Dual Effects of Cyclopentadienyl Ligands on Rh(III)-Catalyzed

Much higher antisymmetric and symmetric CO stretching frequencies of ... These results suggested that the electronic effects of X-Cp ligands play key ...
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Research Article Cite This: ACS Catal. 2018, 8, 8070−8076

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Dual Effects of Cyclopentadienyl Ligands on Rh(III)-Catalyzed Dehydrogenative Arylation of Electron-Rich Alkenes Weidong Lin, Weiwei Li, Dandan Lu, Feng Su, Ting-Bin Wen,* and Hui-Jun Zhang* Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, P. R. China

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S Supporting Information *

ABSTRACT: Despite extensive research on transition metal-catalyzed Fujiwara−Moritani type C−H olefinations, the alkenes used in these transformations are still mainly limited to active acrylate esters and styrenes. Selective aryl C−H olefination with electron-rich alkenes is recognized as a challenging issue. We herein report that simple and readily accessible electron-deficient [CpRh(III)] and [CpCF3Rh(III)] (CpCF3 = C5Me4CF3) complexes are powerful catalysts for dehydrogenative arylation of electron-rich alkenes, including vinyl acetates, enamides, and vinyl ethers. Employing an electron-withdrawing Cp or CpCF3 ligand instead of the privileged Cp* (C5Me5) ligand not only can facilitate the electrophilic aryl C−H rhodation but also can lower the olefin insertion barrier. Both electron-withdrawing and electrondonating directing groups such as -CONR2 and -NHAc could be employed in these reactions, which provides convenient routes toward a series styryl acetates, N-acetylindoles, and aryl methyl ketones. KEYWORDS: half-sandwich rhodium(III) complexes, cyclopentadienyl ligands, C−H activation, electron-rich alkenes, homogeneous catalysis, ligand design

S

mainly be attributed to their much higher insertion barrier into the (aryl)C−metal bond of the metallacycle intermediate during the catalytic cycle.9 Suitable ligands may facilitate the 1,2-migratory insertion of electron-rich alkenes.10 In 2008, Xiao et al. discovered that in Pd-catalyzed classical Heck reactions, using electron-deficient phosphine ligands could reduce the π back-donation from the Pd center to the coordinated electron-rich olefins and therefore lower the olefin insertion barrier.10a In 2001, Svensson et al. found analogous results after they studied the electronic ligand effects on the activity of

teric and electronic properties of ligands have significant effects on the reactivity of transition metal complexes. In recent years, ligand development in C−H activation for pursuing highly efficient catalysts has received a growing amount of attention.1,2 Besides promoting the cleavage of specific C−H bonds, ideal ligands should also have positive effects on the following functionalization steps and even on the poor reaction partners. Since the discovery of the Fujiwara−Moritani reaction in 1967,3 a wide range of transition metal catalysts for oxidative dehydrogenative alkenylations of arenes have been developed.4 However, the alkenes employed in most cases are still limited to active acrylate esters and styrenes. Examples for C−H alkenylation with electron-rich alkenes, such as vinyl acetates,5 vinyl ethers,6 and enamides,7 are rare,8 which can

Scheme 2. Five X-Cp Ligands and Preparation of [X-CpRhI2]2/n Complexes

Scheme 1. Dual Effects of X-Cp Ligands on Rh(III)Catalyzed Fujiwara−Moritani Type Arylation of ElectronRich Alkenes

Received: May 4, 2018 Revised: July 18, 2018 Published: July 19, 2018 © 2018 American Chemical Society

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ACS Catalysis Table 1. Catalyst Screeninga

entry

[X-CpRh(III)]

[Ag]

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

[Cp*RhCl2]2 [CpCF3RhCl2]2 [CpRhI2]n [CpiPrRhI2]2 [CpCO2MeRhI2]2 [CpCOMeRhI2]2 [CpRh(MeCN)3](SbF6)2 [CpRhI2]n [CpCF3RhCl2]2 [CpRhI2]n [CpRhI2]n [CpRhI2]n [CpRhI2]n

