Letter Cite This: Org. Lett. 2018, 20, 888−891
pubs.acs.org/OrgLett
Irradiation-Induced Palladium-Catalyzed Decarboxylative Heck Reaction of Aliphatic N‑(Acyloxy)phthalimides at Room Temperature Guang-Zu Wang,† Rui Shang,*,†,‡ and Yao Fu*,† †
Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Urban Pollutant Conversion, Anhui Province Key Laboratory of Biomass Clean Energy, iChEM, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China ‡ Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan S Supporting Information *
ABSTRACT: It is reported that Pd(PPh3)2Cl2 in combination with 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) under irradiation of blue LEDs efficiently catalyzes a decarboxylative Heck reaction of vinyl arenes and vinyl heteroarenes with aliphatic N-(acyloxy)phthalimides at room temperature. A broad scope of secondary, tertiary, and quaternary carboxylates, including α-amino acid derived esters, can be applied as amenable substrates with high stereoselectivity. The experimental observation was explained by excitation-state reactivity of the palladium complex under irradiation to induce single-electron transfer to activate N-(acyloxy)phthalimides, and to suppress undesired β-hydride elimination of alkyl palladium intermediates.
T
the stabilization of alkyl palladium species4 is incompatible with the harsh condition required for decarboxylation (Figure 1b).5 The activation group strategy using N-hydroxyphthalimide ester in decarboxylative cross-couplings6 reinvigorated interest in using aliphatic carboxylic acid as an alkyl donor under mild reactions, as demonstrated elegantly by Baran et al.,7 in Ni- or Fecatalyzed cross-couplings with various organometallic reagents8 and heteroatoms.9 However, decarboxylative coupling of aliphatic N-(acyloxy)phthalimides with styrene to deliver the Heck type product remains challenging because of the difficulty in reconciling the activation of carboxylate with selective β-H elimination. Nickel catalysts are much less effective in Heck processes because of the reluctance of the system to undergo β-H elimination10 and catalyst regeneration.11 To achieve a decarboxylative Heck reaction between aliphatic N-(acyloxy)phthalimides and styrene, efficient activation of the carboxylate and selective control of β-H elimination after decarboxylation and insertion are considered two key factors. We recently discovered that a Pd catalyst in combination with two types of phosphine ligands under irradiation effectively catalyzes Mizoroki−Heck reaction between a range of alkyl bromides with styrenes at rt.12 Recent elegant work by Gevorgyan et al. also demonstrated irradiation-induced Pd-catalyzed alkyl Heck reaction of functionalized alkyl halides.13 The key enabler for the success of these transformations was the effect of irradiation, which excited the Pd catalyst14 to facilitate a single-electrontransfer type of oxidative addition; furthermore, the irradiationinduced excitation of the alkyl palladium complex formed after
he use of carboxylic acids as a coupling partner in crosscoupling reactions continues to attract the interest of synthetic chemists because of the abundant and environmentally benign nature of these compounds.1 A decarboxylative Mizoroki−Heck reaction to deliver olefin products using aromatic carboxylic acids represents the archetypal example of the synthetic utility of carboxylic acids in Pd-catalyzed crosscouplings2 (Figure 1a). Although under development for years, extending the general scope of the reaction to include aliphatic carboxylic acids as alkyl donor in Heck-type reactions remains challenging because of the lack of d−π interaction in the transition state of redox-neutral decarboxylation3 and because
Figure 1. Challenge of using alkyl carboxylates in ground-state Pdcatalyzed decarboxylative Heck reaction (a, b) and possible solution by applying irradiation excited Pd catalysis (c). © 2018 American Chemical Society
Received: January 4, 2018 Published: January 18, 2018 888
DOI: 10.1021/acs.orglett.8b00023 Org. Lett. 2018, 20, 888−891
Letter
Organic Letters Scheme 1. Study of Controlling Parametersa
Scheme 2. Substrate Scope with Respect to Vinyl (Hetero)arenesa
a
Yields of isolated products. The ratio of stereoisomers was determined by 1H NMR analysis. The reactions were conducted in front of a cooling fan in a room of constant temperature (t = 25 ± 3 °C).
