Electrochemical Semipinacol Rearrangements of Allylic Alcohols

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Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Electrochemical Semipinacol Rearrangements of Allylic Alcohols: Construction of All-Carbon Quaternary Stereocenters Jun-Chen Kang, Yong-Qiang Tu, Jia-Wei Dong, Chao Chen, Jia Zhou, Tong-Mei Ding, Jian-Tao Zai, Zhi-Min Chen,* and Shu-Yu Zhang* Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs & School of Chemistry and Chemical Engineering, Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, P.R. China

Org. Lett. Downloaded from pubs.acs.org by ALBRIGHT COLG on 04/04/19. For personal use only.

S Supporting Information *

ABSTRACT: The first examples of electrochemical trifluoromethylation and sulfonylation/semipinacol rearrangements of allylic alcohols were developed using cheap and stable RSO2Na (R = CF3, Ph) as reagents. Various β-trifluoromethyl and sulfonated ketones were obtained in moderate to excellent yields. This strategy provides a facile, direct, and complementary approach to construct all-carbon quaternary stereocenters. In addition, the reaction has the advantages of being chemical oxidant-free and metal-free and has safe and mild reaction conditions.

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as catalysts.7 In 2014, our group developed Mn(III)-mediated radical azidation/1,2-rearrangement of allylic silyl ethers using a stoichiometric amount of Mn(OAc)3 as an oxidant.8 Despite these advances, metal-free and environmentally friendly radical-initiated 1,2-rearrangements for the construction of all-carbon quaternary stereocenters are still lacking and are in high demand. Electrochemical organic synthesis, an efficient and environmentally friendly synthetic tool, has attracted significant attention. It has a unique characteristic of directly using electric current as the oxidant, so it can allow investigators to avoid the use of stoichiometric chemical oxidants in an electrical environment. During the past decade, the resurgence of organic electrochemistry has promoted the progress of radical chemistry and enabled the development of novel redox organic transformations.9 Numerous synthetic methods for C− H functionalization have been reported (Scheme 1a).10 Recently, Moeller, Lin, Lei, and other groups have made great efforts to explore organic electrochemistry as a new approach for alkene functionalization, avoiding the use of oxidants or expensive transition-metal catalysts.11 The strategy for the electrochemical alkene functionalization is to assist the anode in releasing an electrophilic radical (e.g., halogen radical, azide radical, trifluoromethyl radical, and thiyl radical) followed by addition to the CC bond with the generation of an alkyl radical adduct. For example, Lin’s group reported the diazidation and dichlorination of alkenes using manganese

ll-carbon quaternary stereocenters are common structural motifs in many bioactive natural products and pharmaceuticals (Figure 1).1 Accordingly, the construction of all-

Figure 1. Natural products and pharmaceuticals containing all-carbon quaternary stereocenters.

carbon quaternary centers has received special attention, and a number of efforts to develop synthetic methods have been made.2 Among them, the electrophile-promoted semipinacol rearrangement of allylic alcohols has become a commonly applied and remarkable strategy for the construction of carbonyl compounds with α-quaternary carbon centers.3 A series of methodologies have been developed and successfully applied in the synthesis of natural products and medical molecules.4 Recently, radical-initiated 1,2-rearrangements for the formation of all-carbon quaternary stereocenters were also explored, which further extended the application range of rearrangement reactions.5 In 2013, Wu’s and our group independently developed copper-catalyzed trifluoromethylation/1,2-rearrangements of allylic alcohols using hypervalent Togni’s reagent.6 In 2014, Toste and co-workers described an arylation/1,2-rearrangement reaction of alkenyl cycloalkanols with aryl diazonium salts using Ru(bpy)3(PF6)2 and Ph3PAuCl © XXXX American Chemical Society

Received: January 22, 2019

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DOI: 10.1021/acs.orglett.9b00263 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

