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Cite this: DOI: 10.1039/c5cc10096h Received 8th December 2015, Accepted 23rd December 2015 DOI: 10.1039/c5cc10096h
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Sequential Au(I)/chiral tertiary amine catalysis: a tandem C–H functionalization of anisoles or a thiophene/asymmetric Michael addition sequence to quaternary oxindoles† Zhong-Yan Cao,‡a Yu-Lei Zhao‡a and Jian Zhou*ab
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We report an unprecedented sequential Au(I)/bifunctional tertiary amine catalysis, which enables a tandem C–H functionalization of weak nucleophiles (anisoles or thiophenes) and asymmetric Michael addition for the highly enantioselective synthesis of quaternary oxindoles from diazooxindoles and nitroenynes.
Asymmetric sequential metal/organo catalysis has emerged as a promising strategy to build up molecular complexity from simple materials.1 It nicely takes advantage of metal-catalyzed reactions to generate suitable functionalities for the subsequent asymmetric organocatalytic reactions, which brings into full play the potential of two distinct catalysts to enlarge the reaction types for the development of new tandem sequences.2 While a number of elegant protocols have been developed by this strategy, it is still highly desirable to exploit new catalyst combinations to enable novel one-pot tandem reactions for the efficient creation of all-carbon quaternary carbon stereocenters from easily available substrates in an operationally friendly way, a very challenging task in organic synthesis.3 In this context, the coupling of a metal-catalyzed carbenoid reaction4 with organocatalytic asymmetric reactions is a fruitful strategy to elaborate diazo reagents to create fully substituted carbon stereocenters,5 since the pioneering work of Hu & Gong in merging Rh catalysis with chiral phosphoric acid catalysis to develop a tandem O–H insertion/Mannich sequence.6 When starting from C–H functionalization of arenes, it allows facile construction of all-carbon quaternary stereocenters (Scheme 1). By merging Rh or Pd-catalysis with phosphoric acid catalysis,
a
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, 3663N, Zhongshan Road, Shanghai 200062, China. E-mail: [email protected] b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, P. R. China † Electronic supplementary information (ESI) available: Experimental procedures and characterization data of new compounds. CCDC 1024590 and 1024591. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5cc10096h ‡ These authors contributed equally to this work.
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Scheme 1
Aromatic C–H functionalization based tandem reactions.
Hu et al. exploited several elegant asymmetric multicomponent reactions7 to create chiral quaternary carbons based on C–H functionalization of indoles, pyrroles or N,N-dialkylanilines (A). Gong et al. instead pioneered the combination of Ru or Rh catalysis with chiral bifunctional base catalysis, allowing facile access to quaternary 3-indolyloxindoles from diazooxindole 1 and indole (B).8 Despite progress, weak nucleophiles such as anisoles and thiophenes have not been coupled in such tandem reactions, possibly because they are less active than pyrroles and N,N-dialkylanilines, by at least five orders of magnitude according to Mayr’s nucleophilicity scale.9 In light of this, it is important to identify new catalyst combinations to develop the corresponding reactions based on C–H functionalization of less active aromatics. Here, we wish to report a sequential gold(I)/ chiral tertiary amine catalysis for the highly enantioselective synthesis of quaternary oxindoles 5 and 6 from diazooxindoles 1, anisoles 2 or thiophene 3 and nitroenynes 4.
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Table 1
Condition optimization
Entry Solvent T (1C) Additive Time (d) Yielda (%) Eeb (%) Drc
Scheme 2
1 2 3 4 5
Conditions for C–H functionalization.
