Thiourea–I2 as Lewis Base–Lewis Acid Cooperative Catalysts for

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Cite This: ACS Catal. 2018, 8, 6362−6366

Thiourea−I2 as Lewis Base−Lewis Acid Cooperative Catalysts for Iodochlorination of Alkene with In Situ-Generated I−Cl Takahiro Horibe, Yasutaka Tsuji, and Kazuaki Ishihara* Graduate School of Engineering, Nagoya University, B2-3(611), Furo-cho, Chikusa, Nagoya 464-8603, Japan

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ABSTRACT: Thiourea−I2 as Lewis base−Lewis acid cooperative catalysts are developed for the iodochlorination of alkenes with in situ-generated iodine monochloride (I−Cl). The Lewis base−Lewis acid cooperative system is sufficient to generate I− Cl from I2 with a chlorinating reagent at low temperature. Based on the solid-state structure of the active species, thiourea−I2 cooperatively captures I−Cl. By taking advantage of I−Cl generation and the control of I−Cl at low temperature, the thiourea−I2 cooperative system suppresses side reactions that arise from highly reactive free I−Cl. KEYWORDS: iodochlorination, Lewis base−Lewis acid catalysis, thiourea, iodine, halogen-bonding, dihalogenation

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Table 1. Catalytic Activity of Lewis Base for the Iodochlorination of 1 with In Situ-Generated I−Cla

ecently, 1,2-dihaloalkanes have been recognized as a useful skeleton for marine natural products and related pharmaceutical drugs.1,2 The dihalogenation of alkenes with molecular halogens is one of the most straightforward methods for synthesizing 1,2-dihaloalkanes.3−6 Among these, the iodochlorination of alkenes with iodine monochloride (I−Cl) gives 1,2-iodochloroalkanes, which are used as building blocks for natural products (Scheme 1A).2b,3c,e,7 For example, Vanderwal and colleagues demonstrated the iodochlorination of alkene as a key reaction for the total synthesis of chlorosulfolipid.3c Efficient regioselective iodochlorination of a cis-alkene has been achieved through the judicious choice of substrate alkenes. However, in the case of a similar trans-alkene, a complex mixture of byproducts has been obtained.3e The reactivity and selectivity of iodochlorination strongly depends on the alkene substrates because I−Cl is too reactive. To suppress side reactions that arise from I−Cl, the slow

entry

Lewis base

yield of 4 [%]

yield of 5 [%]

4A:4B

1 2 3 4 5 6 7 8 9

2a 2b 2c 3a 3b 3b 3c without Lewis base without Lewis base

22 33 44 19 84 84b 75 0 57c

0 0 0 0 0 0 0 0 38

67:33 67:33 51:49 47:53 70:30 92:8 68:32 − 96:4

Scheme 1. (A) Iodochlorination of Alkenes with Free I−Cl. (B) Generation of I−Cl in Situ from I2 with a Chlorinating Reagent by a Lewis Base−Lewis Acid Cooperative System a

The reaction was carried out with Lewis base (5 mol %), I2 (1.1 equiv) and NCS (1.0 equiv) in toluene at 25 °C for 5 h. bDCDMH was used in place of NCS. cI−Cl was used instead of I2 and NCS.

generation of I−Cl from metal chloride with iodine (I2) has been developed.8−10 Although these methods have been shown to efficiently give 1,2-iodochloroalkanes, the low solubility of metal chloride requires a high reaction temperature and a long Received: April 23, 2018 Revised: May 15, 2018 Published: June 7, 2018 © XXXX American Chemical Society

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ACS Catalysis

Table 2. Comparison Experiments between Catalysis and Iodochlorination with Free I−Cl

Figure 1. (a) Stoichiometric reaction with 3b with I2 and NCS. (b) Thermal ellipsoids of 6 at the 50% probability level. Hydrogens have been omitted for clarity. (c) Thermal ellipsoids of 7 at the 50% probability level. Hydrogens have been omitted for clarity.

