Oxyfunctionalization of organic substances with singlet molecular oxygen (1O2) has received significant attention, both from the academic and industrial points of view, as it is an important method for the construction of the carbon–oxygen bond . More importantly, 1O2 is a well-established oxidant in photodynamic therapy, which is widely used to treat some malignant cancers . The common method of generating 1O2, dye-sensitized excitation of triplet molecular oxygen, can be challenging when applied to industrial-scale production, because of the need for specially designed photochemical reactors and materials for safe processing . This problem is partially solved by the availability of various chemical sources of 1O2 that also allow for precise control of the 1O2 concentration over the substrate amount [4–6]. Recently, we introduced a novel protocol for the generation of 1O2 via H2O2 decomposition by hypervalent aryl-λ3-iodane phenyliodine bis(trifluoroacetate) (PIFA) 1a (Scheme 1) . A key feature of this protocol is the oxidation of H2O2 with 1a to give a reactive intermediate hydrogen tetraoxide (H2O4), which subsequently undergoes homolytic dissociation to singlet oxygen and H2O2. Inspired by the mechanism underlying 1O2 generation from the PIFA/H2O2 system, we hypothesized that if a hypervalent aryl-λ3-iodane reagent such as an iodosylarene (ArIO) is generated in situ by the oxidation of an iodoarene, subsequent oxidation of H2O2 with this reactive species would produce 1O2. Thus, we needed to develop a protocol in which H2O2 would first oxidize the iodoarene to an iodosylarene and finally reduce iodosylarene to iodoarene, thereby producing 1O2. Unfortunately, direct oxidation of the iodine atom of the arene using H2O2 is not possible, and therefore, H2O2 must be activated by a catalyst for efficient oxygen transfer.
Herein, we report the first example of the methyltrioxorhenium (MTO) -catalyzed in situ generation of an iodosylarene (ArIO) from an iodoarene and H2O2, followed by subsequent reaction with H2O2 to produce 1O2 (eq. 1).
Results and discussion
At the outset of our study, we chose the oxidation of iodobenzene (4a, 1 equiv, Fig. 1) with the MTO/35 % H2O2 (0.05 equiv/10 equiv) system  at ambient temperature in dichloromethane (DCM) as a model reaction in order to monitor 1O2 generation in the presence of 9,10-dimethylanthracene (DMA, 2a) as the chemical trap. The results are summarized in Table 1. The 4a/MTO/35 % H2O2 combination afforded 1O2, as evidenced by the formation of 3a  in 20 % yield (entry 1). It is important to note that in the absence of the 4a, a complex mixture was obtained, indicating that the 4a is essential for the 1O2 generation. To improve the level of conversion through MTO stabilization , a series of tertiary amines – imidazole, 2,6-dimethylpyridine, phenanthroline, N-methylimidazole, and pyridine – were tested for use as the co-catalyst (entries 2–6). The results showed that pyridine had a significant influence on the reaction rate and the yield of 3a (entry 6). Next, attempts were made to further optimize the reaction conditions by screening a series of iodoarenes 4b–n  (Fig. 1).
Depending on the structure of the iodoarene, the conversion and yield varied from 56 to 98 % and from 26 to 86 %, respectively (Table 1, entries 7–18). The highest efficiency in terms of yield and conversion was achieved with 2-iodomesitylene (MesI, 4d) (entry 9). Thus, we concluded that the best choice of iodoarene and co-catalyst would be the 4d and pyridine, respectively, for obtaining endoperoxide 3a in high yield with a high conversion.
It is well known that MTO  is a very efficient catalyst for olefin epoxidation with H2O2 as the oxidant. Hence, we must take into account the chemoselectivity of the present oxidation system in terms of epoxide formation vs. peroxide formation in the presence of olefin substrates. To assess the synthetic feasibility and chemoselectivity of the present oxidation system, oxidation of a set of substrates, including aromatic dienes 2a–d, 1,3-cyclodienes 2e–g, and alkenes 2h,i, was examined. The results are listed in Table 2.
