Metal-free insertion of sulfur dioxide with aryl iodides under ultraviolet irradiation: direct access to sulfonated cyclic compounds

Abstract

Metal-free insertion of sulfur dioxide with aryl iodides and silyl enolates or allylic bromide under ultraviolet irradiation at room temperature is accomplished. This protocol provides a convenient route to sulfonated cyclic compounds under mild conditions. Not only N-(2-iodophenyl)-N-methylmethacrylamides but also 1-iodo-2-allenoxybenzene is workable. A plausible mechanism is proposed, which shows that during the reaction process, aryl radicals formed in situ from aryl iodides under ultraviolet irradiation undergo intramolecular 5-exo-cyclization, with subsequent sulfonylation via insertion of sulfur dioxide. The resulting sulfonyl radicals are further trapped by silyl enolates or allylic bromide giving rise to sulfonated cyclic compounds.

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As part of a program for the generation of sulfonyl-containing drugs and related compounds,¹ we are interested in method development with the insertion of sulfur dioxide by using the sulfur dioxide surrogates of DABCO·(SO₂)₂ (1,4-diazabicyclo[2.2.2]octane-sulfur dioxide) or potassium/sodium metabisulfite.^2,3 Recently, we focused on the radical processes with the insertion of sulfur dioxide, due to the advantages of mild conditions and excellent selectivity.⁴ So far, various radical precursors have been utilized in the transformation, including aryldiazonium tetrafluoroborates, arylhydrazines, aryl iodides/bromides, diaryliodonium salts, 4-substituted Hantzsch esters, and potassium alkyltrifluoroborates.⁴ Among these compounds, aryl iodides are attractive, as they are cheap and easily available.

Recently, rapid progress in the photoinduced reactions of aryl iodides under visible light or ultraviolet irradiation has been witnessed.^5,6 In some cases, the transformations could proceed under metal-free conditions under ultraviolet irradiation.⁷ For instance, Larionov and co-workers described the synthesis of arylboronic acids through a metal- and additive-free, photoinduced borylation of haloarenes under ultraviolet irradiation.^7a Encouraged by these results, we conceived that sulfonylation of aryl halides with the insertion of sulfur dioxide under photoinduced metal-free conditions would be feasible as well. Considering the importance of heterocycles and sulfonyl compounds in pharmaceutical and agrochemical molecules⁸ and prompted by the recent advances in the metal-free coupling of aryl halides under ultraviolet irradiation,⁷ we conceived that the construction of sulfonated cyclic compounds might be achieved via a tandem process through the insertion of sulfur dioxide from aryl iodides under photoinduced metal-free conditions. A proposed synthetic route is designed and presented in Scheme 1.

Scheme 1 Synthesis of sulfonated cyclic compounds via a tandem process through the insertion of sulfur dioxide under photoinduced metal-free conditions.

We envisioned that the C–I bond in compound 1 would be dissociated under ultraviolet irradiation, thus providing aryl radical intermediate A. Consequently, an intramolecular 5-exo radical cyclization and subsequent insertion of sulfur dioxide would occur leading to sulfonyl radical C. Functionalized alkene was designed to trap the resulting sulfonyl radical, affording another carbon radical intermediate D. This carbon radical intermediate D would undergo further transformation giving rise to the final outcome. Although this proposed route seemed feasible under suitable conditions, several competitive pathways involving the direct sulfonylation of aryl halide might be inevitable. Additionally, the resulting aryl radical or sulfonyl radical would be possibly captured by another alkene as well. To verify the practicability of this hypothesis in Scheme 1, we therefore started to explore the feasibility for the synthesis of sulfonated cyclic compounds via a tandem process through the insertion of sulfur dioxide under photoinduced metal-free conditions.

