Chiral Silver Alkoxide Catalyzed Asymmetric Aldol Reaction of Alkenyl Esters with Isatins

Abstract

A catalytic enantioselective aldol reaction of alkenyl esters with isatins was achieved using a DM-BINAP·AgOTf complex as the chiral precatalyst and N,N-diisopropylethylamine as the base precatalyst in the presence of methanol or 2,2,2-trifluoroethanol. Optically active 3-alkylated 3-hydroxy-2-oxindoles having up to 98% ee were diastereoselectively obtained in moderate to high yields not only from cyclic alkenyl esters but also from acyclic ones through the in situ generated chiral silver enolates.

Key words

aldol reaction - alkenyl ester - asymmetric catalysis - isatin - silver

Publication History

Received: 26 March 2021

Accepted after revision: 10 April 2021

Accepted Manuscript online:
12 April 2021

Article published online:
26 April 2021

• References and Notes


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In our previous study on the chiral silver(I)-catalyzed asymmetric allylation of aldehydes, we showed that a considerable amount of an inert 2:1 complex of BINAP/silver(I) salt was formed accompanied by a reactive 1:1 complex when BINAP was added to an equimolar amount of the silver salt in MeOH. In the reaction, a 0.6:1 mixture of BINAP/silver(I) salt was found to produce the desired 1:1 complex without the formation of the 2:1 complex:
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• 8 Typical Experimental Procedure for the Asymmetric Aldol Reaction Catalyzed by (R)-DM-BINAP·AgOTf and (i-Pr)₂NEt: Synthesis of 1-Benzyl-3-hydroxy-3-(1-oxo-1,2,3,4-tetrahydronaphthalen-2-yl)indolin-2-one (3ad, Entry 8 in Table [2], Entry 4 in Table [3], and Entry 2 in Table [4]) A mixture of AgOTf (20.6 mg, 0.08 mmol) and (R)-DM-BINAP (29.4 mg, 0.04 mmol) was dissolved in dry THF (6 mL) under an argon atmosphere with direct light excluded and stirred at room temperature for 20 min. To the resulting solution were added MeOH (40.6 μL, 1.0 mmol) and (i-Pr)₂NEt (17 μL, 0.10 mmol) successively at –40 °C. The mixture was stirred at that temperature for 5 min. Then, alkenyl trifluoroacetate 1a (181.6 mg, 0.75 mmol) and isatin derivative 2d (118.6 mg, 0.5 mmol) were successively added drop by drop to the resulting solution at –40 °C. After stirring for 30 min at that temperature, the mixture was treated with MeOH (3 mL). Then, the mixture was filtered with a glass filter funnel filled with Celite^® and washed with EtOAc, and the combined filtrate and washes were concentrated in vacuo. The residual crude product was purified by column chromatography on silica gel to give corresponding β-hydroxy ketone 3ad (191.7 mg, >99% yield). The anti/syn ratio was determined to be 75:25 by ¹H NMR analysis. The enantiomeric ratio of the anti isomer was determined to be 98% ee by HPLC analysis using a chiral column [Daicel Chiralpak AD-3, hexane–i-PrOH (4:1), flow rate = 1.0 mL/min]: t ₁ = 36.0 min (major), t ₂ = 47.1 min (minor). The enantiomeric ratio of the syn isomer was determined to be 6% ee by HPLC analysis using a chiral column [Daicel Chiralpak AD-3, hexane–i-PrOH (4:1), flow rate = 1.0 mL/min]: t ₁ = 23.1 min (minor), t ₂ = 28.7 min (major). Spectral Data of the Product
