Facile One-Pot Synthesis of Diaryliodonium Salts from Arenes and Aryl Iodides with Oxone.

A straightforward synthesis of diaryliodonium salts is achieved by using Oxone as the stoichiometric oxidant. Slow addition is the key to obtaining good yields and purities of the reaction products, which are highly useful reagents in many different areas of organic synthesis.

Diaryliodonium salts are well‐investigated compounds in organic synthesis. They are typically prepared from the corresponding iodoarenes by using oxidative reaction conditions, but iodine(III) compounds have also been used for the generation of diaryliodonium salts and most of these reactions have been summarized in Review articles.1 These compounds have found many applications in synthesis,2 but also in polymerizations as photoinitiators, and some of these compounds even possess biologic activities. Many different strategies have been reported for the synthesis of this compound class by various research groups3 and us.4

Herein, we describe the straightforward and facile synthesis of symmetrically and unsymmetrically substituted diaryliodonium salts from arenes and iodoarenes in the presence of trifluoroacetic acid with Oxone as stoichiometric oxidant. Similar procedures have been reported by using peroxodisulfates as oxidants,5 but Oxone is more economically viable. We have already investigated the use of Oxone in the presence of trifluoroacetic acid and found it to be more convenient in the synthesis of [bis(trifluoroacetoxy)iodo]arenes.6 Oxone has also been used by Yakura et al. for the oxidation of iodine(I).7 The experimental procedure used to obtain the reaction products from iodoarenes 1 and arenes 2 is very simple. Initially, iodoarene 1 is reacted with Oxone in the presence of trifluoroacetic acid to generate [bis(trifluoroacetoxy)iodo]arenes before arene 2 is added to the reaction mixture (method A). The workup only consists of an extraction followed by recrystallization of the reaction product. The diaryliodonium compounds are obtained as the trifluoroacetate salts in very good yields (Scheme 1, Table 1). The yields of the overall transformation could be improved when arene 2 was added slowly over a period of several hours (method B) to the preformed [bis(trifluoroacetoxy)iodo]arenes. In other cases, the speed of arene addition did not have an influence on the outcome of the synthesis of diaryliodonium salts 3.

Scheme 1

Synthesis of diaryliodonium trifluoroacetates using Oxone as a stoichiometric oxidant.

Table 1

Synthesis of diaryliodonium trifluoroacetates.

Entry	Iodoarene 1	Arene 2	Product 3	Yield 3 [%] (method)[a]
1	PhI	PhMe		49 (A)[b] 64 (B)[b]
2	PhI	1,2‐Me2C6H4		80 (A)
3	PhI	1,3,5‐Me3C6H3		92 (A) 94 (B)
4	PhI	PhOMe		81 (B)
5	3‐CF3C6H4I	PhMe		87 (B)[b]
6	3‐CF3C6H4I	1,4‐Me2C6H4		86 (B)
7	3‐CF3C6H4I	1,3,5‐Me3C6H3		89 (A) 87 (B)
8	3‐CF3C6H4I	4‐MeOC6H4Br		94 (B)
9	C6F5I	PhMe		62 (B)[b]
10	C6F5I	1,4‐Me2C6H4		59 (B)
11	C6F5I	1,3,5‐Me3C6H3		74 (A) 80 (B)
12	C6F5I	PhOMe		91 (B)

[a] Method A: Addition of 2 directly after formation of [bis(trifluoroacetoxy)iodo]arene. Method B: Addition of 2 over 6 h after formation of [bis(trifluoroacetoxy)iodo]arene. [b] Small amounts (2–5 %) of ortho‐isomer were observed.

By exchanging trifluoroacetic acid with trifluoromethanesulfonic acid, the corresponding diaryliodonium triflates are accessible. However, slow addition of a mixture of iodoarene 1 and arene 2 to the Oxone with trifluoromethanesulfonic acid is required for good yields in this process. Fast addition will lead to dark‐colored solutions and to the formation of decomposition products. The reactions leading to symmetrical as well as unsymmetrical diaryliodonium triflates are summarized in Table 2. Recently, similar protocols have been reported for the synthesis of unsymmetrical diaryliodonium tosylates, although the synthesis consisted of two steps: firstly the oxidation to iodine(III) and secondly the generation of the diaryliodonium species.8

Table 2

Synthesis of diaryliodonium triflates.