AgSbF6 AgSbF6 AgSbF6 AgSbF6 AgSbF6 AgSbF6 − − − AgOAc AgBF4 AgNTf2 AgSbF6

cationic RhIII complexes11 featuring electron-deficient cyclopentadienyl (X-Cp) ligands would be ideal catalyst candidates for Fujiwara−Moritani type arylation of electron-rich alkenes because the electron-withdrawing X-Cp ligands may promote both electrophilic metalation of aromatic C−H bonds12 and 1,2-migratory insertion of electron-rich alkenes into (aryl)C− metal bonds13,14 (Scheme 1). Over the past decade, with pioneering contributions of Miura and Sato,15 Rovis,16 Cramer,17 and Tanaka,18 a number of substituted cyclopentadienyl ligands, such as C5HPh4, Cpt, CpiPr, CpCF3, Cp*tBu, Cp*Cy, Ind*, chiral CpX,19 CpCy, CpE, and CpA, have been identified as outstanding alternatives for the privileged Cp* ligand in Rh(III)-catalyzed C−H functionalization reactions.20,21 Nevertheless, the most simple and electrondeficient unsubstituted cyclopentadienyl (Cp) ligand has not received adequate attention in this field.22,23 Herein, we describe the facile synthesis of electron-deficient [X-CpRhI2]2/n complexes that enabled us to develop a powerful X-CpRh(III)-catalyzed dehydrogenative arylation of electron-rich alkenes. To start our investigation, five simple and sterically different X-Cp ligands (Cp, CpCF3, CpiPr, CpCO2Me, and CpCOMe) were chosen (Scheme 2). Much higher antisymmetric and symmetric CO stretching frequencies of corresponding [X-CpRh(CO)2] complexes compared to those of [Cp*Rh(CO)2] (2016/1948 cm−1) indicated the electron-withdrawing properties of these X-Cp ligands (Scheme 2 and Table S1).2a Notably, the introduction of a -CO2Me or -COMe group as a strong

yield (%)b 21 82 86 79 37 73 87 0 0 0 10 88 22

(13:1) (14:1) (10:1) (6:1) (19:1) (15:1) (10:1)

(9:1) (10:1) (16:1)

a

Reaction conditions: 1a (0.2 mmol), 2a (2.0 mmol), [X-CpRh(III)] (5 mol %), [Ag] (10 mol %), Cu(OAc)2·H2O (1.1 equiv), THF (1 mL), 80 °C, in air for 12 h. bIsolated yields (E/Z ratios are shown in parentheses). cIn the absence of Cu(OAc)2·H2O.

cationic phenylpalladium(II)diimine complexes using density functional theory calculations.10b We then reasoned that active Table 2. Scope of Benzamides 1a,b

a Reaction conditions: 1 (0.2 mmol), 2a (2.0 mmol), [CpRhI2]n or [CpCF3RhCl2]2 (5 mol % [Rh]), AgSbF6 (10 mol %), Cu(OAc)2·H2O (1.1 equiv), THF (1 mL), 80 °C, in air for 12 h. bIsolated yields of 3 under the catalysis of [CpRh] and [CpCF3Rh] (ratios of the amount of C2 alkenylation product to the amount of C6 or C8 alkenylation product are shown in parentheses). cAt 100 °C for 24 h. d2a (2 equiv).

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ACS Catalysis Table 3. Scope of Alkenesa,b

a Reaction conditions: 1 (0.2 mmol), 2 (2.0 mmol), [CpRhI2]n or [CpCF3RhCl2]2 (5 mol % [Rh]), AgSbF6 (10 mol %), Cu(OAc)2·H2O (1.1 equiv), THF (1 mL), 80 °C, in air for 12 h. bIsolated yields of 3 under the catalysis of [CpRh] and [CpCF3Rh] (yields for the reactions catalyzed by [Cp*RhCl2]2 are shown in parentheses). c2 (2 equiv). d2 (5 equiv). eAt 100 °C for 24 h. fAcetone instead of THF.