10
Recently, with our interest in oxindole chemistry, we tried using diazooxindoles 1 to develop asymmetric reactions, a research in its infancy,11 and developed a highly stereoselective olefin cyclopropanation of 1 catalyzed by Au(I).11a During this study, we noticed that in the presence of Au(I), C–H insertion of anisole occurred. Investigation revealed that under the catalysis of only 1.0 mol% Ph3PAuOTf, the C–H functionalization of anisole 2a using diazooxindole 1a could complete within 2 h at 10 1C, and that of 3,4-dimethylthiophene 3 finished within 0.5 h at room temperature, which represented the first Au-catalyzed C–H bond insertion of thiophene using diazo reagents (Scheme 2).12 In contrast, Rh, Pd and Ru catalysts that Hu and Gong used all failed in both reactions, although Ag(I) catalyzed both reactions slowly, so did Cu(I) in the reaction of 3 (for details, see the ESI†). The efficiency of this low catalyst loading method not only showed the potential of Au catalysis in C–H functionalization of less active aromatics,13 but also encouraged us to consider an unexplored sequential Au(I)/chiral tertiary amine catalysis for tandem synthesis of quaternary oxindoles from diazooxindoles.14 First, chiral tertiary amines were powerful in deprotonative activation of 3-aryloxindoles for asymmetric syntheses of quaternary oxindoles, a privileged scaffold in natural products and drugs.15 Second, while Dixon reported the incompatibility of the two catalysts in their pioneering work of sequential chiral tertiary amine/Au(I) catalysis,16 we speculated that the presence of 1.0 mol% Au(I) might have no severe detrimental effect on the potency of a chiral tertiary amine that was typically used in 10 mol%, based on our work in multicatalyst promoted tandem reactions,17 which was later confirmed by control experiments.18 With this hypothesis, we first tried combining a Michael reaction using nitroenynes14c with the C–H functionalization for sequential synthesis of quaternary oxindoles, as the conjugate addition of 3-substituted oxindole to nitroenynes has not been attempted, although this synthetic valuable reaction had been intensively studied.19 A merit of using nitroenynes was to install an alkyne moiety as a versatile synthetic handle for further elaboration of the Michael adducts. Condition screenings suggested CH2Cl2 as the best solvent for the Au-catalyzed C–H functionalization, and our cinchonidine
Chem. Commun.
CH2Cl2 Et2O Et2O Et2O Et2O
10 10 40 40 40
— — — 4 Å MS 5 Å MS
1.0 1.0 3.0 3.0 3.0
95 90 89 94 93
97 98 98 98 99
2:1 4:1 5:1 6:1 7:1
a Isolated yield. b Determined by chiral phase HPLC analysis. c Determined by 1H NMR analysis of the crude reaction mixture.
based bifunctional phosphoramide C120 was suitable for Michael reaction of oxindole 7b to nitroenyne 4a (for details, see the ESI†). However, the merging of the two distinct catalytic reactions into one-pot operation was not trivial as it first appeared. After careful optimization, it turned out that solvent CH2Cl2 must be removed after the initial step finished, as the use of Et2O as solvent was important to obtain higher diastereoselectivity in the Michael addition (entry 1 vs. 2, Table 1). Further lowering the temperature to 40 1C, with the addition of 5 Å MS, the dr value increased to 7 : 1 (entries 3–5). Finally, a simple procedure for the one-pot tandem sequence was established. After the initial C–H functionalization step finished, CH2Cl2 was removed under vacuum, followed by the successive addition of Et2O, MS 5 Å, 10 mol% C1 and nitroolefin. Then the reaction was run at indicated temperature till completion. By this procedure, we evaluated the substrate scope with respect to differently substituted aromatics and nitroenynes, as shown in Table 2. Very impressively, under gold(I) catalysis, anisole and its derivatives even with a bromo or an iodo group worked well with diazooxindoles 1 to produce 3-aryloxindoles 7 for the next Michael addition catalyzed by phosphoramide C1 to furnish quaternary oxindoles 5 in good to excellent yield and good dr value, with 99% ee in all the cases. However, while Michael adduct 6 derived from 3,4-dimethylthiophene was also obtained in 82% yield and 99% ee, the diastereoselectivity was poor. By replacing Ph3PAuOTf with (2,4-tBu2C6H3O)3PAuSbF6,12c acetanilide derived quaternary oxindole 9 was also prepared via this sequence in good yield, dr and 93% ee. On the other hand, the alkyne substituent of nitroenynes could be varied from an alkyl, aryl to trimethylsilyl (TMS) group. The relative and absolute configuration of product 5h was determined by X-ray analysis. In addition to nitroenynes, ordinary nitrostyrenes also worked well in this tandem sequence, as shown by the synthesis of compound 10 in 84% yield, 6 : 1 dr and 97% ee. The synthetic value of the resulting adducts 5 was exemplified by the facile cyclization of 5g to give spirocyclic oxindole 11 in the presence of 1 mol% of Ph3PAuOTf, without erosion of the ee value. Our method constituted a new method for the
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Table 2
Tandem sequence to quaternary oxindoles
For details, ESI.
synthesis of spirocyclic oxindoles,15 a type of privileged scaffold in bioactive compounds.
In addition, an Au(I)-catalyzed hydration could be coupled to form a one-pot triple asymmetric sequence for the facile synthesis of quaternary oxindole 12 with a ketone moiety in excellent dr and ee values. The enrichment of the dr value was due to the fact that the minor diastereomer of 5a was difficult to be hydrated.
