Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)
Letter
Photocatalytic regio- and stereo-selective [2+2] cycloaddition of styrene derivatives using a heterogeneous organic photocatalyst Run Li, Beatriz Chiyin Ma, Wei Huang, Lei Wang, Di Wang, Hao Lu, Katharina Landfester, and Kai A. I. Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00490 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Photocatalytic Regio- and Stereo-Selective [2+2] Cycloaddition of Styrene Derivatives Using a Heterogeneous Organic Photocatalyst Run Li, Beatriz Chiyin Ma, Wei Huang, Lei Wang, Di Wang, Hao Lu, Katharina Landfester and Kai. A. I. Zhang* Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ABSTRACT: Photocatalytic [2+2] cycloaddition is a useful tool for the synthesis of cyclobutane derivatives, which contributes a large part to natural compound production. Usually, the [2+2] cycloadditions are catalyzed by transition metal complexes. Here we report the use of a pure organic photocatalyst based on a conjugated microporous polymer network for the [2+2] cycloaddition of styrene derivatives under the irradiation of visible light. Both unsymmetrical and homocycloaddition could be obtained with high conversion and selectivity. In addition, natural products, such as di-Omethylendiandrin A and endiandrin A are also synthesized with a high catalytic efficiency.
KEYWORDS: [2+2] cycloaddition, metal-free photocatalysis, organic photocatalyst, visible light, conjugated microporous polymer Strained carbocyclic molecules, especially cyclobutane derivatives, have gained increasing attentions in the past decades due to their role as important molecular building motifs and intermediates for the synthesis of natural products and pharmaceuticals.1-3 Certain cyclobutane derivatives such as endiandrin A, di-O-methylendiandrin A and magnosalin are regarded as potent binders for glucocorticoid receptor, which plays a critical role in the treatment of inflammatory conditions.4 Up to now, various preparative strategies have been developed to obtain cyclobutane derivatives. Beside the ring expansion of cyclopropanes,5,6 [2+2] cycloaddition reaction of alkenes has been widely applied for the synthesis of cyclobutanecontaining products. Conventionally, [2+2] cycloaddition is activated under thermal conditions, which also involves precisely designed catalysts such as Lewis acids,7 amines,8 or transition metals.9 However, a disadvantage of the thermally activated [2+2] cycloaddition is that high reactions yields can only be obtained using polarized alkenes. More specifically, the two alkenes as cycloaddition partners should either be electrophile or nucleophile, respectively, to match the frontier molecular orbitals of reactants.10 This requirement limits a wilder application field of the thermal cycloaddition reactions. Photocycloaddition, in comparison, has been successfully employed as a useful tool for the synthesis of cyclobutane-containing products in recent years.11,12 Especially low energetic visible light-driven [2+2] cycloaddition has been reported either for electron-rich alkenes,13 or electron-deficient olefins.14 So far, the photocatalytic systems used were mainly based on transition metal.15,16 Very few molecular organic photocatalysts have been reported for
[2+2] cycloaddition reaction.17 Nevertheless, considerable drawbacks are still associated to the noble metalcontaining systems including rareness in the nature, toxicity, high price and common low stability due to photobleaching effect for small molecular metal or non-metal catalytic systems. To overcome these limitations, an alternative photocatalytic system is indeed needed. Organic semiconductor-based photocatalysts offer a more sustainable and environmentally friendly alternative due to their pure organic nature, highly tunable electronic and optical properties via molecular design or morphology engineering. Recent studies showed that either small molecules18-20 or macromolecular21-32 systems have been employed as efficient photocatalysts for visible lightpromoted reactions. Nevertheless, cycloaddition reactions using organic semiconductors are still unknown. Herein, we report the use of a pure organic photocatalyst based on a conjugated microporous polymer network as photocatalyst for the [2+2] cycloaddition of styrene derivatives under the irradiation of visible light. Both homo- and unsymmetrical cycloaddition reactions could be performed with high conversion and selectivity. A mechanistic study on the reaction insights is described. In addition, some natural products, such as di-Omethylendiandrin A and endiandrin A are also synthesized with a high catalytic selectivity. In this study, a cross-linked poly(benzothiadiazole) (B-BT) network was chosen as pure organic photocatalyst (Figure 1a). B-BT was synthesized via SonogashiraHagihara cross-coupling of 4,7-dibromobenzo[c][1,2,5]thiadiazole with 1,3,5-triethynylbenzene
ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
similar to a previous report.21 The scanning electron (SEM) and transmission electron microscopy (TEM) images (Figure 1b and 1c) showed a fiber-shaped structure with a diameter of ca. 200 nm. The FTIR spectrum (Figure 1d) showed typical signals at 1348, 1377, 1567, and 1577 cm1 , which are characteristic for C=N and N-S stretching modes in the benzothiadiazole moiety. The signals at around 2200 cm-1 and 3300 cm-1 indicate the C≡C and alkynyl C-H stretching modes, respectively. The signals at 1440, 1495, and 1600 cm-1 can be assigned to the skeleton vibration of aryl rings in the polymer. 13C solid-state crosspolarization magic angle spinning (CP/MAS) NMR spectrum (Figure S1) showed characteristic signals at 154 ppm, which can be assigned to the adjacent carbon to nitrogen atoms in the benzothiadiazole unit. The resonance peak between 110 and 140 ppm are attributed to aromatic carbons, and the signals from 80 to 100 ppm can be assigned to the carbon of the triple bond. The Brunauer-EmmetTeller (BET) surface area of B-BT was found to be 120 m2/g with a pore volume of 0.12 m3/g (Figure S2).