  anti Isomer ¹H NMR (400 MHz, CDCl₃): δ = 8.10 (dd, J = 8.0, 1.2 Hz, 1 H), 7.49 (td, J = 7.5, 1.4 Hz, 1 H), 7.41 (dd, J = 7.3, 0.8 Hz, 1 H), 7.18–7.35 (m, 8 H), 7.06 (td, J = 7.5, 0.8 Hz, 1 H), 6.71 (d, J = 7.9 Hz, 1 H), 6.23 (s, 1 H), 4.87 (dd, J = 29.8, 15.7 Hz, 2 H), 3.21 (dd, J = 13.2, 4.7 Hz, 1 H), 2.83–2.98 (m, 2 H), 1.80–1.97 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 199.8, 176.8, 143.9, 143.6, 135.5, 134.1, 132.5, 130.0, 128.7 (2 C), 128.6, 128.4, 127.9, 127.6, 127.3 (2 C), 127.0, 123.9, 123.2, 109.5, 78.3, 51.6, 43.8, 28.9, 24.2. IR (neat): 3300, 2959, 1681, 1614, 1496, 1467, 1433, 1389, 1358, 1321, 1264, 1221, 1183, 1156, 1073, 1015, 998, 932 cm^–1. MS (ESI): m/z calcd for [C₂₅H₂₁O₃NNa]⁺ ([M + Na]⁺): 406.1414; found: 406.1409; [α]_D ^23.8 –51.0 (c 1.0, CHCl₃, 98% ee); mp 178–180 °C. syn Isomer ¹H NMR (400 MHz, CDCl₃): δ = 8.13 (dd, J = 7.9, 1.1 Hz, 1 H), 7.50 (td, J = 7.5, 1.3 Hz, 1 H), 7.27–7.37 (m, 7 H), 7.14–7.20 (m, 2 H), 6.93 (td, J = 7.6, 0.9 Hz, 1 H), 6.73 (d, J = 7.9 Hz, 1 H), 6.23 (s, 1 H), 5.00 (d, J = 15.5 Hz, 1 H), 4.86 (d, J = 15.7 Hz, 1 H), 3.45 (dd, J = 13.8, 4.4 Hz, 1 H), 3.01–3.10 (m, 1 H), 2.79 (dt, J = 16.7, 3.4 Hz, 1 H), 1.83–1.89 (m, 1 H), 1.40 (qd, J = 13.2, 4.3 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 201.9, 174.7, 144.3, 143.0, 135.4, 134.6, 132.2, 129.6, 129.5, 128.8 (2 C), 128.7, 127.7, 127.4, 127.3 (2 C), 126.9, 124.6, 123.3, 109.5, 78.6, 51.9, 43.9, 28.5, 24.7. IR (neat): 3399, 1685, 1615, 1492, 1456, 1371, 1226, 1172, 1073, 940 cm^–1. MS (ESI): m/z calcd for [C₂₅H₂₁O₃NNa]⁺ ([M + Na]⁺): 406.1414; found: 406.1407; [α]_D ^23.9 +6.3 (c 0.99, CHCl₃, 6% ee); mp 147–148 °C
• 9 Libman J, Sprecher M, Mazur Y. Tetrahedron 1969; 25: 1679

The anti/syn ratio of 3fd was determined by comparison with reported ¹H NMR data:
• 10a Zhao H, Meng W, Yang Z, Tian T, Sheng Z, Li H, Song X, Zhang Y, Yang S, Li B. Chin. J. Chem. 2014; 32: 417

See also:
• 10b Mao Z, Zhu X, Lin A, Li W, Shi Y, Mao H, Zhu C, Cheng Y. Adv. Synth. Catal. 2013; 355: 2029 ; the anti/syn ratios of other aldol products 3 were determined by analogy
• 11 Use of small amount of ROH decreases the rate of alcoholysis of silver alkoxide 5 resulting in low yield of the desired product 3, while excess amount of ROH accelerate the protonation of chiral silver enolate 4 and reduces the yield of 3.

Two examples of the synthesis of achiral silver alkoxides have been reported:
• 12a Edworthy IS, Rodden M, Mungur SA, Davis KM, Blake AJ, Wilson C, Schröder M, Arnold PL. J. Organomet. Chem. 2005; 690: 5710
• 12b Reisinger A, Himmel D, Krossing I. Angew. Chem. Int. Ed. 2006; 45: 6997