This sequential Au(I)/chiral tertiary amine catalysis constituted a flexible strategy to prepare 3,3-disubstituted oxindoles with C3 heteroatom substitution as well, a prominent structural motif in drugs and bioactive compounds. Basically, the reaction of 3-aryloxindoles with a heteroatom based electrophile, or the reaction of 3-heteroatom substituted oxindoles with carbon
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Sequential catalysis for 3-heteroatom oxindoles
electrophiles would lead to this subunit. This was exemplified by a tandem C–H insertion/amination sequence and an O–H insertion/Michael sequence to aminooxindole 13 and 3-alkoxyoxindole 14, bearing a C3 tetrasubstituted carbon stereocenter, respectively (Table 3). In addition, it was possible to synthesize oxindole derivatives with two C3 heteroatoms, as shown by highly enantioselective synthesis of oxindole O,N-ketal 15 via a tandem O–H insertion/amination sequence. It is worth mentioning that the insertion/amination sequence was a novel type of tandem sequence starting from diazo reagents (entries 1 and 3). By choosing a suitable chiral tertiary amine catalyst, it was possible to obtain the desired amination products21 in high to excellent enantioselectivity. This result further demonstrates the flexibility of sequential Au(I)/chiral tertiary amine catalysis in elaborating diazo reagents to value added compounds featuring a tetrasubstituted carbon stereocenter. In conclusion, we have developed an asymmetric sequential gold(I)/chiral tertiary amine catalysis as a flexible strategy for the one-pot enantioselective synthesis of 3,3-disubstituted oxindoles from diazooxindoles. Importantly, this research further highlights the potency of cationic Au(I) catalysis in C–H functionalization of weak nucleophiles such as anisoles and thiophenes, which plays a crucial role in developing these tandem reactions. The use of this new catalyst combination to develop other types of asymmetric tandem reactions is now in progress in our laboratory. We thank the 973 program (2015CB856600) and NSFC (21222204, 21472049) for financial support.
Notes and references 1 For reviews: (a) J. Zhou, Multicatalyst system in asymmetric catalysis, John Wiley & Sons, New York, 2014; (b) M. Rueping, R. M. Koenigs and I. Atodiresei, Chem. – Eur. J., 2010, 16, 9350; (c) J. Zhou, Chem. – Asian J., 2010, 5, 422; (d) N. T. Patil, V. S. Shinde and B. Gajula, Org. Biomol. Chem., 2012, 10, 211; (e) Z. Du and Z. Shao,
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Cite this: DOI: 10.1039/c5cc10096h Received 8th December 2015, Accepted 23rd December 2015 DOI: 10.1039/c5cc10096h
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Sequential Au(I)/chiral tertiary amine catalysis: a tandem C–H functionalization of anisoles or a thiophene/asymmetric Michael addition sequence to quaternary oxindoles† Zhong-Yan Cao,‡a Yu-Lei Zhao‡a and Jian Zhou*ab
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We report an unprecedented sequential Au(I)/bifunctional tertiary amine catalysis, which enables a tandem C–H functionalization of weak nucleophiles (anisoles or thiophenes) and asymmetric Michael addition for the highly enantioselective synthesis of quaternary oxindoles from diazooxindoles and nitroenynes.
Asymmetric sequential metal/organo catalysis has emerged as a promising strategy to build up molecular complexity from simple materials.1 It nicely takes advantage of metal-catalyzed reactions to generate suitable functionalities for the subsequent asymmetric organocatalytic reactions, which brings into full play the potential of two distinct catalysts to enlarge the reaction types for the development of new tandem sequences.2 While a number of elegant protocols have been developed by this strategy, it is still highly desirable to exploit new catalyst combinations to enable novel one-pot tandem reactions for the efficient creation of all-carbon quaternary carbon stereocenters from easily available substrates in an operationally friendly way, a very challenging task in organic synthesis.3 In this context, the coupling of a metal-catalyzed carbenoid reaction4 with organocatalytic asymmetric reactions is a fruitful strategy to elaborate diazo reagents to create fully substituted carbon stereocenters,5 since the pioneering work of Hu & Gong in merging Rh catalysis with chiral phosphoric acid catalysis to develop a tandem O–H insertion/Mannich sequence.6 When starting from C–H functionalization of arenes, it allows facile construction of all-carbon quaternary stereocenters (Scheme 1). By merging Rh or Pd-catalysis with phosphoric acid catalysis,
a
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, 3663N, Zhongshan Road, Shanghai 200062, China. E-mail: [email protected] b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, P. R. China † Electronic supplementary information (ESI) available: Experimental procedures and characterization data of new compounds. CCDC 1024590 and 1024591. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5cc10096h ‡ These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2016
Scheme 1
Aromatic C–H functionalization based tandem reactions.
