ISRN Organic ChemistryVolume 2012 (2012), Article ID 814247, 4 pagesdoi:10.5402/2012/814247

Research Article

Catalyzed C–C Coupling of Aryl Iodides and Boronic Acids

Payal Malik and Debashis Chakraborty

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India

Received 28 August 2012; Accepted 14 September 2012

Academic Editors: S. Bellemin-Laponnaz, T. C. Dinadayalane, L. Minuti, and T. Ogiku

Copyright © 2012 Payal Malik and Debashis Chakraborty. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

An efficient La₂O₃-catalyzed new route for the carbon-carbon bond formation in particular, symmetrical and unsymmetrical biphenyls has been developed, which proceeds through carbon-carbon coupling reaction of aryl iodides with boronic acids. The reaction provided the desired products in moderate-to-good yields with a wide range of functional group tolerance.

1. Introduction

The formation of new carbon-carbon bonds is of central importance in organic and medicinal chemistry [1, 2]. The development of new methods for carbon-carbon bond formation is a well-growing area in organic chemistry [3]. In the past decades, tremendous efforts have been devoted into the transition-metal catalyzed cross-coupling reactions [4]. The transition metals have played an important role in organic chemistry and this has led to the development of a large number of transition metal-catalyzed reactions for the formation of C–C and carbon-heteroatom bonds in organic synthesis [5, 6]. In the literature, a variety of nontransition and transition metals like palladium [7–9], copper [10, 11], iron [12], nickel [13, 14], cobalt [15], zinc [16], indium [17], solid supported catalyst [18], and metal nanoparticle [19] have been used for the coupling reactions. In fact palladium-catalyzed Suzuki-type cross-coupling reactions are very well explored and frequently used in organic synthesis and medicinal chemistry [1]. Organoborane and boronic acids have been utilised as arylating agent for the C–C bond formation [7–15].

Metal oxides represent one of the most important and widely used solid catalysts, either as active phases or as supports. The metal oxides are the largest family of catalysts in heterogeneous catalysis due to the acid-base and redox properties [20–23]. The outer electron configuration of the transition and noble group metals made them the most frequently used catalysts [24]. These metal oxides have been proved as efficient catalysts for the coupling reaction. Hell and coworkers reported copper-free Sonogashira reaction of alkynes and aryl halides by using Pd/MgLa mixed oxide [25]. Herein, we have developed a La₂O₃ catalyzed C–C coupling by using aryl halide and boronic acids.

2. Experimental Section

2.1. General Considerations

All the substrates used in this study were purchased from Aldrich and used as received. All the solvents were purchased from Ranchem, India and purified using standard methods. The products are characterized by recording ¹H, ¹³C NMR, and ESI-MS by using Bruker Avance 400 MHz instrument and JEOL JMS GC-mate II instrument.

2.2. Typical Procedure for C–C Coupling Reaction

To a stirred solution of boronic acid (1 mmol) and La₂O₃ (10 mol%) in DMSO (3 mL) was added aryl iodides (1 mmol) followed by trans-1,2-diaminocyclohexane (10 mol%) and KO-t-Bu (2 equiv.). The reaction mixture was heated to 150°C and the progress of reaction was monitored by TLC. After completion, the reaction mixture was washed with EtOAc-H₂O and the organic phase was separated and dried over Na₂SO₄. The EtOAc was evaporated, and the further purification was done by using column chromatography.

3. Result and Discussion

To optimize the reaction conditions, different bases and solvents were screened in the presence of La₂O₃ as catalyst for the C–C coupling reactions. For the initial studies, we chose phenyl iodide and phenyl boronic acid as model substrates and various ligands and bases were screened (Table 1). The results revealed that the trans-1,2-diamino cyclohexane (L2) was the best ligand for the coupling reaction. N,N′-dimethylethane-1,2-diamine (L4) was proved to be an effective ligand in the coupling reaction; in fact, the reaction took a little longer time to complete as compared to the L2 (entry 3 versus entry 16). On the basis of base optimization results, KO-t-Bu was found to be the best base among the rest of the bases which have been used for optimization (Table 1, entry 3). Among the different solvents, DMSO gave the best results (Table 1, entry 3). On the ground of optimization results, we concluded that phenyl iodide and phenyl boronic acid in combination of La₂O₃ (10 mol%), KO-t-Bu (2 equiv.), trans-1,2-diaminocyclohexane (L2) (10 mol%), and DMSO as solvent at 150°C is the most efficacious reaction condition.

