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Article Cite This: ACS Omega 2018, 3, 1409−1415

Catalyst-Free 1,6-Conjugate Addition/Aromatization/Sulfonylation of para-Quinone Methides: Facile Access to Diarylmethyl Sulfones Teng Liu,*,† Jianjun Liu,† Shubiao Xia,† Jie Meng,† Xianfu Shen,† Xiufang Zhu,*,‡ Wenchang Chen,† Chengke Sun,† and Feixiang Cheng† †

Center for Yunnan-Guizhou Plateau Chemical Functional Materials and Pollution Control, Qujing Normal University, Qujing 655011, P. R. China ‡ School of Chemical Science and Technology, Yunnan University, Kunming 650091, P. R. China S Supporting Information *

ABSTRACT: An efficient, catalyst-free strategy to construct diarylmethyl sulfones via 1,6-conjugate addition/aromatization/ sulfonylation reaction of para-quinone methides with sulfonyl hydrazides under mild and environmentally benign conditions has been developed. The established protocol provided a highly chemo- and regioselectivity synthesis of a diverse array of novel diarylmethyl sulfones with excellent yields, and the reaction could be scaled up.

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INTRODUCTION In recent years, developing green and sustainable chemical procedures to achieve the targets have attracted the attention of more and more chemists. Many great achievements of “green and sustainable chemistry” have been made, such as metal-free catalysis1−5 and catalyst-free synthesis.6−11 For the past few years, exploring green and sustainable methodologies for constructing the C−S bond have emerged as a significant field of research in organic synthesis12,13 and medical chemistry.14−18 Diarylmethyl sulfones as one of the most important sulfur-containing compounds, which are often valuable intermediates in organic transformations19−21 such as construction of triarylmethane derivatives, are widely present in medicinal chemistry and materials science.22−24 Recently, several methods for their preparation have been reported, such as the oxidation of corresponding sulfides (Figure 1(i)) 25 and copper-catalyzed nitrogen loss of sulfonylhydrazones (Figure 1(ii)).26 In the past few years, transition metal-catalyzed arylation of 1 with aryl halide via nucleophilic substitution reaction to construct diarylmethyl sulfones has been well-documented (Figure 1(iii)).21,27−30 For example, in 2014, Nambo et al.21 reported a Pd-catalyzed C−H arylation of monoarylated sulfones with iodoarenes to provide diarylmethyl sulfones. Unfortunately, heteroaromatic substrates were not well-tolerated by the arylation reaction and formed products with lower yields (30−42%). In addition, to obtain the diarylmethyl sulfones, [{PdCl(allyl)}2] (5 mol %), P(tBu)3· © 2018 American Chemical Society

HBF4 (20 mol %), and KOtBu (3.0 equiv) and a high temperature were required. The common points of the aforementioned transition metal-catalyzed methods, high acidity of the benzylic proton and strong base, are often required.21,27−30 To the best of our knowledge, phenylsulfinate ion (PhSO2−) presents two tautomers, sulfone and sulfinate, and therefore has the selective of the O-attack or S-attack of carbocations to provide sulfones (Ar2CH−SO2Ph) or esters (Ar 2 CH−OS(O)Ph).31 Notably, most of the methods described above often suffer from the common limitations of using toxic and odorous reagents, hazardous and costly catalysts, rigorous reaction conditions, poor selectivity, limited substrates, and low functional group compatibility. To address these limitations, developing efficient, green, and sustainable strategies to construct diarylmethyl sulfones remains important and is challenging. In continuation of our interest in constructing functionalized molecules,32−36 herein, we report a catalyst-free, facile, and efficient route for the synthesis of diarylmethyl sulfones from the readily available starting materials, sulfonyl hydrazides9,37 and p-quinone methides (pQMs),38−53 which are widely used in modern organic synthesis because of the assembly of carbonyl and olefinic moieties (Figure 1b). Received: November 7, 2017 Accepted: January 17, 2018 Published: February 2, 2018 1409

DOI: 10.1021/acsomega.7b01745 ACS Omega 2018, 3, 1409−1415

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ACS Omega

Figure 1. Synthesis of diarylmethyl sulfones: (a) previous methods and (b) our strategy.

