Chiral Cyclopentadienyl Ligands Enable a Rhodium(III)-Catalyzed Enantioselective Access to Hydroxychromanes and Phthalides

Dedicated to K. Peter C. Vollhardt for his pioneering contributions to chiral cyclopentadienyl ligands.

Abstract

The demand for efficient chiral cyclopentadienyl ligands (Cp^x) has increased significantly in recent years, partly because Cp*Rh(III) species have been developed as powerful catalysts for directed C–H functionalization reactions. However, a lack of suitable Cp^x ligands has hampered the development of the corresponding enantioselective processes. We report expansions of the libraries of two generations of Cp^x ligands and their corresponding rhodium(I) complexes. The potential of the rhodium complexes as catalysts was evaluated in enantioselective C–H functionalizations involving cyclizations across tethered aldehydes. The mild reaction conditions permit the syntheses of hydroxychromanes and phthalides in good yields and high enantioselectivities.

Key words

asymmetric catalysis - ligands - rhodium - cyclizations - heterocycles

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• 20 (4S)-4-Hydroxy-N-isopropoxychromane-5-carboxamide (8b): Typical ProcedureA solution of catalyst 2c (3.10 mg, 5.00 µmol) and (BzO)₂ (1.20 mg, 5.00 µmol) in anhyd DCE (1.0 mL) was stirred for 15 min at 23 °C and then transferred to a vial containing 7b (26.0 mg, 0.10 mmol). The resulting solution was stirred at 23 °C for 14 h. Volatiles were removed under reduced pressure and the residue was loaded onto a silica gel column and subjected to gradient elution with hexane–EtOAc (5:1 to 1:1) to give a colorless film; yield: 21.0 mg (80%; 92:8 er); [α]_D ²⁰ –123 (c 1.0, CH₂Cl₂); R_f = 0.30 (hexane–EtOAc, 1:1); IR (ATR): 3198, 2976, 2932, 2886, 1638, 1592, 1515, 1476, 1449, 1406, 1375, 1354, 1291, 1250, 1214, 1181, 1158, 1146, 1112, 1081, 1046, 1022, 980, 950, 906, 890, 840, 819, 803, 786, 760 cm^–1; ¹H NMR (400 MHz, acetone-d ₆): δ = 10.66 (s, 1 H), 7.26–7.19 (m, 1 H), 7.08 (dd, J = 7.4, 1.2 Hz, 1 H), 6.95 (dd, J = 8.3, 1.2 Hz, 1 H), 4.71 (t, J = 3.1 Hz, 1 H), 4.35–4.18 (m, 3 H), 2.02 (q, J = 2.5 Hz, 1 H), 2.00 – 1.91 (m, 1 H), 1.29 (d, J = 6.2 Hz, 3 H), 1.26 (d, J = 6.2 Hz, 3 H); ¹³C NMR (100 MHz, acetone-d ₆): δ = 167.5, 155.0, 135.2, 128.7, 124.4, 120.0, 119.7, 77.4, 61.2, 59.7, 20.2, 20.1; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₃H₁₈NO₄: 252.1230; found: 252.1230; HPLC: Chiralpak IC (4.6 × 250 mm); 20% iPrOH–hexane (1.0 mL/min); 254 nm; t_{r (minor)} = 17.7 min, t_{r (major)} = 15.4 min; 92:8 er.
• 21 Crystallographic data for compound 9f have been deposited with the accession number CCDC 1056306 and can be obtained free of charge from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: [phone]; E-mail: [e-mail]; Web site: www.ccdc.cam.ac.uk/conts/retrieving.html.