of both photoactivation and C−C bond formation proceeding with a single metal catalyst.18 The optimized reaction conditions are shown in Scheme 1, entry 1. The selectivity of this reaction is remarkable given that alkyl Heck reactions deliver product as an E/Z mixture accompanied by a double insertion byproduct.19 The key results obtained by studying the parameters of this reaction are summarized in Scheme 1 (see the Supporting Information for details). Both PPh3 and Xantphos are essential for this reaction. The reaction did not proceed when Pd(Xantphos)Cl2 was used alone without PPh3. Various photoredox catalysts20 were ineffective (entries 4−6), revealing that a simple radical redox pathway is not feasible. The wavelength of the irradiation source was crucial. White LEDs were less effective (entry 14). Green LEDs of a lower energy (wavelength 520−525 nm) were completely ineffective (entry 15). Purple LEDs (wavelength 395−405 nm) with higher irradiation energy than blue LEDs (wavelength 450−455 nm) resulted in a lower yield (entry 16). The use of high-energy UV light (254 nm) disturbed the desired reaction, and none of the desired product was detected. Ligand screening revealed the unique efficacy of Xantphos as a ligand and suggested the ligand structure may be related to the excitation of the Pd catalyst (Scheme 1b). The dual ligand system may play an essential role to facilitate single-electron transfer from a Pd catalyst to activate a redox active ester. Testing other activation groups on the redox active ester revealed Nhydroxyphthalimide to be uniquely effective (Scheme 1c). The reaction did not continue after the irradiation source was removed, showing that irradiation is not only for Pd(0) generation.13 Aromatic N-(acyloxy)phthalimide was ineffective under the established condition.
a
Styrene (0.2 mmol), aliphatic carboxylate (0.3 mmol), Pd(PPh3)2Cl2 (5 mol %), Xantphos (6 mol %), K2CO3 (120 mol %), H2O (100 mol %) in DMA (2 mL), irradiation by 36 W blue LEDs at room temperature for 24 h under an Ar atmosphere. Yield and ratio of stereoisomers were determined by gas chromatography using biphenyl as an internal standard.
oxidative addition suppressed undesired β-H elimination before insertion.12 We envisioned that the irradiation-excited Pd complex may also be suitable for activation of N-(acyloxy)phthalimide through single-electron transfer because alkyl bromide (e.g., for n-BuBr, E1/2 = −1.22 V vs SCE)15 and N-(acyloxy)phthalimide (E1/2 = −1.26 to −1.37 V vs SCE) have similar redox potential,16 even though the activation mode should be different (Figure 1c). In this work, we discovered that, upon irradiation by blue LEDs, commercially available Pd(PPh3)2Cl2 in combination with Xantphos efficiently catalyzes a decarboxylative Heck reaction between N-(acyloxy)phthalimide and a range of styrene derivatives. The reaction enables the use of aliphatic carboxylates as an alkyl source in Heck type reactions. The method also demonstrates a new activation mode of N-(acyloxy)phthalimide by using a photoexcited Pd catalyst, thereby offering new opportunities to apply aliphatic N-(acyloxy)phthalimide in Pdcatalyzed coupling reactions. Although it is known that redox active esters can be activated by a low-valent transition-metal catalyst (e.g., Ni or Fe)7,8 and photoredox catalyst,17 the Pd catalytic system demonstrated herein represents a rare example 889
DOI: 10.1021/acs.orglett.8b00023 Org. Lett. 2018, 20, 888−891
Letter
Organic Letters Scheme 3. Substrate Scope with Respect to Aliphatic Carboxylatesa
Figure 3. Mechanistic studies: (a) A radical clock experiment; (b) intermolecular kinetic isotope experiment; (c) UV−vis absorption spectrum of reaction mixture using stoichiometric amount of Pd(PPh3)2Cl2 and Xantphos.
unlikely. Several unreactive substrates are also reported at the bottom of Scheme 2. We then established the substrate scope of the reaction regarding aliphatic carboxylates (Scheme 3). Gratifyingly, a variety of quaternary, tertiary, and secondary aliphatic carboxylates could be used as tertiary, secondary, and primary alkyl donors. Even for substrates delivering tertiary alkyl groups with eliminable β-hydrogen, the reactions still proceeded in excellent yield (17, 18, 21). The successful coupling of these tertiary alkyls demonstrates that β-H elimination on the Pd center is suppressed by the irradiation excitation.12 The stereoselectivity of coupling with tertiary alkyl is excellent. For the products of coupling with tertiary and secondary carboxylates, the stereoselectivity was slightly decreased, but remained high (E/Z = 92:8 in the lowest case). α-Amino acid and α-alkoxy acid-derived redox esters were also suitable substrates, generating allylic amine and allylic ether derivatives (25, 26, 27, 28). Notably, both terminal aliphatic alkene (30) and aliphatic alkyne (32) were well tolerated. When a chlorinated aliphatic carboxylate was used, the alkyl chloride moiety remained intact after the reaction and could be used for further cross-coupling (33).21 Several competition reactions were conducted to gain mechanistic information on this reaction. First, competition among quaternary, tertiary, and secondary carboxylates revealed that the carboxylate delivering a more stable alkyl radical reacts faster, which is consistent with a radical decarboxylation mechanism (Figure 2a). The competition experiment between styrene and 1-octene showed the overwhelming selectivity of styrene. We did not detect any side product of the 1-octene trapping cyclohexyl radical (Figure 2b). This reactivity may be associated with the alkyl-palladium species in its excited state as a Pd-intercepted alkyl radical. The radical properties of the alkylcoupling partner are further evidenced by the results of radical clock experiments (Figure 3a).22 The results of intermolecular kinetic isotope effect (KIE) experiments showed that β-H elimination is not the productdetermining step (Figure 3b). Similar to our recently reported Heck reaction with alkyl bromide, we also observed a UV−vis
a
Yields of isolated products. The ratio of stereoisomers was determined by 1H NMR analysis. The reactions were conducted in front of a cooling fan in a room of constant temperature (t = 25 ± 3 °C). bAliphatic carboxylates (0.4 mmol), time = 36 h.
