive elimination might selectively afford ketimines 5.
Since α-diimines 3 are expected to be important precursors for the synthesis of nitrogen-containing heterocyclic compounds, an attempt was also made to synthesize nitrogen-containing heterocycles without purification of α-diimines 3 prepared by the Pd(OAc)2-catalyzed diarylation of isocyanides with triarylbismuthines. Hence, we next examined the synthesis of quinoxaline derivatives (Scheme 6).
After the catalytic diarylation of t-BuNC 2a with BiAr3 1a was complete, the reaction mixture was filtered through a Celite pad. The filtrate was then treated with 1 N HCl aq., followed by the addition of o-phenylenediamine 8a. The mixture was heated at 60 °C for 12 h to successfully afford the corresponding quinoxaline derivative 9a in high yield. 4,5-Dimethyl-substituted o-phenylenediamine 8b also reacted with the α-diimine 3aa formed in situ to give the quinoxaline 9b in high yield. In the case of o-phenylenediamine 8c, which has an electron-withdrawing nitro group, the corresponding quinoxaline 9c was formed in moderate yield. Moreover, when BiAr3 compounds with p-methyl- or p-fluoro-groups were employed for this cascade synthesis, the corresponding quinoxalines 9d and 9e, respectively, were obtained in good yields. The present method of quinoxaline synthesis is very convenient, because the α-diimines 3 formed in situ can be used directly without purification.

3. Materials and Methods

3.1. General Comments

All solvents were distilled before use. Triphenylbismuthine (1a) was purchased form a commercial source. The other bismuthines were prepared according to the literature. All aliphatic isocyanides and 2,6-xylylisocyanide (2f) were purchased from a commercial source. The other isocyanides were prepared according to the literature. N,N’-dialkyl α-diimines were isolated by recycle GPC (eluent: CHCl3). N,N’-diaryl α-diimines were isolated by preparative TLC (eluent: hexane/ethyl acetate). 1H NMR spectra were recorded on JEOL JNM-ECX400 (400 MHz) FT NMR or JEOL JNMECS400 (400 MHz) FT NMR in CDCl3 with Me4Si as an internal standard. 13C{1H} NMR spectra were recorded on JEOL JNM-ECX400 (100 MHz) FT NMR or JEOL JNM-ECS400 (100 MHz) FT NMR in CDCl3.

3.2. Typical Reaction Procedure for Ketimine Synthesis

In a dried 10 mL Schlenk test tube, norbornadiene rhodium(I) chloride dimer (0.04 mmol) and (p-MeOC6H4)3P (0.08 mmol) were dissolved in benzene (1.0 mL), and the mixture was stirred for 10 min at room temperature under Ar atmosphere. Then, triarylbismuthine (1; 0.4 mmol) and isocyanides (2; 0.4 mmol) were added to the reaction mixture. The resulting mixture was heated at 70 °C for 18 h. After the reaction, the crude product was filtered through a Celite pad using AcOMe as the eluent. All volatiles were evaporated under reduced pressure, and the yields of corresponding ketimines were determined by 1H NMR spectroscopy (solv.: CDCl3, internal standard: 1,3,5-trioxane) [43].
In this synthetic method, ketimine can be synthesized in a highly selective manner, and the purity is high even in the crude state. However, when treated with recycled GPC to remove unreacted starting substrates, the ketimine undergoes hydrolysis to produce a small amount of the corresponding ketone. Therefore, in order to use this ketimine synthesis method effectively, it is recommended to use it in one pot without isolating the ketimine. Ketimine 5da and 5fa could be isolated by recycled GPC (CH2Cl2), and their characterization data are shown as follows (1H and 13C{1H} NMR spectra are included in the Supplementary Materials):
N-tert-butyl-1,1-bis(4-(trifluoromethyl)phenyl)methanimine (5da). 43% yield (63.8 mg); white solid, m.p. 80.0–81.0 °C; 1H NMR (400 MHz, CDCl3) δ 7.70 (d, J = 8.2 Hz, 2H), 7.61 (d, J = 8.2 Hz, 2H), 7.54 (d, J = 8.6 Hz, 2H), 7.34 (d, J = 8.2 Hz, 2H), 1.17 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3) δ 160.6, 144.3, 143.1, 131.5 (d, J = 32.6 Hz), 130.7 (d, J = 32.6 Hz), 130.3 (overlapped), 128.8, 128.3, 125.2 (d, J = 3.8 Hz), 125.1 (d, J = 3.9 Hz), 57.7, 31.6; HRMS (EI) m/z calcd for C19H17F6N [M]+: 373.1265, found: 373.1268.
N-tert-butyl-1,1-bis(4-chlorophenyl)methanimine (5fa) [CAS: 27126-15-4] [43]. 60% yield (73.0 mg); white solid, m.p. 104.0–105.0 °C; 1H NMR (400 MHz, CDCl3) δ 7.44 (d, J = 8.6 Hz, 2H), 7.39 (d, J = 8.6 Hz, 2H), 7.24 (d, J = 8.6 Hz, 2H), 7.12 (d, J = 8.2 Hz, 2H), 1.16 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3) δ 161.2, 140.1, 137.8, 135.8, 134.2, 129.8, 129.4, 128.4, 128.2, 57.3, 31.6.