electronic effects of X-Cp ligands play key roles in this transformation. Moreover, without the addition of AgSbF6, no desired product could be formed in the presence of polymeric [CpRhI2]n or dimeric [CpCF3RhCl2]2 (entry 8 or 9, respectively). Other silver salts such as AgOAc, AgBF4, and AgNTf2 were also tested (entries 10−12, respectively). Only AgNTf2 proved to be comparable to AgSbF6 (entry 12). These results disclosed that the real catalyst for this alkenylation should be a cationic rhodium(III) species without halides. It is interesting to note that the reaction performed in the absence of Cu(OAc)2·H2O also provided 3aa in 22% yield (entry 13). After determining the optimal reaction conditions, we set out to investigate the scope of this alkenylation reaction by examining various benzamide derivatives under the catalysis of [CpRh(III)] and [CpCF3Rh(III)] (Table 2). A wide range of amide groups containing diverse substituents, such as -Me, -Et, -Cy, -Bn, -(CH2)5-, -OMe, and -2,6-Me2Ph, could be employed as suitable directing groups, and the corresponding products 3ba−ia were all produced in good yields (62−98%). Thereafter, the reactions of a series of ortho- and para-substituted benzamides (1j−p) and especially several heteroaromatic substrates (1q−s) with vinyl acetate all led to good yields of alkenylated products (3ja−sa). Both electron-donating (-Me and -OMe) and electron-withdrawing groups (-F, -Cl, -Br, -Ph, and -CO2Me) were well tolerated. In most cases, the slightly more electrondeficient [CpRh(III)] catalyst delivered much better results than

electron-withdrawing substituent on the Cp ring does not decrease its electron density dramatically. The corresponding half-sandwich complexes [X-CpRhI2]2/n were then synthesized through a very convenient and practical method starting from commercially available [Rh(cod)Cl]2 and X-CpNa (Scheme 2).24−26 Importantly, through this pathway, the polymeric [CpRhI2]n could be isolated cleanly. Having the desired X-CpRh(III) complexes in hand, we first investigated their catalytic activities in the coupling between rather electron-poor tertiary benzamide 1a and vinyl acetate 2a (Table 1). For the purpose of comparison, the reaction catalyzed by [RhCp*Cl2]2 (2.5 mol %) and AgSbF6 (10 mol %) was performed, which afforded the corresponding alkenylation product 3aa in 21% yield (entry 1). Only a trace amount of ortho C−H vinylation product was observed;14 75% of benzamide 1a was recovered. Gratifyingly, employing the X-Cp-based rhodium(III) catalyst precursors resulted in much improved yields (37−86%, entries 2−6). The sterically different but electronically similar Cp and CpCF3 ligands are both optimal for the alkenylation that allowed the formation of 3aa in high yields (entries 2 and 3). Notably, two electronically similar precatalysts, [CpCO2MeRhI2]2 and [CpCOMeRhI2]2, exhibited rather different catalytic activities, which is still not fully understood (entries 5 and 6, respectively). To exclude the halide effect, preformed catalyst [CpRh(MeCN)3][SbF6]2 was employed affording 3aa in 87% yield (entry 7). These results suggested that the 8072

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ACS Catalysis Table 4. Reaction of Anilides with Electron-Rich Alkenesa,b

a Reaction conditions: 4 (0.2 mmol), 2 (2.0 mmol), [CpRhI2]n or [CpCF3RhCl2]2 (5 mol % [Rh]), AgSbF6 (10 mol %), Cu(OAc)2·H2O (1.1 equiv), THF (1 mL), 80 °C, in air for 12 h. bIsolated yields of 5 under the catalysis of [CpRh] and [CpCF3Rh] (yields for the reactions catalyzed by [Cp*RhCl2]2 are shown in parentheses). cCu(OAc)2·H2O (0.3 equiv). d2 (5 equiv). eAcetone instead of THF. fAgSbF6 (5 mol %). g 2 (4 equiv), 120 °C.

