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S41467-018-07534-X.Pdf ARTICLE DOI: 10.1038/s41467-018-07534-x OPEN Differentiation between enamines and tautomerizable imines in the oxidation reaction with TEMPO Xiaoming Jie1, Yaping Shang1, Zhe-Ning Chen 1, Xiaofeng Zhang1, Wei Zhuang1 & Weiping Su1 Enamine and imine represent two of the most common reaction intermediates in syntheses, and the imine intermediates containing α-hydrogen often exhibit the similar reactivity to 1234567890():,; enamines due to their rapid tautomerization to enamine tautomers. Herein, we report that the minor structural difference between the enamine and the enamine tautomer derived from imine tautomerization results in the different chemo- and regioselectivity in the reaction of cyclohexanones, amines and TEMPO: the reaction of primary amines furnishes the formal oxygen 1,2-migration product, α-amino-enones, while the reaction of secondary amines under similar conditions generates exclusively arylamines via consecutive dehydrogenation on the cyclohexyl rings. The 18O-labeling experiment for α-amino-enone formation revealed that TEMPO served as oxygen transfer reagent. Experimental and computational studies of reaction mechanisms revealed that the difference in chemo- and regioselectivity could be ascribed to the flexible imine-enamine tautomerization of the imine intermediate con- taining an α-hydrogen. 1 State Key Laboratory of Structural Chemistry, Center for Excellence in Molecular Synthesis, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao Road West, Fuzhou 350002, China. Correspondence and requests for materials should be addressed to W.Z. (email: [email protected]) or to W.S. (email: [email protected]) NATURE COMMUNICATIONS | (2018) 9:5002 | DOI: 10.1038/s41467-018-07534-x | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-07534-x namines and imines are both key reactive intermediates of imines, which are generated in-situ from the condensation of Enumerous classic organic reactions1,2 and the recently devel- cyclohexanones with primary amines, bring about oxygen oped aminocatalysis for transformations of aldehydes and transfer from TEMPO to α-carbon atom of imines and furnish α- ketones3–6. Due to the diverse reactivity of enamine amino enone products, whereas enamines generated in situ from intermediates, aminocatalysis not only accelerates α-functionaliza- the condensation of cyclohexanones with secondary amines tion of aldehydes and ketones with a variety of electrophiles undergo dehydrogenative aromatization to produce aryl amines. by enhancing nucleophilicity of carbonyl α-carbon atoms3–10, but also enables use of various nucleophiles or radical trapping reagents for α-functionalization11–15 and even β- Results functionalization16,17 of aldehydes and ketones via oxidation of Discovery and development of α-amino enone formation enamines to generate delocalized radical cations or iminium ions. reaction. We started our investigation by examining the reaction On the other hand, imines containing α-hydrogen can rapidly of 4-tert-butyl-cyclohexanone (1a) with 4-chloro-aniline (2a) (1.5 tautomerize to their enamine tautomers, and therefore often behave equiv.) and TEMPO (1.5 equiv.), in which imine intermediate was – as enamines18 22. In this context, the tautomerizable imines have expected to be generated in-situ from the condensation of been reported to participate, through their enamine tautomers, in cyclohexanone with aniline (Fig. 2). Initially, the reaction was 18 aldol-type condensation with aldehydes (Fig. 1a) , Michael addi- carried out in the presence of Cu(OAc)2 and 2,2´-bipyridine tion to electrophilic olefins19, Pd-catalyzed intramolecular α-C-H ligand (bpy), which is similar to the reaction conditions estab- arylation of imines via imine α-palladation (Fig. 1b)20,21, lished for the α-aminoxylation of aldehydes with TEMPO via Cu-catalyzed aerobic oxygenation at the position α to imine enamine intermediate24. Interestingly, the reaction of cyclohex- group22. The similarity in reactivity between enamine and tauto- anone with aniline produced a formal oxygen 1,2-migration merizableimineledustoconsiderwhatistheexactdifferencein compound, α-amino enone (3a), in 63% yield (entry 1) (the reactivity between the enamine tautomer derived from tautomer- structure of this product was also established by single crystal izable imine and the real enamine. The effort to address this X-ray diffraction analysis). After removing bpy ligand from the question would develop the chemistry of imine and enamine and reaction system, this α-amino enone formation reaction obtained gain an insight into their structure-reactivity relationship. a higher yield (entry 2). Considering that the ketone-amine Recently, a series of α-aminoxylation reactions of carbonyl condensation to generate imine would be a prerequisite step in – compounds14,15,23 30 using 2,2,6,6-tetramethylpiperidine-1-oxyl this reaction, we turned our attention to using Lewis acid catalyst (TEMPO) as oxidant has been established, some of which for accelerating the ketone-amine condensation. As expected, the involved metal-promoted oxidation of the enamine intermediates use of Lewis acid catalysts such as Yb(OTf)3, Zn(OTf)2 and AlCl3 generated in-situ from the condensation of aldehydes with sec- in place of Cu(OAc) effected the desired reaction, albeit in lower – 2 ondary amines14,15,23 28. These elegant studies provided a good yields (entries 3–5). 