Page 2 of 6
The EPR spectra showed an enhanced signal after light irradiation compared to the signal reordered in dark (Figure S5), indicating the existence of photo-generated single electrons (i.e. radicals).34 The photocurrent test illustrated a response for light-on and -off event, indicating an efficient light-induced electronic conductivity, as well as charge separation and charge transfer inside B-BT (Figure S6). Furthermore, thermal gravimetric analysis (TGA) revealed that B-BT could remain intact up to 400 0C at nitrogen atmosphere (Figure S7). The residual weight of 75% might be attributed to the carbonized material. To test the visible light-promoted photocatalytic activity of B-BT, the [2+2] cycloaddition between (E)-1methoxy-4-(prop-1-en-1-yl) benzene (commercial name: trans-anethole) was chosen as model reaction. Both homo- and unsymmetrical cycloaddition either with or without the addition of styrene as coupling partner were investigated respectively. As listed in Table 1, both homoand unsymmetrical cycloaddition reactions showed a high conversion and selectivity (entries 1 and 7). Control experiments conducted in dark (entries 3 and 9), or without using B-BT as photocatalyst led only to trace conversions, demonstrating the essential roles of light and the photocatalyst. Furthermore, various solvents (entries 11 -14) were investigated and nitromethane was found with highest conversion while in dimethyl sulfoxide and toluene no product was detected. Table 1. Screening and control experiments of the [2+2] cycloaddition using B-BT as photocatalyst.a B-BT, CH3NO2
+ 1
entry
Figure 1. (a) Chemical structure, (b) SEM and (c) TEM image, (d) FTIR, (e) UV/Vis DR spectra, (f) Kubelka-Munk plot, and (g) HOMO and LUMO band positions of B-BT.
The UV/vis diffuse reflectance spectrum (DRS) of B-BT (Figure 1e) showed a maximum absorption at 395 nm with a broad absorption range up to 800 nm. An optical band gap of 2.4 eV could be derived from the Kubelka-Munk function (Figure 1f). Cyclic voltammetry measurement revealed the lowest occupied molecular orbital (LUMO) position of B-BT) at -0.67 eV vs SCE (Figure S3). The highest unoccupied molecular orbital (HOMO) position at +1.73 eV vs SCE could be thus derived from the optical band gap. Additionally, the HOMO level in the excited state was determined via ultraviolet photoelectron spectroscopy (UPS) (Figure S4).33 An ionization potential could be calculated to be 7.07 eV, resulting into a HOMO level at 2.39 eV vs SCE in the excited state. This value was higher than that obtained from CV measurement, indicating a string oxidizing character of the organic photocatalyst B-BT. To study the photo-induced electron/hole charge separation, we then conducted electron paramagnetic resonance (EPR) and photocurrent measurements.
+
visible light, r. t.
O 2
1
O
O
O
3
2
light
reaction condition variations
4
ratio (3/4)
conv. (%)b
1
+
-
+
-
-
86
2
+
-
+
no B-BT
-
5
3
+
-
-
in dark
-
trace
4
+
-
+
in N2
-
40
-
38
5c
+
-
+
electron scavenger
d
6
+
-
+
hole scavenger
-
trace
7
+
+
+
-
> 15:1
92
8
+
+
+
no B-BT
-
4
9
+
+
-
in dark
-
trace
10
+
+
+
in N2
15:1
60
11
+
+
+
in dichloromethane
>20:1
10
12
+
+
+
in acetonitrile
> 20:1
15
13
+
+
+
in dimethyl sulfoxide
-
trace
14
+
+
+
in toluene
-
trace
15c
+
+
+
electron scavenger
> 15:1
56
16d
+
+
+
hole scavenger
-
trace
ACS Paragon Plus Environment
Page 3 of 6
ACS Catalysis a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Standard reaction conditions: [anethole]=0.1 M, [styrene]=0.5 M, [B-BT]= 1 mg/ml, CH3NO2: 2.4 ml, blue LED 2 b lamp (460 nm, 0.26 W/cm ), room temperature, air, 8h. c Conversion determined by GC-MS. CuCl2 as electron scavd enger. KI as hole scavenger.