At the outset, the reaction of N-(2-iodophenyl)-N-methylmethacrylamide 1a, DABCO·(SO₂)₂, and trimethyl((1-phenylvinyl)oxy)silane 2a was selected for method development. At the beginning, the reaction was performed in MeCN at room temperature under ultraviolet irradiation (600 W, Table 1, entry 1). To our delight, the corresponding product 3a was generated in 26% yield. However, further screening of solvents could not provide an improved result. The yield was decreased to 6% when the reaction occurred in 1,4-dioxane (Table 1, entry 2). No reaction took place when THF or DMSO was used instead (Table 1, entries 3 and 4). Only a trace amount of the product was observed when the reaction was performed in DMF (Table 1, entry 5). The desired product 3a was afforded in 13% yield when toluene was utilized as the solvent (Table 1, entry 6). We also explored the reaction by using potassium metabisulfite or sodium metabisulfite as the source of sulfur dioxide (Table 1, entries 7 and 8). However, the results were inferior. With the addition of tetrabutylammonium iodide (TBAI) in the reaction system, the yield was increased to 45% (Table 1, entry 9). Changing the concentration of the mixture would result in the formation of compound 3a with higher yields (Table 1, entries 10 and 11). No better results were obtained when other additives such as TBAB, KI (Table 1, entries 12 and 13), NaI and I₂ (data not shown in Table 1) were used. Although the exact role of TBAI in this reaction was not clear, the presence of TBAI would improve the solubility of DABCO·(SO₂)₂ and thus increase the transmission rate. The reaction was hampered when the light conditions were changed to white 65 W LED light irradiation (Table 1, entry 14). Additionally, only a trace amount of the product was observed when aryl bromide was used as a replacement.

Table 1 Initial studies for the reaction of N-(2-iodophenyl)-N-methylmethacrylamide 1a, DABCO·(SO₂)₂, and trimethyl((1-phenylvinyl)oxy)silane 2a ^a

Entry	Solvent	Additive	“SO2”	Yield^b (%)
a Reaction conditions: N-(2-Iodophenyl)-N-methylmethacrylamide 1a (0.2 mmol), DABCO·(SO2)2 (0.2 mmol), trimethyl((1-phenylvinyl)oxy)silane 2a (0.3 mmol), solvent (4.0 mL), rt, N2. b Isolated yield based on N-(2-iodophenyl)-N-methylmethacrylamide 1a. c In the presence of MeCN (6.0 mL). d In the presence of MeCN (8.0 mL). e Under irradiation with white 65 W LED light.
1	MeCN		DABCO·(SO2)2	26
2	1,4-Dioxane		DABCO·(SO2)2	6
3	THF		DABCO·(SO2)2	NR
4	DMSO		DABCO·(SO2)2	NR
5	DMF		DABCO·(SO2)2	Trace
6	Toluene		DABCO·(SO2)2	13
7	MeCN		K2S2O5	17
8	MeCN		Na2S2O5	19
9	MeCN	TBAI	DABCO·(SO2)2	45
10^c	MeCN	TBAI	DABCO·(SO2)2	64
11^d	MeCN	TBAI	DABCO·(SO2)2	81
12^d	MeCN	TBAB	DABCO·(SO2)2	31
13^d	MeCN	KI	DABCO·(SO2)2	21
14^e	MeCN	TBAI	DABCO·(SO2)2	NR

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To explore the scope generality of this method, the metal-free reaction of (2-iodophenyl)-N-methylmethacrylamides 1, DABCO·(SO₂)₂ and silyl enolates under the above optimized conditions was then examined. As shown in Table 2, various (2-iodophenyl)-N-methylmethacrylamides 1 were workable with the insertion of sulfur dioxide, leading to the corresponding products. Although different functional groups were compatible under ultraviolet irradiation, the yields were not satisfactory in most cases. For instance, the ester-substituted product 3h was afforded in 46% yield, and the nitro-containing product 3i was produced in 30% yield. Although these results were not as good as expected, they also provided a convenient route to sulfonated cyclic compounds under mild conditions. The structure of compound 3b was confirmed by X-ray diffraction analysis (see the ESI†),⁹ which showed excellent chemoselectivity during the reaction process.

