Photoinduced, Copper-Promoted Regio- and Stereoselective

Letter pubs.acs.org/OrgLett

Photoinduced, Copper-Promoted Regio- and Stereoselective Decarboxylative Alkylation of α,β-Unsaturated Acids with Alkyl Iodides Chao Wang,† Yingjie Lei,† Mengzhun Guo, Qinyu Shang, Hong Liu, Zhaoqing Xu,* and Rui Wang* Key Laboratory of Preclinical Study for New Drugs of Gansu Province, The Institute of Pharmacology, School of Basic Medical Science, Lanzhou University, 199 West Donggang Road, Lanzhou 730000, China S Supporting Information *

ABSTRACT: The first example of UV light-induced, coppercatalyzed regio- and stereoselective decarboxylative coupling of α,β-unsaturated acids with alkyl iodides was reported. Under standard conditions, the 1°, 2°, and 3° alkyl iodides proceeded smoothly with the E-selective alkenes obtained in uniformly good yields and high stereoselectivities. Scheme 1. Decarboxylative Alkylation of α,β-Unsaturated Acids

A

lkyl-substituted alkenes are important structural motifs that frequently appear in chemicals, materials, natural products, and pharmaceuticals.1 In this context, preparation of alkylated alkenes is among the most important parts in organic synthetic chemistry. The Pd-catalyzed cross couplings between alkenyl halides and alkylmetal reagents provide direct ways to synthesize alkyl-substituted alkenes.2 However, the sterically specific alkenyl halides are normally expensive and not easily obtained. In the meantime, alkylmetal reagents suffer from instability and lack of functional group compatibility, and the preparation procedures from alkyl halides always require relatively restricted conditions. Alkyl halides are commercially available or readily accessible, and another benefit is their structural diversity and functional group tolerance. It is ideal to directly use alkyl halides as the coupling partner for alkene alkylation reactions instead of alkylmetal reagents. Unfortunately, the couplings of alkyl halides with olefins are normally problematic, due to the decreased rates of oxidative addition using sp3-hybridized electrophiles3 and the predisposition of putative alkyl palladium species to undergo dehydrohalogenation.4 In the past few years, transition-metalcatalyzed Heck-type radical alkylation reactions provided alternative routes to realize the alkylation of alkenes by directly using alkyl halides. However, limitations such as high temperature and long reaction time, low to moderate stereoselectivities, limited substrate scopes (α-carbonyl alkyl halides or benzyl halides), or using of stoichiometric amount of Grignard reagent as activators still exist.5 α,β-Unsaturated acids are stable and readily available in high stereoselectivities, which were successfully applied in decarboxylative cross couplings for the construction of C(sp2)−C(sp2) bonds.6 Recently, transition-metal-catalyzed alkyl radical addition−elimination processes of α,β-unsaturated acids were developed. Using this strategy, highly stereoselective alkylation of alkenes could be realized. However, the oxidative conditions and high temperature lead to narrow substrate scopes or poor regioselectivities of the alkyl groups, which restrict its synthetic applications (Scheme 1).7 Very recently, Chen and co-workers © 2017 American Chemical Society

reported a novel Ru-catalyzed, visible light-induced chemo- and regioselective decarboxylative alkylation of α,β-unsaturated acids by using stoichiometric amount of hypervalent iodine as the mediator. This protocol required the preparation of alkyl trifluoroborate reagents which are still difficult in most cases.8a Additionally, Duan8b and Wang8c recently successively reported the coupling of aryl carboxylic acids with redox-active ester of alkyl carboxylic acid induced by visible light. Wang8d and Wu8e described copper-catalyzed decarboxylative trifluoroethylation of cinnamic acids. However, in view of atom- and step-economy, the direct use of alkyl halides as the highly regio- and chemoselective alkylation reagents for decarboxylative alkylation of α,β-unsaturated acids in a catalytic manner is highly desirable but yet not realized. In the past few years, Ir- and Ru-catalyzed photoredox were reported to form alkyl radicals from alkyl halides in the present of reductants and widely applied in alkyl radical coupling reactions.9 In contrast, Cu has been used scarcely as photoredox catalysts for organic transformations,10 although early work by the groups of Received: October 22, 2017 Published: November 20, 2017 6412