Hu et al. exploited several elegant asymmetric multicomponent reactions7 to create chiral quaternary carbons based on C–H functionalization of indoles, pyrroles or N,N-dialkylanilines (A). Gong et al. instead pioneered the combination of Ru or Rh catalysis with chiral bifunctional base catalysis, allowing facile access to quaternary 3-indolyloxindoles from diazooxindole 1 and indole (B).8 Despite progress, weak nucleophiles such as anisoles and thiophenes have not been coupled in such tandem reactions, possibly because they are less active than pyrroles and N,N-dialkylanilines, by at least five orders of magnitude according to Mayr’s nucleophilicity scale.9 In light of this, it is important to identify new catalyst combinations to develop the corresponding reactions based on C–H functionalization of less active aromatics. Here, we wish to report a sequential gold(I)/ chiral tertiary amine catalysis for the highly enantioselective synthesis of quaternary oxindoles 5 and 6 from diazooxindoles 1, anisoles 2 or thiophene 3 and nitroenynes 4.
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Table 1
Condition optimization
Entry Solvent T (1C) Additive Time (d) Yielda (%) Eeb (%) Drc
Scheme 2
1 2 3 4 5
Conditions for C–H functionalization.
10
Recently, with our interest in oxindole chemistry, we tried using diazooxindoles 1 to develop asymmetric reactions, a research in its infancy,11 and developed a highly stereoselective olefin cyclopropanation of 1 catalyzed by Au(I).11a During this study, we noticed that in the presence of Au(I), C–H insertion of anisole occurred. Investigation revealed that under the catalysis of only 1.0 mol% Ph3PAuOTf, the C–H functionalization of anisole 2a using diazooxindole 1a could complete within 2 h at 10 1C, and that of 3,4-dimethylthiophene 3 finished within 0.5 h at room temperature, which represented the first Au-catalyzed C–H bond insertion of thiophene using diazo reagents (Scheme 2).12 In contrast, Rh, Pd and Ru catalysts that Hu and Gong used all failed in both reactions, although Ag(I) catalyzed both reactions slowly, so did Cu(I) in the reaction of 3 (for details, see the ESI†). The efficiency of this low catalyst loading method not only showed the potential of Au catalysis in C–H functionalization of less active aromatics,13 but also encouraged us to consider an unexplored sequential Au(I)/chiral tertiary amine catalysis for tandem synthesis of quaternary oxindoles from diazooxindoles.14 First, chiral tertiary amines were powerful in deprotonative activation of 3-aryloxindoles for asymmetric syntheses of quaternary oxindoles, a privileged scaffold in natural products and drugs.15 Second, while Dixon reported the incompatibility of the two catalysts in their pioneering work of sequential chiral tertiary amine/Au(I) catalysis,16 we speculated that the presence of 1.0 mol% Au(I) might have no severe detrimental effect on the potency of a chiral tertiary amine that was typically used in 10 mol%, based on our work in multicatalyst promoted tandem reactions,17 which was later confirmed by control experiments.18 With this hypothesis, we first tried combining a Michael reaction using nitroenynes14c with the C–H functionalization for sequential synthesis of quaternary oxindoles, as the conjugate addition of 3-substituted oxindole to nitroenynes has not been attempted, although this synthetic valuable reaction had been intensively studied.19 A merit of using nitroenynes was to install an alkyne moiety as a versatile synthetic handle for further elaboration of the Michael adducts. Condition screenings suggested CH2Cl2 as the best solvent for the Au-catalyzed C–H functionalization, and our cinchonidine
Chem. Commun.