To reveal the mechanistic insights and the specific roles of the photo-generated electron/hole pair during the [2+2] cycloaddition process, electron and hole scavengers were added into the reaction mixture for both homo- and asymmetry cycloadditions. With the addition of KI as hole scavenger, only trace conversion was observed for both reactions (entry 6 and 16), indicating the essential role of hole for formation of product.35 By adding CuCl2 as electron scavenger, reduced conversions were observed (entries 5 and 15). The same phenomenon was observed by excluding oxygen from the reaction atmosphere (entries 4 and 10). The result indicates that the electronactivated superoxide radical (O2•‒) as active species could played an accelerating role for the cycloaddition, similar to previous reports.36 An additional control experiment using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as superoxide radical trapping agent showed typical EPR patterns for DMPO-O2•‒ adducts(Figure 2a).
Figure 2. (a) EPR spectra of DMPO-O2•‒ adducts with B-BT as photocatalyst in dark or under blue light irradiation 2 (λ=460 nm, 1.2 W/cm ). (b) EPR spectra using PBN as radical trapping agent for the radical intermediate of anethole under light irradiation. Pure PBN (black); PNB with anethole and B-BT under N2 atmosphere (red); or under O2 (blue); simulated EPR pattern of the trapped radical (green).
Cyclic voltammetry measurement showed that the oxidation potential of anethole lay at 1.17 eV vs. SCE, which
makes its direct oxidation by the photo-generated hole of B-BT (Eoxi. = 1.73 V vs. SCE) possible.37 Based on the observations, we proposed a reaction mechanism similar to previous reports.13 As illustrated in Figure 3, the catalytic process could occur via three different routes. Under visible light irradiation, trans-anethole was oxidized by the photo-generated hole of B-BT to form its cationic radical intermediate a, which then reacts with another equivalent olefin (anethole for homo- or styrene for unsymmetrical [2+2] cycloaddition) to generate the radical intermediate b. In the following three possible routes, the intermediate b can either lose one electron to the photogenerated hole (i), the activated superoxide O2•‒ (ii), or another anethole molecule (iii), and thus lead to the formation of the final product. According to previous reports,13,38,39 the reaction between the cationic radical a and alkene derivatives, here styrene, should first form a trapezoidal cyclobutane cation radical intermediate with a long-bond structure, where the σ bond was formed preferred to the β carbon rather than the α carbon of styrene. Therefore, the cycloaddition between the cation radical a and styrene is sterically more favored than that with another anethole unit. This would explain the favored selective formation of the unsymmetrical cycloaddition product in contrast to the minimal amount of the homo-cycloaddition of anethole. In addition, oxygen could also stabilize some specific intermediates which might part of reason that oxygen could facilitate the reaction conversion.40 Further proof of the formation of the radical intermediate a could be observed using N-tertbutyl-α-phenylnitrone (PNB) as radical trapping agent (Figure 2b). A typical pattern for PBN-trapped radical could be recorded, which corresponded well to its simulation. The enhanced signal intensity taken under oxygen atmosphere indicates that oxygen played an important role within the photocatalytic cycle.
Figure 3. Proposed reaction mechanisms for the photocatalytic [2+2] cycloaddition using B-BT via 3 possible routes.
To further test the general feasibility of B-BT as photocatalyst, an extensive investigation of [2+2] cycloaddition with various styrene derivatives was conducted with the optimized reaction conditions. As illustrated in Figure 4, high conversion and selectivity were obtained in most cases. Both electron-donating substitution groups on the
ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
phenyl rings of the substrates such as methyl, methoxy, or dimethoxyl and electron- withdrawing substitution groups such as ester or halides did not affect the conversion and selectivity. In addition, cycloaddition of anethole with methyl-substituted styrene on ortho, meta, or para positions generally occurred with high efficiency. Cycloaddition of other anethole derivatives with different substitution groups on the alkene such as isomethyleugenol also achieved with high conversion and selectivity. It is worth to note that the catalytic efficiency of the organic photocatalyst B-BT was comparable with that of the traditional transition metal complexes such as Ru(bpy)3Cl2 and fac-Ir(ppy)3.13 Moreover, the repeating experiment was also conducted for both homo- and crossed cycloaddition reactions with a slight decrease of conversion, which might be caused by the catalyst lose during recovery process (Figure S8). No considerable changes from the FTIR (Figure S9), SEM and TEM images (Figure S10-S11) were observed, showing the stability of B-BT.
