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Tandem Cyclization/Hydroarylation of #,#-Dienes Triggered by Scandium-Catalyzed C-H Activation Bin Tang, Xiaoyan Hu, Chunli Liu, Tao Jiang, Fakhre Alam, and Yanhui Chen ACS Catal., Just Accepted Manuscript • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018
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ACS Catalysis
Tandem Cyclization/Hydroarylation of ,-Dienes Triggered by Scandium-Catalyzed C−H Activation Bin Tang, Xiaoyan Hu, Chunli Liu, Tao Jiang, Fakhre Alam, and Yanhui Chen* College of Chemical Engineering and Materials Science, Tianjin University of Science and Technology, 29 13th. Avenue, TEDA, Tianjin, 300457 P. R. China. KEYWORD: ring-closure, CH activation, dienes, hydroarylation, rare earths ABSTRACT: A highly regio- and diastereoselective cyclization/hydroarylation reaction of 1,5- and 1,6-dienes with aromatic ethers and tertiary anilines was established by a cationic 2-picoline-tethered-half-sandwich scandium alkyl catalyst, which constitutes a process for producing a diverse array of cis-1,3-disubstituted arylated and trans-1,2-disubstituted benzylated methylcyclopentane derivatives in one step with 100% atom-efficiency. Furthermore, a mechanism involving a scandium-catalyzed aromatic ortho-C−H activation and alkene-insertion cascade was also proposed.
The exploration of new annulation methods continues to be a highly active area of research due to the prevalence of carbocyclic units in biologically relevant molecules.1, 2 In this regard, catalytic cyclization of α,ω-dienes represents a powerful conversion of simple, readily accessible substrates into versatile carbocyclic skeletons.2-7 Besides the formation of carbocyclic alkenes via cycloisomerization,2, 3 diene cyclization combined with various transformations such as silylation, boration, and carbometalation also produces functionalized carbocycles for further elaboration.1g, 4-7 With the increasing progress in C−H functionalization, especially in C−H addition to monoalkenes, and its extensive recognition as a step- and atom-economical synthesis strategy,8, 9 thereby in principle, a cascade C−H addition sequence into dienes would also allow one-step construction of C-substituted carbocyclic derivatives. However, such a C−H-activation-mediated cyclization of dienes has been hardly achieved up to date. Transition metals such as Rh and Ni have been reported to catalyze C−H transformation towards dienes, but in most cases they preferred to furnish linear adducts rather than cyclization products.10 We recently studied a class of mono(phosphinoamide) rareearth dialkyl complexes, and found that cationic scandium alkyl species such as [N-P-Sc]+ could effectively facilitate a cascade addition of pyridine ortho-C−H across 1,5-dienes to form 2-(3-methylcyclopentyl)pyridines with a distinctive cisgeometry.9f The success of this transformation may be ascribed to the high activity of cationic scandium catalyst for C−H activation as well as for diene cyclization.11 Relevantly, cationic half-sandwich scandium alkyl [Cp*-Sc]+ was also reported to be active for hydrothiomethylative cyclization of 1,5-dienes catalytically.9g To our knowledge, none of previous works were reported to catalyze hydroarylative cyclization of dienes, especially for 1,6-dienes. As an extension of our work in this field, herein we report a highly regio- and diastereoselective and scandium-catalyzed hydroarylative cyclization of 1,5- and 1,6-dienes with aromatic ethers and tertiary anilines. A broad range of substrates were subjected to a novel cationic 2-picoline- tethered-half-sandwich scandium
Table 1. Optimization of Reaction Conditions a
Entry [Ln] [B] Yield [%]b Cis:transc 1 [Ph3C][B(C6F5)4] 8 (6) 98 : 2 N-P-Sc 2 [Ph3C][B(C6F5)4] 9 (7) 92 : 8 Cp*-Sc 3 [Ph3C][B(C6F5)4] 88 (84) 75 : 25 CpO-Sc 4 [Ph3C][B(C6F5)4] 97 (92) > 98 : 2 CpN-Sc 5 [Ph3C][B(C6F5)4] 15 (13) 80 : 20 CpN-Y 6 [Ph3C][B(C6F5)4] 1 CpN-Lu 7d [Ph3C][B(C6F5)4] 96 (94) > 98 : 2 CpN-Sc e N 8 [Ph3C][B(C6F5)4] 87 (83) > 98 : 2 Cp -Sc a Reaction conditions: 1a (1.0mmol), 2a (1.5mmol), [Ln] (2.0mol%), [Ph3C][B(C6F5)4] (2.0mol%), at 70oC, 1h, in C6H5Cl (2.0mL). bGC Yield, isolated yield was in parentheses. cStereoselectivity was determined by 1H NMR spectrum. dIn toluene (2.0mL). eIn benzene (2.0mL).