Table 1. Optimization of the Reaction Conditiona

entry

catalystb

solvent

t (h)

yieldc (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23d 24e

HOAc TFA PhCO2H TEA i-Pr2NH TMG DBU DABCO DMAP t-BuOK Na2CO3 K2CO3

EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH MeOH toluene CHCl3 THF CH3CN EtOAc DMF H2O EtOH/H2O (3:1, v/v) EtOH/H2O (3:1, v/v) EtOH/H2O (3:1, v/v)

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 8 8 64

30 15 45 77 80 30 42 55 44 35 trace trace 81 70 60 65 72 50 25 N.D. 60 90 80 75

a The reaction was performed with 2a (0.1 mmol), 3a (0.11 mmol), and the solvent (2.0 mL). bCatalyst (20 mol %). cIsolated yield based on p-QM 2a. dTemperature = 70 °C. eRoom temperature (23 °C).

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ACS Omega Table 2. Catalyst-Free Synthesis of Diarylmethyl Sulfones 4aa−4aqa

entry

R

t (h)

4

yieldb (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17c

Ph 4-Me-C6H4 4-MeO-C6H4 2,4,6-(Me)3-C6H2 4-F-C6H4 4-Cl-C6H4 4-Br-C6H4 4-NO2-C6H4 4-CF3-C6H4 4-Ph-C6H4 α-naphthyl β-naphthyl 2-thienyl Bn Et n-Bu (−)-10-camphoryl

8 18 6 20 3 3.5 4 16 1 8 2 5 3 10 24 12.5 16

4aa 4ab 4ac 4ad 4ae 4af 4ag 4ah 4ai 4aj 4ak 4al 4am 4an 4ao 4ap 4aq

90 82 80 80 93 92 92 81 96 88 94 91 92 88 90 90 85

a

All reactions were performed with 2a (0.1 mmol) and 3 (0.11 mmol) in the solvent (2.0 mL). bIsolated yield based on p-QM 2a. cCompound 4aq with 1.3:1 diastereoselectivity.

Table 3. Catalyst-Free Synthesis of Diarylmethyl Sulfones 4ba−4bpa

a

entry

Ar

t (h)

4

yieldb (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

4-Me-C6H4 4-MeO-C6H4 3,4-(MeO)2-C6H3 4-tBu-C6H4 4-F-C6H4 4-Cl-C6H4 4-Br-C6H4 3-Br-C6H4 2-Br-C6H4 4-I-C6H4 4-CN-C6H4 4-NO2-C6H4 4-CF3-C6H4 α-naphthyl 2-thienyl 2-pyridyl

7 24 11 12 7 8 10 7.5 12 8 11 20 3 12 24 9

4ba 4bb 4bc 4bd 4be 4bf 4bg 4bh 4bi 4bj 4bk 4bl 4bm 4bn 4bo 4bp

90 82 83 90 95 95 95 93 92 95 88 80 94 91 85 90

All reactions were performed with 2 (0.1 mmol) and 3a (0.11 mmol) in the solvent (2.0 mL). bIsolated yield based on p-QM 2.

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RESULTS AND DISCUSSION We commenced our study by selecting p-QM 2a and sulfonyl hydrazide 3a as model substrates and ethanol as the solvent to optimize the reaction conditions (Table 1). Pleasingly, the reaction smoothly occurred in the numerous organic acidic

catalysts or organic basic catalysts, offering the desirable product 4aa in 15−80% yield (Table 1, entries 1−10). Furthermore, better results were observed in the organic basic catalysts. By contrast, only a trace of the product was observed in the presence of the inorganic bases (Table 1, 1411