Table 1: La₂O₃-catalyzed C–C coupling of aryl halides with phenyl boronic acid.

Entry
Base
Ligand
Catalyst
Solvent
Time (h)a
Yield (%)b

1
NaOH
L2
La₂O₃
DMSO
18
65

2
Na₂CO₃
L2
La₂O₃
DMSO
16
23

3
KO-t-Bu
L2
La₂O₃
DMSO
17
84

4
Cs₂CO₃
L2
La₂O₃
DMSO
18
70

5
K₂CO₃
L2
La₂O₃
DMSO
16
72

6
KOH
L2
La₂O₃
DMSO
15
70

7
KO-t-Bu
L2
La₂O₃
DMF
18
73

8
KO-t-Bu
L2
La₂O₃
THFc
19
15

9
KO-t-Bu
L2
La₂O₃
Toluenec
21
40

10
KO-t-Bu
L2
La₂O₃

23
22

11
KO-t-Bu
L2
La₂O₃
CH₃CNc
23
51

12
KO-t-Bu
L2
La₂O₃
EtOAcc
24
10

13
KO-t-Bu
L2
La₂O₃
1,4-dioxanec
24
15

14
KO-t-Bu
L1
La₂O₃
DMSO
21
32

15
KO-t-Bu
L3
La₂O₃
DMSO
24
73

16
KO-t-Bu
L4
La₂O₃
DMSO
20
80

17
KO-t-Bu
L5
La₂O₃
DMSO
25
20

18
KO-t-Bu
L6
La₂O₃
DMSO
20
51

aMonitored using TLC. bIsolated yield after column chromatography of the crude product. cThe reaction mixture was set to reflux.

After optimizing the reaction conditions, we have explored the substrate scope by carrying out the reaction with various aryl halides and the results are illustrated in Table 2.

Table 2: La₂O₃-catalyzed coupling of substituted phenyl iodide with substituted aryl boronic acids.

Entry
Boronic acid
Aryl halide
Product
Time (h)a
Yield (%)b

1

17
84

2

14
86 (12)

3

13
80 (11)

4

17
81 (15)

5

12
90 (5)

6

20
80 (12)

7

19
85 (10)

8

24
80 (12)

9

18
82 (10)

10

19
80 (12)

11

13
81 (14)

12

12
85 (12)

13

16
81 (15)

aMonitored using TLC. bIsolated yield after column chromatography of the crude product. cYield in parenthesis is the homocoupling product from boronic acid.

Alkyl substituted halide substrates (Table 2, entries 2–5) gave good yield as shown in Table 2. In case of phenyl boronic acid and phenyl iodide coupling reaction, the observed yield was good, since the homocoupled and coupled product is biphenyl. Electron donating substrates (Table 1, entries 2–5) provided biaryl in shorter reaction time as compared to the electron withdrawing substrates (Table 2, entries 2–5 versus entries 6–8). As a matter of fact, a small amount of homocoupled products were observed in the reaction mixtures. There is a report which discusses the formation of homo-coupled product from boronic acids under similar conditions [26]. To confirm this, we have performed the reaction without phenyl iodide and observed 5–15% of the homo-coupled product in the reaction mixture.

4. Conclusion

In summary, we have developed an efficient La₂O₃-catalyzed new route for the carbon-carbon bond formation. The developed catalytic system shows the wide range substrate applicability and functional group tolerance. A small amount of homo-coupled aryls as side product was observed.

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