3.3. Typical Reaction Procedure for α-Diimine Synthesis (Schemes 3 and 4)

Triphenylbismuthine (1a; 0.4 mmol) and tert-butyl isocyanide (2a; 0.4 mmol) were dissolved in benzene (2 mL) in a dried two-necked test tube under a N2 atmosphere. Palladium diacetate (0.08 mmol) was added to the mixture. The resulting mixture was stirred for 18 h at 70 °C. After the reaction, the crude product was filtered through a Celite pad. All volatiles were evaporated under reduced pressure, and the NMR spectrum was measured (solv.: CDCl3). Dioxane was used as an internal standard. 1H and 13C{1H} NMR spectra are included in the Supplementary Materials.
N,N’-Bis(1,1-dimethylethyl)-1,2-diphenylethane-1,2-diimine (3aa) [CAS: 38015-77-9] [42]. 88% yield (56.3 mg); white solid; m.p. 107.5–109.0 °C; 1H NMR (400 MHz, CDCl3) δ 7.79–7.76 (m, 4H), 7.36–7.29 (m, 6H), 1.24 (s, 18H).
N,N’-Bis(1,1,3,3-tetramethylpropyl)-1,2-diphenylethane-1,2-diimine (3ab) [CAS: 1800598-80-4] [42]. 78% yield (67.4 mg); white solid; m.p. 108.0–109.0 °C; 1H NMR (400 MHz, CDCl3) δ 7.78–7.76 (m, 4H), 7.35–7.29 (m, 6H), 1.50 (s, 4H), 1.37 (s, 6H), 1.16 (s, 6H), 1.04 (s, 18H).
N,N’-Dicyclohexyl-1,2-diphenylethane-1,2-diimine (3ac) [CAS: 20586-41-8] [42]. 73% yield (54.3 mg); white solid; m.p. 89.5–91.0 °C; 1H NMR (400 MHz, CDCl3) δ 7.82–7.73 (m, 4H), 7.39–7.30 (m, 6H), 3.23 (tt, J = 9.6, 4.1 Hz, 2H), 1.90–1.82 (m, 2H), 1.75–1.17 (m, 16H), 1.15–0.98 (m, 2H).
N,N’-Dipentyl-1,2-diphenylethane-1,2-diimine (3ad) [CAS: 906560-91-6] [42]. 85% yield (59.2 mg); pale yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.75–7.73 (m, 4H), 7.41–7.32 (m, 6H), 3.33 (t, J = 7.3 Hz, 4H), 1.73–1.65 (m, 4H), 1.36–1.24 (m, 8H), 0.86 (t, J = 7.3 Hz, 6H).
N,N’-Bis(4-methoxyphenyl)-1,2-diphenylethane-1,2-diimine (3ae) [CAS: 32349-49-8] [42]. 80% yield (33.6 mg); yellow solid; 1H NMR (400 MHz, CDCl3) δ 7.87–7.84 (m, 4H), 7.42–7.33 (m, 6H), 6.68–6.62 (m, 8H), 3.71 (s, 6H).
N,N’-Bis(1,1-dimethylethyl)-1,2-bis(4-methylphenyl)ethane-1,2-diimine (3ba) [CAS: 956375-94-3] [42]. 58% yield (40.4 mg); white solid; m.p. 83.5–85.0 °C; 1H NMR (400 MHz, CDCl3) δ 7.65 (d, J = 8.2 Hz, 4H), 7.11 (d, J = 8.2 Hz, 4H), 2.33 (s, 6H), 1.23 (s, 18H).
N,N’-Bis(1,1-dimethylethyl)-1,2-bis(4-fluorophenyl)ethane-1,2-diimine (3ca) [CAS: 884050-44-6] [42]. 27% yield (19.2 mg); white solid; m.p. 99.5–102.0 °C; 1H NMR (400 MHz, CDCl3) δ 7.77–7.72 (m, 4H), 7.03–6.98 (m, 4H), 1.23 (s, 18H).
N,N’-Bis(4-methoxyphenyl)-1,2-di-p-tolylethane-1,2-diimine (3be) [CAS: 86980-71-4] [44,45]. 47% yield (21.1 mg); 1H NMR (400 MHz, CDCl3) δ 7.75 (d, J = 8.2 Hz, 4H), 7.16 (d, J = 8.2 Hz, 4H), 6.62 (s, 8H), 3.72 (s, 6H), 2.36 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3) δ 163.3, 157.1, 142.6, 141.1, 134.9, 129.4, 128.1, 122.1, 113.6, 55.3, 21.5.
N,N’-1,2-Tetrakis(4-methoxyphenyl)ethane-1,2-diimine (3ee) [CAS: 130440-66-3] [45]. 54% yield (25.9 mg); 1H NMR (400 MHz, CDCl3) δ 7.81 (d, J = 9.2 Hz, 4H), 6.87 (d, J = 9.2 Hz, 4H), 6.62 (s, 8H), 3.82 (s, 6H), 3.72 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3) δ 162.8, 161.6, 157.0, 142.7, 130.4, 129.8, 122.1, 114.0, 113.6, 55.3.