Page 4 of 6
Figure 4b, di-O-methylendiandrin A could be obtained with a moderate conversion but with high selectivity via the homo-cycloaddition of (E)-1,2-dimethoxy-4-(prop-1en-1-yl)benzene (5), which has not been reported far. In addition, endiandrin A could be synthesized from the homo-coupled product of trans-anethole, following by aryl bromination, demethylation and Ullmann-type coupling with a totally product yield of 47 %, similar to literature17 (see SI). The visible light-driven approach offers a high regio- and stereo-selectivity for the synthesis of those cyclobutanes in a greener and novel way to obtain natural products. In summary, we demonstrated a visible light-promoted homo- and unsymmetrical [2+2] cycloaddition of styrene derivatives using a conjugated microporous polymer network as pure organic photocatalyst. The cycloaddition was achieved with high efficiency and regio- and stereoselectivity. Additionally, cyclobutane-containing natural products such as di-O-methylendiandrin A and endiandrin A could be obtained via developed metal-free reaction route. We envisioned that the new class of organic photocatalysts ought not to be limited only on [2+2] cycloaddition. A broader range of cycloaddition reactions such as [3+2] or [4+2] cycloaddition reactions could also be catalyzed via the pure organic photocatalytic pathway.
ASSOCIATED CONTENT Supporting Information. Additional experimental details and characterization such as 13 solid state C CP/MAS NMR, Cyclic voltammetry, UPS, EPR, 1 13 Photocurrent, TGA, H NMR, C NMR, N2 sorption isotherms and pore size distribution are shown in supporting information
AUTHOR INFORMATION Corresponding Author *
[email protected] Notes All authors declare no competing financial interests.
ACKNOWLEDGMENT The authors acknowledge the Max Planck Society for financial support. R.L., W.H., L.W. and D.W. thank the China Scholarship Council (CSC) for funding.
REFERENCES
Figure 4. (a) Scope of the [2+2] cycloaddition reactions; the value in parentheses indicates the selectivity of unsymmetrical products compared with dimers. (b) some natural products obtained.
Furthermore, the facile visible light-promoted photocatalytic protocol also allowed the preparation of important cyclobutane-containing natural products such as endiandrin A and di-O-methylendiandrin A. As showed in
(1) Namyslo, J. C.; Kaufmann, D. E. Chem. Rev. 2003, 103, 1485-1538. (2) Xu, Y.; Conner, M. L.; Brown, M. K. Angew. Chem. Int. Ed. 2015, 54, 11918-11928. (3) Sergeiko, A.; Poroikov, V. V.; Hanuš, L. O.; Dembitsky, V. M. Open Med. Chem. J. 2008, 2, 26-37. (4) Davis, R. A.; Carroll, A. R.; Duffy, S.; Avery, V. M.; Guymer, G. P.; Forster, P. I.; Quinn, R. J. J. Nat. Prod. 2007, 70, 1118-1121. (5) Kleinbeck, F.; Toste, F. D. J. Am. Chem. Soc. 2009, 131, 9178-9179. (6) Alberti, G.; Bernard, A. M.; Frongia, A.; Piras, P. P.; Secci, F.; Spiga, M. Synlett 2006, 14, 2241-2245.
