Synthesis of arylbenziodoxoles using pseudocyclic benziodoxole triflate and arenes

An acid activated pseudocyclic hypervalent iodine reagent, 2-[hydroxy(trifluoromethanesufonyloxy)]- iodobenzoic acid, can easily react with various arenes in the presence of trifluoromethanesulfonic acid to produce pseudocyclic diaryliodonium triflate salts. This synthetic procedure proceeds under mild conditions to afford the respective iodonium salts in moderate to good yields. Several pseudocyclic diaryliodonium triflate salts structures have been confirmed by X-ray crystallography. Obtained products can be easily converted to cyclic hypervalent iodine(III) compounds, arylbenziodoxolones, in moderate to good yields under basic conditions


Introduction
Hypervalent iodine compounds have been used as efficient oxidizing reagents and employed in many conversion reactions in organic synthesis.3][14][15][16][17] Diaryliodonium salts are also used as benzyne precursor reagents, and many benzyne-mediated reactions utilizing these reagents have been reported. 18,19While numerous reaction examples using diaryliodonium salts are well-known, a number of synthetic studies on them have also been reported. 12A typical synthetic method of diaryiodonium salts is to combine common hypervalent iodine(III) reagents with aromatic compounds under appropriate conditions.As a developed synthetic example, the diaryliodonium compounds can be prepared from iodoarenes and aromatic compounds by treatment with suitable oxidants.
However, the reactivity towards other arenes using IBA-TfOH has not been reported.In the present study, we report the optimization and scope for the preparation of pseudocyclic diaryliodonium triflates from IBA-TfOH reagents with various arenes in the presence of trifluoromethanesulfonic acid.The obtained products can be further converted to the cyclic hypervalent iodine(III) compounds, arylbenziodoxolones, under mild basic conditions. 41,42

Results and Discussion
We investigated the preparation of pseudocyclic diaryliodonium triflate 3a using IBA-TfOH 1 and benzene 2a in different solvents based on our previous results (Table 1).In the initial study, the reaction was performed in 2,2,2-trifluoroethanol (TFE) in the absence of TfOH under our previously reported condition resulting in no desired product 3a, and the reagent 1 was recovered from the reaction mixture (entry 1).The reaction using excess amount of benzene 2a in TFE or dichloromethane also did not produce the desired product 3a (entries and 3).Addition of TfOH as an additive in dichloromethane dramatically improved the reaction to give the desired product 3a in quantitative yield (entry 4).Screening of various solvents in the presence of TfOH has demonstrated that dichloromethane is the best solvent in this transformation reaction (entries 4-10).Decreasing the amount of benzene 2a from 56.0 to 4.0 equivalents did not affect the yield of product 3a (entries 4, 11-13).Further decreasing the amount of benzene 2a led to a reduced yield of product 3a (entries 14-16).a Reaction conditions: IBA-TfOH 1 (1 equiv.),benzene 2a (1-56 equiv.)and TfOH (0-1.2 equiv.) in various solvent (2.0 mL) at room temperature for 24 hours.b Yield of isolated product.c In the absence of TfOH.d IBA-TfOH 1 was recovered from the reaction mixture.
By using the optimal conditions, we have investigated the conversion of various substituted arenes 2 to the respective pseudocyclic diaryliodonium triflates 3.In general, the reaction of arenes with either electrondonating or electron-withdrawing substituents gave the corresponding desired products 3a-l in moderate to good yields.The reaction with sterically bulky ortho-substituted arenes also gave the pseudocyclic diaryliodonium triflates 3 in moderate to good yields (Table 2).Structures of pseudocyclic diaryliodonium triflates 3a, 3f and 3k were established by X-ray crystallography (Figure 2).According to the X-ray data, the © AUTHOR(S) pseudocyclic diaryliodonium structures with strong intramolecular interaction between iodine and oxygen atoms were observed. 20,30,31,37,43The triflate oxygen atom was also involved in a weak intermolecular interaction with iodine atom resulting in a pseudo-square planar coordination of hypervalent iodine center.Table 2. Preparation of pseudocyclic diaryliodonium triflates 3 using IBA-TfOH 1 with various arene 2 a a Reaction conditions: IBA-TfOH 1 (1 equiv.),arene 2 (4 equiv.)and TfOH (1.2 equiv.) in dichloromethane at room temperature for 24 hours.Next, we investigated a one-pot preparation of pseudocyclic diaryliodonium triflate 3a from benzene 2a and IBA-TfOH 1 generated in situ from 2-iodosylbenzoic acid 4a with trifluoromethanesulfonic acid (Scheme 2).This reaction gave the pseudocyclic diaryliodonium triflate 3a in quantitative yield (eq. 1). 37Compared to the previously reported method for preparation of pseudocyclic diaryliodonium triflates, our one-pot procedure was able to afford these products in comparable yields. 44As expected, the reaction of substituted iodosylbenzoic acids 4b,c under one-pot method resulted in the corresponding compounds 3m,n in good yields (eq.1). 45Furthermore, we have found that 3a could be prepared in moderate yield from 2-iodobenzoic acid and benzene using m-chloroperoxybenzoic acid in the presence of trifluoromethanesulfonic acid (eq.2). 41heme 2. One-pot synthesis of pseudocyclic diaryliodonium triflates.
Finally, we have demonstrated that the obtained pseudocyclic diaryliodonium triflates could be converted to arylbenziodoxolones. 41,42,46,47 In particular, the treatment of 3a with sodium bicarbonate could lead to phenylbenziodoxolone 6a in quantitative yield.Following this strategy, we performed the reaction of substituted pseudocyclic diaryliodonium triflates 3a-n resulting in the desired arylbenziodoxolones 6a-n in moderate to good yields (Table 3).Table 3. Preparation of arylbenziodoxolones 6 from pseudocyclic diaryliodonium triflates 3 a a Reaction conditions: Pseudocyclic diaryliodonium triflates 3 in saturated NaHCO 3 aqueous -dichloromethane (1:2) at room temperature for 1 hour.b The isomeric ortho and para products were detected.