3.4. Typical Reaction Procedure for Cascade Synthesis of 2,3-Diarylquinoxalines (Scheme 4)

Triphenylbismuthine (1a; 0.4 mmol) and tert-butyl isocyanide (2a; 1.2 mmol) were dissolved in MeCN (2 mL) in a dried two-necked test tube under air. Palladium diacetate (0.024 mmol) was added to the mixture. The resulting mixture was stirred for 4 h at 70 °C. After the reaction, the crude product was filtered through a Celite pad. All volatiles were evaporated under reduced pressure, after which dioxane (5 mL) and 1 N HCl (5 mL) were added. The mixture was stirred at 60 °C for 6 h, then α-diamine (0.6 mmol) was added and the reaction was stirred at 60 °C for 12 h. The mixture was extracted three times with EtOAc (10 mL), dried over MgSO4, and the desired product was purified by silica gel chromatography (eluent: hexane/EtOAc). 1H NMR spectra are included in the Supplementary Materials.
2,3-Diphenylquinoxaline (9a) [CAS: 1684-14-6] [46]. 84% yield (142.1 mg); white solid; m.p. 124–125 °C. 1H NMR (400 MHz, CDCl3) δ 8.20–8.16 (m, 2H), 7.79–7.75 (m, 2H), 7.53–7.51 (m, 4H), 7.39–7.31 (m, 6H).
6,7-Dimethyl-2,3-diphenylquinoxaline (9b) [CAS: 13362-56-6] [46]. 82% yield (152.5 mg); white solid; m.p. 174–175 °C; 1H NMR (400 MHz, CDCl3) δ 7.92 (s, 2H), 7.51–7.48 (m, 4H), 7.36–7.29 (m, 6H), 2.51 (s, 6H).
6-Nitro-2,3-diphenylquinoxaline (9c) [CAS: 7466-45-7] [46]. 55% yield (107.9 mg); yellow solid; m.p. 188–189 °C. 1H NMR (400 MHz, CDCl3) δ 9.08 (d, J = 2.7 Hz, 1H), 8.53 (dd, J = 9.1, 2.7 Hz, 1H), 8.30 (d, J = 9.5 Hz, 1H), 7.58–7.55 (m, 4H), 7.45–7.35 (m, 6H).
2,3-di-p-tolylquinoxaline (9d) [CAS: 3719-84-4] [47]. 76% yield (141.4 mg); m.p. 142–144 °C; 1H NMR (400 MHz, CDCl3): δ 8.16–8.13 (m, 2H), 7.74–7.71 (m, 2H), 7.43 (d, J = 8.2 Hz, 4H), 7.14 (d, J = 8.2 Hz, 4H), 2.36 (s, 6H).
2,3-Bis(4-fluorophenyl)quinoxaline (9e) [CAS: 148186-43-0] [48]. 63% yield (120.2 mg); m.p. 134–136 °C; 1H NMR (400 MHz, CDCl3) δ 8.16 (dd, J = 6.3, 3.2 Hz, 2H), 7.78 (dd, J = 6.3, 3.6 Hz, 2H), 7.52–7.49 (m, 4H), 7.05 (t, J = 8.6 Hz, 4H).