Next, we investigated the scope of alkenes in this Rhcatalyzed C−H alkenylation reaction (Table 3). A series of vinyl esters 2b−e as well as geminal-substituted vinyl acetates 2f−j were suitable substrates for this transformation. In most cases, the sterically less demanding [CpRh(III)] catalyst displayed activities higher than those of [CpCF3Rh(III)], which is consistent with the results shown in Table 2. As a sterically bulky substrate, 1,2-disubstituted alkene 2k reacted with N,N-dimethylbenzamide 1b in the presence of [CpRh(III)] providing not only expected β-arylation product 3bk (23%) but also a 1,1-diphenylethylene derivative 3bk′ (59%). Furthermore, both catalysts could be employed in the alkenylation of 1a with 2-vinylisoindoline-1,3-dione 2l, which afforded desired product 3al in good yields ([CpRh], 89%; [CpCF3Rh], 88%). The selective α-arylation of vinyl ether 2m with 1a in the presence of [CpCF3Rh(III)] also led to the formation of product 3am′ in moderate yield.27 The reaction of 2l or 2m with 1a could also be catalyzed by [Cp*Rh(III)], albeit affording 3al and 3am′ in much lower yields (28 and 29%, respectively). The much higher catalytic activities of [CpRh(III)] and [CpCF3RhIII] compared with that of [Cp*Rh(III)] were further indicated in the direct couplings between anilides 4 and electron-rich alkenes (Table 4). The reactions of N-(m-tolyl)acetamide 4b with vinyl acetate 2a as well as geminal-substituted vinyl acetates 2f−h all afforded the corresponding products 5ba and 5bf−bh in good yields, whereas the cationic [Cp*Rh(III)] catalyst is ineffective for the dehydrogenative arylation of geminal-substituted vinyl acetates. For example, the reaction of 4b with 2f in the presence of 5 mol % [CpRh(III)] afforded

Scheme 3. Facile Synthesis of 1,2-Diarylethene 7 and Indole 8

the [CpCF3Rh(III)] catalyst. Although it seems that the steric bulkiness of the CpCF3 ligand may have adverse effects on the reaction efficiency, it is beneficial for regioselectivity. For instance, in the presence of [CpRh(III)], the alkenylation of N,N-diisopropyl-2-naphthamide 1t with vinyl acetate afforded two regioisomeric products (87% yield, 1:1 C2:C8). When using bulky [CpCF3Rh(III)] as the catalyst, an only 71% yield of the C8-alkenylated product was formed. Moreover, the reactions of meta-substituted benzamides 1u−w with 2a in the presence of [CpRh(III)] all gave both C2- and C6-alkenylated products 3ua−wa, respectively, whereas using [CpCF3Rh(III)] as the catalyst, C6 alkenylation of 1v and 1w was preferred. For 3-methoxybenzamide 1u, because of the coordination ability of the -OMe group to Rh(III),8e employing [CpCF3Rh(III)] instead of [CpRh(III)] resulted in only increased C6-alkenylated product 3ua (2:1 to 1:1 C2:C6). When the meta substituent is bulky enough as in 1x, C6 alkenylation product 3xa could be formed selectively no matter which catalyst was employed. 8073

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ACS Catalysis Scheme 4. Mechanistic Studiesa

For the sake of clarity, hydrogens and the SbF6− counterion have been omitted from the ORTEP view of rhodacycle 9.

a

under acidic conditions led to the formation of N-acetylindoles 8 in 69% yield (entry 2). To gain insight into the mechanism, we conducted several experiments (Scheme 4). A competition experiment between 1-OMe and 1-F under the standard reaction conditions highlighted the higher reactivity of more electron-rich benzamide (Scheme 4a), which indicated an electrophilic type C−H activation mode. It is also implied that electron-deficient [X-CpRh] should be more suitable than electron-rich [Cp*Rh] for this C−H rhodation step. Then, we explored the hydrogen/ deuterium (H/D) exchange reactions of 1a in the presence of acetic acid-d4 (Scheme 4b; for more details, see the Supporting Information). When the reaction was performed under the catalysis of [CpRh] in the absence of 2a for 6 h, 59% deuterium incorporation at both ortho positions of the phenyl ring was observed. Still under the catalysis of [CpRh], if the reaction was conducted in the presence of 2 equiv of 2a, there was almost no deuterium incorporation observed in the recovered 1a. These results suggest that the C−H rhodation with [CpRh] is irreversible in the presence of an alkene. In contrast, under the catalysis of [Cp*Rh], the H/D exchange at the two ortho positions of 1a was independent of the alkene, which indicated that the migratory insertion of 2a might be accelerated by using the electron-deficient Cp ligand instead of the Cp* ligand. Thereafter, the stoichiometric reaction of 1b with [CpRhI2]n/AgSbF6 and vinyl acetate in acetone was performed at room temperature, which afforded expected rhodacycle 9 in 80% yield (Scheme 4c). The structure of 9 was confirmed by single-crystal X-ray diffraction (Scheme 4c).31 The formation of rhodacycle 9 was

