Metal- and base-free tandem sulfonylation/cyclization of 1,5-dienes with aryldiazonium salts via the insertion of sulfur dioxide.

A metal- and base-free 5-endo-trig sulfonylative cyclization between 1,5-dienes, aryldiazonium salts and SO2 (from SOgen) is presented. This method could successfully produce sulfonylated pyrrolin-2-ones in one pot with excellent regioselectivity and good-to-excellent yields. This strategy features mild reaction conditions and broad substrate scope. Moreover, a scale-up reaction and three synthetic applications demonstrate the practicality of this method. Lastly, control experiments indicate that the 5-endo-trig sulfonylative cyclization may proceed in a radical pathway. This journal is © The Royal Society of Chemistry.

Introduction

Pyrrolin-2-ones and their N-heterocyclic compound derivatives, widely exist in natural plants,[1] pharmaceuticals[2] and bioactive molecules.[3] Similarly, sulfonyl groups are frequently found in pharmaceuticals[4] and photoelectric materials[5] due to their unique chemical properties. Numerous studies have indicated that the incorporation of sulfonyl groups into heterocycles could enhance their pharmacological activity.[6] Therefore, great efforts have been devoted to explore efficient and straightforward methods to build sulfone-containing N-heterocyclic frameworks.

Radical cascade cyclization reactions represent a powerful strategy for the synthesis of functionalized cyclic structure, characterized by multiple C–C/C–X bond-forming in one step.[7] The incorporation of sulfonyl group into heterocycles by radical cascade cyclization reactions has aroused extensive interest among scientists.[8] In recent years, many sulfone-containing heterocyclic frameworks have been constructed by radical cascade cyclization reactions, such as sulfonylindoles,[9] sulfonylindolins,[10] sulfonylated pyrrolidines,[11] sulfonylated phenanthridines,[12] sulfonylated benzofurans,[13] sulfonated oxazolines,[14] sulfonylated spirocycles[15] and others.[16] In 2021, sulfonylated pyrrolinones were synthesized via sulfonylation/cyclization of 1,5-dienes with sulfonyl chlorides or sodium sulfinates by Wang and co-workers (Fig. 1a and b).[17] However, due to the limited accessibility of sulfonyl chlorides and sodium sulfinates, these two methods suffered from a narrow range of substrates. Besides, transition metal (Cu and Ag), base and elevated temperature were essential in these transformations.

Fig. 1

Overview of tandem sulfonylative cyclization of 1,5-dienes: (a) sulfonyl chlorides as sulfonylation reagents; (b) sodium sulfinates as sulfonylation reagents; (c) SOgen as sulfonylation reagents (this work).

On the other hand, direct insertion of sulfur dioxide (SO2) provides an alternative and efficient approach to introduce sulfonyl moiety into molecules.[18,19] Recently, a cheap and bench-stable SO2 surrogate (SOgen) has been developed by our group, which has been successfully applied in several sulfonylation reactions.[20] Inspired by Wang's work and our continuous interests in SO2 chemistry, we herein attempt to construct sulfonylated pyrrolinones using SOgen as SO2 surrogate (Fig. 1c). This transformation features metal- and base-free conditions and could proceed smoothly at room temperature to form sulfonylated pyrrolinones with excellent regioselectivity and good to excellent yields.

Results and discussion

We started the studies by evaluating the reaction between 1,5-diene (1a), 4-methylbenzenediazonium tetrafluoroborate (2a) and SO2 gas (from SOgen) under metal- and base-free conditions. Pleasingly, when the reaction was carried out in NMP at room temperature for 24 h, desired product 3a was successfully obtained in 91% yield with excellent regioselectivity (Table 1, entry 1).

Optimization of reaction conditionsa

Entry	Variation from std conditions	Yield of 3ab (%)
1	None	91 (87)c
2	THF instead of NMP	0
3	MeCN instead of NMP	0
4	DCM instead of NMP	0
5	Toluene instead of NMP	0
6	MeOH instead of NMP	0
7	DMSO instead of NMP	21
8	DMF instead of NMP	20
9	DMA instead of NMP	56
10	3.5 equiv. of SO2	86
11	3.0 equiv. of SO2	80
12	2.5 equiv. of SO2	74
13	Air instead of Ar	54
14d	DABSO as SO2 surrogate	71
15d	Na2S2O5 as SO2 surrogate	53
16d	K2S2O5 as SO2 surrogate	52
17d	HOCH2SO2Na·H2O as SO2 surrogate	0

Standard conditions: chamber A, SOgen (0.80 mmol), 1-methyl-4-vinylbenzene (0.81 mmol), tetradecane (1.0 mL), at 100 °C for 10 min; chamber B, 1a (0.2 mmol, 1.0 equiv.), 2a (0.44 mmol, 2.2 equiv.), NMP (1.0 mL), at room temperature for 24 h under argon atmosphere.

Yields were determined by 1H-NMR analysis using 1,3,5-trimethoxybenzene as an internal standard.

Isolated yield in the parentheses.

The reaction was set up in a 4 mL vial.

Then we explored the influence of other solvents on this reaction, the target product (3a) was not obtained in most solvents, such as THF, MeCN, DCM, toluene and MeOH (Table 1, entries 2–6). When the solvent was DMA, DMF and DMSO, 3a was formed in only poor yields (Table 1, entries 7–9). Next, the amount of SO2 (from 2.5 equiv. to 4.0 equiv.) was investigated, and the results indicated that 4.0 equiv. was the best choice (Table 1, entries 10–12). Although this reaction could work under an air atmosphere, argon atmosphere proved to be more beneficial for the transformation (Table 1, entry 13). Finally, other sulfur dioxide surrogates were examined. The use of DABSO and inorganic SO2 surrogates (Na2S2O5 and K2S2O5) could both lead to the formation of product 3a but with lower yields (Table 1, entries 14–16). Unfortunately, rongalite reagent (HOCH2SO2Na·H2O) would hamper the reaction (Table 1, entry 17).