ACS Paragon Plus Environment
Page 5 of 6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
(7) Boxer, M. B.; Yamamoto, H. Org. Lett. 2005, 7, 3127-3129. (8) Duan, G.-J.; Ling, J.-B.; Wang, W.-P.; Luo, Y.C.; Xu, P.-F. Chem. Commun. 2013, 49, 4625-4627. (9) Sakai, K.; Kochi, T.; Kakiuchi, F. Org. Lett. 2013, 15, 1024-1027. (10) Schultz, D. M.; Yoon, T. P. Science 2014, 343, 1239176. (11) Poplata, S.; Tröster, A.; Zou, Y.-Q.; Bach, T. Chem. Rev. 2016, 116, 9748-9815. (12) Kärkäs, M. D.; Porco, J. A.; Stephenson, C. R. J. Chem. Rev. 2016, 116, 9683-9747. (13) Ischay, M. A.; Ament, M. S.; Yoon, T. P. Chem. Sci. 2012, 3, 2807-2811. (14) Du, J.; Yoon, T. P. J. Am. Chem. Soc. 2009, 131, 14604-14605. (15) Ischay, M. A.; Anzovino, M. E.; Du, J.; Yoon, T. P. J. Am. Chem. Soc. 2008, 130, 12886-12887. (16) Du, J.; Skubi, K. L.; Schultz, D. M.; Yoon, T. P. Science 2014, 344, 392-396. (17) Riener, M.; Nicewicz, D. A. Chem. Sci. 2013, 4, 2625-2629. (18) Neumann, M.; Fuldner, S.; Konig, B.; Zeitler, K. Angew. Chem. Int. Ed. 2011, 50, 951-954. (19) Ghosh, I.; Ghosh, T.; Bardagi, J. I.; König, B. Science 2014, 346, 725-728. (20) Wang, L.; Huang, W.; Li, R.; Gehrig, D.; Blom, P. W.; Landfester, K.; Zhang, K. A. Angew. Chem. Int. Ed. 2016, 55, 9783-9787. (21) Zhang, K.; Kopetzki, D.; Seeberger, P. H.; Antonietti, M.; Vilela, F. Angew. Chem. Int. Ed. 2013, 52, 14321436. (22) Kang, N.; Park, J. H.; Ko, K. C.; Chun, J.; Kim, E.; Shin, H. W.; Lee, S. M.; Kim, H. J.; Ahn, T. K.; Lee, J. Y.; Son, S. U. Angew. Chem. Int. Ed. 2013, 52, 6228-6232. (23) Zhang, K.; Vobecka, Z.; Tauer, K.; Antonietti, M.; Vilela, F. Chem. Commun. 2013, 49, 11158-11160. (24) Luo, J.; Zhang, X.; Zhang, J. ACS Catal. 2015, 5, 2250-2254. (25) Wang, Z. J.; Ghasimi, S.; Landfester, K.; Zhang, K. A. I. Adv. Mater. 2015, 27, 6265-6270. (26) Sprick, R. S.; Jiang, J.-X.; Bonillo, B.; Ren, S.; Ratvijitvech, T.; Guiglion, P.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. J. Am. Chem. Soc. 2015, 137, 3265-3270. (27) Ghasimi, S.; Prescher, S.; Wang, Z. J.; Landfester, K.; Yuan, J.; Zhang, K. A. I. Angew. Chem. Int. Ed. 2015, 54, 14549-14553. (28) Yang, C.; Ma, B. C.; Zhang, L.; Lin, S.; Ghasimi, S.; Landfester, K.; Zhang, K. A. I.; Wang, X. Angew. Chem. Int. Ed. 2016, 55, 9202-9206. (29) Sprick, R. S.; Bonillo, B.; Clowes, R.; Guiglion, P.; Brownbill, N. J.; Slater, B. J.; Blanc, F.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Angew. Chem. Int. Ed. 2016, 55, 17921796. (30) Zhang, G.; Lan, Z.-A.; Wang, X. Angew. Chem. Int. Ed. 2016, 55, 15712-15727. (31) Wang, Z. J.; Ghasimi, S.; Landfester, K.; Zhang, K. A. I. J. Mater. Chem. A 2014, 2, 18720-18724. (32) Su, F.; Mathew, S. C.; Möhlmann, L.; Antonietti, M.; Wang, X.; Blechert, S. Angew. Chem. Int. Ed. 2011, 50, 657-660. (33) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z. Science 2015, 347, 970. (34) Zhang, J.; Zhang, G.; Chen, X.; Lin, S.; Mohlmann, L.; Dolega, G.; Lipner, G.; Antonietti, M.; Blechert, S.; Wang, X. Angew. Chem. Int. Ed. 2012, 51, 3183-3187.
(35)
Studer, A.; Curran, D. P. Nat. Chem. 2014, 6,
765-773. (36) Su, F.; Mathew, S. C.; Lipner, G.; Fu, X.; Antonietti, M.; Blechert, S.; Wang, X. J. Am. Chem. Soc. 2010, 132, 16299-16301. (37) Yueh, W.; Bauld, N. L. J. Phys. Org. Chem. 1996, 9, 529-538. (38) Bauld, N. L.; Bellville, D. J.; Pabon, R.; Chelsky, R.; Green, G. J. Am. Chem. Soc. 1983, 105, 2378-2382. (39) Schepp, N. P.; Johnston, L. J. J. Am. Chem. Soc. 1994, 116, 6895-6903. (40) Stevenson, S. M.; Shores, M. P.; Ferreira, E. M. Angew. Chem. Int. Ed. 2015, 54, 6506-6510.
ACS Paragon Plus Environment
ACS Catalysis
Page 6 of 6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment
6