the corresponding 5bf in 79% yield. No vinylation and double olefination products were observed. Also, the alkenylation of 1,1-dimethyl-3-phenylurea 4c with 2a could not be catalyzed by [Cp*Rh(III)], but it works smoothly in the presence of the [CpRh(III)] or [CpCF3Rh(III)] catalyst and provides 5ca in good yield. Then the reactions of acetamide 4a with other two representative electron-rich alkenes, 2l and 2m, were performed. Both of the desired products, 5al and 5am′, could be formed in good yields.28 Moreover, under [CpCF3Rh(III)] catalysis, the reaction of 4b with 2m afforded acetylation product 5bm′ in 90% yield. Apparently, compared with the classical Friedel−Crafts acylation reactions, this direct C−H acetylation with vinyl ethers may provide a much cleaner route to aryl methyl ketones. Surprisingly, the reaction of 1,2-disubstituted alkene 2k with 4a selectively led to the formation of a C−H/ C−O cross-coupling product 6ak ([CpRh(III)], 72%; [Cp*Rh(III)], 12%). Expected β-arylation product 5ak was not observed. The significance of [CpRh(III)]-catalyzed dehydrogenative arylation of electron-rich alkenes was demonstrated by the facile construction of 1,2-diarylethene and indole skeletons (Scheme 3). (E)-1,2-Diarylethene product 7 was formed in 58% yield through [CpRh]-catalyzed selective C−H/C−O crosscoupling between styryl acetate product 3aa (14:1 E:Z) and acetanilide 4a (entry 1). For further applications, the amide group could be chemoselectively reduced to aldehyde29 or transformaed to ketone.30 Direct regioselective ortho alkenylation of acetanilide 4d with 2f followed by hydrolysis and cyclization 8074