In order to minimize epoxide formation and shift the balance in favor of iodine oxidation, an essential step for 1O2 generation, the reaction conditions were reoptimized to include the 4d/MTO/35 % H2O2/pyridine in 4:0.1:15:0.5 ratio relative to substrate 2. When 2a was exposed to the chemical peroxidation conditions, complete conversion of the starting material was observed, and the corresponding endoperoxide 3a  was obtained in 94 % yield (entry 1). With 2b, fast oxygenation occurred, as indicated by the disappearance of the intense color of the starting material and the formation of 3b  (entry 2). The relatively less reactive substrate 2c was oxygenated to 3c  but with a slightly lower yield of 48 % at 60 % conversion (entry 3). The phenolic substrate 2d (entry 4) gave a low yield of hydroperoxide 3d , with 45 % conversion. Oxygenation of α-terpinene (2e) at complete consumption gave ascaridole (3e)  in 75 % yield, along with p-cymene (5a) in 80:20 ratio (entry 5). Likewise, 1,3-cyclohexadiene (2f) was transformed into the corresponding endoperoxide 3f  in high yield (72 %), along with benzene (5b) as the byproduct (entry 6). Aromatic products 5a,b were formed by oxidation of the starting material with the in situ generated iodosylmesytilene (MesIO, 10)  species, as confirmed by a control experiment involving 1,3-cyclodienes 2e,f and 10. Notably, no trace of epoxide resulting from the starting materials 2e,f or products 3e,f was detected by NMR analysis. Oxidation of 2g gave endoperoxide 3g  in high yield (87 %), with an appreciable amount of epoxide 6a  in 93:7 ratio (entry 7). An electron-rich alkene, 2,3-dimethyl-2-butene (2h), when subjected to the established reaction conditions afforded a mixture of oxidized products consisting of allylic hydroperoxide 3h , allylic alcohol 7a , and epoxide 6b  (entry 8). The formation of 7a can be rationalized in terms of the reduction of 3h under the reaction conditions. The observed allylic oxygenation products 3h and 7a, which resulted from the 1O2 ene reaction, and the 6b ratio (37:63) indicated that present protocol shows no preference for the oxidation of the 4d by MTO/H2O2 over that of double-bond oxidation when an electron-rich alkene such as 2h is employed as the substrate. Likewise, oxidation of substrate 2i bearing distinct allylic hydrogens afforded the corresponding allylic oxidation products 3i,k , 7b , 8 , and epoxide 6c  in 28 and 47 % yields, respectively (entry 9), in 40:60 ratio.
A plausible mechanism for 1O2 generation from the 4d/MTO/35 % H2O2 system is as follows (Scheme 2). First, the 4d is oxidized to the active aryl-λ3-iodane species the 10 by the action of the MTO peroxy complex 9. Subsequent reaction of H2O2 with the 10 generates mesitylene-λ3-peroxyiodane 11 [7, 27], which rapidly undergoes homolytic dissociation to hydroperoxyl radical and the 4d. Hydroperoxyl radical combines with another one to form the reactive intermediate H2O4 , decomposing to give 1O2 and H2O2.
To corroborate the involvement of the 10 in the 1O2 generation, we carried out the reaction of 4 equiv of pure sample of the 10 (relative to the singlet oxygen acceptor 2a) with 15 equiv of 35 % H2O2 in DCM (Scheme 3). After the usual work-up, the mixture was analyzed by NMR, which confirmed the formation of the corresponding endoperoxide 3a in 95 % yield and at 99 % conversion. Similarly, the oxygenation of α-terpinene (2e) under aforementioned conditions afforded ascaridole (3e) in 76 % yield, with 99 % conversion.
In conclusion, we have presented a novel protocol for 1O2 generation from H2O2, induced by in situ generated hypervalent aryl-λ3-iodanes from iodoarenes and MTO/35 % H2O2 in the presence of pyridine as the co-catalyst.
General aspects and product identification
400 MHz 1H NMR and 100 MHz 13C NMR spectra were acquired on Bruker and Varian spectrometers. Chemical shifts are reported relative to tetramethylsilane (TMS) and chloroform-d, which were used as internal standards. Substrates 2a–i, iodoarenes 4a–m, MTO, and tertiary amines were used as received. Compounds 4n and 10 were prepared according to the reported procedures. All the oxidation products are known in the literature and were identified by comparison with the previously reported NMR data.
Optimizing reaction conditions for 1O2 generation from the ArI/MTO/35 % H2O2 system (main text, Table 1)
In a 25-mL round-bottomed flask equipped with a magnetic stirrer were placed CH2Cl2(5 mL), iodoarene (1 mmol), 35 % H2O2 (0.86 mL, 10 mmol), and a co-catalyst (0.25 mmol), as specified in Table 1. The reaction was initiated by the addition of a solution of methyltrioxorhenium (MTO) (12.5 mg, 0.05 mmol) and 9,10-dimethylanthracene (2a, 0.207 g, 1 mmol) in CH2Cl2 (2 mL) via a syringe over 2 h at room temperature. The resulting mixture was further stirred for 4 h. After the addition of diphenylmethane (168 mg, 1 mmol) as an internal standard, the reaction mixture was diluted with water (5 mL), extracted with CH2Cl2 (2×5 mL), and dried over Na2SO4. The solvent was removed under reduced pressure (15 °C, 25 Torr), and the conversion and yield were directly determined by 1H NMR analysis of the crude mixture.
