Homocoupling of heteroaryl/aryl/alkyl Grignard reagents: I2-promoted, or Ni- or Pd- or Cu- or nano-Fe-based salt catalyzed

Xing Li*, Dongjun Li, Yingjun Li, Honghong Chang, Wenchao Gao and Wenlong Wei*
Department of Chemistry and Chemical Engineering, Taiyuan University of Technology, 79 West Yingze Street, Taiyuan 030024, People's Republic of China. E-mail: lixing@tyut.edu.cn

Received 13th July 2016 , Accepted 7th September 2016

First published on 8th September 2016


Abstract

Five efficient processes for the homo-coupling of various Grignard reagents including aryl, heteroaryl and aliphatic ones in the presence of I2, Pd(OAc)2, Ni(OAc)2, CuI, and nano-Fe3O4 were developed, respectively.


The symmetrical biaryl unit is found in a number of natural products, pharmaceuticals, optical materials and functional molecules.1 Oxidative homo-coupling reactions of Grignard reagents have gained great attention as they provide an easy and efficient access for the construction of such compounds. Several kinds of transition metal promoters including cobalt salts,2 TlBr,3 TiCl4,4 vanadium salts,5 copper salts,6 etc.7 have been used in the presence of a reoxidant to carry out the oxidative coupling process in most cases.8 In these reactions, however, a stoichiometric or excess amount of transition metal is used for smooth transformation. Consequently, with the urgent demand for green chemistry process and sustainable development, the exploration of the suitable catalytic systems with efficient catalysts and inexpensive oxidants obviously becomes one of the most important topics. On the other hand, some common and easily available transition metal catalysts have been successfully used in the cross-coupling reactions of Grignard reagents with organic electrophiles and possessed the synthetic advantages such as high selectivity, broad substrate scopes and mild reaction conditions in recent years.9 In contrast, the catalyzed routes to symmetrical biaryls from Grignard reagents about them have not received as much attention although there have been several examples carried out on manganese-,10 iron-,11 cobalt-,12 ruthenium-13 and copper-catalyzed14 homo-coupling reactions. Therefore, we anticipate that similar Pd- and Ni-catalyzed methods can also be realized under certain conditions, and to the best of our knowledge, there have been no reports on I2-mediated, nano-Fe3O4- and CuI-catalyzed oxidative homo-coupling so far. Herein, we will report Pd(OAc)2-, Ni(OAc)2-, CuI-, and nano-Fe3O4-catalyzed, and I2-promoted routes to symmetrical biaryls from Grignard reagents.

First, when phenylmagnesium bromide was treated with 0.2 equiv. of I2 in toluene at 110 °C, the desired product was obtained in 37% yield (Table 1, entry 1). The yield was improved greatly by increasing the amount of I2 and the highest yield was provided when 0.8 equiv. I2 was used (Table 1, entry 4 vs. 2, 3 and 5).

Table 1 The optimization of the homocoupling of Grignard reagents with I2a

image file: c6ra17859f-u1.tif

Entry I2 (equiv.) Yieldb (%)
a Reaction conditions: phenylmagnesium bromide (0.3 mmol), I2, toluene (1.5 mL), temperature (110 °C), time (48 h).b Isolated yield.
1 0.2 37
2 0.4 65
3 0.6 80
4 0.8 96
5 0.9 92


With the promising results, the scope of Grignard reagents was next investigated and the results are shown in Table 2. The results indicated that a variety of Grignard reagents could also be quickly transformed into the corresponding products in the presence of I2. Grignard reagents having electron-poor or -rich groups on benzene ring underwent smoothly transformation to give products in good to high yields (Table 2, entries 1–7). The benzylmagnesium bromide could afford the corresponding product in 54% yield, although a longer time and higher temperature was required (Table 2, entry 8). Heteroaryl Grignard reagents such as pyridylmagnesium bromide and thienylmagnesium bromide were found be suitable substrates to give the homo-coupled products in good yields (Table 2, entries 9–11). Homo-couplings of arylmagnesium chloride also proceeded well under similar conditions (Table 2, entries 12–14). Excitedly, 1,4-diphenylbuta-1,3-diyne was provided in 62% yield when (phenylethynyl)magnesium bromide was used as a substrate (Table 2, entry 15). However, no product was detected for styrylmagnesium bromide (Table 2, entry 16).