reaction time. Moreover, the generation of Lewis acidic metal residues limits functional group tolerance and a wider substrate scope. Our group has previously reported chiral Lewis basepromoted enantioselective halocyclization.11,12 Activation of the halogenating reagent has been achieved by nucleophilic attack of a chiral Lewis base, which gives a reactive halonium cation−Lewis base complex.11a,13 Recently, we developed a chiral Lewis base−Lewis acid cooperative system for enantioselective iodocyclization.12d,14,15 In the proposed reaction cycle, I2 is activated by chiral phosphate and Nchlorophthalimide (NCP) cooperatively (Scheme 1B). As a result, reactive I−Cl is generated by halogen exchange, which promotes iodolactonization at −40 °C. In this regard, the Lewis base−Lewis acid cooperative system should be effective for the iodochlorination of alkenes with in situ-generated I−Cl at low temperature. To obtain fundamental information regarding the generation and reactivity of I−Cl, we began to screen various Lewis base catalysts for iodochlorination of 1 (Table 1). In the presence of 5 mol % of Lewis base catalyst, I2 and N-chlorosuccinimide (NCS) were used for the iodochlorination of 1 in toluene at 25 °C for 5 h. 2a−2c and 3a−3c were selected as Lewis base catalysts having chalcogen atoms. As a result, 2a−2c and 3a gave the corresponding product in moderate yield with moderate regioselectivity (4A:4B = 47:53−67:33) (entries 1 to 4). In the case of thiourea Lewis base 3b, the catalytic activity was dramatically improved (entry 5). The corresponding product was obtained in 84% yield with moderate regioselectivity (4A:4B = 70:30). When 1,3-dichloro-5,5-dimethylhydantoin (DCDMH) was used instead of NCS, the regioselectivity was improved (4A:4B = 92:8) (entry 6). This result suggests that the Lewis acid controls the regioselectivity of the products. In sharp contrast, no product was obtained in the absence of a Lewis base catalyst (entry 8). Moreover, when free I−Cl was used in the absence of a Lewis base, undesired 5 was obtained in 38% yield.16 These results suggest that the Lewis base−Lewis acid cooperative system not only efficiently generated I−Cl in situ but also controlled the reactivity of iodochlorination. To reveal the detailed mechanism of the generation and control of I−Cl, we investigated the active species of the Lewis base−Lewis acid cooperative system. Treatment of 3b with 2

a

The reaction was carried out with 3b (5 mol %), I2 (1.1 equiv) and NCS (1.0 equiv). bThe reaction was carried out with free I−Cl (1.0 equiv) in the absence of Lewis base. cA:B = 73:27. dThe yield was determined by 1H NMR analysis using 1,3-dinitrobenzene as an internal standard. eA:B = 77:23. fA:B = 95. gA:B = 86:14. h DCDMH was used in place of NCS. idr = 76:24.

equiv of I2 afforded 6 quantitatively in 5 min at room temperature (Figure 1a). The solid-state structure of 6 revealed that the thiourea moiety coordinated to I2 [I(1)−I(2)] (Figure 1b). Moreover, halogen-bonding interaction of I(1)−I(2) with I(3)−I(4) was observed, which suggested that I(3)−I(4) acted as a Lewis acid for I(1)−I(2). As a result of the cooperative activation for I(1)−I(2) by the Lewis base−Lewis acid, the bond length of I(1)−I(2) (3.06 Å) was longer than the distance for I2 in the solid state (2.72 Å).17 Subsequently, addition of NCS provided 7 quantitatively in 5 min by halogen exchange between NCS and I2 of 6 (Figure 1c). In the solid-state structure of 7, the generation of I−Cl was unambiguously observed. The resulting I−Cl was captured between thiourea− I2 as a Lewis base−Lewis acid. Therefore, bond elongation was observed for I(5)−C(1) (2.86 Å) in comparison with the solidstate structure of free I−Cl (2.35 and 2.44 Å).18 These results strongly suggested that I−Cl is generated and controlled by the Lewis base−Lewis acid cooperative system. 6363

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ACS Catalysis Scheme 2. Catalytic Iodochlorination of Alkenesa

Scheme 3. Proposed Catalytic Cycle

thiourea−I2 catalysis exhibited efficient iodochlorination without producing byproducts. These results strongly suggested that the Lewis base−Lewis acid cooperative system controlled the reactivity of I−Cl. Next, the scope of substrate was examined (Scheme 2). The thiourea−I2 cooperative catalyst was effective for iodochlorination with various substrates. A variety of substrates with a sterically hindered aromatic group, electron-withdrawing and -donating aromatic groups could also be used, and the corresponding products 18−27 were obtained in high yield with high regioselectivity (A:B = 98:2−88:12). Notably, a 2.0 gscale (6.0 mmol) synthesis of 22 was examined in the presence of 0.5 mol % of 3b. Moreover, the reaction was completed in 80 min when 1 mol % of 3b was used. In this case, the observed turnover frequency was 61.5 h−1 (82% yield at 80 min). Whereas heteroatoms often coordinate to free I−Cl and decrease the yield, our catalyst system tolerated hetero aromatic-substituted substrates (see 28 and 29). Substrates with an acidic proton, such as alcohol and amide, were also suitable for use in this system (see 30 and 31). Our proposed catalytic cycle is shown in Scheme 3. Halogenbonding interaction between 3b and two molar equiv of I2 affords 6. The resulting polarized I2 in complex 6 should undergo halogen exchange with NCS, which provides I−Cl in situ. The generated I−Cl in 7 reacts with alkenes that afford iodiranium intermediate 7′. In 7′, Lewis base coordinates to iodiranium cation while −Cl coordinates to I2. These halogenbonding interactions should be effective for suppressing side reactions that arise from free I−Cl. As a result of nucleophilic attack of −Cl to cationic carbon of iodiranium intermediate, the corresponding 1,2-iodochloroalkane is obtained. In the final step, 6 is regenerated by halogen-bonding interaction with another molar equivof I2. In conclusion, we have developed thiourea−I2 as Lewis base−Lewis acid cooperative catalysts for the iodochlorination of alkenes with in situ-generated I−Cl. Because I−Cl is controlled by the Lewis base−Lewis acid, various substrates with functional groups are well-tolerated. X-ray diffraction