Table 2 Scope exploration for the reaction of N-(2-iodoaryl)acrylamides 1, DABCO·(SO₂)₂, and silyl enolates 2 under ultraviolet irradiation^a

a Isolated yield based on N-(2-iodoaryl)acrylamides 1.

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We further explored the reaction of (2-iodophenyl)-N-methylmethacrylamides 1, DABCO·(SO₂)₂ and allylic bromide under the standard conditions (Scheme 2). Interestingly, the corresponding products 4, 5 and 6 were afforded in 52%, 47% and 56% yields, respectively. Furthermore, the reaction of N-butyl-N-(2-iodophenyl)methacrylamide 7 with DABCO·(SO₂)₂ and trimethyl((1-phenylvinyl)oxy)silane 2a was examined, which afforded the corresponding 1-butyl-3-methyl-3-(((2-oxo-2-phenylethyl)sulfonyl)methyl)indolin-2-one 8 in 45% yield. However, no reaction occurred when N-benzyl-N-(2-iodophenyl)methacrylamide was employed under the conditions (data not shown in Scheme 2). Interestingly, 1-iodo-2-allenoxybenzene 9 could be used as a replacement for (2-iodophenyl)-N-methylmethacrylamide 1a in the reaction of DABCO·(SO₂)₂ and trimethyl((1-phenylvinyl)oxy)silane. This transformation was effective, giving rise to the expected benzofuran 10 in 50% yield (Scheme 2). Since we hypothesized that this route might be a radical process, we then examined the model reaction in Table 1 with the addition of 3.0 equivalents of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) under the standard conditions (data not shown in Scheme 2). Consequently, only a trace amount of the product 3a was observed. Additionally, the related product could be obtained in 31% yield (determined by NMR) when ethene-1,1-diyldibenzene was used as the radical trapper.

Scheme 2 Further exploration.

As suggested by the previous reports⁷ and the above experimental results, we postulated a plausible route for the outcomes, as shown in Scheme 3. We reasoned that under ultraviolet irradiation, aryl radical A would be formed from aryl iodide via the cleavage of the C–I bond, which prefers to undergo intramolecular 5-exo-cyclization leading to alkyl radical intermediate B. Subsequently, sulfonylation would occur through the insertion of sulfur dioxide to provide sulfonyl radical intermediate C. In the presence of silyl enolate or allylic bromide, sulfonyl radical C would be trapped by the double bond, giving rise to intermediate D or F respectively. Radical intermediate D would then couple with the iodo radical, affording compound E. However, other possible routes could not be excluded. For example, intermediate E would be formed by trapping intermediate D with iodine generated in situ from two iodo radicals. Alternatively, in the presence of iodine, intermediate D would be oxidized to a cation. This cation would be subsequently trapped with iodide to afford compound E. Further desilylation would take place to produce the corresponding product 3, 8 or 10. For the radical intermediate F, the sulfonated cyclic compound 4–6 would be furnished with the release of a bromo radical.

Scheme 3 Plausible mechanism.

In conclusion, we have reported a metal-free process for the insertion of sulfur dioxide with aryl iodides and silyl enolates or allylic bromide under ultraviolet irradiation at room temperature. This protocol provides a convenient route to sulfonated cyclic compounds under mild conditions. Not only N-(2-iodophenyl)-N-methylmethacrylamides but also 1-iodo-2-allenoxybenzene is workable. This pathway is highly selective, and several competitive routes are not observed. A plausible mechanism is proposed, which shows that during the reaction process, aryl radicals formed in situ from aryl iodides under ultraviolet irradiation undergo intramolecular 5-exo-cyclization, with subsequent sulfonylation via insertion of sulfur dioxide. The resulting sulfonyl radicals are further trapped by silyl enolates or allylic bromide giving rise to sulfonated cyclic compounds.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the National Natural Science Foundation of China (No. 21672037 and 21532001) is gratefully acknowledged.

Notes and references

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9. CCDC 1938512 contains the supplementary crystallographic data (compound 3b) for this paper.†.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 1938512. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9qo01274e

‡ These authors contributed equally.

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