DOI: 10.1021/acs.orglett.7b03289 Org. Lett. 2017, 19, 6412−6415

Letter

Organic Letters Kutal and Mitani described it.11 Recently, the using of ultraviolet (UV) light as an efficient energy source in organic synthesis has received more and more attention.12 Very recently, Fu, Peters, and Ackermann reported the copper salt catalyzed radical couplings of organic halides with amide, cyano, and heterocycles under UV light promotion.13 In the reactions, the excited state of CuI was oxidatively quenched by organic halides and formed the R•. Inspired by these precedents, we envisioned that under UV light irradiation, by using a suitable single electron reductant, alkyl halide could also be converted to alkyl radical through the Cu-catalyzed oxidative quenching cycle without the assistance of expensive Ru or Ir photocatalysts. Herein, we report the first example of copper-promoted decarboxylative alkylations of α,βunsaturated carboxylic acids with 100 W UVC compact fluorescent light bulb irradiation under mild conditions.14 The direct using of readily alkyl iodides as alkylation reagents avoids the preparation of alkylmetals. Furthermore, the 1°, 2°, and 3° alkyl iodides proceeded smoothly, and the E selective alkenes were obtained in uniformly good yields with high chemo- and regioselectivities. Moreover, the absence of oxidants and strong inorganic bases provided good functional group tolerance. Our investigation was initiated by using 4-fluorocinnamic acid and cyclohexyl iodide as model substrates and irradiation with 100 W UVC compact fluorescent light bulbs under room temperature. DIPEA was reported as a single-electron-transfer agent for reducing CuII to CuI in photoinduced oxidative quenching cycles.15 We assumed that DIPEA might also suitable for our purpose (Table 1, entry 1; see the Supporting Information for details). When the reaction mixture was irradiated with longer wavelength light (365 nm), little coupling product was observed. Reducing the light bulbs to 50 W led to a low conversion. The control experiments indicated that UV irradiation and DIPEA were essential for this transformation

(entries 2 and 3). Although cupric acetylacetonate (Cu(acac)2) was unnecessary, the yield of desired product was more sharply decreased than under standard conditions (entry 4). Other copper salts could not promote this approach efficiently (entries 5). Replacing DIPEA with other tertiary amines, secondary amines, or inorganic bases led to significantly lower yields (entries 6 and 7). Solvents screening indicated that acetone was the best choice (entry 8). It should be noted that when cyclohexyl bromide was tested under the established conditions, the yield of cyclohexyl-substituted alkene was 58% (entry 9). Reducing the amount of Cu(acac)2 resulted in substantial drop of yields (entries 10 and 11). Furthermore, decreasing the loading of cyclohexyl iodide or DIPEA resulted negative effects on reaction yields, respectively (entries 12 and 13). The reaction was sensitive to oxygen (entry 14). Further improving the yield by extending the reaction time to 5 h was not effective, whereas a small decrease of E/Z ratio was noticed, which might be caused by the strong absorption of alkene product at 254 nm (entry 15, see the Supporting Information).16,17 Finally, under optimal conditions, the decarboxylative alkylation product 3a was obtained in 81% isolated yield with E/Z > 20:1 (entry 1). With the optimized conditions in hand, we examined the scope of α,β-unsaturated carboxylic acids. As shown in Scheme 2, aryl Scheme 2. Scope of Various α,β-unsaturated Carboxylic and Alkyl Iodidesa

Table 1. Optimization of Reaction Conditionsa

entry

change from the “standard conditions”

yieldb (%)