catalyst [CpN-Sc]+, yielding a variety of cis-1,3-disubstituted arylated and trans-1,2-disubstituted benzylated methylcyclopentane derivatives via one-step process with noticeable efficiency. Our project began with the exploration of C−H alkylation of anisole (1a) towards 1,5-hexadiene (2a) by employing various rare-earth complexes in conjunction with equimolar borate compound [Ph3C][B(C6F5)4] for optimization (See Table 1).12 After the reaction proceeded at 70oC for 1h in chlorobenzene (C6H5Cl), phosphinoamido-ligated scandium complex N-PSc9f delivered the cyclization product of 3aa in 8% yields (Table 1, entry 1), and the extension of reaction time (for 36h) or elevation of reaction temperature (at 100oC) failed to increase the yields of 3aa significantly. Likewise, the reported half-sandwich scandium complex Cp*-Sc9g was also proved less effective (Table 1, entry 2). Fortunately, the tBuOSiMe2bonded-half-sandwich scandium complex CpO-Sc afforded
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Table 2. Scope for Aromatic Ethers a, b, c
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Table 3. Scope for Tertiary Anilines a, b, c
a a
Reaction conditions: 1 (1.0mmol), 2a (1.5mmol), CpN-Sc (2.0mol%), [Ph3C][B(C6F5)4] (2.0mol%), at 70oC, in C6H5Cl (2.0mL). bIsolated yield. c Stereoselectivity was determined based on 1H NMR spectrum.
3aa in 88% yield, but with 75% cis-selectivity (Table 1 entry 3). Instead, when 2-picoline-tethered-half-sandwich scandium complex CpN-Sc was used, the yield of 3aa remarkably improved to 97% and with 98% cis-selectivity (Table 1, entry 4). Moreover in this case, none of linear alkylation byproducts were observed. Under the same conditions, analogous yttrium complex CpN-Y and lutetium complex CpN-Lu showed rather poor activity (Table 1 entries 5, 6). These results suggest that rare earth metal as well as ancillary ligand affects catalyst activity significantly. In addition to chlorobenzene, the CpNSc/[Ph3C][B(C6F5)4] catalytic system in toluene or benzene also gives good results (Table 1, entries 7-8). No reaction took place in the absence of CpN-Sc or [Ph3C][B(C6F5)4], implying the essential role of cationic scandium species for this transformation.9f-g The stereochemistry of cis-3aa was confirmed by NOE measurements, and was in sharp contrast with the trans-disubstituted cyclopentanes generated from cyclization/functionalization of 1,5-dienes by other catalysts.1g, 4a-b, 5, 6a-b
With these results in hand, various aromatic ethers were subsequently scrutinized with 2a as a partner (See Table 2). In the case of 4-substituted anisoles, substituents such as Me, tBu, allyl, Ph, even halogen were well-tolerated, leading to high yields of alkylation products 3ba-3ia with excellent cisselectivity (77-97% yield, > 97 cis). For 3-substituted anisoles that have two different ortho-C−H bonds, 3-methylanisole provided a regioisomeric mixture of cis-3ja and cis-3ja (90% combined yield, > 98 cis), while 3-phenylanisole or 3-Cl(Br)anisole produced a single regioisomer (See 3ka-3ma, Table 2). This is probably due to the electronic effect of substituent. 1,4Benzodioxane, bearing two O-directing groups, quantitatively offered dialkylation product cis-3na through a bis-C−H functionalization (90% yield, > 97 cis). Noticeably, coupling with cis-cyclization of 2a, the C−H alkylation of 1,4benzoxathian (1o) or N-methylbenzomorpholine (1p) also occurred exactly at ortho-position of O rather than S or N atom, implying the stronger oxophilicity of metal center of catalyst (See 3oa-3pa, Table 2). It was noteworthy that the C−S bond of 1o survived intact over the course of C−H
Reaction conditions: 1 (1.0mmol), 2a (1.1mmol), CpN-Sc (2.0mol%), [Ph3C][B(C6F5)4] (2.0mol%), at 50oC, in C6H5Cl (2.0mL). bIsolated yield. c Stereoselectivity was determined based on 1H NMR spectrum.