Table 2 The scope of the homocoupling of Grignard reagents with I2a

image file: c6ra17859f-u2.tif

Entry Grignard reagent Product t (h) Yieldb (%)
a Reaction conditions: Grignard reagent (0.3 mmol), I2 (0.8 equiv.), toluene (1.5 mL), temperature (110 °C).b Isolated yield.c 140 °C was used.d N.D. = not detected.
1 image file: c6ra17859f-u3.tif image file: c6ra17859f-u4.tif 48 96
2 image file: c6ra17859f-u5.tif image file: c6ra17859f-u6.tif 50 83
3 image file: c6ra17859f-u7.tif image file: c6ra17859f-u8.tif 48 88
4 image file: c6ra17859f-u9.tif image file: c6ra17859f-u10.tif 48 92
5 image file: c6ra17859f-u11.tif image file: c6ra17859f-u12.tif 48 86
6 image file: c6ra17859f-u13.tif image file: c6ra17859f-u14.tif 50 84
7 image file: c6ra17859f-u15.tif image file: c6ra17859f-u16.tif 50 91
8 image file: c6ra17859f-u17.tif image file: c6ra17859f-u18.tif 48 54c
9 image file: c6ra17859f-u19.tif image file: c6ra17859f-u20.tif 48 82
10 image file: c6ra17859f-u21.tif image file: c6ra17859f-u22.tif 48 76
11 image file: c6ra17859f-u23.tif image file: c6ra17859f-u24.tif 48 81
12 image file: c6ra17859f-u25.tif image file: c6ra17859f-u26.tif 48 89
13 image file: c6ra17859f-u27.tif image file: c6ra17859f-u28.tif 50 82
14 image file: c6ra17859f-u29.tif image file: c6ra17859f-u30.tif 48 87
15 image file: c6ra17859f-u31.tif image file: c6ra17859f-u32.tif 48 62
16 image file: c6ra17859f-u33.tif image file: c6ra17859f-u34.tif 48 N.D.d


Subsequently, our studies focused on the Pd-catalyzed homo-coupling of various types of Grignard reagents. It was found that through the use of Pd(OAc)2,15 a wide range of Grignard reagents, including arylmagnesium bromide, heteroarylmagnesium bromide, and arylmagnesium chloride could be effectively transformed in good to excellent yields in the presence of LiClO4 (Table 3, entries 1–7 and 9–14).16 Finally, benzylmagnesium bromide also represented a compatible substrate under the reflux conditions (Table 3, entry 8).

Table 3 The scope of the homocoupling of Grignard reagents with Pd(OAc)2a

image file: c6ra17859f-u35.tif

Entry Grignard reagent Product t (h) Yieldb (%)
a Reaction conditions: Grignard reagent (0.3 mmol), Pd(OAc)2 (10 mol%), LiClO4 (0.3 mmol, 1.0 equiv.), toluene (2.0 mL), temperature (100 °C).b Isolated yield.c 120 °C was used.
1 image file: c6ra17859f-u36.tif image file: c6ra17859f-u37.tif 15 98
2 image file: c6ra17859f-u38.tif image file: c6ra17859f-u39.tif 17 87
3 image file: c6ra17859f-u40.tif image file: c6ra17859f-u41.tif 15 93
4 image file: c6ra17859f-u42.tif image file: c6ra17859f-u43.tif 15 96
5 image file: c6ra17859f-u44.tif image file: c6ra17859f-u45.tif 15 87
6 image file: c6ra17859f-u46.tif image file: c6ra17859f-u47.tif 17 86
7 image file: c6ra17859f-u48.tif image file: c6ra17859f-u49.tif 15 98
8 image file: c6ra17859f-u50.tif image file: c6ra17859f-u51.tif 24 61c
9 image file: c6ra17859f-u52.tif image file: c6ra17859f-u53.tif 15 86
10 image file: c6ra17859f-u54.tif image file: c6ra17859f-u55.tif 15 85
11 image file: c6ra17859f-u56.tif image file: c6ra17859f-u57.tif 15 88
12 image file: c6ra17859f-u58.tif image file: c6ra17859f-u59.tif 15 92
13 image file: c6ra17859f-u60.tif image file: c6ra17859f-u61.tif 16 86
14 image file: c6ra17859f-u62.tif image file: c6ra17859f-u63.tif 15 89


To prove the generality of Ni(OAc)2 as a catalyst in the homo-coupling of Grignard reagents, various substrates were investigated. Optimized reaction conditions17 were next applied to prepare a variety of homo-coupled products. For the aryl Grignard reagents having electron-poor or -rich groups on benzene ring (Table 4, entries 2–7), the coupling reactions provided high yields (85–97%). The steric hindrance also played an important role (Table 4, entries 4 vs. 2 and 7 vs. 6). The longer reaction time was required for the reaction with benzylmagnesium bromide, giving relatively low yields (58%) (Table 4, entry 8). Heteroarylmagnesium bromide with Ni(OAc)2 as a catalyst provided somewhat lower yields than that with Pd(OAc)2 (Table 4, entries 9–11). Similar yields were obtained with arylmagnesium chloride (Table 4, entries 12–14).