Preparative oxygenation of various substrates with the 4d/MTO/35 % H2O2 system (main Text, Table 2)
In a 25-mL round-bottomed flask equipped with a magnetic stirrer were placed CDCl3 (5 mL), 2-iodomesitylene (4d, 0.98 g, 4 mmol), 35 % H2O2 (1.29 mL, 15 mmol), and pyridine (40 μL, 0.5 mmol). The reaction was initiated by the addition of a solution of MTO (25.0 mg, 0.1 mmol) and substrate (1 mmol) in CDCl3 (2 mL) via a syringe over 2 h at room temperature. The resulting mixture was additionally stirred for 4 h. After the addition of diphenylmethane (168 mg, 1 mmol) as an internal standard, the reaction mixture was diluted with water (5 mL), extracted with CDCl3 (2×5 mL), and dried over Na2SO4. The conversion, yield, and product distribution were directly determined by 1H NMR analysis of the crude mixture. After removal of the solvent (15 °C, 25 Torr), the residue was loaded on a silica gel column and eluted with a 90:10 mixture of hexane and Et2O. The first fractions gave 2-iodomesitylene (4d). Further elution afforded the corresponding oxygenation product (main text, Table 2, entries 1–7).
Procedure for the oxidation of 9,10-dimethylanthracene (2a) and α-terpinene (2e) with the MesIO–H2O2 system (main text, Scheme 3)
To a solution of substrate (2, 1 mmol) and 35 % H2O2 (1.29 mL, 15 mmol) in CH2Cl2 (5 mL) at room temperature was added MesIO (10, 1.05 g, 4.0 mmol) in portions over 2 h. The mixture was additionally stirred for 2 h. After the addition of diphenylmethane (168 mg, 1 mmol) as an internal standard, the mixture was extracted with CH2Cl2 (2×10 mL), washed with water (10 mL), and dried over Na2SO4. After removal of the solvent at reduced pressure (15 °C, 25 Torr), the conversion and yield were directly determined by 1H NMR analysis of the crude mixture.
9,10-Dimethyl-4a,9,9a,10-tetrahydro-9,10-epidioxyanthracene (3a): Isolated yield: 94 %; 1H NMR (CDCl3): 7.41–7.37 (AA′ part of AA′BB′ system, 4H), 7.29–7.26 (BB′ part of AA′BB′ system, 4H), 2.14 (s, 6H). 13C NMR (CDCl3): 140.8, 127.4, 120.6, 79.5, 13.7.
Benzene-1,2-diylbis(phenylmethanone) (3b): Isolated yield: 95 %; 1H NMR (CDCl3): 7.71–7.36 (m, 14H). 13C NMR (CDCl3): 196.6, 140.0, 137.2, 133.0, 130.3, 129.8, 129.6, 128.3.
9,10-Diphenyl-9,10-dihydro-9,10-epidioxyanthracene (3c): Isolated yield: 48 %; 1H NMR (CDCl3): 7.71–7.69 (m, 4H), 7.65–7.61 (m, 4H), 7.56–7.52 (m, 2H), 7.22–7.16 (m, 8H). 13C NMR (CDCl3): 140.2, 133.0, 128.3, 128.2, 127.6, 127.5, 123.5, 84.1.
4-Hydroperoxy-2,4,6-trimethylcyclohexa-2,5-dien-1-one (3d): Isolated yield: 20 %; 1H NMR (CDCl3): 8.02 (s, 1H), 6.64 (s, 2H), 1.92 (s, 6H), 1.37 (s, 3H). 13C NMR (CDCl3):186.6, 144.2, 136.8, 78.6, 23.0, 16.0.
1-Methyl-4-(propan-2-yl)-2,3-dioxabicyclo[2.2.2]oct-5-ene (3e): Isolated yield: 70 %; 1H NMR (CDCl3): 6.50 (d, 3JH,H= 8.6 Hz, A part of AB system, 1H), 6.42 (d, 3JH,H = 8.6 Hz, B part of AB system, 1H), 2.05–1.98 (m, 2H), 1.96–1.89 (m, 1H), 1.53–1.51 (m, 2H), 1.38 (s, 3H), 1.0 (d, J = 7.0 Hz, 6H). 13C NMR (CDCl3): 136.4, 133.0, 79.8, 74.4, 32.1, 29.5, 25.6, 21.4, 17.2, 17.2.