(Langlois reagent), which is a stable, inexpensive, and practical CF3 source that has previously been used successfully in electrochemistry.15,11e Herein, we present the development of successful metal-free electrochemical trifluoromethylation and sulfonylation/semipinacol rearrangements of allylic alcohols under mild reaction conditions. Through cyclic voltammetry studies, 1a demonstrates a weak oxidation peak at Ep/2 = 2.1 V vs SCE compared to CF3SO2Na Ep/2 = 1.4 V vs SCE, suggesting that 1a is stable enough to carry out a trifluoromethylation/semipinacol rearrangement reaction (for details, see the SI). Accordingly, allylic alcohol 1a was chosen as the model substrate. We first identified the process for 1a, which involved constant-current electrolysis using a carbon anode and a Pt plate cathode, in an undivided cell containing a mixed electrolyte solution of CF3SO2Na and LiClO4 in acetonitrile and acetic acid (10:1) at room temperature. To our delight, the desired product 2a was produced in 40% yield (Table 1, entry 1). To improve the

Scheme 1. Design of Electrochemical Semipinacol Rearrangement

Table 1. Optimization studies.a

species or a radical scavenger to trap the radical adduct.11c−h Additionally, intramolecular trapping of the radical adduct by an aromatic ring is another pathway for heterocycle synthesis, and elegant work in this area has been reported by Zeng and Lei (Scheme 1b).12 However, alkyl radicals can also be overoxidized anodically to afford cations, followed by a nucleophilic addition with oxygen-/nitrogen-related nucleophile trapping.13 To the best of our knowledge, the electrochemical semipinacol rearrangement of allylic alcohols to synthesize α-quaternary carbon centers of β-functionalized ketones is unexplored. Our long-standing interest and experience with semipinacol rearrangements14 led us to hypothesize that the 1,2-alkyl/aryl rearrangement process is also likely to be involved in the electrochemical pathway (Scheme 1c). However, to successfully achieve an electrochemical semipinacol rearrangement, the following three main challenges should be carefully considered: (1) the identification of suitable allylic alcohol substrate and radical source, since some allylic alcohol substrates are easily decomposed under high oxidant potential; (2) as mentioned above, the alkyl radical intermediate may be trapped through different pathways, and if it is trapped through a radical migration process, the diastereoselectivity of the products could be problematic; and (3) because the migration process is slower than oxygen-trapping or deprotonation, which will yield byproducts 4aa or 4ab, undesired processes or products must be avoided. We envisioned that using 1-(1phenylvinyl)cyclobutan-1-ol (1a) as the substrate, since it was stable under electrochemical conditions, and radical adduct 3a might be easily oxidized to carbocation, which subsequently would undergo a cation 1,2-rearrangement process. Meanwhile, the migration ability of cyclobutanol, which has been proven, was prior to that of acyclic alkyl or aryl groups, which could contribute to avoiding the undesired products 4aa or 4ab. Additionally, considering the importance of compounds containing trifluoromethyl groups, we first chose CF3SO2Na

entry

(+)/(−)

solvent

CF3SO2Na (equiv)

yieldb (%)

1 2 3 4 5 6 7 8 9d 10 11f 12g 13h 14i

C/Pt C/Pt C/Pt C/Pt C/Pt C/Pt C/Pt C/Pt C/Pt Pt/Pt C/Pt C/Pt C/Pt C/Pt

MeCN/AcOH = 10:1 MeCN/TFE = 10:1 MeCN/HFIP = 10:1 MeCN/MeOH = 10:1 MeCN/H2O = 10:1 MeCN/H2O = 10:1 MeCN/H2O = 10:1 MeCN/H2O = 10:1 MeCN/H2O = 2:1 MeCN/H2O = 2:1 MeCN/H2O = 2:1 MeCN/H2O = 2:1 MeCN/H2O = 2:1 MeCN/H2O = 2:1

3 3 3 3 3 2.5 2 1.5 2 2 2 2 2 2

40 48 48 53 55 71 87c 73 91 (85)e 21 35 85 76 0

a Reaction conditions: allylic alcohol 1a (0.4 mmol), LiClO4 (3.0 equiv) in MeCN (8 mL) and cosolvent in an undivided cell with carbon anode and Pt plate cathode, 15 mA, 2 h. bYield was determined by crude 19F-NMR. c10% of 4aa was found by crude 19FNMR. dAnode potential starts from 1.42 V vs SCE, Ecell range from 2.4 to 4 V. eIsolated yield. f10 mA for 3 h. g3.0 equiv of TBAPF6 as electrolyte. hThe reaction was conducted at a constant potential 1.45 V vs SCE. iNo electricity.