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ACS Catalysis

Org. Chem. 2013, 78, 8927−8955. (c) Yang, Y.-F.; Hong, X.; Yu, J.Q.; Houk, K. N. Experimental-Computational Synergy for Selective Pd(II)-Catalyzed C−H Activation of Aryl and Alkyl Groups. Acc. Chem. Res. 2017, 50, 2853−2860. (2) (a) Piou, T.; Romanov-Michailidis, F.; Romanova-Michaelides, M.; Jackson, K. E.; Semakul, N.; Taggart, T. D.; Newell, B. S.; Rithner, C. D.; Paton, R. S.; Rovis, T. Correlating Reactivity and Selectivity to Cyclopentadienyl Ligand Properties in Rh(III)-Catalyzed C−H Activation Reactions: An Experimental and Computational Study. J. Am. Chem. Soc. 2017, 139, 1296−1310. (b) Wang, P.; Verma, P.; Xia, G.; Shi, J.; Qiao, J. X.; Tao, S.; Cheng, P. T. W.; Poss, M. A.; Farmer, M. E.; Yeung, K.-S.; Yu, J.-Q. Ligand-Accelerated Non-Directed C−H Functionalization of Arenes. Nature 2017, 551, 489−493. (c) Wang, X.-C.; Gong, W.; Fang, L.-Z.; Zhu, R.-Y.; Li, S.; Engle, K. M.; Yu, J.-Q. Ligand-Enabled meta-C−H Activation Using a Transient Mediator. Nature 2015, 519, 334−338. (d) Wang, D.-H.; Engle, K. M.; Shi, B.F.; Yu, J.-Q. Ligand-Enabled Reactivity and Selectivity in a Synthetically Versatile Aryl C−H Olefination. Science 2010, 327, 315−319. (e) Zhang, Y.-H.; Shi, B.-F.; Yu, J.-Q. Pd(II)-Catalyzed Olefination of Electron-Deficient Arenes Using 2,6-Dialkylpyridine Ligands. J. Am. Chem. Soc. 2009, 131, 5072−5074. (3) Jia, C.; Piao, D.; Oyamada, J.; Lu, W.; Kitamura, T.; Fujiwara, Y. Efficient Activation of Aromatic C-H Bonds for Addition to C-C Multiple Bonds. Science 2000, 287, 1992−1995. (4) (a) Satoh, T.; Miura, M. Oxidative Coupling of Aromatic Substrates with Alkynes and Alkenes under Rhodium Catalysis. Chem. - Eur. J. 2010, 16, 11212−11222. (b) Le Bras, J.; Muzart, J. Intermolecular Dehydrogenative Heck Reactions. Chem. Rev. 2011, 111, 1170−1214. (c) Patureau, F. W.; Wencel-Delord, J.; Glorius, F. Cp*Rh-Catalyzed C−H Activations: Versatile Dehydrogenative Cross-Couplings of Csp2 C−H Positions with Olefins, Alkynes, and Arenes. Aldrichimica Acta 2012, 45, 31−41. (d) Kozhushkov, S. I.; Ackermann, L. Ruthenium-Catalyzed Direct Oxidative Alkenylation of Arenes through Twofold C−H Bond Functionalization. Chem. Sci. 2013, 4, 886−896. (e) Zhou, L.; Lu, W. Towards Ideal Synthesis: Alkenylation of Aryl C-H Bonds by a Fujiwara−Moritani Reaction. Chem. - Eur. J. 2014, 20, 634−642. (5) Zhang, H.-J.; Lin, W.; Su, F.; Wen, T.-B. Rhodium-Catalyzed βSelective Oxidative Heck-Type Coupling of Vinyl Acetate via C−H Activation. Org. Lett. 2016, 18, 6356−6359. (6) Li, Y.; Xue, D.; Lu, W.; Fan, X.; Wang, C.; Xiao, J. 3-Acylindoles via Palladium-Catalyzed Regioselective Arylation of Electron-Rich Olefins with Indoles. RSC Adv. 2013, 3, 11463−11466. (7) Pankajakshan, S.; Xu, Y.-H.; Cheng, J. K.; Low, M. T.; Loh, T.-P. Palladium-Catalyzed Direct C-H Arylation of Enamides with Simple Arenes. Angew. Chem., Int. Ed. 2012, 51, 5701−5705. (8) (a) Dai, H.; Yu, C.; Wang, Z.; Yan, H.; Lu, C. SolventControlled, Tunable β-OAc and β-H Elimination in Rh(III)Catalyzed Allyl Acetate and Aryl Amide Coupling via C−H Activation. Org. Lett. 2016, 18, 3410−3413. (b) Gigant, N.; Bäckvall, J.-E. Access to Cinnamyl Derivatives from Arenes and Allyl Esters by a Biomimetic Aerobic Oxidative Dehydrogenative Coupling. Org. Lett. 2014, 16, 1664−1667. (c) Pan, D.; Yu, M.; Chen, W.; Jiao, N. PdII-Catalyzed Highly Selective Arylation of Allyl Esters via C-H Functionalization of Unreactive Arenes with Retention of the Traditional Leaving Group. Chem. - Asian J. 2010, 5, 1090−1093. (d) Li, Z.; Zhang, Y.; Liu, Z.-Q. Pd-Catalyzed Olefination of Perfluoroarenes with Allyl Esters. Org. Lett. 2012, 14, 74−77. (e) Huang, L.; Wang, Q.; Qi, J.; Wu, X.; Huang, K.; Jiang, H. Rh(III)-Catalyzed ortho-Oxidative Alkylation of Unactivated Arenes with Allylic Alcohols. Chem. Sci. 2013, 4, 2665−2669. (f) Shi, Z.; Boultadakis-Arapinis, M.; Glorius, F. Rh(III)-Catalyzed Dehydrogenative Alkylation of (Hetero)arenes with Allylic Alcohols, Allowing Aldol Condensation to Indenes. Chem. Commun. 2013, 49, 6489− 6491. (9) Yeung, C. S.; Dong, V. M. Catalytic Dehydrogenative CrossCoupling: Forming Carbon−Carbon Bonds by Oxidizing Two Carbon−Hydrogen Bonds. Chem. Rev. 2011, 111, 1215−1292.