4. Conclusions

In this study, we describe in detail the transition-metal-catalyzed diarylation of isocyanides with triarylbismuthines. When rhodium complexes were used as the catalyst in the diallylation reaction, ketimines (with one molecule of isocyanide incorporated) were highly selectively formed, whereas when palladium-based catalyst was used, α-diimines (with two molecules of isocyanide incorporated) were formed preferentially. For the purpose of further elucidating the details of this catalytic system, the effects of catalyst, solvent, and reaction temperature on the diarylation were investigated in detail to optimize the reaction conditions and determine the byproducts. Furthermore, the palladium-catalyzed diarylation was successfully applied to the cascade synthesis of quinoxalines via the formation of α-diimines as key intermediates.

Supplementary Materials

The following are available online at https://0-www-mdpi-com.brum.beds.ac.uk/article/10.3390/ma14154271/s1, Figure S1: Copies of 1H and 13C{1H} NMR spectra.

Author Contributions

Investigation, S.K., Y.Y., Y.K., H.M., C.C.T., S.-i.K., A.N., and A.O.; formal analysis, S.K., Y.Y., Y.K., H.M., C.C.T.; resources, A.N. and A.O.; writing—original draft preparation, S.K., Y.Y., C.C.T., and A.O.; writing—review and editing, S.K., Y.Y., S.-i.K., A.N., and A.O.; funding acquisition, S.-i.K., A.N., and A.O. All authors have read and agreed to the published version of the manuscript.

Funding

C.C.T. thanks the Graduate Course for System-inspired Leaders in Material Science (SiMS) Scholarship Program. This work was supported by JSPS KAKENHI (19K17746; B, 19H02791; B, 19H02756) from the Japan Society for the Promotion of Science (JSPS), and also supported by the Nanotechnology Platform Program of the Nara Institute of Science and Technology (NAIST).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in the article and in its Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest.