yield, we first screened the cosolvent, and the yield was enhanced to 55% when a combination of acetonitrile and water (10:1) was used (entry 5). It was determined that the amount of CF3SO2Na was an important factor in this reaction (entries 6−8). We observed competitive trifluoromethylation of the arenes when 3.0 equiv of CF3SO2Na was used (see the SI). Therefore, the yield increased to 87% when the amount of CF3SO2Na was decreased to 2.0 equiv. Meanwhile, 4aa14f was also produced with a yield of approximately 10% under the conditions. Interestingly, after the ratio of H2O to acetonitrile was increased, we were surprised to find that the byproduct 4aa could be suppressed and the yield further improved to B

DOI: 10.1021/acs.orglett.9b00263 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters 91%, which suggested that increasing the polarity of the solvent was able to promote the migration process (entry 9). It should be noted that the electricity is 2.8 F and Faraday efficiency is 71% under the conditions (entry 9). In comparison, use of the Pt anode or lowering the current dramatically decreased the formation of 2a (entries 10 and 11). In addition, using tetrabutylammonium hexafluorophosphate (TBAPF6) as an electrolyte instead of LiClO4 afforded a similar yield (entry 12). The constant potential experiment was also conducted and gave the desired product with 76% yield (entry 13). The control experiments demonstrated that electricity was crucial for this reaction (entry 14). As a result, the reaction conditions of entry 9 were selected as the optimal procedure, and the isolated yield was 85%. With optimized conditions in hand, the reaction scope was investigated with a variety of styrene derivatives. Overall, the desired all-carbon α-quaternary centers of β-trifluoromethyl ketones were obtained with moderate to good yields (Scheme 2). Compared with the model substrate 1a, the orthosubstitution (1b) led to an evidently decreased yield, while electron-deficient substituents or a methyl group in the metaor para-position did not significantly affect the yields (2c−g). It is worth mentioning that although competing reactions may occur on aryl rings when the substrates with electron-rich substituents were used, by controlling the reaction time and properly lowering the current, we could also obtain the corresponding products (2h−j) in moderate yields. However, the electron-withdrawing substituents did not work well. The indene-type substrates (1l,m) were also tolerant, which afforded the corresponding products (2l,m) with spiro structures in moderate yields and moderate to good diastereoselectivities. However, an increasing amount of CF3SO2Na was needed for the full conversion of those substrates. The relative configuration of product 2l was determined by X-ray crystallography, and thus, 2m was assigned by analogy. Cyclopentanol and 9-fluorenol were also tested in the reaction and gave the desired products (2n and 2o, respectively) in reasonable yields. It should be noted that, except for the case of ring expansion, 1,2-aryl migration was also accessible under the standard conditions. Allylic alcohol 1q, which is a nonactivated alkene, afforded product 2q in moderate yield. In contrast, we determined that the desired product 2p was obtained only in 33% yield along with an epoxide byproduct 4pa in the reaction of substrate 1p. Finally, in order to evaluate the synthetic utility of this reaction, a gram scale (5 mmol) trifluoromethylation/rearrangement of substrate 1a was performed under the standard electrochemical reaction conditions, and the desired product 2a was obtained in 65% yield. Encouraged by the above studies, we assumed that more versatile transformations could be realized via semipinacol rearrangement of allylic alcohols under electrochemical oxidation conditions. Organic sulfone compounds are very important synthetic intermediates and are also widespread in many medical molecules and functional materials.16 Although some traditional methods have been documented, the development of metal-free,17 efficient, and eco-friendly synthetic methods for the preparation of sulfone-containing compounds is still highly desirable.18 We noted that aryl sulfinate was also readily oxidized into sulfonyl radicals under electrochemical conditions. Aryl sulfonyl radicals may be trapped by alkenes prior to SO2 extrusion. Therefore, we began to study the electrochemical sulfonylation/semipinacol rear-