Figure 2. Competition experiments. Standard conditions: Pd(PPh3)2Cl2 (5 mol %), Xantphos (6 mol %), K2CO3 (120 mol %) in DMA (2 mL), after 36 h under an Ar atmosphere at room temperature. GC yields using biphenyl as an internal standard.
The substrate scope with respect to an alkene coupling partner is demonstrated in Scheme 2. A variety of vinyl arenes and vinyl heteroarene (14) were amenable substrates. Both electron-rich and -deficient styrenes reacted well, and a variety of functional groups such as halide (4, 5, 6, 9, 12), amine (7), boronate (8), acetal (11), ketone (16), and esters (16) could be well tolerated. Besides vinyl arenes, 1-phenyl styrene was also an amenable substrate (15). Notably, 1,2,3,4,5-pentafluoro-6-vinylbenzene (12) and 2-vinylpyridine (14) were amenable substrates, suggesting that the involvement of benzylic carbenium is 890
DOI: 10.1021/acs.orglett.8b00023 Org. Lett. 2018, 20, 888−891
Letter
Organic Letters
Spiewak, A. M.; Johnson, K. A.; DiBenedetto, T. A.; Kim, S.; Ackerman, L. K. G.; Weix, D. J. J. Am. Chem. Soc. 2016, 138, 5016. (c) Xue, W.; Oestreich, M. Angew. Chem., Int. Ed. 2017, 56, 11649. (7) (a) Qin, T.; Cornella, J.; Li, C.; Malins, L.; Edwards, J. T.; Kawamura, S.; Maxwell, B. D.; Eastgate, M. D.; Baran, P. S. Science 2016, 352, 801. (b) Edwards, J. T.; Merchant, R. R.; McClymont, K. S.; Knouse, K. W.; Qin, T.; Malins, L. R.; Vokits, B.; Shaw, S. A.; Bao, D.-H.; Wei, F.-L.; Zhou, T.; Eastgate, M. D.; Baran, P. S. Nature 2017, 545, 213. (8) (a) Cornella, J.; Edwards, J. T.; Qin, T.; Kawamura, S.; Wang, J.; Pan, C.-M.; Gianatassio, R.; Schmidt, M.; Eastgate, M. D.; Baran, P. S. J. Am. Chem. Soc. 2016, 138, 2174. (b) Wang, J.; Qin, T.; Chen, T.-G.; Wimmer, L.; Edwards, J. T.; Cornella, J.; Vokits, B.; Shaw, S. A.; Baran, P. S. Angew. Chem., Int. Ed. 2016, 55, 9676. (c) Toriyama, F.; Cornella, J.; Wimmer, L.; Chen, T.-G.; Dixon, D. D.; Creech, G.; Baran, P. S. J. Am. Chem. Soc. 2016, 138, 11132. (9) Li, C.; Wang, J.; Barton, L. M.; Yum, S.; Tian, M.; Peters, D. S.; Kumar, M.; Yu, A. W.; Johnson, K. A.; Chatterjee, A. K.; Yan, M.; Baran, P. S. Science 2017, 356, eaam7355. (10) Koga, N.; Obara, S.; Kitaura, K.; Morokuma, K. J. Am. Chem. Soc. 1985, 107, 7109. (11) Lin, B.-L.; Liu, L.; Fu, Y.; Luo, S.-W.; Chen, Q.; Guo, Q.-X. Organometallics 2004, 23, 2114. (12) Wang, G.-Z.; Shang, R.; Cheng, W.-M.; Fu, Y. J. Am. Chem. Soc. 2017, 139, 18307. (13) Kurandina, D.; Parasram, M.; Gevorgyan, V. Angew. Chem., Int. Ed. 2017, 56, 14212. It should be mentioned that n-decyl iodide and cyclohexyl iodide were also demonstrated as substrates in this work. (14) For irradiation-excited Pd catalysis, see: (a) Parasram, M.; Chuentragool, P.; Sarkar, D.; Gevorgyan, V. J. Am. Chem. Soc. 2016, 138, 6340. (b) Parasram, M.; Chuentragool, P.; Wang, Y.; Shi, Y.; Gevorgyan, V. J. Am. Chem. Soc. 2017, 139, 14857. (15) Isse, A. A.; Lin, C. Y.; Coote, M. L.; Gennaro, A. J. Phys. Chem. B 2011, 115, 678. (16) Pratsch, G.; Lackner, G. L.; Overman, L. E. J. Org. Chem. 2015, 80, 6025. (17) (a) Schnermann, M. J.; Overman, L. E. Angew. Chem., Int. Ed. 2012, 51, 9576. (b) Cheng, W.-M.; Shang, R.; Fu, Y. ACS Catal. 2017, 7, 907. (c) Cheng, W.-M.; Shang, R.; Fu, M.