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ACS Omega Scheme 1. Two Representative Examples to be Scaled Up

substituted p-QM was well-tolerated by the transformation to give product 4bn in 91% yield (Table 3, entry 14). Notably, heteroaromatic-substituted p-QMs underwent the 1,6-conjugate addition reaction with sulfonyl hydrazide 3a smoothly, giving the desired diarylmethyl sulfones in 85−90% yield (Table 3, entries 15−16). To further demonstrate the potential utility of our strategy, we selected phenylsulfonyl hydrazide 3a and n-butylsulfonyl hydrazide 3p as the sulfonyl precursors to react with p-QM 2a as the two representative examples to be scaled up (5.0 mmol 2a). To our delight, the yield of the corresponding products remained reasonable with 88% (4aa) and 86% (4ap) yields, respectively (Scheme 1). The chemical structures of diarylmethyl sulfones 4 were fully characterized by proton nuclear magnetic resonance (1H NMR), carbon nuclear magnetic resonance (13C NMR), and high-resolution mass spectrometry (HRMS) spectroscopies. To further verify the structure of the targets, 4aa was selected as a representative compound and unequivocally confirmed by Xray diffraction analysis, as shown in Figure 2 (CCDC 1583037).

entries 11−12). On the basis of the aforementioned results, we envisioned whether the reaction could occur in neutral catalysts. To our delight, the reaction could work well without any catalyst, obtaining 4aa with a satisfactory yield. Notably, the yield of 4aa was remarkably increased when the mixed solvents (EtOH/H2O, 3:1, v/v) were applied (Table 1, entries 14−22). The result revealed that water played an important role in the reaction, although the substrates have poor solubility in water. Furthermore, the higher or lower temperature was obviously adverse to the reaction (Table 1, entries 23−24). Thus, the optimum conditions were achieved by employing 2a (0.1 mmol) and 3a (0.11 mmol) in EtOH/H2O (3:1, v/v) under 50 °C for 8 h (Table 1, entry 22). Notably, high chem- and regioselectivities were observed, and the corresponding ester, 1,2- or 1,4-addition product was not detected in any case. With the optimal reaction conditions in hand, the scope of the 1,6-conjugate addition/aromatization/sulfonylation reaction of p-QMs to sulfonyl hydrazides was investigated. First, a set of sulfonyl hydrazides 3 were tested by keeping pQM 2a constant. For sulfonyl hydrazides 3, the substituents on the aromatic ring, whether with electron-donating groups [Me, OMe, 2,4,6-(Me)3, and Ph] or electron-withdrawing groups (F, Cl, Br, NO2, and CF3), afforded the corresponding diarylmethyl sulfones with excellent yields 80−96% (Table 2, entries 1−10). Notably, the sterically hindered mesitylenesulfonohydrazide was also well-tolerated by the conjugate addition reaction to give product 4ad in 80% yield (Table 2, entry 4), however, the substrate could not participate in the sulfonylation reaction in Patil’s report.54 Subsequently, α-naphthylsulfonyl hydrazide and β-naphthylsulfonyl hydrazide were tested, and the desired products 4ak−4al were obtained in 94 and 91% yields, respectively (Table 2, entries 11−12). More importantly, thienylsulfonyl hydrazide and aliphaticsulfonyl hydrazides also worked satisfactorily under the optimal reaction conditions to give the corresponding targets in 88−90% yields (Table 2, entries 13−16). It is interesting to observe that (−)-10camphorsulfonyl hydrazide also survived, and in this transformation, product 4aq was obtained in 85% yield with 1.3:1 diastereoselectivity (Table 2, entry 17). Next, we investigated the utility of p-QMs for the reaction under the standard reaction conditions. As shown in Table 3, various stable p-QMs with electron-rich and electron-deficient substituents at the ortho, meta, and para positions of the aryls were compatible with the reaction, providing the desired products with excellent yields and high chem- and regioselectivities (Table 3, entries 1−13). Furthermore, α-naphthyl-

Figure 2. Oak Ridge thermal ellipsoid plot diagram of 4aa; ellipsoids are drawn at the 30% probability level.

