Subscriber access provided by EPFL | Scientific Information and Libraries
Communication
Ag-Catalyzed Long-distanced Aryl Migration from Carbon Center to Nitrogen Center Taigang Zhou, Fei-Xian Luo, Ming-Yu Yang, and Zhang-Jie Shi J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 2, 2015
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.
Journal of the American Chemical Society 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
Journal of the American Chemical Society
Ag-Catalyzed Long-distanced Aryl Migration from Carbon Center to Nitrogen Center Taigang Zhou,a Fei‐Xian Luo,a Ming‐Yu Yang,a Zhang‐Jie Shi* a,b a
Beijing National Laboratory of Molecular Sciences and Key Laboratory of Bioorganic Chemistry and Molecular En‐ gineering of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 and bState Key Laboratory of Elemento‐Organic Chemistry, Nankai University, Tianjin 300071, China Supporting Information ABSTRACT: Selective cleavage of inert C‐C bond followed by C‐O/N formation through a long‐distanced aryl migration from carbon to nitrogen center was reported via Ag‐catalysis. The migrating products were easily converted into γ‐hydroxy amines and tetrahydroquinoline derivatives in quantitative yields. Preliminary mechanistic studies indicated a radical pathway.
Selective C‐C bond activation/cleavage has attracted much 1 attention in recent years. Not only is it one of the most chal‐ lenging themes in fundamental organic chemistry, but it also represents a powerful, straightforward, and atom‐economic strategy for constructing new organic compounds through the completely new pathway based on the skeleton reorgani‐ zation of easily available compounds, differentiating from 1 conventional organic syntheses. During the past few decades, many achievements in transition‐metal‐catalyzed C‐C cleav‐ age have been made starting from strained and unstrained 2,3 compounds. In the absence of transition metal catalysts, C‐ 4 C could also been cleaved and transformed through radical 5 and cationic intermediates. Among different strategies to approach direct C‐C cleavage of unstrained molecules, the migration of carbon‐based groups is common and important 6 to facilitate the C‐C cleavage and new C‐C formation.
asymmetric C‐N formation through desymmetrization via Ag catalysis with chiral ligands. Accidentally, we found phenyl migration product 2a, which was unambiguously confirmed by X‐ray crystallography (Scheme 2b). We proposed that, both annulation and migration products might be generated from the same N‐centered radical or cation as a key interme‐ diate. From those intermediates, the dearomatization pro‐ cess would take place to form the ‐complexes (either ‐ radical or ‐cation complex) when presenting aryl group at the proper position. which could induce a completely differ‐ ent chemistry from the same starting materials. Indeed, both intra‐ and inter‐ molecular dearomatization of the heterocycles as well as electron‐rich phenol derivatives through radical or cationic processes have been well studied, thus offering the possibility for our hypothesis although the dearomatization of common benzene derivatives was scarce‐ 10 ly reported. Therefore, we turned out to develop this long‐ distanced aryl migration.
Scheme 1. New Development of C‐C Cleavage via Ag‐ Catalyzed Long‐distanced Aryl Migration under Oxi‐ dative Condition
In general, group migration only takes place at carbon cen‐ 5 6c‐f 3a ters adjacent to carbon cations, radicals or carbenes (Scheme 1). Beautiful examples to present long‐distanced (over four carbons) migrations, were also reported by 6g 6h 6i 6j Pohmakotr , Liang , Tchabanenko , Uneyama , and other 6a,b groups . However, group migration from carbon to nitro‐ 7 gen center is scarcely reported. In particular, long‐distanced aryl migration from carbon to nitrogen center has never been approached. Herein we demonstrated an unusual Ag‐ catalyzed long‐distanced aryl migration from carbon to ni‐ trogen center (Scheme 1d). In our previous studies we observed direct annulation 2 8 through sp C‐H amidation via Ag catalysis (Scheme 2a). Yokoyama and coworkers also reported the formation of tetrahydroquinoline derivatives from the similar triflic amide 9 promoted by irradiation. Later on, we planned to explore the
Scheme 2. Different Chemistry of Triflic Amides
ACS Paragon Plus Environment
Journal of the American Chemical Society
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
Page 2 of 6
DCE, dioxane, and so on, were screened while only DCE worked well (Table S5, entry 2). To our interest, the addition of PhCl as co‐solvent dramatically improved the yield to 75% (entry 19).
Table 1. Ag‐catalyzed Long‐distanced Aryl Migration of 1a under Different Conditions.a
With the optimized conditions in hand, we first evaluated different functionalities on nitrogen atom (Scheme 3), in‐ cluding free amine (3a), carbamates (3b and 3c), acetamide (3d), and sulfonamides (3e, 3f and 1a). However, we found that only the sulfonamides performed the migration while Ts (3e) or Ms (3f) protected amides only gave poor conversions. Presumably, the strong electron‐withdrawing ability of Tf group enhanced the acidity of the NH group, which could be easily deprotonated by the proper bases and further coordi‐ nated to the Ag center. Such an Ag complex could be oxi‐ dized to Ag (II) (maybe Ag (III)), which further generated the electron‐deficient N‐centered radical. This radical attacked the phenyl ring to form the 5‐membered spiro radical ‐ complexes. To prove the importance of N‐H in amide, the substrate 3g was tested and no desired product was obtained at all.
