Manganese Catalyzed Asymmetric Transfer Hydrogenation of Ketones Using Chiral Oxamide Ligands

Published as part of the 30 Years SYNLETT – Pearl Anniversary Issue

Abstract

The asymmetric transfer hydrogenation of ketones using isopropyl alcohol (IPA) as hydrogen donor in the presence of novel manganese catalysts is explored. The selective and active systems are easily generated in situ from [MnBr(CO)₅] and inexpensive C ₂-symmeric bisoxalamide ligands. Under the optimized reaction conditions, the Mn-derived catalyst gave higher enantioselectivity compared with the related ruthenium catalyst.

Key words

transfer hydrogenation - asymmetric - manganese - chiral ­ligands - ketones

• References and Notes


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• 13 Ligands L3a, L3c, and L7 were synthesized analogously to the procedure described for related compounds, see: Woods BP, Orlandi M, Huang CY, Sigman MS, Doyle AG. J. Am. Chem. Soc. 2017; 139: 5688 ; The corresponding amino alcohol (1 mmol) and dimethyl oxalate (1 mmol) were added under a flow of argon into a flame-dried 25 mL Schlenk tube containing a PTFE-coated stirring bar. Toluene (10 mL) was added by using a syringe and the suspension was heated to 90 °C. After 3 h, the mixture was allowed to cool to room temperature and the volatiles were removed in vacuo. The resulting solid was washed with cold toluene (2 × 2 mL) to give the analytically pure product.Analytical data found for L3c ¹H NMR: δ = 8.05 (d, J = 9.8 Hz, 2 H), 3.74–3.40 (m, 6 H), 0.87 (s, 18 H).¹³C NMR (75 MHz, DMSO-d ₆): δ = 160.21, 59.90, 59.41, 33.93, 26.86.MS (ESI-TOF): m/z calcd 289.2127 [M+H]⁺, 311.1941 [M+Na]⁺; found: 289.2120 [M+H]⁺, 311.1944 [M+Na]⁺.L7 ¹H NMR (300 MHz, DMSO-d ₆): δ = 9.10 (s, 2 H), 7.30–6.94 (m, 8 H), 5.16–5.03 (m, 2 H), 4.58–4.41 (m, 2 H), 3.21–3.12 (m, 2 H), 2.78–2.69 (m, 2 H).¹³C NMR: (75 MHz, DMSO-d ₆): δ = 160.60, 141.16, 139.79, 127.66, 126.63, 124.66, 123.59, 76.91, 61.43, 40.20 (shoulder of DMSO signal).MS (ESI-TOF): m/z calcd 352.1423 [M]⁺; found: 352.1429 [M]⁺
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• 17 General procedure for the ATH of prochiral ketones: Ligand L4a (6.6 mg, 0.02 mmol, 2 mol%) and MnBr(CO)₅ (16.2 mg, 0.06 mmol, 6 mol%) were placed in a flame-dried 25 mL Schlenk tube equipped with a PTFE-coated stirring bar, followed by anhydrous degassed isopropyl alcohol (2 mL). The suspension was stirred for 10 min at room temperature. A solution of potassium tert-butoxide (22.4 mg, 0.2 mmol, 20 mol% in 2 mL iPrOH) was added and the resulting yellowish solution was stirred for a further 10 min at room temperature. A solution of the desired ketone (1 mmol in 2 mL iPrOH) was then added and the mixture was heated to 80 °C and kept at this temperature for 20 h. The reaction solution was allowed to cool to room temperature and filtered through a plug of silica and washed with iPrOH (3 × 5 mL). Hexadecane (20 mg) was added to the reaction solution. The yield of the desired alcohol was determined by GC analysis using hexadecane as internal standard, and the ee was determined either by GC or HPLC analysis using an appropriate separation method (see the Supporting Information for further information).