Entry	Iodoarene 1	Arene 2	Product 4	Yield 4 [%]
1	PhI	PhH		31
2	PhI	PhMe		68[a]
3	PhI	1,4‐Me2C6H4		52
4	PhI	1,3,5‐Me3C6H3		82
5	3‐CF3C6H4I	PhH		33
6	3‐CF3C6H4I	PhMe		51[a]
7	3‐CF3C6H4I	1,4‐Me2C6H4		51
8	3‐CF3C6H4I	1,3,5‐Me3C6H3		84
9	4‐ClC6H4I	PhMe		61[a]
10	4‐ClC6H4I	1,3,5‐Me3C6H3		53
11	4‐BrC6H4I	PhMe		60[a]
12	4‐BrC6H4I	1,3,5‐Me3C6H3		61
13	3‐I‐C5H4N	1,3,5‐Me3C6H3		68

[a] Small amounts (2–5 %) of ortho‐isomer were observed.

The yields in this reaction are moderate to good and all products have been purified by recrystallization from ethyl acetate/ hexane (1:10). The reaction is limited to arenes with electron‐donating substituents. Even in the case of benzene as an unsubstituted arene, the yield of the aryliodonium salt is quite low, as compound 4 a was isolated in only 31 % yield (Table 2, entry 1). Entry 13 (Table 2) shows that also heteroaromatic iodides such as 3‐iodopyridine can be employed in this reaction, leading to the diaryliodonium compound 4 m in good yield.

The reaction described here is operationally very simple, uses readily available reagents and starting materials and provides pure reaction products in good yields. Synthetic applications of the diaryliodonium compounds are not provided here; electronically9 or sterically very different substituents can react selectively, whereas similar aryl moieties often lead to mixed results in aryl‐transfer reactions.10 Recently, even catalytic approaches involving diaryliodonium compounds have been published.11

In summary, we report a practically very simple, high‐yielding process to make diaryliodonium triflates and trifluoroactetates in good yields by using Oxone as a cheap stoichiometric oxidant.

Experimental Section

Synthesis of Diaryliodonium Trifluoroacetates

Method A

To a solution of iodoarene 1 (1.0 mmol) in a mixture of trifluoroacetic acid (1.5 mL) and chloroform (0.5 mL), Oxone (400 mg, 1.3 mmol) was added whilst stirring at room temperature. After complete disappearance of the iodoarene (monitored by TLC), arene 2 (1.7–3.5 mmol) was added and the solution was stirred for approximately 20 h (see supporting information). A change to a dark brown color was observed. After addition of CH2Cl2 (5 mL) and H2O (5 mL), the organic layer was separated, washed with H2O (5 mL), dried (Na2SO4), and the organic solvent removed under vacuum. The crude products 3 were further purified by recrystallization from EtOAc/hexane (1:10).

Method B

To a solution of iodoarene 1 (1.0 mmol) in a mixture of trifluoroacetic acid (1.5 mL) and chloroform (0.5 mL), Oxone (400 mg, 1.3 mmol) was added whilst stirring at room temperature. After complete disappearance of the iodoarene (monitored by TLC), arene 2 (1.05–1.50 mmol) in CHCl3 (2 mL) was added over 6 h by using a syringe pump, and the solution was stirred for approximately 20 h (see the Supporting Information). A change to a light brown color was observed. After addition of CH2Cl2 (5 mL) and H2O (5 mL), the organic layer was separated, washed with H2O (5 mL), dried (Na2SO4), and the organic solvent removed under vacuum. The crude products 3 were further purified by recrystallization from EtOAc/hexane (1:10).

Synthesis of Diaryliodonium Triflates

To a solution of trifluoromethanesulfonic acid (2.26 mmol, 339 mg, 200 μL) and Oxone (614 mg, 2 mmol) in CH2Cl2 (2 mL), a solution of iodoarene 1 (1.0 mmol) and arene 2 (1.2 mmol) in acetonitrile (2 mL) was added over 6 h by using a syringe pump. Then, Oxone (307 mg, 1 mmol) was added to the reaction mixture. The reaction mixture was stirred at room temperature overnight and a color change from red–orange to pale yellow was observed. After addition of CH2Cl2 (5 mL) and H2O (5 mL), the organic layer was separated, washed with H2O (5 mL), dried (Na2SO4), and the organic solvent removed under vacuum. The crude products 4 were further purified by recrystallization from EtOAc/hexane (1:10).

Conflict of interest

The authors declare no conflict of interest.

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Supplementary

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