Table 4 The scope of the homocoupling of Grignard reagents with Ni(OAc)2a

image file: c6ra17859f-u64.tif

Entry Grignard reagent Product t (h) Yieldb (%)
a Reaction conditions: Grignard reagent (0.3 mmol), Ni(OAc)2 (10 mol%), Ag2O (0.3 mmol, 1.0 equiv.), CH2Cl2 (1.5 mL), temperature (25 °C).b Isolated yield.c 13% yield was obtained in the absence of Ni(OAc)2.
1 image file: c6ra17859f-u65.tif image file: c6ra17859f-u66.tif 18 95c
2 image file: c6ra17859f-u67.tif image file: c6ra17859f-u68.tif 20 90
3 image file: c6ra17859f-u69.tif image file: c6ra17859f-u70.tif 18 93
4 image file: c6ra17859f-u71.tif image file: c6ra17859f-u72.tif 18 97
5 image file: c6ra17859f-u73.tif image file: c6ra17859f-u74.tif 17 90
6 image file: c6ra17859f-u75.tif image file: c6ra17859f-u76.tif 20 85
7 image file: c6ra17859f-u77.tif image file: c6ra17859f-u78.tif 18 92
8 image file: c6ra17859f-u79.tif image file: c6ra17859f-u80.tif 24 58
9 image file: c6ra17859f-u81.tif image file: c6ra17859f-u82.tif 18 82
10 image file: c6ra17859f-u83.tif image file: c6ra17859f-u84.tif 18 77
11 image file: c6ra17859f-u85.tif image file: c6ra17859f-u86.tif 18 81
12 image file: c6ra17859f-u87.tif image file: c6ra17859f-u88.tif 18 83
13 image file: c6ra17859f-u89.tif image file: c6ra17859f-u90.tif 18 86
14 image file: c6ra17859f-u91.tif image file: c6ra17859f-u92.tif 18 90


To demonstrate the efficiency and scope of the method with CuI as a catalyst,18 we applied the catalytic system to a variety of Grignard reagents (Table 5). Various substrates including arylmagnesium bromide possessing methyl, methoxy and fluro groups, heteroarylmagnesium bromide, and arylmagnesium chloride were smoothly converted to the desired products in good to excellent yields (Table 5, entries 1–7 and 9–14). Treatment of benzylmagnesium bromide also provided 57% yield at 120 °C (Table 5, entry 8).

Table 5 The scope of the homocoupling of Grignard reagents with CuIa

image file: c6ra17859f-u93.tif

Entry Grignard reagent Product t (h) Yieldb (%)
a Reaction conditions: Grignard reagent (0.3 mmol), CuI (15 mol%), toluene (2.0 mL), temperature (100 °C), air.b Isolated yield.c 120 °C was used.
1 image file: c6ra17859f-u94.tif image file: c6ra17859f-u95.tif 15 95
2 image file: c6ra17859f-u96.tif image file: c6ra17859f-u97.tif 18 95
3 image file: c6ra17859f-u98.tif image file: c6ra17859f-u99.tif 15 95
4 image file: c6ra17859f-u100.tif image file: c6ra17859f-u101.tif 15 96
5 image file: c6ra17859f-u102.tif image file: c6ra17859f-u103.tif 15 95
6 image file: c6ra17859f-u104.tif image file: c6ra17859f-u105.tif 18 86
7 image file: c6ra17859f-u106.tif image file: c6ra17859f-u107.tif 18 98
8 image file: c6ra17859f-u108.tif image file: c6ra17859f-u109.tif 24 57c
9 image file: c6ra17859f-u110.tif image file: c6ra17859f-u111.tif 15 88
10 image file: c6ra17859f-u112.tif image file: c6ra17859f-u113.tif 15 82
11 image file: c6ra17859f-u114.tif image file: c6ra17859f-u115.tif 15 87
12 image file: c6ra17859f-u116.tif image file: c6ra17859f-u117.tif 15 89
13 image file: c6ra17859f-u118.tif image file: c6ra17859f-u119.tif 16 79
14 image file: c6ra17859f-u120.tif image file: c6ra17859f-u121.tif 16 84


Nano Fe3O4 was also found to be a good catalyst.19 Various Grignard reagents turned out to be suitable substrates and worked well (Table 6, entries 1–5 and 9–14) even though the materials bearing electron-withdrawing groups (Table 6, entries 6 and 7). Benzylmagnesium bromide could afford the corresponding product in 50% yield (Table 6, entry 8).