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

no change no hv no i-Pr2NEt no Cu(acac)2 other copper salts instead of Cu(acac)2 other amines instead of i-Pr2NEt inorganic bases instead of i-Pr2NEt other solvents instead of acetone CyBr instead of CyI 10 mol % of Cu(acac)2 5 mol % of Cu(acac)2 3.0 equiv of CyI 3.0 equiv of i-Pr2NEt Under O2 atmosphere 5h

84 (81) 0 0 35 18−52 0−74 0 0−75 58 64 47 70 72 36 84

a

Standard conditions were carried out by using 4-fluorocinnamic acid (0.1 mmol), cyclohexyl iodide (5.0 equiv), Cu(acac)2 (20 mol %), iPr2NEt (4.0 equiv), under Ar, and stirred at room temperature for 2 h under UV light irradiation. bYield was determined by 19F NMR using benzotrifluoride as an internal standard, and the number in parentheses refers isolated yield. cE/Z = 15:1.

a

Reaction conditions: vinyl carboxylic acid 1 (0.2 mmol), alkyl iodide 2 (1.0 mmol), Cu(acac)2 (0.04 mmol), i-Pr2NEt (0.8 mmol), under Ar, and stirred at room temperature for 2 h under 100 W UVC. bE/Z ratio was determined by analysis of 1H NMR. 6413

DOI: 10.1021/acs.orglett.7b03289 Org. Lett. 2017, 19, 6412−6415

Letter

Organic Letters vinyl carboxylic acids with various substituted groups including electron-withdrawing and electron-donating groups at the p-, m-, or o-position provided good to excellent yields (3aa−ma, 68− 86%) with high stereoselectivities (E/Z > 20:1). Bracingly, heteroaromatic α,β-unsaturated carboxylic acid also reacted smoothly under the standard conditions and gave desired product 3na in 65% yield. We next studied the generality of alkyl iodides with different structures. The primary, secondary, and tertiary iodides were all tolerant, whereas in cases of using primary alkyl iodides, a small decrease of E/Z ratio was noted (3kb−ee, E/Z ≥ 11:1). It should be noted that alkyl iodides bearing sensitive functional groups such as ether, triple bonds, and N-Boc groups all proceeded well under the standard conditions with good yields (3kb−nh, 55−75%). Moreover, the reactions of halogen-substituted aromatic vinyl carboxylic acids efficiently proceeded to form the corresponding alkylated alkenes in good yields, with the halogen substituents untouched during the reaction, which render the coupling products good candidates for further transformations such as transition-metalcatalyzed functionalization of the C−halogen bonds (3ca, see the SI for details). The sterically demanding 1-adamantyl iodide 2i was efficiently reacted and the yields were up to 76% (3gi and 3ei). Acyl-substituted vinyl carboxylic acid 1r and the conjugated diene substrate 1s gave the corresponding products 3ri and 3si in moderate yields, respectively. We were pleased to find that under the stated conditions perfluorobutyl iodide smoothly reacted to furnish the perfluorobutyl-substituted alkene 3ej in good yield and high stereoselectivity. To demonstrate the synthetic utility of this methodology, this catalytic system was applied to the decarboxylative alkylation and perfluoroalkylation of estrone derivative 1t (Scheme 3). The

Scheme 4. Mechanistic Studies

Scheme 5. Proposed Mechanism

generation of CuI and an amine radical cation.15 Under UV light irradiation, CuI was excited to its triplet state [CuI]*, which engages in an electron-transfer reaction with alkyl iodide to furnish alkyl radical and I− along with recycling of CuII.13a,d On the other hand, the carboxylate anion A was formed from α,βunsaturated acids under basic conditions. A radical addition of alkyl radical R• to A took place and gave the radical intermediate B.18 The loss of carbon dioxide results in intermediate C, which was oxidized by amine radical cation, and alkyl-substituted alkene 3 was obtained.19,20 In summary, we describe a novel copper-promoted decarboxylative coupling of α,β-unsaturated acids with alkyl iodides under UV light irradiation. This strategy could efficiently construct 1°, 2°, and 3° alkyl-substituted alkenes containing various sensitive groups on both aromatic rings of α,β-unsaturated acids and alkyl iodides. The coupling reactions were found to be stereoselective, with the trans-alkenes (E/Z from 11:1 to >20:1) formed in good to excellent yields. Moreover, the acyl-substituted vinyl carboxylic acid, the conjugated diene substrate carboxylic acid, as well as perfluorobutyl iodide also proceeded well under standard conditions.