activation. Upon replacing methyl unit at O atom in 1a with phenyl or ethyl group, ortho-C−H alkylation followed by the cis-cyclization of 2a also proceeds well (See 3qa-3ra, Table 2). The scope of CH substrate was then expanded to include tertiary anilines (See Table 3). Typically, with N,N-dimethylaniline (1s), trapping the cyclization of 2a (1.5 equiv.) at 70oC for 1h resulted in the formation of ortho-(3-methylcyclopentyl)aniline (3sa) as a single cis-isomer along with minor bis-C−H alkylation byproduct. To avoid over alkylation, the reaction was optimized at 50oC with the use of 1.1 equiv. of 2a, in which cis-3sa was isolated in 87% yield. Under these new conditions, a range of N,N-dimethylanilines were examined and good results were obtained as well (See 3ta3aaa, Table 3). Notably, N-ethyl-N-methylaniline (1ab), Nmethyl-1,2,3,4-tetrahydroquinoline (1ac), and 1-phenylpyrrolidine (1ad) could also be successfully ortho-C−H alkylated in a similar manner (See 3aba-3ada, Table 3). The cis-geometry of 3ta was unambiguously confirmed by X-ray crystallographic analysis of its HPF6 salt.13 Next, we checked the cyclization of various 1,5-dienes with N,N,4-trimethylaniline (1t) as C−H partner at 50oC in C6H5Cl with 4.0mol% catalyst loading (See Table 4). Similar to 2a, the reaction of substituted 1,5-dienes also performed well and was found highly selective without the contamination of linear adducts. Usually, the initial 2,1-addition of Sc−Ph bond prefers to occur at a less hindered alkene, and followed by intramolecular cis-cyclization to produce arylated 3-methylcyclopentane derivatives. Thus, 2-methyl-1,5-hexadiene (2b) afforded the expected racemic carbocyclic product 3tb in 94% yield. E/Z-hepta-1,5-diene was converted to carbocycle 3tc in 95% yield and with 67% cis-selectivity. To our surprise, racemic-2-methyl-4-phenyl-1,5-diene (2d) was cyclized to diastereoisomer 3td containing 63% cis-isomer. The excess of cis-3td presumably arises from minimizing the steric repulsion between catalyst and substituents of 2d in the second insertion event. 1,5-Dienes 2e and 2f, bearing a bulky cycloalkane unit, were also found suitable for this annulation, affording 100% cis-fused bicylo [3,3,0] and bicycle [4,3,0] products 3te and 3tf in 96% and 70% yields, respectively. The cyclization of 3-
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ACS Catalysis Table 4. Scope for 1,5-Dienes a, b, c
Scheme 1. Spirobicylization of Trienes 2s-2t with 1t
a
Reaction conditions: 1t (1.0mmol), 2 (1.1mmol), CpN-Sc (4.0mol%), [Ph3C][B(C6F5)4] (4.0mol%), at 50oC, in C6H5Cl (2.0mL). bIsolated yield; c Stereoselectivity was determined based on 1H NMR spectrum.