also observed under the catalytic conditions by using 10 mol % [CpRh] (see the Supporting Information). The alkenylation of 1b with 2a in the presence of 5 mol % 9 was then conducted, which afforded the desired 3ba in 86% yield (Scheme 4d). Therefore, the Rh(III)-catalyzed dehydrogenative arylation of vinyl acetate may indeed proceed through a generally proposed cycle that includes directed electrophilic C−H rhodation, olefin insertion, and subsequent β-H elimination.9,13 In summary, electron-deficient [CpRh(III)] and [CpCF3Rh(III)] complexes exhibit catalytic activities much higher than that of the privileged [Cp*Rh(III)] in dehydrogenative arylation of electron-rich alkenes. The arylation of vinyl acetates provides a facile and convenient route toward various styryl acetates and N-acylindoles. Synthetically useful aryl methyl ketones are prepared readily through the direct arylation of vinyl ether under mild conditions. [CpRh(III)] could also be used to catalyze the chemoselective C−H/C−O cross-coupling between styryl acetates and acetanilides, which may provide an atom- and step-economic approach toward (E)-1,2diarylethenes. The cheap and readily available features of electron-rich alkenes and their versatile reactivities as well as the convenient synthetic method toward dimeric or polymeric [X-CpRhI2]2/n make this alkenylation reaction a valuable tool for C−C bond formation. More importantly, the successful use of dual effects of electron-withdrawing Cp and CpCF3 ligands in this reaction provides new insights for catalyst and ligand design in C−H functionalizations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b01753. General and experimental information about the preparation of [X-CpRhI2]2/n complexes, optimization studies and procedures for dehydrogenative arylation of electron-rich alkenes, details about mechanism studies, and characterization data (1H NMR, 13C NMR, 19F NMR, and HRMS) for all new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Hui-Jun Zhang: 0000-0001-9567-3010 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the financial support from the Natural Science Foundation of China (21572188, 21772162, and 21302157) and Fundamental Research Funds for the Central Universities (20720160049). H.-J.Z. thanks Prof. Pierre H. Dixneuf for valuable discussions and corrections.

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REFERENCES

(1) (a) Piou, T.; Rovis, T. Electronic and Steric Tuning of a Prototypical Piano Stool Complex: Rh(III) Catalysis for C−H Functionalization. Acc. Chem. Res. 2018, 51, 170−180. (b) Engle, K.; Yu, J.-Q. Developing Ligands for Palladium(II)-Catalyzed C−H Functionalization: Intimate Dialogue between Ligand and Substrate. J. 8075