Scheme 3. Alkylation and Perfluoroalkylation of Estrone Derivative 1t

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reactions afforded corresponding products 3ti and 3tj in 72% and 67% yields with high stereoselectivities (E/Z > 20:1), respectively. These excellent performances illustrated the potential application values of our methodology in drug discovery and synthesis of natural products. To gain some mechanistic insight into this decarboxylative coupling, a radical-trapping experiment was carried out (Scheme 4). When radical scavenger TEMPO was added to the standard reaction, the reaction was completely shut down. In the meanwhile, the radical trapping product 7 was isolated in 45% yield, which indicated the formation of alkyl radical. To explore the influence of bases, all types of inorganic bases were used instead of DIPEA. The negative results demonstrated the importance of organic amines in this UV-promoted decarboxylative coupling process. Although multiple scenarios can be envisaged, on the basis of the above investigations and previous reports,10 a possible mechanism was proposed in Scheme 5. As a single electrontransfer agent, DIPEA could reduce CuII with concomitant

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03289. Experimental procedures and full analytical data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Zhaoqing Xu: 0000-0001-7663-6249 Rui Wang: 0000-0002-4719-9921 Author Contributions †

C.W. and Y.L. contributed equally.

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DOI: 10.1021/acs.orglett.7b03289 Org. Lett. 2017, 19, 6412−6415

