Asymmetric Intramolecular C–H Insertion Promoted by Dirhodium(II) Carboxylate Catalyst Bearing Axially Chiral Amino Acid Derivatives

Abstract

A dirhodium(II) carboxylate catalyst bearing axially chiral amino acid derivatives is prepared. X-ray crystal structure analysis reveals that four of the bridging ligands are aligned in a C ₂-symmetry-like conformation. This catalyst is effective in the asymmetric intramolecular C–H insertion of α-aryl-α-diazoacetates to α-aryl-β-substituted γ-lactones with a reasonable level of diastereo- and enantioselectivity, especially in the reaction of phenyl- and β-naphthyl-substituted substrates.

Key words

asymmetric synthesis - C–H insertion - chiral catalyst - axial chirality - γ-lactone - artificial amino acid

• References and Notes


For reviews, see:
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For a review, see:
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• 2b See also ref. 1d.
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For examples of axially chiral Rh carboxylate catalysts, see:
• 6a Hikichi K, Kitagaki S, Anada M, Nakamura S, Nakajima M, Shiro M, Hashimoto S. Heterocycles 2003; 129: 391
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Axially chiral binaphthyl phosphate was also employed as a bridging ligand, see:
• 6c McCarthy N, McKervey MA, Ye T, McCann M, Murphy E, Doyle MP. Tetrahedron Lett. 1992; 33: 5983
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For isolation and synthesis of rubioncolin B (8), see:
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For isolation of trigolutesin A (9), see:
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For examples, see:
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• 10b See also ref. 1a.

Enantioselective version of this transformation to α-phenyl-β,β-disubstituted γ-lactone, see:
• 11a Doyle MP, May EJ. Synlett 2001; 967