a

The reaction was carried out with 3b (5 mol %), I2 (1.1 equiv) and NCS (1.0 equiv) in toluene at 0 °C. b0.5 mol % of 3b was used c1.0 mol % of 3b was used.

The captured I−Cl in 7 was suggested to have different properties than those of free I−Cl. Therefore, we conducted comparison experiments between Lewis base−Lewis acid cooperative catalysis and iodochlorination with free I−Cl (Table 2). Whereas Lewis base−Lewis acid cooperative catalysis gave the desired 1,2-iodochloroalkanes, free I−Cl was generated as a byproduct. For example, in the case of alkenes with a secondary alkyl group, free I−Cl gave byproducts such as 9 and 11 (entries 1 and 2, respectively). In sharp contrast, the catalysis provided desired iodochlorination products (8 and 10) in high yield. When a substrate has an intramolecular nucleophile, free I−Cl promotes iodocyclization because of the highly polarized I−Cl bond.19 Therefore, in the case of free I−Cl with a substrate that has a nucleophilic aromatic group (entry 3), iodocyclization mainly proceeded as a side reaction. However, our catalytic system gave the corresponding iodochlorination product in good yield. Trimethylsilyl (TMS)-substituted aromatic group was welltolerated in our catalyst system (entry 4). However, free I−Cl promoted the iodination of aryltrimethylsilane to give 15. In general, TMS-protected alcohols are not stable in the presence of Cl−. Therefore, free I−Cl induced deprotection of TMSether quantitatively. Our catalysis provided the corresponding product in good yield with intact TMS-ether (entry 5). Overall, 6364