To further assess this annulation protocol, spirobicyclization of trienes were probed with 1t as a C−H partner. Under the optimized conditions, trienes 2s and 2t underwent a sequential cyclization to give expected arylated spiro[4.4]nonan 3ts and benzylated spiro[4.5]decane 4tt in 88% and 94% yields upon isolation (See Scheme 1). Scheme 2. Deuterium-Labeling Experiments a
phenyl-1,5-diene (2g) and 3-methyl-3-phenyl-1,5-diene (2h) were also proved efficient, affording diastereomeric products 3tg and 3th, quantitatively. As indicated in Table 5, a series of 1,6-dienes were then evaluated using this annulation protocol. Unlike the reported Ti (or Y)-catalyzed cyclization/functionalization of 1,6-dienes that usually yields the expected cyclohexane motifs,1g, 4a-b, 5, 6a-b hydroarylative cyclization of 1,6-dienes using this scandium catalyst, surprisingly, produce unusual 1,2-disubstitued benzylated methylcyclopentane derivatives as major products. Furthermore, the cyclization reaction proceeded with extremely high trans-selectivity. The X-ray diffraction analysis of crystal 4sl clearly highlights its trans-geometry.13 In this way, cyclizations of hepta-1,6-diene (2i), 4-phenyl-1,6diene (2j), sterically hindered 1,6-dienes (2k-2n), and even diallyldialkylsilanes (2o-2r) with 1t were all well achieved (See 4ti-4tr 42-95% yield, > 98 trans).14 As was the case for Pd-catalyzed hydrosilylative cyclization of 1,6-dienes,2c, 4c-e the current ring-closing pattern maybe stem from a cascade 1,2- and 2,1-insertion of alkene sequence. Table 5. Scope for 1,6-Dienes a, b, c
a
Reaction conditions: 1t (1.0mmol), 2 (1.1mmol), CpN-Sc (4.0mol%), [Ph3C][B(C6F5)4] (4.0mol%), at 50oC, in C6H5Cl (2.0mL). bIsolated yield; c Stereoselectivity was determined based on 1H NMR spectrum.
To get some mechanistic insight into this annulation protocol, deuterium-labeling experiments were conducted, as shown in Scheme 2. The ortho-D of aniline 1s-D5 was transferred to the methyl group of carbocycle in 3sa-D5 or 4soD5, which implies an amino-directing C−H bond cleavage process in this catalysis (Eq. 1, Eq. 2). The inter- and intramolecular kinetic isotopic competition experiments gave kinetic isotope effect (KIE) of kH/kD = 2.1 and 8.1, which suggests that C−H activation might be involved in the rate-determining steps over the catalytic cycle (Eq. 3, Eq. 4). Therefore, the more sluggish rate of C−H activation relative to that of intramolecular cyclization of dienes maybe account for the high cycloselectivity of this catalysis.9f A possible mechanism for scandium-catalyzed regio- and diastereoselective hydroarylative cyclization of 1,5- and 1,6dienes, as exemplified by the conversion of 2a and 2o to cis3sa and trans-4so, is proposed in Scheme 3. Coordination of N atom in 1s to cationic scandium catalyst, generated in situ by the reaction of CpN-Sc and [Ph3C][B(C6F5)4],9a-g, 11 assists in deprotonation of ortho-C−H in 1s, forming metallacycle A (C−H activation).9d, 15 The diastereoselective alkene 2,1-
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Scheme 3. Postulated Mechanism
insertion of 2a9f or alkene 1,2-insertion of 2o2c, 4c-e into Sc−Ph bond to form alkylscandium olefin B or D, followed by rapid intramolecular 2,1-insertion of remained alkene into Sc−C bond in B or D to form cyclopentylmethylscandium C or E (cyclization). Upon activating ortho-C−H of another 1s, C or E regenerates A whilst liberating cis-3sa or trans-4so. In summary, we have realized the first example of hydroarylative cyclization of 1,5- and 1,6-dienes with aromatic ethers and tertiary anilines as C−H partners using CpN-Sc in combination with [Ph3C][B(C6F5)4] as catalyst and proposed the possible reaction mechanism. The reaction is a green process, and generally proceeds under mild conditions to yield a variety of cis-1,3-disubstituted arylated and trans1,2-disubstituted benzylated methylcyclopentane derivatives containing monocyclic, bicyclic, spirocyclic, and heterocyclic skeletons in one step with high regio- and diasteroselectivity. The simple staring materials, broad scope of substrates, 100% atom-economy, unusual stereoselectivity and ring-closing pattern may enable this reaction highly valuable in view of the wealth of biologically active and naturally occurring carbocycles. Further study on asymmetric version of this methodology using chiral rare-earth catalysts is in progress.