Diastereoselective version of this C–H insertion to α-aryl-β-substituted γ-lactones, see:
• 11b Villalobos MN, Wood JL. Tetrahedron Lett. 2009; 50: 6450
• 12 It has been confirmed that racemization does not occur during the Curtius rearrangement and subsequent ester hydrolysis in the preparation of (R)-3 and 4, see ref. 7b.
• 13 Preparation of Catalyst 1 from (R)-12 through a Ligand Exchange Reaction with Rh₂(OAc)₄ (Scheme 1) A solution of (R)-12 (46 mg, 0.12 mmol) and Rh₂(OAc)₄·2MeOH (11 mg, 22 μmol) in chlorobenzene (10 mL) was slowly evaporated under reflux conditions for 4 h. The green residue was diluted with EtOAc (20 mL), washed with sat. aq NaHCO₃, brine, and dried over Na₂SO₄. Then it filtered and concentrated in vacuo to give a residue. The residue was purified by column chromatography (SiO₂, hexane–EtOAc = 10:1 to 5:1 to 2:1 to 3:2) to afford catalyst 1 (28 mg, 75%) as green powder. Purified catalyst 1 was recrystallized from EtCN to give purple prisms. X-ray crystal structure analysis revealed the crystals of catalyst 1 includes six equivalents of EtCN relative to 1. Analytical Data for Catalyst 1 mp >300 °C (green powder without EtCN). [α]_D ²⁰ +98.2 (c 1.0, CHCl₃, catalyst 1 without EtCN). ¹H NMR (400 MHz, CDCl₃, catalyst 1 with EtCN): δ = 1.40 (t, J = 7.8 Hz, 12 H, CH ₃CH₂CN), 2.48 (q, J = 7.8 Hz, 8 H, CH₃CH ₂CN), 3.00 (br s, 12 H), 6.09 (s, 4 H), 6.37 (d, J = 8.7 Hz, 4 H), 6.83 (d, J = 8.7 Hz, 4 H), 6.85–6.91 (m, 4 H), 7.10 (ddd, J = 1.4, 7.0, 8.6 Hz, 4 H), 7.15 (t, J = 7.3 Hz, 4 H), 7.23 (d, J = 8.7 Hz, 4 H), 7.35–7.45 (m, 4 H), 7.73–7.78 (m, 8 H), 7.84 (d, J = 8.2 Hz, 4 H), 7.92 (d, J = 8.7 Hz, 4 H), 8.13 (br s, 4 H). ¹³C NMR (150 MHz, CDCl₃, catalyst 1 without EtCN): δ = 51.7, 120.8, 124.5, 125.5, 126.0, 126.6, 126.9, 127.0, 127.5, 127.6, 127.80, 127.84, 127.9, 128.2, 130.1, 130.30, 130.33, 132.1, 133.0, 133.1, 134.7, 154.3, 184.4. MS–FAB: m/z = 1686 [M]⁺, 1709 [M + Na]⁺. HRMS–FAB: m/z calcd for C₉₂H₆₄N₄O₁₆Rh₂ [M]⁺: 1686.2427; found: 1686.2412. Crystallographic Data for Catalyst 1·6EtCN C₁₁₀H₉₄N₁₀O₁₆Rh₂, M = 2017.77, 0.03 × 0.01 × 0.01 mm³, triclinic, P1 (#1), a = 10.4116(4), b = 15.2831(3), c = 16.3622(5) Å, α = 71.61°, β = 89.08°, γ = 85.19°, V = 2461.74(13) ų, Z = 1, ρ _calcd = 1.361 g cm^–3, 2θ _max = 26.00°, T = 103(2) K, 27189 reflections measured, 18783 unique. The final R ₁ and wR were 0.0726 and 0.1146 (all data). The residual electron densities (peak and hole) were 0.480 and –0.676 e Å^–3. These data has been deposited with the Cambridge Crystallographic Data Center as CCDC 1510840. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/getstructures.
• 14 This crystal structure is similar to that of the dirhodium carboxylate catalyst with axially chiral biphenyl-type bridging ligands prepared by Hashimoto, see ref. 6a.
• 15 Hashimoto S, Watanabe N, Ikegami S. Tetrahedron Lett. 1992; 33: 2709
• 16 Spectroscopic data of trans-5a have been reported, see: Seizert CA, Ferreira EM. Chem. Eur. J. 2014; 20: 4460
• 17 Typical Procedure for Asymmetric C–H Insertion of 6a (Table 1, Entry 2) To a refluxing solution of catalyst 1 (10 mg, 6.0 μmol, 2.0 mol%) in CH₂Cl₂ (3.0 mL), CH₂Cl₂ solution (1.5 mL) of 6a (65 mg, 0.3 mmol) was added dropwise over 1.5 h under an Ar atmosphere via a syringe pump. The mixture was further stirred for 30 min under reflux, and then the solvent was evaporated in vacuo to give a residue. The residue was purified by preparative TLC (SiO₂, hexane–EtOAc = 5:1) to afford a mixture of cis-5a and trans-5a (30 mg, 52%) and cyclopropanated product 13a (7.8 mg, 14%). The ratio for cis-5a/trans-5a (6.0:1) was determined by ¹H NMR integration of the purified mixture of cis- and trans-5a. These isomers were separated by preparative HPLC to determine the enantiomeric excess. Analytical Data for cis-5a Colorless oil. [α]_D ²⁰ +9.1 (c 0.3, CHCl₃, 74% ee). ¹H NMR (400 MHz, CDCl₃): δ = 3.43–3.49 (m, 1 H), 4.00 (d, J = 8.9 Hz, 1 H), 4.23–4.26 (m, 1 H), 4.48–4.52 (m, 1 H), 5.00–5.20 (m, 2 H), 5.09 (d, J = 16.5 Hz, 1 H), 5.27–5.36 (m, 1 H), 7.14–7.16 (m, 2 H), 7.27–7.37 (m, 3 H). ¹³C NMR (150 MHz, CDCl₃): δ = 45.6, 50.5, 70.4, 118.6, 127.6, 128.7, 129.1, 133.2, 133.3, 176.9. IR (neat): 3065, 3031, 2985, 2907, 1773, 1497, 1370, 1211, 1149, 1015, 925, 704 cm^–1. ESI-HRMS: m/z calcd for C₁₂H₁₂O₂Na [M + Na]⁺: 211.0730; found: 211.0727. Analytical data for trans-5a, see ref. 16.
• 18a Wamser M, Bach T. Synlett 2014; 25: 1081
• 18b Fu L, Wang H, Davies HM. L. Org. Lett. 2014; 16: 3036
• 19 The structure of cis-5e was confirmed by transformation under catalytic hydrogenation to cis-α-phenyl-β-ethyl γ-lactone, which is identical with the reduced product from cis-5a, see the Supporting Information.
• 20 The enantioselectivity of cyclopropane derivative 13b was measured and found to be low (8% ee).
• 21 Palomo C, Vera S, Mielgo A, Gomez-Bengoa E. Angew. Chem. Int. Ed. 2006; 45: 5984