Scheme 2. Substrate Scope of the Electrochemical Tandem Trifluoromethylation/Semipinacol Rearrangementa

a

Allylic alcohol (0.4 mmol), CF3SO2Na (2.0 equiv), and LiClO4 (3.0 equiv) in MeCN/H2O = 8:4 mL in an undivided cell with carbon anode and Pt plate cathode under constant current 15 mA for 2 h. b12 mA for 2.2 h. c2.5 equiv of CF3SO2Na was added. d20 mA and 3.0 equiv of CF3SO2Na were added.

rangement. To our delight, the reaction proceeded smoothly, and the desired product 3a was obtained in 78% yield after simply screening the reaction conditions. As shown in Scheme 3, numerous electron-withdrawing, electron-rich substituents on the benzene ring of cycloalkanol-substituted styrene derivatives were well-tolerated and afforded the corresponding products (3b−k) in good yields. Subsequently, the 2-naphthylsubstituted substrate was also subjected to the reaction and gave product 3l in good yield. Of note, the reaction with substrate 1m, derived from 1-indanone, was suitable and afforded the product 3m with good yield and excellent diastereoselectivity, suggesting that this reaction was a cation 1,2-migration process. The relative configuration of the product 3m was also assigned by X-ray crystallography. Finally, we were pleased to find that several sodium benzenesulfonate salts were also able to promote this reaction in moderate yields (3aa−ac). C

DOI: 10.1021/acs.orglett.9b00263 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 3. Substrate Scope of Electrochemical Tandem Sulfonylation/Semipinacol Rearrangementa

Scheme 4. Proposed Mechanistic Pathway

preparation of all-carbon α-quaternary centers of β-functionalized ketones with moderate to excellent yields. The transformations can be achieved via a 1,2-migration process. The requirements of stoichiometric oxidants, transition-metal catalysts, and harsh reaction conditions were avoided. Studies of the application of these reactions and other electrochemical semipinacol rearrangements are underway in our group and will be reported in due course.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00263. Experimental details, cyclic voltammetry experiment, electricity, X-ray crystallography structure of compounds 2l and 3m, analytical data for new compounds, NMR spectra of new compounds (PDF) Accession Codes

CCDC 1880038−1880039 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

a Allylic alcohol (0.4 mmol), ArSO2Na (3.0 equiv), and LiClO4 (3.0 equiv) in MeCN/H2O = 5:5 mL in an undivided cell with a carbon anode and Pt plate cathode under constant current of 10 mA for 3 h.

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To gain mechanistic insight into the reaction, some preliminary mechanistic experiments were performed for this tandem trifluoromethylation/semipinacol rearrangement. After the addition of TEMPO as a radical scavenger, the reaction was inhibited. No desired product 2a was detected, but we did detect a TEMPO-trapped CF3 adduct by GC−MS. Cyclic voltammetry experiments showed the oxidation peak of CF3SO2Na, indicating the initial CF3 radical. In addition, during the solvent filtration, we found the formation of methanol-trapped species when methanol was employed as cosolvent, suggesting the involvement of a cation intermediate. Based on the above mechanistic studies and previous reports, we proposed a plausible mechanism for trifluoromethylation, as shown in Scheme 4. The SET oxidation process of CF3SO2Na under anode oxidation would give the sulfonyl radical followed by a fast extrusion of SO2 to generate a CF3 radical. Upon the addition of the CF3 radical to the CC bond, the generated benzyl radical 5 would be further oxidized to form cation 6. After ring expansion and deportation, 2a would be formed. Predictably, the sulfonylation process is similar. In conclusion, we have developed the first electrochemical trifluoromethylation and sulfonylation/semipinacol rearrangements of allylic alcohols that provide modular strategies for the

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhi-Min Chen: 0000-0002-6988-8955 Shu-Yu Zhang: 0000-0002-1811-4159 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “Thousand Youth Talents Plan”, NSFC (21672145, 51733007, 21702135, and 21871178), the Shuguang program (16SG10) from SEDF & SMEC, STCSM (17JC1403700), and the Drug Innovation Major Project of the Ministry of Science and Technology of China (2018ZX09711001-005-002)

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DOI: 10.1021/acs.orglett.9b00263 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.9b00263 Org. Lett. XXXX, XXX, XXX−XXX