ASSOCIATED CONTENT Supporting Information Experiment detail and NMR data. These materials are available free of charge via the internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *
[email protected]. Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 21371131).
REFERENCES
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ACS Catalysis of Substituted 1,5- and 1,6-Dienes. J. Am. Chem. Soc. 1992, 114, 3123-3125. (b) Widenhoefer. R. A.; Vadehra, A.; Cheruvu, P. K. Cyclization/Hydrogermylation of Functionalized 1,6-Dienes Catalyzed by Cationic Palladium Complexes. Organomet. 1999, 18, 46144618. (c) Aϊssa, C.; Ho, K. Y. T.; Tetlow, D.,J.; Pin-Nό, M. Diastereoselective Carbocyclization of 1,6-Heptadienes Triggered by Rhodium-Catalyzed Activation of an Olefinic CH Bond. Angew. Chem. Int. Ed. 2014, 53, 4209-4212. (d) Hu, Y.; Yu, C.; Ren, D.; Hu, Q.; Zhang, L.; Cheng, D. One-Step Synthesis of the Benzocyclo [penta- to octa]isoindole Core. Angew. Chem. Int. Ed. 2009, 48, 5448-5451. (e) Lee, D.-Y.; Kim, I.-J.; Jun, C.-H. Synthesis of Cycloalkanones from Dienes and Allylamines through C−H and C−C Bond Activation Catalyzed by a Rhodium (I) Complex. Angew. Chem. Int. Ed. 2002, 41, 3031-3033. (8) Some reviews for C−H functionalization: (a) Dong, Z.; Ren, Z.; Thompson, S. J.; Xu, Y.; Dong, G. Transition-Metal-Catalyzed C–H Alkylation using Alkenes. Chem. Rev. 2017, 117, 9333-9403. (b) Hummel, J. R.; Boerth, J. A.; Ellman, J. A. Transition-MetalCatalyzed C–H Bond Addition to Carbonyls, Imines, and Related Polarized π Bonds. Chem. Rev. 2017, 117, 9163-9227. (c) WencelDelord, J.; Glorius, F. C–H Bond Activation Enables the Rapid Construction and Late-Stage Diversification of Functional Molecules. Nat. Chem. 2013, 5, 369-375. (d) 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. Rew. 2012, 41, 3651-3678. (e) Yamaguchi, J.; Yamaguchi, A. D.; Itami, K. C–H Bond Functionalization: Emerging Synthetic Tools for Natural Products and Pharmaceuticals. Angew. Chem., Int. Ed. 2012, 51, 8960-9009. (f) Crabtree, R. H. Alkane C–H Activation and Functionalization with Homogeneous Transition Metal Catalysts: A Century of Progress—A New Millennium in Prospect. J. Chem. Soc. Dalton Trans. 2001, 2437-2450. (g) Godula, K.; Sames, D. C–H Bond Functionalization in Complex Organic Synthesis. Science 2006, 312, 67-72. (9) Recent examples for C−H functionalization by rare-earth-catalyst: (a) Guan, B.; Hou, Z. Rare-Earth-Catalyzed C–H Bond Addition of Pyridines to Olefins. J. Am. Chem. Soc. 2011, 133, 18086-18089. (b) Guan, B.; Wang, B.; Nishiura, M.; Hou, Z. Yttrium‐Catalyzed Addition of Benzylic C–H Bonds of Alkyl Pyridines to Olefins. Angew. Chem. Int. Ed. 2013, 52, 4418-4421. (c) Oyamada, J.; Hou, Z. Regioselective C–H Alkylation of Anisoles with Olefins Catalyzed by Cationic Half‐Sandwich