After determining the optimal reaction conditions, we began to explore the substrate scope of this reaction, and the results are summarized in Scheme 1. We first investigated the functional group compatibility of aryldiazonium tetrafluoroborates in the transformation. Alkyl substituted aryldiazonium tetrafluoroborates at the meta- or para-position of the phenyl ring proceeded well and afforded corresponding products 3b–3d in good yields (83–93%). While a methyl substituent at the ortho-position could lead to a lower yield (67%, 3e), possibly due to the steric hindrance. In addition, substrates with methoxy or phenoxy group delivered desired products 3f and 3g in 83% and 86% yield, respectively. Substrates bearing a biphenyl or 2-naphthyl group showed good reactivity, producing expected products (3h and 3i) in excellent yields. Notably, halogen groups were found to be well tolerated under the standard conditions (3j–3q). Moreover, substrates with electron-withdrawing groups such as MeCO–, PhCO–, CF3O–, MeSO2– were subject to the reaction conditions, and gave corresponding products (3r–3v) in good yields. In addition, heterocyclic diazonium salt was found to be compatible in the transformation (3w). Finally, aryldiazonium tetrafluoroborates with complicated substituent structures could also work smoothly to afford 3x and 3y in 92% and 90% yield, respectively.

Scheme 1

Substrate scope. Reaction conditions: chamber A, SOgen (0.80 mmol), 1-methyl-4-vinylbenzene (0.81 mmol), tetradecane (1.0 mL), at 100 °C for 10 min; chamber B, 1 (0.2 mmol, 1.0 equiv.), 2 (0.44 mmol, 2.2 equiv.), NMP (1.0 mL), at rt for 24 h under argon atmosphere. All yields are isolated yields.

Next, the substrate scope of 1,5-dienes was investigated. The results showed that halogen groups (–Cl, –Br and –I) on 1,5-dienes had little effect on the reaction, and the corresponding products 3z–3ae were formed in 80–93% yields. Notably, 1,5-diene with a strongly electron-withdrawing group (–NO2) could deliver desired products 3af in an excellent yield (90%). Meanwhile, the one with an electron-donating group (–Me) could also give products 3ag in a similar yield (91%). Naphthalene ring was well tolerated, achieving 3ah in 92% yield and the configuration of compound 3ah was confirmed by X-ray crystallography. Pyridine moiety was also adapted to the reaction conditions and generated 3ai in 80% yield. In addition, it was found that benzodioxole moiety (3aj) could be well tolerated under the standard conditions. When R4 group was replaced by other substituents, such as benzyl, phenyl, n-butyl and –CH2COOEt, desired products (3ak–3ap) could still be made in good-to-excellent yields. When R1 group was alkyl, the sulfonylation reaction could still proceed, demonstrated by two successful examples (3aq and 3ar). It was worth noting that compound 3ar contained two isomers (3ar-1 : 3ar-1 = 1 : 1).

The practicality of this methodology was successfully illustrated by the production of 3a with 90% yield in a scale-up reaction (Scheme 2(1)). To further demonstrate the synthetic utility of this method, 3a was then applied in subsequent transformations. In the presence of diethylamine, the acetyl group on the nitrogen atom could be easily removed in a quantitative yield (Scheme 2(2)). Then, different substituents could be introduced on the N atom. For example, N–H could be transferred to N–Me (5) in 90% yield in a mixture of sodium hydride and iodomethane (Scheme 2(3)). In addition, the amide moiety of 3a could be reduced to hydroxy amine (6) via ring-opening by NaBH4 in excellent yield (Scheme 2(4)).

Scheme 2

Scale-up reaction and synthetic applications.

In order to understand the mechanism of this reaction, three control experiments with radical scavengers were carried out. Firstly, in the presence of radical scavenger (TEMPO), the desired product (3a) was totally quenched and TEMPO adduct 7 was identified by LC-MS (Scheme 3a(1)). Secondly, when 1,1-diphenylethylene or BHT was added, the reaction showed similar result and corresponding aryl radicals adduct (8 or 10) sulfonyl radicals adduct (9 or 11) were identified, respectively (Scheme 3(2) and (3)). These results indicated that this transformation might proceed through a radical pathway.

Scheme 3

(a) Control experiments and (b) proposed mechanism.

Base on the control experiments and literature,[20] a plausible reaction mechanism is proposed herein (Scheme 3b). One of lone-pair electrons on the N atom of 1,5-diene is transferred to aryldiazonium tetrafluoroborate, which leads to the formation of nitrogen radical cation species (A) and aryl radical. Then aryl radical is trapped by sulfur dioxide and gives aryl sulfone radical (B). Sulfonyl radical B selectively adds to the double bond of 1,5-diene and produces alkyl radical species C. Subsequently, intramolecular 5-endo-trig cyclization produces intermediate D, which has an equilibrium with E. Finally, the desired product (3) is produced via tautomerization from E.

Conclusions

In conclusion, a metal- and base-free sulfonylative cyclization of 1,5-dienes with aryldiazonium salts via the insertion of SO2 (from SOgen) has been developed. This method can work under mild conditions and provide the desired products in good yields with excellent regioselectivity. In addition, this approach greatly expands the substrates scope compared with previous reported work. Preliminary mechanism studies indicate that this 5-endo-trig sulfonylative cyclization may proceed in a radical pathway.

Conflicts of interest

There are no conflicts to declare.