DOI: 10.1021/acscatal.8b01753 ACS Catal. 2018, 8, 8070−8076

Research Article

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Synthesis and Application of Chiral Spiro Cp Ligands in RhodiumCatalyzed Asymmetric Oxidative Coupling of Biaryl Compounds with Alkenes. J. Am. Chem. Soc. 2016, 138, 5242−5245. (20) Hong, S. Y.; Jeong, J.; Chang, S. [4 + 2] or [4 + 1] Annulation: Changing the Reaction Pathway of a Rhodium-Catalyzed Process by Tuning the Cp Ligand. Angew. Chem., Int. Ed. 2017, 56, 2408−2412. (21) (a) Wencel-Delord, J.; Patureau, F. W.; Glorius, F. Rh(III)- and Ir(III)-Catalyzed C-C Bond Cross Coupling from C-H Bonds. Top. Organomet. Chem. 2015, 55, 1−27. (b) Colby, D. A.; Tsai, A. S.; Bergman, R. G.; Ellman, J. A. Rhodium Catalyzed Chelation-Assisted C−H Bond Functionalization Reactions. Acc. Chem. Res. 2012, 45, 814−825. (c) Song, G.; Wang, F.; Li, X. C−C, C−O and C−N Bond Formation via Rhodium(III)-Catalyzed Oxidative C−H Activation. Chem. Soc. Rev. 2012, 41, 3651−3678. (22) Higher CO stretching frequencies of [CpRh(CO)2] compared with those of [CpCF3Rh(CO)2] suggested that Cp might be even more electron-withdrawing than CpCF3 (for details, see Table S1). (23) Loginov, D. A.; Muratov, D. V.; Nelyubina, Y. V.; Laskova, J.; Kudinov, A. R. μ-Borole Triple-Decker Complexes as Catalysts for Oxidative Couplingof Benzoic Acid with Alkynes. Structure of a Hybrid Rhodacyclopentadienyl/borole Triple-Decker Complex. J. Mol. Catal. A: Chem. 2017, 426, 393−397. (24) The polymeric [CpRhCl2]n complex cannot be isolated cleanly as prepared from the metal chloride hydrate salts; using Kudinov’s strategy can resolve this problem. See: Loginov, D. A.; Vinogradov, M. M.; Starikova, Z. A.; Petrovskii, P. V.; Kudinov, A. R. Arene Complexes [(η-C5H5)M(η-C6R6)]2+ (M = Rh, Ir). Russ. Chem. Bull. 2004, 53, 1949−1953. (25) For synthesis of NaCpCO2Me, see: Hart, W. P.; Macomber, D. W.; Rausch, M. D. A New, General Route to Functionally Substituted η5-Cyclopentadienyl Metal Compounds. J. Am. Chem. Soc. 1980, 102, 1196−1198. (26) The dimeric [CpCF3RhCl2]2 complex was synthesized by the reaction of 1,2,3,4-tetramethyl-5-(trifluoromethyl)cyclopenta-1,3diene with RhCl3·XH2O. See: Gassman, P. G.; Mickelson, J. W.; Sowa, J. R., Jr. 1,2,3,4-Tetramethyl-5-(trifluoromethyl)cyclopentadienide: a unique ligand with the steric properties of pentamethylcyclopentadienide and the electronic properties of cyclopentadienide. J. Am. Chem. Soc. 1992, 114, 6942−6944. (27) The electrical properties of two olefin carbons in vinyl ether strongly influence the olefin insertion step, which results in the regioselective α-arylation of 2m. Acetylation product 3am′ is probably derived from in situ hydrolysis of the α-arylation intermediate. Also see ref 6. (28) For the reaction between 4a and 2m, an N-acetylindole 5am″ was formed as a byproduct ([CpRh], 6%; [CpCF3Rh], 7%), which may be derived from the hydrolysis of a β-arylation intermediate followed by cyclization.

(29) (a) Xiao, P.; Tang, Z.; Wang, K.; Chen, H.; Guo, Q.; Chu, Y.; Gao, L.; Song, Z. Chemoselective Reduction of Sterically Demanding N,N-Diisopropylamides to Aldehydes. J. Org. Chem. 2018, 83, 1687− 1700. (b) White, J. M.; Tunoori, A. R.; Georg, G. I. A Novel and Expeditious Reduction of Tertiary Amides to Aldehydes Using Cp2Zr(H)Cl. J. Am. Chem. Soc. 2000, 122, 11995−11996. (30) Huang, P.-Q.; Wang, Y.; Xiao, K.-J.; Huang, Y.-H. A General Method for the Direct Transformation of Common Tertiary Amides into Ketones and Amines by Addition of Grignard Reagents. Tetrahedron 2015, 71, 4248−4254. (31) For X-ray single-crystal structure analysis of 9 CCDC 1829538, see the Supporting Information.

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DOI: 10.1021/acscatal.8b01753 ACS Catal. 2018, 8, 8070−8076