4-Methylanthranilic acid, which proved to starting point for our investigation of the structure-reactivity be effective catalyst for ketone-amine condensation31,32, could relationship of enamine and tautomerizable imine. also promote α-amino enone formation in 37% yield (entry 6). Herein, we demonstrate the distinct difference between Adding 3 Å molecular sieve (400 mg) to the reaction system, in enamines and tautomerizable imines in the metal-free oxidation conjunction with 10 mol% 4-methylanthranilic acid, significantly reactions with TEMPO as oxidant (Fig. 1c): the tautomerizable improved the reaction (77% yield) (entry 7), most likely because Previous work : a Similarity : nucleophilic addition of imine to aldehyde via imine tautomerization to enamine 2 R CH3 N 1 1) cat. CoCl2 H3C R + R2 O 2) H2O/HOAc H O 2 R CH3 1 2 H O/HOAc H3C R R O 2 1 N 1 R = t-octyl H H NR b Similarity : intramolecular -C-H arylation of imine via imine tautomerization to enamine Pd(OAc) CH3 cat. Pd(OAc) 2 R N R N R N R N H HOAc H This work : c Dissimilarity : -oxygenation of imine by TEMPO versus consecutive dehydrogenation of enamine R2 N NHR2 Primary O N O amine O Oxygenation R1 R1 2 3 2 3 R R O N R R Secondary N N R1 amine Consecutive dehydrogenation R1 R1 Fig. 1 Similarity and Dissimilarity in Reactivity between Enamine and Tautomerizable Imine. In both nucleophilic addition reaction (a) and Pd-catalysed intramolecular α-C-H arylation reaction of α-hydrogen-containing imine (b), imines participate in reaction through taotumerization to their enamine tautomers and show similar reactivity to enamine. The reaction of cyclohexanones, amines and TEMPO depends on the reaction intermediates and affords α-amino enones from primary amines and arylamines from secondary amines (c), exhibiting the dissimilarity in reactivity between enamine and α-hydrogen-containing imine 2 NATURE COMMUNICATIONS | (2018) 9:5002 | DOI: 10.1038/s41467-018-07534-x | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-07534-x ARTICLE Cl Cl O NH2 Catalyst, TEMPO (1.5 equiv) HN N + O Toluene (1.0 mL), Additive + N tBu Cl H 120 °C, N2, 24 h tBu tBu 1a 2a 3a 4 5 Catalyst Yield of 3a Conv. of TEMPO Yield of 4 Yield of 5 Entry Additive (10 mol%) (%)b (%)c (%)b (%)c 1 Cu(OAc)2, bpy - 63 96 trace 94 2Cu(OAc)2 - 69 97 trace 94 3 Yb(OTf) 3 - 9713815 4Zn(OTf)2 - 10 71 34 17 73 5AlCl3 - 48 95 trace NH2 6 - 37 78 20 43 H3C CO2H NH 2 3Å MS 7 77 100 11 92 (400 mg) H3C CO2H NH2 3Å MS d 86 100 trace 91 8 (400 mg) H3C CO2H Fig. 2 Optimization of α-amino-enone formation from primary aminea. a Reaction conditions: 1a (0.3 mmol), 2a (0.2 mmol), catalyst (10 mol%), solvent b c (1.0 mL), N2, 120 °C for 24 h. Yields were determined by GC analysis using dodecane as an internal standard. Conversion of TEMPO and yield of 5 were based on the amount of TEMPO and determined by GC analysis. d 15 mol% catalyst was used molecular sieve absorbed water generated in the ketone-amine spite of a broad substrate scope with respect to cyclic ketone, condensation step to push the reversible ketone-amine con- α-substituted cyclohexanones did not participate in this reac- densation towards imine formation. Increasing loading of tion. Unfortunately, four-membered cyclic ketone did not work 4-methylanthranilic acid to 15 mol% further enhanced the yield for this reaction either. to 86% (entry 8). Our general method for synthesis of α-amino-enones from cyclohexanones and primary amines is attractive because Scope of α-amino enone formation reaction.Wenextexamined traditional syntheses of α-amino-enones invoke multi-step the scope of the metal-free method for facile syntheses of α- procedures33. Due to their dual electronic attitude, α-amino- amino enones (Fig. 3). With respect to primary amine coupling enones proved to be versatile building blocks in organic – partners, a broad range of primary amines served as competent synthesis33 36. In this context, α-amino-enones have recently substrates. A variety of anilines bearing electron-withdrawing been used to prepare several important classes of heterocycles (3a, 3b, 3c, 3d, 3e, 3f, 3g) and electron-donating (3h, 3i)groups such as oxazines, azaspirones, quinolinones and quinolones in afforded the corresponding products in generally good yields. regio- and chemoselective fashion via controllable annulations33. The positions of substituents on the phenyl rings of anilines did Moreover, α-Amino-enones are amenable to intramolecular not affect the reaction outcomes, as exemplified by 3-methyl oxidative C-C bond formation to construct substituted tetrahy- aniline (3j) and 2-methyl aniline (3k).
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