2,3-Dioxabicyclo[2.2.2]oct-5-ene (3f): Isolated yield: 65 %; 1H NMR (CDCl3): 6.69 (dd, 3JH,H = 4.4, 3.3 Hz, 2H), 4.68–4.64 (m, 2H), 2.31–2.25 (m, 2H, AA′ part of AA′BB′ system), 1.51–1.47 (m, 2H, BB′ part of AA′BB′ system).13C NMR (CDCl3): 132.1, 70.8, 21.5.
6,7-Dioxabicyclo[3.2.2]non-8-ene (3g): Isolated yield: 74 %; 1H NMR (CDCl3): 6.40 (dd, 3JH,H = 4.9, 3.2 Hz, 2H), 4.76–4.73 (m, 2H), 2.00–1.86 (m, 4H), 1.61–1.55 (m, 1H), 1.46–1.30 (m, 1H). 13C NMR (CDCl3): 127.9, 77.3, 31.7, 18.4.
8-Oxabicyclo[5.1.0]oct-2-ene (6a): 1H NMR (CDCl3): 5.91–5.88 (m, 1H), 5.81–5.77 (m, 1H), 3.44–3.43 (m, 1H), 3.27–3.25(m, 1H), 2.25–2.23 (m, 2H), 1.99–1.98 (m, 2H), 1.60–1.59 (m, 2H).
2,3-Dimethylbut-3-en-2-yl hydroperoxide (3h): 1H NMR (CDCl3): 7.41 (s, 1H), 5.02–5.01 (m, 1H), 4.97–4.96 (m, 1H), 1.82 (dd, 3JH,H = 1.4, 0.8 Hz, 3H), 1.37 (s, 6H). 13C NMR (CDCl3): 148.0, 112.0, 84.4, 23.9, 18.8.
2,3-Dimethylbut-3-en-2-ol (7a): 1H NMR (CDCl3): 5.00 (m, 1H), 4.77 (m, 1H), 1.81 (m, 3H), 1.54 (bs, 1H), 1.35 (s, 6H). 13C NMR (CDCl3): 151.9, 108.4, 73.1, 28.8, 19.2.
Tetramethyloxirane (6b): 1H NMR (CDCl3): 1.32 (s, 12H). 13C NMR (CDCl3): 62.0, 21.1.
2-Methylpent-1-en-3-yl hydroperoxide (3i): 1H NMR (CDCl3): 7.73 (s, 1H), 5.04 (m, 1H), 5.02 (m, 1H), 4.25 (t, 3JH,H = 6.9, 1H), 1.73 (s, 3H), 1.67–1.44 (m, 2H), 0.92 (t, 3JH,H = 7.4, 3H). 13C NMR (CDCl3): 143.4, 114.4, 91.1, 23.7, 17.1, 10.0.
(3E)-2-Methylpent-3-en-2-yl hydroperoxide (3k): 1H NMR (CDCl3): 7.84 (s, 1H), 5.73 (m, 1H), 5.57 (d, 3JH,H = 1.4, 1H), 1.74 (d, 3JH,H = 1.4, 3H), 1.32 (s, 6H). 13C NMR (CDCl3): 134.3, 127.0, 82.2, 24.2, 14.1.
2-Methylpent-1-en-3-ol (7b): 1H NMR (CDCl3): 4.94–4.92 (m, 1H), 4.86–4.84 (m, 1H), 3.99 (t, 3JH,H = 6.5 Hz, 1H), 1.72 (t, 3JH,H= 1.3 Hz, 3H,), 1.69–1.67 (m, 1H), 1.66–1.50 (m, 2H), 0.89 (t, 3JH,H= 7.5 Hz, 3H).
3-Ethyl-2,2-dimethyloxirane (6c): 1H NMR (CDCl3): 2.72 (t, 3JH,H = 7.0 Hz, 1H), 1.53 (m, 2H), 1.35 (s, 3H), 1.30 (s, 3H), 1.00 (t, J = 7.0 Hz, 3H). 13C NMR (CDCl3): 65.1, 57.6, 24.9, 22.4, 18.5, 10.6.
2-Methylpent-1-en-3-one (8): 1H NMR (CDCl3): 5.95 (m, 1H), 5.75 (m, 1H), 2.72 (q, 3JH,H = 7.3, 2H), 1.88 (s, 3H), 1.10 (t, 3JH,H = 7.3, 3H). 13C NMR (CDCl3): 202.6, 144.3, 124.1, 30.6, 17.7, 8.4.
Financial support by the Scientific and Technological Research Council of Turkey (TUBITAK) (TBAG-109T161) is gratefully acknowledged.
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