Rare Earth Alkyl Complexes. Angew. Chem. Int. Ed. 2012, 51, 12828-12832. (d) Song, G.; Luo, G.; Oyamada, J.; Luo, Y.; Hou, Z. ortho-Selective C–H Addition of N,N-Dimethyl Anilines to Alkenes by a Yttrium Catalyst. Chem. Sci. 2016, 7, 52655270. (e) Nako, A. E.; Oyamada, J.; Nishiura, M.; Hou, Z. ScandiumCatalysed Intermolecular Hydroaminoalkylation of Olefins with Aliphatic Tertiary Amines. Chem. Sci. 2016, 7, 6429-6434. (f) Chen, Y.; Song, D.; Li, J.; Hu, X.; Bi, X.; Jiang, T. Hou, Z. Diastereoselective Cyclization of 1,5-Dienes with the C−H Bond of Pyridine Catalyzed by a Cationic Mono(phosphinoamide) Alkyl Scandium Complex. ChemCatChem 2018, 10, 159-164. (g) Luo, Y.; Ma, Y.; Hou, Z. α-C– H Alkylation of Methyl Sulfides with Alkenes by a Scandium Catalyst. J. Am. Chem. Soc. 2018, 140, 114-117. (h) Kundu, A.; Inoue, M.; Nagae, H, Tsurugi, H.; Mashima, K. Direct ortho-C–H Aminoalkylation of 2-Substituted Pyridine Derivatives Catalyzed by Yttrium Complexes with N,N'-Diarylethylenediamido Ligands. J. Am. Chem. Soc. 2018, 140, 7332-7342. (10) Examples for C−H transformation towards dienes by transition metal catalyst: (a) Potter, T. J.; Ellman, J. A. Rh (III)-Catalyzed C–H Bond Addition/Amine-Mediated Cyclization of Bis-Michael Acceptors. Org. Lett. 2016, 18, 3838-3841. (b) Jun, C.-H.; Hong, J.-B.; Kim, Y.-H.; Chung, K.-Y. The Catalytic Alkylation of Aromatic Imines by Wilkinson's Complex: The Domino Reaction of Hydroacylation and ortho‐Alkylation. Angew. Chem. Int. Ed. 2000, 39, 3440-3442. (c) Nakao, Y.; Yamada, Y.; Kashihara, N.; Hiyama, T. Selective C-4 Alkylation of Pyridine by Nickel/Lewis Acid Catalysis. J. Am. Chem. Soc. 2010, 132, 13666-13668. (d) Lim, Y.-G.; Kang, J.-B.; Koo, B. T. Rhodium-Catalyzed Coupling Reaction of 2-Vinylpyridines with 1,5Hexadiene: Effects of Phosphine Ligands. Tetrahedron Lett. 1999, 40, 7691-7694.
(11) (a) Nishiura, M.; Guo, F.; Hou, Z. Half-Sandwich Rare-EarthCatalyzed Olefin Polymerization, Carbometalation, and Hydroarylation. Acc. Chem. Res. 2015, 48, 2209-2220. (12) The synthesis and characterization of rare-earth complexes CpOSc and CpN-Ln (Ln = Sc, Y, and Lu), See Supporting Information. (13) The detail for preparation and X-ray crystallography study of compounds cis-3taHPF6 (CCDC 1880298) and trans-4sl (CCDC 1880297), See Supporting Information, Figure S2. (14) None of acyclic byproduct was observed in all cases, and a small amount of 1,3-disubstituted arylated methylcyclohexanes 3 were also formed in the cyclization/hydroarylation reactions of 1,6-dienes 2i-2j with 1t, See Supporting Information. (15) The formation of intermediate A was validated by 1H NMR spectrum through the reaction in-situ of CpN-Sc with [PhNMe2H] [B(C6F5)4] in C6D5Cl, See Supporting Information.
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