Table 6 The scope of the homocoupling of Grignard reagents with Fe3O4a

image file: c6ra17859f-u122.tif

Entry Grignard reagent Product t (h) Yieldb (%)
a Reaction conditions: Grignard reagent (0.3 mmol), nano-sized Fe3O4 (10 mol%), AgNO3 (0.36 mmol, 1.2 equiv.), toluene (2.0 mL), temperature (100 °C).b Isolated yield.c 9% yield was obtained in the absence of Nano-Fe3O4.d 120 °C was used.
1 image file: c6ra17859f-u123.tif image file: c6ra17859f-u124.tif 20 93c
2 image file: c6ra17859f-u125.tif image file: c6ra17859f-u126.tif 24 87
3 image file: c6ra17859f-u127.tif image file: c6ra17859f-u128.tif 20 89
4 image file: c6ra17859f-u129.tif image file: c6ra17859f-u130.tif 20 94
5 image file: c6ra17859f-u131.tif image file: c6ra17859f-u132.tif 20 90
6 image file: c6ra17859f-u133.tif image file: c6ra17859f-u134.tif 18 76
7 image file: c6ra17859f-u135.tif image file: c6ra17859f-u136.tif 18 83
8 image file: c6ra17859f-u137.tif image file: c6ra17859f-u138.tif 24 50d
9 image file: c6ra17859f-u139.tif image file: c6ra17859f-u140.tif 18 80
10 image file: c6ra17859f-u141.tif image file: c6ra17859f-u142.tif 20 77
11 image file: c6ra17859f-u143.tif image file: c6ra17859f-u144.tif 20 83
12 image file: c6ra17859f-u145.tif image file: c6ra17859f-u146.tif 20 86
13 image file: c6ra17859f-u147.tif image file: c6ra17859f-u148.tif 20 82
14 image file: c6ra17859f-u149.tif image file: c6ra17859f-u150.tif 20 87


To clarify the possible reaction mechanism I2-promoted, the homocoupling of iodobenzene with the I2/toluene system, cross coupling of iodobenzene with phenyl magnesium bromide in the presence of I2, and phenyl magnesium bromide with PhI(OAc)2/toluene system were carried out (Scheme 1). The experimental results showed that the reaction process may not involve the generation of an iodobenzene (eqn (1) and (2)). The fact that biphenyl was observed in 57% yield when PhI(OAc)2 was used instead of I2 indicated that hypervalent iodine may play an important role in this process, which suggested that iodine serves not only as the promoter but also as the oxidant (eqn (3)). On the basis of the observations and reported literatures,20 a possible mechanism is postulated as follows (Scheme 2): partial iodine may be first transformed into some hypervalent iodine A in the reaction system. Then a low-valent iodine species B is formed through reduction of A using the Grignard reagent as a strong reducing agent. Subsequently, the species B reacts with the Grignard reagent in the presence of I2 as an oxidant to generate intermediate C, which gives the homo-coupling product D by reductive elimination and releases A.


image file: c6ra17859f-s1.tif
Scheme 1 Controlled experiments.

image file: c6ra17859f-s2.tif
Scheme 2 A proposed mechanism for the I2-promoted homocoupling.

In conclusion, we have described the homo-coupling reactions of various Grignard reagents in the presence of Pd(OAc)2, Ni(OAC)2, CuI, nano-Fe3O4, and I2, respectively. The five synthetic methods worked very well and were applicable to the homo-coupling of various aryl and heteroaryl Grignard reagents. Similar good yields were obtained regarding nano-Fe3O4, and I2. Pd-, Ni-, and Cu-based catalyst systems provided higher yields than them. Notably, the first two systems are more green.

Notes and references

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  15. Extensive screening showed that the optimized reaction conditions were 0.3 mmol Grignard reagent, 10 mol% Pd(OAc)2 and 1.0 equiv. of LiClO4 in toluene under N2 atmosphere at 100 °C. (See Table 1 in ESI).
  16. LiClO4 served as an oxidant.
  17. The optimal reaction conditions screened were 0.3 mmol Grignard reagent, 10 mol% Ni(OAc)2 and 1.0 equiv. of Ag2O which was used as an oxidant in 1.5 mL CH2Cl2 under N2 atmosphere at 25 °C. (See Table 2 in ESI).
  18. The optimized reaction conditions were 0.3 mmol Grignard reagent and 15 mol% CuI in 2.0 mL toluene under air atmosphere at 100 °C. (See Table 3 in ESI).
  19. The optimal reaction conditions were 0.3 mmol Grignard reagent, 10 mol% nano-Fe3O4 and 1.2 equiv. of AgNO3 utilized as an oxidant in 2.0 mL toluene under N2 atmosphere at 100 °C. (See Table 4 in ESI).
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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17859f

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