CH2Cl2 Et2O Et2O Et2O Et2O
10 10 40 40 40
— — — 4 Å MS 5 Å MS
1.0 1.0 3.0 3.0 3.0
95 90 89 94 93
97 98 98 98 99
2:1 4:1 5:1 6:1 7:1
a Isolated yield. b Determined by chiral phase HPLC analysis. c Determined by 1H NMR analysis of the crude reaction mixture.
based bifunctional phosphoramide C120 was suitable for Michael reaction of oxindole 7b to nitroenyne 4a (for details, see the ESI†). However, the merging of the two distinct catalytic reactions into one-pot operation was not trivial as it first appeared. After careful optimization, it turned out that solvent CH2Cl2 must be removed after the initial step finished, as the use of Et2O as solvent was important to obtain higher diastereoselectivity in the Michael addition (entry 1 vs. 2, Table 1). Further lowering the temperature to 40 1C, with the addition of 5 Å MS, the dr value increased to 7 : 1 (entries 3–5). Finally, a simple procedure for the one-pot tandem sequence was established. After the initial C–H functionalization step finished, CH2Cl2 was removed under vacuum, followed by the successive addition of Et2O, MS 5 Å, 10 mol% C1 and nitroolefin. Then the reaction was run at indicated temperature till completion. By this procedure, we evaluated the substrate scope with respect to differently substituted aromatics and nitroenynes, as shown in Table 2. Very impressively, under gold(I) catalysis, anisole and its derivatives even with a bromo or an iodo group worked well with diazooxindoles 1 to produce 3-aryloxindoles 7 for the next Michael addition catalyzed by phosphoramide C1 to furnish quaternary oxindoles 5 in good to excellent yield and good dr value, with 99% ee in all the cases. However, while Michael adduct 6 derived from 3,4-dimethylthiophene was also obtained in 82% yield and 99% ee, the diastereoselectivity was poor. By replacing Ph3PAuOTf with (2,4-tBu2C6H3O)3PAuSbF6,12c acetanilide derived quaternary oxindole 9 was also prepared via this sequence in good yield, dr and 93% ee. On the other hand, the alkyne substituent of nitroenynes could be varied from an alkyl, aryl to trimethylsilyl (TMS) group. The relative and absolute configuration of product 5h was determined by X-ray analysis. In addition to nitroenynes, ordinary nitrostyrenes also worked well in this tandem sequence, as shown by the synthesis of compound 10 in 84% yield, 6 : 1 dr and 97% ee. The synthetic value of the resulting adducts 5 was exemplified by the facile cyclization of 5g to give spirocyclic oxindole 11 in the presence of 1 mol% of Ph3PAuOTf, without erosion of the ee value. Our method constituted a new method for the
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Table 2
Tandem sequence to quaternary oxindoles
For details, ESI.
synthesis of spirocyclic oxindoles,15 a type of privileged scaffold in bioactive compounds.
In addition, an Au(I)-catalyzed hydration could be coupled to form a one-pot triple asymmetric sequence for the facile synthesis of quaternary oxindole 12 with a ketone moiety in excellent dr and ee values. The enrichment of the dr value was due to the fact that the minor diastereomer of 5a was difficult to be hydrated.
This sequential Au(I)/chiral tertiary amine catalysis constituted a flexible strategy to prepare 3,3-disubstituted oxindoles with C3 heteroatom substitution as well, a prominent structural motif in drugs and bioactive compounds. Basically, the reaction of 3-aryloxindoles with a heteroatom based electrophile, or the reaction of 3-heteroatom substituted oxindoles with carbon
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Communication Table 3
Sequential catalysis for 3-heteroatom oxindoles
electrophiles would lead to this subunit. This was exemplified by a tandem C–H insertion/amination sequence and an O–H insertion/Michael sequence to aminooxindole 13 and 3-alkoxyoxindole 14, bearing a C3 tetrasubstituted carbon stereocenter, respectively (Table 3). In addition, it was possible to synthesize oxindole derivatives with two C3 heteroatoms, as shown by highly enantioselective synthesis of oxindole O,N-ketal 15 via a tandem O–H insertion/amination sequence. It is worth mentioning that the insertion/amination sequence was a novel type of tandem sequence starting from diazo reagents (entries 1 and 3). By choosing a suitable chiral tertiary amine catalyst, it was possible to obtain the desired amination products21 in high to excellent enantioselectivity. This result further demonstrates the flexibility of sequential Au(I)/chiral tertiary amine catalysis in elaborating diazo reagents to value added compounds featuring a tetrasubstituted carbon stereocenter. In conclusion, we have developed an asymmetric sequential gold(I)/chiral tertiary amine catalysis as a flexible strategy for the one-pot enantioselective synthesis of 3,3-disubstituted oxindoles from diazooxindoles. Importantly, this research further highlights the potency of cationic Au(I) catalysis in C–H functionalization of weak nucleophiles such as anisoles and thiophenes, which plays a crucial role in developing these tandem reactions. The use of this new catalyst combination to develop other types of asymmetric tandem reactions is now in progress in our laboratory. We thank the 973 program (2015CB856600) and NSFC (21222204, 21472049) for financial support.
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