-C.; Fu, Y. Chem. - Eur. J. 2017, 23, 2537. (18) For irradiation-induced copper-catalyzed C−N bond and C−S bond formation, see: (a) Creutz, S. E.; Lotito, K. J.; Fu, G. C.; Peters, J. C. Science 2012, 338, 647. (b) Bissember, A. C.; Lundgren, R. J.; Creutz, S. E.; Peters, J. C.; Fu, G. C. Angew. Chem., Int. Ed. 2013, 52, 5129. (c) Ziegler, D. T.; Choi, J.; Munoz-Molina, J. M.; Bissember, A. C.; Peters, J. C.; Fu, G. C. J. Am. Chem. Soc. 2013, 135, 13107. (d) Uyeda, C.; Tan, Y.; Fu, G. C.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 9548. (19) (a) Zou, Y.; Zhou, J. S. Chem. Commun. 2014, 50, 3725. (b) McMahon, C. M.; Alexanian, E. J. Angew. Chem., Int. Ed. 2014, 53, 5974. For examples of a cobalt-catalyzed alkyl Heck reaction, see: (c) Ikeda, Y.; Nakamura, T.; Yorimitsu, H.; Oshima, K. J. Am. Chem. Soc. 2002, 124, 6514. (20) (a) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013, 113, 5322. (b) Skubi, K. L.; Blum, T. R.; Yoon, T. P. Chem. Rev. 2016, 116, 10035. (c) Romero, N. A.; Nicewicz, D. A. Chem. Rev. 2016, 116, 10075. (21) Nakamura, M.; Matsuo, K.; Ito, S.; Nakamura, E. J. Am. Chem. Soc. 2004, 126, 3686. (22) Noda, D.; Sunada, Y.; Hatakeyama, T.; Nakamura, M.; Nagashima, H. J. Am. Chem. Soc. 2009, 131, 6078. (23) For references on irradiation-induced transition-metal catalysis, see: (a) Weiss, M. E.; Kreis, L. M.; Lauber, A.; Carreira, E. M. Angew. Chem., Int. Ed. 2011, 50, 11125. (b) Weiss, M. E.; Carreira, E. M. Angew. Chem., Int. Ed. 2011, 50, 11501. (c) Kreis, L. M.; Krautwald, S.; Pfeiffer, N.; Martin, R. E.; Carreira, E. M. Org. Lett. 2013, 15, 1634. (d) Parasram, M.; Gevorgyan, V. Chem. Soc. Rev. 2017, 46, 6227. (e) Kurandina, D.; Rivas, M.; Radzhabov, M.; Gevorgyan, V. Org. Lett. 2018, DOI: 10.1021/acs.orglett.7b03591.
absorption onset of the mixture of stoichiometric reaction around 450 nm (Figure 3c).12 The absorption is affected by the absence of Xantphos, but not affected by the absence of substrates. N-(Acyloxy)phthalimide itself has no absorption in the wavelength range of blue LED irradiation. The UV−vis absorption experiments show that the Pd intermediate is indeed excited by photoirradiation of blue LEDs.23 In summary, we discovered a Pd catalyst that can be photoexcited by irradiation with blue LEDs to enable the decarboxylative alkyl Heck reaction of N-(acyloxy)phthalimide with vinyl (hetero)arenes at rt. With this reaction in hand, a broad scope of aliphatic carboxylic acids can now be used as alkyl donors in alkyl Heck type reactions. We also discovered a new activation mode of aliphatic N-(acyloxy)phthalimide by a photoexcited Pd catalyst to generate alkyl palladium species. These new discoveries, along with the irradiation effect to suppress undesired β-H elimination, are expected to inspire the exploration of new alkylation reactions catalyzed by palladium, such as C−H alkylation and alkylation of heteroatoms.
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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.8b00023. Experimental details and characterization data (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Rui Shang: 0000-0002-2513-2064 Yao Fu: 0000-0003-2282-4839 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the NSFC (21325208, 21572212), MOST (2017YFA0303500), FRFCU, and PCSIRT. REFERENCES
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DOI: 10.1021/acs.orglett.8b00023 Org. Lett. 2018, 20, 888−891