Letter

Organic Letters Notes

(10) For reviews, see: (a) Paria, S.; Reiser, O. ChemCatChem 2014, 6, 2477. (b) Reiser, O. Acc. Chem. Res. 2016, 49, 1990. (c) HernandezPerez, A. C.; Collins, S. K. Acc. Chem. Res. 2016, 49, 1557. (11) (a) Grutsch, P. A.; Kutal, C. J. Am. Chem. Soc. 1979, 101, 4228. (b) Mitani, M.; Kato, I.; Koyama, K. J. Am. Chem. Soc. 1983, 105, 6719. (12) Some recent examples of UV-promoted reactions: (a) Liu, W.; Li, L.; Li, C.-J. Nat. Commun. 2015, 6, 6526. (b) Li, L.; Liu, W.; Zeng, H.; Mu, X.; Cosa, G.; Mi, Z.; Li, C.-J. J. Am. Chem. Soc. 2015, 137, 8328. (c) Li, L.; Mu, X.; Liu, W.; Wang, Y.; Mi, Z.; Li, C.-J. J. Am. Chem. Soc. 2016, 138, 5809. (d) Liu, W.; Chen, Z.; Li, L.; Wang, H.; Li, C.-J. Chem. Eur. J. 2016, 22, 5888. (e) Yang, X.; Liu, W.; Li, L.; Wei, W.; Li, C.-J. Chem. - Eur. J. 2016, 22, 15252. (f) Mfuh, A. M.; Doyle, J. D.; Chhetri, B.; Arman, H. D.; Larionov, O. V. J. Am. Chem. Soc. 2016, 138, 2985. (g) Mfuh, A. M.; Nguyen, V. T.; Chhetri, B.; Burch, J. E.; Doyle, J. D.; Nesterov, V. N.; Arman, H. D.; Larionov, O. V. J. Am. Chem. Soc. 2016, 138, 8408. (h) Seo, H.; Katcher, M. H.; Jamison, T. F. Nat. Chem. 2016, 9, 453. (i) Xie, J.; Zhang, T.; Chen, F.; Mehrkens, N.; Rominger, F.; Rudolph, M.; Hashmi, A. S. K. Angew. Chem., Int. Ed. 2016, 55, 2934. (j) Xie, J.; Li, J.; Wurm, T.; Weingand, V.; Sung, H.-L.; Rominger, F.; Rudolph, M.; Hashmi, A. S. K. Org. Chem. Front. 2016, 3, 841. (13) (a) Ratani, T. S.; Bachman, S.; Fu, G. C.; Peters, J. C. J. Am. Chem. Soc. 2015, 137, 13902. (b) Uyeda, C.; Tan, Y.; Fu, G. C.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 9548. (c) Ziegler, D. T.; Choi, J.; MuñozMolina, J. M.; Bissember, A. C.; Peters, J. C.; Fu, G. C. J. Am. Chem. Soc. 2013, 135, 13107. (d) Do, H.-Q.; Bachman, S.; Bissember, A. C.; Peters, J. C.; Fu, G. C. J. Am. Chem. Soc. 2014, 136, 2162. (e) Yang, F.; Koeller, J.; Ackermann, L. Angew. Chem., Int. Ed. 2016, 55, 4759. (14) Four 25 W UVC bulbs were used, which can be purchased from the supermarket ($∼9 each). (15) (a) Baralle, A.; Fensterbank, L.; Goddard, J.-P.; Ollivier, C. Chem. Eur. J. 2013, 19, 10809. (b) Michelet, B.; Deldaele, C.; Kajouj, S.; Moucheron, C.; Evano, G. Org. Lett. 2017, 19, 3576. (16) UV-promoted Heck-type radical coupling of styrenes and alkyl iodides in low stereoselectivities (Z/E = 1:1−2.9:1) under strong base conditions (10.8 equiv of NaOtBu) and using a highly excess amount of alkyl halides (10 equiv): Liu, W.; Li, L.; Chen, Z.; Li, C.-J. Org. Biomol. Chem. 2015, 13, 6170. (17) An excess amount of cyclohexyl iodide was necessary for a rapid conversion, and the residue was collected after the reaction without significant side reactions. (18) Xu, Y.; Tang, X.; Hu, W.; Wu, W.; Jiang, H. Green Chem. 2014, 16, 3720. (19) Although the tertiary amine functions as a catalyst, a stoichiometric amount of DIPEA was required for good conversions. The requirement for a high concentration of amine may be a reflection of its dual roles as an electron-transfer agent and a base. For other examples using stoichiometric amount of tertiary amines as electrontransfer agents in photoredox reactions, see ref 15 and: (a) Jeffrey, J. L.; Petronijević, F. R.; MacMillan, D. W. C. J. Am. Chem. Soc. 2015, 137, 8404. (b) Petronijević, F. R.; Nappi, M.; MacMillan, D. W. C. J. Am. Chem. Soc. 2013, 135, 18323. (c) Amador, A. G.; Sherbrook, E. M.; Yoon, T. P. J. Am. Chem. Soc. 2016, 138, 4722. (20) In the process of conditional optimization, 35% of 3aa was obtained in the absence of Cu(acac)2. Tertiary amine might provide a single electron to alkyl iodide with concomitant generation of alkyl radical R• and I− under UV light irradiation. (a) Cossy, J.; Ranaivosata, J.-L.; Bellosta, V. Tetrahedron Lett. 1994, 35, 8161. (b) Kropp, P. J.; Adkins, R. L. J. Am. Chem. Soc. 1991, 113, 2709. For further discussion and experimental details, see the Supporting Information.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the grants from the NSFC (No. 21432003) and the Fundamental Research Funds for the Central Universities (lzu-jbky-2016-216).

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DOI: 10.1021/acs.orglett.7b03289 Org. Lett. 2017, 19, 6412−6415