Scheme 3. Evaluating Different Amides a
a
a
Reaction condition: substrate (0.1 mmol), catalyst (20 mol%), ligand (20 mol%), oxidant (2.0 equiv.), additive (2.0 equiv.), dichloroethane (DCE) (2.0 mL), 120 oC, 1 h. b The yield was determined by 1H NMR of the crude reaction mixture with CH2Br2 as internal standard. cIsolated yields. More details see table S1‐S5 in SI.
With the idea in mind, we first screened the conditions to promote such a migration with 1a as a model (Table 1). Ac‐ cording to our studies, such a long‐distanced aryl migration only took place under the oxidative conditions while Ag cata‐ lyst was critical to promote the efficacy. Thus, different oxi‐ dants were tested in the presence of AgOAc as the catalyst and 2,2'‐bipyridine (L1) as the ligand (entries 2‐5). PhI(OTFA)2 was found to be the only oxidant to produce the desired migrating product 2a in 31% yield (entry 5). Other metal catalysts, including Pd, Cu and Au, were also screened while most of them were found to be insufficient or in a quite low efficacy (entries 6‐8). Different Ag salts were examined and others did not show better performance (entries 9‐11). To promote the efficacy, various ligands were evaluated (entries 12‐14). 4,4'‐Di(tert‐butyl)‐2,2'‐bipyridine L3 was found the best and the desired product 2a was obtained in 38% yield (entry 13). Different bases were also screened (entries 15‐18). Lithium carbonate and potassium carbonate were found effi‐ cient (entries 17 and 18) while cesium carbonate and potassi‐ um tert‐butoxide diminished the efficiency (entries 15 and 16). These results indicated that, probably the proper base effi‐ ciently deprotonated N‐H to facilitate the oxidation of amide to N radical or cation. Different solvents, including PhCl,
Reaction condition: substrate (0.1 mmol), AgOAc (20 mol%), L3 (20 mol%), PhI(OTFA)2 (2.0 equiv.), K2CO3 (2.0 equiv.), PhCl/DCE (1.0 mL/1.0 mL), 120 oC, 1 h; The yield was determined by 1H NMR of the crude reaction mixture with CH2Br2 as internal standard.
Table 2. Ag‐catalyzed Aryl Migration of Symmetrical γ,γ‐Diaryl Substituted Triflic Amidesa
a Reaction condition: substrate (0.1 mmol), AgOAc (20 mol%), L3 (20 mol%), PhI(OTFA)2 (2.0 equiv.), K2CO3 (2.0 equiv.), PhCl/DCE (1.0 mL/1.0 mL), 120 oC, 1 h; Yields of isolated products. bYields of isolated corresponding alcoholysis products, due to these compounds were easily alcoholysis when they were purified by flash column chromatography (silica gel). c 120 oC, 10 h. d The d.r ratio was determined by 1H NMR.
We further extended such an aryl migration to various symmetric γ,γ‐diaryl substituted triflic amides (Table 2). Substrates bearing electron‐rich groups at para‐position of t aryl ring, including Me (1b), Bu (1c), Ph (1d) were submitted and the desired migrating products were afforded in moder‐ ate to good yields (2b‐2d). However, strong electron‐ donating groups containing heteroatoms, for example MeO and Me2N, induced the decomposition of starting materials
ACS Paragon Plus Environment
Page 3 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
Journal of the American Chemical Society
under this oxidation condition. Meanwhile, substrates bear‐ ing electron‐deficient groups at para‐position, including F (1e), Cl (1f), Br (1g), CF3 (1h), gave the desired migrating products in comparable efficacy (2e‐2h). Substrates with the meta‐substituents were also workable as migration groups (1i‐1l), giving the corresponding products with moderate yields (2i‐2l). More bulky substrate with an ortho‐tolyl mi‐ grating group (1m) also provided the migrating product (2m) in an acceptable yield. These results indicated that the steric hindrance on the aryl ring had a slight effect while the elec‐ tronic property did not have regular effects. Moreover, the migration of 10 with β‐methyl substituent of triflic amide performed well and 74% yield of 2o was obtained. However, the yield of 1n with methyl group adjacent to the amide was reduced to 45% albeit in the longer time, further proving the steric effect on this migration.
Table 3. Ag‐catalyzed Aryl Migration of Unsymmet‐ rical γ,γ‐Diaryl Substituted Triflic Amidesa
phenyl