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Highly Selective Synthesis of Halomon, Plocamenone, and Isoplocamenone. J. Am. Chem. Soc. 2015, 137, 12784−12787. (4) For a review on stereoselective dihalogenation, see: Cresswell, A. J.; Eey, S. T.-C.; Denmark, S. E. Catalytic, Stereoselective Dihalogenation of Alkenes: Challenges and Opportunities. Angew. Chem., Int. Ed. 2015, 54, 15642−15682. (5) For enantioselective dihalogenation, see: (a) Nicolaou, K. C.; Simmons, N. L.; Ying, Y.; Heretsch, P. M.; Chen, J. S. Enantioselective Dichlorination of Allylic Alcohols. J. Am. Chem. Soc. 2011, 133, 8134− 8137. (b) Hu, D. X.; Shibuya, G. M.; Burns, N. Z. Catalytic Enantioselective Dibromination of Allylic Alcohols. J. Am. Chem. Soc. 2013, 135, 12960−12963. (c) Hu, D. X.; Seidl, F. J.; Bucher, C.; Burns, N. Z. Catalytic Chemo-, Regio-, and Enantioselective Bromochlorination of Allylic Alcohols. J. Am. Chem. Soc. 2015, 137, 3795−3798. (d) Soltanzadeh, B.; Jaganathan, A.; Yi, Y.; Yi, H.; Staples, R. J.; Borhan, B. Highly Regio- and Enantioselective Vicinal Dihalogenation of Allyl Amides. J. Am. Chem. Soc. 2017, 139, 2132−2135. (6) (a) Kamada, Y.; Kitamura, Y.; Tanaka, T.; Yoshimitsu, T. Dichlorination of Olefins with NCS/Ph3P. Org. Biomol. Chem. 2013, 11, 1598−1601. (b) Ren, J.; Tong, R. Convenient in Situ Generation of Various Dichlorinating Agents from Oxone and Chloride: Diastereoselective Dichlorination of Allylic and Homoallylic Alcohol Derivatives. Org. Biomol. Chem. 2013, 11, 4312−4315. (c) Swamy, P.; Reddy, M. M.; Kumar, M. A.; Naresh, M.; Narender, N. Vicinal Dichlorination of Olefins Using NH4Cl and Oxone®. Synthesis 2014, 46, 251−257. (d) Cresswell, A. J.; Eey, S. T.-C.; Denmark, S. E. Catalytic, Stereospecific Syn-dichlorination of Alkenes. Nat. Chem. 2015, 7, 146−152. (e) Fu, N.; Sauer, G. S.; Lin, S. Electrocatalytic Radical Dichlorination of Alkenes with Nucleophilic Chlorine Sources. J. Am. Chem. Soc. 2017, 139, 15548−15553. (7) Voorhees, V.; Skinner, G. S. Some New Derivatives of Barbituric Acid. J. Am. Chem. Soc. 1925, 47, 1124−1127. (8) (a) Baird, W. C., Jr.; Surridge, J. H.; Buza, M. Halogenation with Copper(II) Halides. Synthesis of Chloro Iodo Alkanes. J. Org. Chem. 1971, 36, 2088−2092. (b) Baird, W. C., Jr.; Surridge, J. H.; Buza, M. Halogenation with Copper(II) Halides. Halogenation of Olefins with Complexed Copper(II) Halides. J. Org. Chem. 1971, 36, 3324−3330. (c) Uemura, S.; Fukuzawa, S.; Okano, M.; Sawada, S. The Chloroiodination of Deactivated Olefins with Antimony(V) Chloride−iodine and Iodine Monochloride. Bull. Chem. Soc. Jpn. 1980, 53, 1390−1392. (9) (a) Uemura, S.; Okazaki, H.; Onoe, A.; Okano, M. Chlorination and Chloroiodination of Acetylenes with Copper(II) Chloride. J. Chem. Soc., Perkin Trans. 1 1977, 1977, 676−680. (b) Ho, M. L.; Flynn, A. B.; Ogilvie, W. W. Single-isomer Iodochlorination of Alkynes and Chlorination of Alkenes Using Tetrabutylammonium Iodide and Dichloroethane. J. Org. Chem. 2007, 72, 977−983. (10) (a) Mohanakrishnan, A. K.; Prakash, C.; Ramesh, N. A Simple Iodination Protocol via in Situ Generated ICl Using NaI/FeCl3. Tetrahedron 2006, 62, 3242−3247. (b) Emmanuvel, L.; Shukla, R. K.; Sudalai, A.; Gurunath, S.; Sivaram, S. NaIO4/KI/NaCl: A New Reagent System for Iodination of Activated Aromatics Through in Situ Generation of Iodine Monochloride. Tetrahedron Lett. 2006, 47, 4793−4796. (c) Lista, L.; Pezzella, A.; Napolitano, A.; d’Ischia, M. Mild and Efficient Iodination of Aromatic and Heterocyclic Compounds with the NaClO2/NaI/HCl System. Tetrahedron 2008, 64, 234−239. (d) Yamamoto, T.; Toyota, K.; Morita, N. An Efficient and Regioselective Iodination of Electron-rich Aromatic Compounds Using N-Chlorosuccinimide and Sodium Iodide. Tetrahedron Lett. 2010, 51, 1364−1366. (11) For recent examples of Lewis base-catalyzed electrophilic halocyclization, see: (a) Denmark, S. E.; Burk, M. T. Lewis Base Catalysis of Bromo- and Iodolactonization, and Cycloetherification. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 20655−20660. (b) Zhou, L.; Tan, C. K.; Jiang, X.; Chen, F.; Yeung, Y.-Y. Asymmetric Bromolactonization Using Amino-thiocarbamate Catalyst. J. Am. Chem. Soc. 2010, 132, 15474−15476. (c) Tripathi, C. B.; Mukherjee, S. Catalytic Enantioselective Iodoetherification of Oximes. Angew. Chem., Int. Ed. 2013, 52, 8450−8453. (d) Chen, F.; Tan, C. K.; Yeung,

analyses suggest the details of the mechanism of the generation and control of I−Cl.

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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/acscatal.8b01565. Experimental procedure, characterization data, copies of 1 H NMR and 13C NMR spectra of all new compounds (PDF) X-ray data for 6 (CIF) X-ray data for 7 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Takahiro Horibe: 0000-0002-4692-4642 Kazuaki Ishihara: 0000-0003-4191-3845 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported in part by JSPS KAKENHI (Grant Numbers 15H05755, 15H06266, 17K14484, and 15H05810), Program for Leading Graduate Schools “IGER program in Green Natural Sciences,” the Toyoaki Scholarship Foundation, the Society of Iodine Science and MEXT, Japan. We thank Ms. Mie Torii for her assistance with NMR.

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REFERENCES

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