Furthermore, several controlled experiments were conducted to get a deep insight into the mechanism of the sulfonylation process. Initially, 2,2,6,6-tetramethyl-1-piperidinyloxy as the radical scavenger was employed to elucidate whether the reaction involves radical species under standard reaction conditions. The radical trapping experiments revealed that the transformation did not proceed via a free-radical pathway, affording 4aa in 65% yield (Scheme 2, a). Subsequently, to clarify the source of hydrogen in the phenolic hydroxyl group of the product, EtOD/D2O and toluene/D2O were used as the reaction media under standard conditions, respectively 1412


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ACS Omega Scheme 2. Mechanistic Investigations of the Sulfonylation Process

a

Toluene was strictly distilled with sodium.

Scheme 3. Mechanism Hypotheses for the Synthesis of Target Compounds 4aa

ionization) was performed on an Agilent LC/MSD TOF instrument. All chemicals and solvents were used as received without further purification, unless otherwise noted. Column chromatography was performed on silica gel (200−300 mesh). p-QMs 2 and sulfonyl hydrazides 3 were prepared according to a procedure described in the literature.9,37−39,41 The structure of diarylmethyl sulfones 4 were confirmed by 1H NMR, 13C NMR, and HRMS spectra. General Procedure for the Synthesis of Compounds 4. A 10 mL round-bottom flask was charged with p-QMs 2 (0.1 mmol), sulfonyl hydrazides 3 (0.11 mmol), EtOH/H2O (1.5− 0.5 mL, v/v), and the solution was stirred for 2−24 h under 50 °C until p-QMs 2 were completely consumed, as indicated by thin-layer chromatography. Then, the crude products were purified by flash silica gel chromatography (petroleum ether/ EtOAc = 10:1−6:1), which afforded the pure product 4 in 80− 95% yields.

(Scheme 2, b and c). Interestingly, we did not detect the deuterated product, and the result indicated that the origin of hydrogen of phenolic hydroxyl was not derived from H2O and EtOH. Thus, we speculated that hydrogen may be derived from benzenesulfonyl hydrazide itself. On the basis of the aforementioned results and previous reports,9,37,55,56 it is reasonable to speculate a possible reaction pathway, which is depicted in Scheme 3. First, sulfonylhydrazide 3a is quickly transformed into sulfinyl anion 6, which can resonate with sulfur-centered anion 7 in the presence of water, generating hydronium ions and releasing N2. Then, sulfur-centered anion 7 is selectively added to p-QM 2a via the 1,6-conjugate addition reaction to form intermediate 8. Finally, the target product 4aa was obtained by proton transfer from hydronium ions and aromatization reaction.

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CONCLUSIONS In summary, we report a catalyst-free 1,6-conjugate addition/ aromatization/sulfonylation reaction of p-QMs, a facile and efficient route to synthesis diarylmethyl sulfones. The established protocol provided a concise, rapid, and environmentally friendly vision to prepare a diverse array of diarylmethyl sulfones. The reaction has attractive features, including mild conditions, environmentally friendly, high chemo- and regioselectivities, broad scope of substrates, excellent yields, and scalability. The controlled experiments indicated that hydrogen of phenolic hydroxyl may be derived from benzenesulfonyl hydrazide. The mechanistic details and the potential utility of diarylmethyl sulfones in organic synthesis are currently underway.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01745. Spectroscopic and analytical data as well as the original copy of 1H and 13C NMR spectra of all new compounds (PDF) X-ray crystallographic data of compound 4aa (CCDC 1583037) (CIF)

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EXPERIMENTAL SECTION General Methods. All received reagents and solvents were used without further purification, unless otherwise stated. Melting points were determined on a XT-4A melting point apparatus and are uncorrected. NMR spectra were recorded on a Bruker 400 (1H: 400 MHz, 13C: 100 MHz) with CDCl3 as the solvent. The chemical shifts (δ) are expressed in parts per million relative to the residual deuterated solvent signal, and coupling constants (J) are given in hertz. HRMS (electrospray

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone and Fax: +86 8748998658 (T.L.). *E-mail: [email protected] (X.Z.). ORCID

Teng Liu: 0000-0002-7480-0444 Notes

The authors declare no competing financial interest. 1413


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ACKNOWLEDGMENTS This work was supported by the Program for the National Natural Science Foundation of China (nos. 51764048, 21261019).

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