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Materials 14 04271 sch001 550
Scheme 1. Transition-metal-catalyzed diarylation of isocyanides.
Scheme 1. Transition-metal-catalyzed diarylation of isocyanides.
Materials 14 04271 sch001
Materials 14 04271 sch002 550
Scheme 2. Substrate scope of the Rh-catalyzed diarylation of aliphatic isocyanides 2 with BiAr3 1.
Scheme 2. Substrate scope of the Rh-catalyzed diarylation of aliphatic isocyanides 2 with BiAr3 1.
Materials 14 04271 sch002
Materials 14 04271 sch003 550
Scheme 3. Substrate scope of the Pd-catalyzed diarylation of isocyanides 2 with BiPh3 1a.
Scheme 3. Substrate scope of the Pd-catalyzed diarylation of isocyanides 2 with BiPh3 1a.
Materials 14 04271 sch003
Materials 14 04271 sch004 550
Scheme 4. Substrate scope of the Pd-catalyzed diarylation of isocyanides 2 with BiAr3 1.
Scheme 4. Substrate scope of the Pd-catalyzed diarylation of isocyanides 2 with BiAr3 1.
Materials 14 04271 sch004
Materials 14 04271 sch005 550
Scheme 5. A possible pathway for Pd-catalyzed diarylation.
Scheme 5. A possible pathway for Pd-catalyzed diarylation.
Materials 14 04271 sch005
Materials 14 04271 sch006 550
Scheme 6. Application to cascade synthesis of 2,3-diarylquinoxalines.
Scheme 6. Application to cascade synthesis of 2,3-diarylquinoxalines.
Materials 14 04271 sch006
Table
Table 1. Influence of reaction conditions on 5aa/3aa selectivity.
Table 1. Influence of reaction conditions on 5aa/3aa selectivity.
Materials 14 04271 i001
EntryCat. M (mol%)Solv.Time (h)Yields (%) a
5aa3aa
1Pd(OAc)2 (20)C6H618990
2Pd(PPh3)4 (20)C6H618trace23
3PdCl2 (20)C6H618571
4Pd(PPh3)2Cl2 (20)C6H6181162
5 bPd(OAc)2 (20)C6H618566
6Pd2(dba)3·CHCl3 (10)C6H618trace56
7noneC6H61802
8 cPd(OAc)2 (20)C6H618trace58
9 dPd(OAc)2 (20)C6H6181082
10 ePd(OAc)2 (20)C6H618859
11Pd(OAc)2 (10)C6H618465
12Pd(OAc)2 (5)C6H618451
13Pd(OAc)2 (20)THF182163
14Pd(OAc)2 (20)EtOH18trace49
15Pd(OAc)2 (20)MeCN18084
16Pd(OAc)2 (20)PhMe18973
17Pd(OAc)2 (20)C6H64181
18 fPd(OAc)2 (20)C6H618677
a Determined by 1H NMR. Calculated based on the amount of 2a; b triphenylphosphine (40 mol%) was used as a ligand; c loading of 1a was 0.1 mmol; d air; e room temp; f the reaction was conducted under Ar.
Table
Table 2. Rh-catalyzed diarylation of t-BuNC with BiPh3.
Table 2. Rh-catalyzed diarylation of t-BuNC with BiPh3.
Materials 14 04271 i002
EntryRh Catalyst (mol%)Yield of 5aa (%) a
1 b[RhCl(nbd)]2 (10)50
2 b[RhCl(nbd)]2 (5)32
3 b,c[RhCl(nbd)]2 (10)75
4 bRhH(CO)(PPh3)3 (20)20
5[RhCl(nbd)]2/(p-MeO-C6H4)3P (10/20)51
6 d[RhCl(nbd)]2/(p-MeO-C6H4)3P (10/20)61
7 e[RhCl(nbd)]2/(p-MeO-C6H4)3P (10/20)71
8 f[RhCl(nbd)]2/(p-MeO-C6H4)3P (10/20)57
9 e,g[RhCl(nbd)]2/(p-MeO-C6H4)3P (10/20)66
10RhCl(PPh3)3 (10)12
11RhH(PPh3)3 (10)12
12RhBr(PPh3)3 (10)17
13trans-RhCl(CO)(PPh3)3 (10)55
14[Rh(dppp)(cod)]+BF4− (10)0
15RhCl3 (10)2
16[Rh(OAc)2]2 (10)6
a Determined by 1H NMR. 1,3,5-Trioxane was used as an internal standard; b 1a (0.2 mmol), t-BuNC (0.2 mmol), and benzene (2.0 mL) were used; c loading of 1a was 0.4 mmol; d C6H6 (2.0 mL); e C6H6 (1.0 mL); f C6H6 (0.5 mL); g loading of 1a was 0.6 mmol.
Table
Table 3. Pd-catalyzed diarylation of t-BuNC with Bi(C6H4-F-p)3.
Table 3. Pd-catalyzed diarylation of t-BuNC with Bi(C6H4-F-p)3.
Materials 14 04271 i003
Entry2a (mmol)1c (Equiv.)Yields (%) a
3ca5ca
10.21.03748
20.20.754543
30.20.5742
a Determined by 1H NMR; calculated based on the amount of 2a.
Table
Table 4. Influence of oxidants on Pd-catalyzed diarylation of t-BuNC with BiPh3.
Table 4. Influence of oxidants on Pd-catalyzed diarylation of t-BuNC with BiPh3.
Materials 14 04271 i004
EntryOxidant (Equiv.)Cat. (mol%)2a (mmol)1a (Equiv.)Yields (%) a
1none (N2 atm.)50.4127
2air50.4185
3O250.4177
4Cu(OAc)2∙H2O (1/2)50.410
5CuCO3∙Cu(OH)2∙H2O (1/4)50.4119
6air50.41/385 b
7air11.51/369 b
8air21.51/381 b
a Determined by 1H NMR. Calculated based on the amount of 2a; b calculated based on the amount of the phenyl moieties on bismuth atom.
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Kodama, S.; Yamamoto, Y.; Kobiki, Y.; Matsubara, H.; Tran, C.C.; Kawaguchi, S.-i.; Nomoto, A.; Ogawa, A. Transition-Metal-Catalyzed Diarylation of Isocyanides with Triarylbismuthines for the Selective Synthesis of Imine Derivatives. Materials 2021, 14, 4271. https://0-doi-org.brum.beds.ac.uk/10.3390/ma14154271

AMA Style

Kodama S, Yamamoto Y, Kobiki Y, Matsubara H, Tran CC, Kawaguchi S-i, Nomoto A, Ogawa A. Transition-Metal-Catalyzed Diarylation of Isocyanides with Triarylbismuthines for the Selective Synthesis of Imine Derivatives. Materials. 2021; 14(15):4271. https://0-doi-org.brum.beds.ac.uk/10.3390/ma14154271

Chicago/Turabian Style

Kodama, Shintaro, Yuki Yamamoto, Yohsuke Kobiki, Hitomi Matsubara, Cong Chi Tran, Shin-ichi Kawaguchi, Akihiro Nomoto, and Akiya Ogawa. 2021. "Transition-Metal-Catalyzed Diarylation of Isocyanides with Triarylbismuthines for the Selective Synthesis of Imine Derivatives" Materials 14, no. 15: 4271. https://0-doi-org.brum.beds.ac.uk/10.3390/ma14154271

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