Conclusions
In summary, we have prepared the pseudocyclic diaryliodonium triflates from IBA-TfOH and arenes in the presence of TfOH.The structure of several products 3 was confirmed by X-ray crystallography.The combination of 2-iodosylbenzoic acid and trifluoromethanesulfonic acid, or 2-iodobenzoic acid and mchloroperoxybenzoic acid in the presence of trifluoromethanesulfonic acid generates IBA-TfOH in-situ, which can be used for the preparation of pseudocyclic diaryliodonium triflates.Furthermore, the produced pseudocyclic diaryliodonium triflates can be easily converted to the respective arylbenziodoxolones under mild basic condition.

Experimental Section
General.All reactions were performed under dry argon atmosphere with flame-dried glassware.Dichloromethane was distilled from CaH 2 immediately prior to use.All commercial reagents were ACS reagent grade and used without further purification.Melting points were determined in an open capillary tube with a Mel-temp II melting point apparatus.Infrared spectra were recorded on a PerkinElmer 1600 series FT-IR spectrophotometer.NMR spectra were recorded on a Varian Inova 500, 300 MHz and Bruker 400 MHz NMR spectrometer ( 1 H NMR and 13 C NMR).X-ray crystal analysis was performed by Rigaku RAPID II XRD Image Plate using graphite-monochromated Cu or Mo Kradiation (( = 1.54187 or 0.71073 Å) at 125 or 173 K. Please see the cif file for more detailed crystallography information.Hypervalent iodine reagents, IBA-TfOH 1, 39 , 2-Iodosylbenzoic acids 4a-b 48 , and 4c 49 were prepared according to the reported procedure.
2-Carboxyphenyl(phenyl)iodonium triflate (3a).Reaction of 2-iodosylbenzoic acid 4a (53 mg, 0.20 mmol) according to general procedure afforded 95 mg (100%) of product 3a isolated as a white solid identical to the same from previous experiment.One-pot preparation of pseudocyclic diaryliodonium triflates 3a using 2-iodobenzoic acid 5 and benzene 2a using TfOH with mCPBA.Benzene 2a (62 mg, 0.80 mmol), trifluoromethanesulfonic acid (66 mg, 0.44 mmol), and mCPBA (76 mg, 0.44 mmol) was added to a solution of 2-iodobenzoic acid 5 (50 mg, 0.20 mmol) in dichloromethane (1 mL).The reaction was stirred at room temperature for 24 hours.After completion of the reaction, the solvent was removed under reduced pressure and the solid product was washed with diethyl ether several times then dried in vacuum to give the pure compound 3a; 64 mg (68%) isolated as a brown solid identical to the same from previous experiment.

4-Methyl-2-carboxyphenyl(phenyl)iodonium triflate (3m
General procedure for preparation of arylbenziodoxolones 6 from pseudocyclic diaryliodonium triflates 3. Pseudocyclic diaryliodonium triflates 3 was added to dichloromethane (2 mL) and saturated NaHCO 3 (1 mL).The reaction was stirred at room temperature for 1 hour.After completion of the reaction, the solvent was removed under reduced pressure and the solid product was washed with diethyl ether several times then dried in vacuum to give the pure compound 6.

Table 1 .
Optimization of synthesis of pseudocyclic diaryliodonium triflate 3a a