Bis(oxazolinyl)phenylrhodium(III) Aqua Complex: Efficiency in Enantioselective Addition of Methallyltributyltin to Aldehydes under Aerobic Conditions

Abstract

A simple and general protocol is described for the enantio­selective addition of methallyltributyltin to aldehydes catalyz­ed by chiral (Phebox)RhCl₂(H₂O) complexes 1 [Phebox = 2,6-bis(oxazolinyl)phenyl]. The reaction can be performed even unde­r aerobic conditions to afford the corresponding optically activ­e homoallyl­ic alcohols in good yields with high enantio­selectivities (90-99% ee).

Key words

aerobic conditions - allylation - asymmetric catalysis - rhodium - pincer complex

References

• 1a

  New address: Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580, Japan.

• 1b

  New address: Department of Applied Chemistry, Graduate School of Engeering, Nagoya University, Chikusa, Nagoya 464-8603, Japan.

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4

In the presence of MS 4 Å, this catalytic reaction was slightly accelerated but the enantioselectivity was not changed, see ref. [3b]

6

General Procedure for the Catalytic Enantioselective Addition of Methallyltributyltin 3 to Aldehydes Catalyzed by i -Pr-1. To a suspension of MS 4A (125 mg) in CH₂Cl₂ (1 mL) were added (i-Pr-Phebox)RhCl₂(H₂O) complex i-Pr-1 (6.1 mg, 0.0125 mmol, 5 mol%), aldehyde (0.25 mmol) and methallyltributyltin (0.375 mmol, 1.5 equiv) at 25 °C. After stirring the reaction mixture for 12 h at that temperature, the reaction mixture was concentrated under reduced pressure. The residue was dissolved 5 mL of Et₂O, and this solution was treated with a mixture of 1 N HCl (5 mL) and solid KF (ca. 0.5 g) at r.t. for 30 min. The resultant precipitate was filtered off, the filtrate was dried over MgSO₄ and concentrated under reduced pressure. Purification by silica gel chromatography (hexane-Et₂O = 3:1, then EtOAc as eluent for recovering i-Pr-1) gave homoallylic alcohol, the enantioselectivity was determined by chiral HPLC analysis. 5a: Daicel CHIRALCEL OD-H, UV Detector 254 nm, hexane-i-PrOH = 20:1, flow rate 0.5 mL/min. t _R = 13.1 min (S), 14.6 min (R); 5b: Daicel CHIRALCEL OJ, UV Detector 254 nm, hexane-i-PrOH = 30:1, flow rate 0.5 mL/min. t _R = 16.9 min (S), 19.1 min (R); 5c: Daicel CHIRALCEL OD-H, UV Detector 230 nm, hexane-i-PrOH = 30:1, flow rate 0.5 mL/min. t _R = 20.9 min (R), 22.9 min (S); 5d: Daicel CHIRALCEL OD-H, UV Detector 230 nm, hexane-i-PrOH = 40:1, flow rate 0.5 mL/min. t _R = 18.1 min (S), 19.1 min (R); 5e: Daicel CHIRALCEL OD-H, UV Detector 254 nm, hexane-i-PrOH = 20:1, flow rate 0.5 mL/min. t _R = 13.5 min (S), 19.2 min (R); 5f: Daicel CHIRALCEL OD-H, UV Detector 254 nm, hexane-i-PrOH = 20:1, flow rate 1.0 mL/min. t _R = 10.9 min (R), 20.8 min (S); 5g: The %ee was determined after converting to the benzoate ester. Daicel CHIRALPAK AD, UV Detector 254 nm, hexane-i-PrOH = 200:1, flow rate 0.5 mL/min. t _R = 9.4 min (R), 11.2 min (S).

7

5a: [α]_D ²³ -48.7 (c 0.63, Et₂O); lit. [8a] [α]_D ²⁰ -19.70° (c 9.90, Et₂O) for 40% ee (S); 5b: [α]_D ²² -44.5 (c 0.77, benzene); lit. [8b] [α]_D ²¹ -40.4° (c 3.15, benzene) for 88% ee (S); 5c: [α]_D ²³ -67.9 (c 0.99, CHCl₃); 5d: [α]_D ²¹ -49.2° (c 0.61, CHCl₃); 5e: [α]_D ²² +15.8° (c 0.85, CHCl₃); lit. [8b] [α]_D ²³ +16.6 (c 2.66, CHCl₃) for 67% ee (R); 5f: [α]_D ²² -19.4 (c 0.26, benzene); lit. [8c] [α]_D +19.0 (c 1.44, benzene) for 84% ee (R); 5g: [α]_D ²⁰ -3.30 (c 0.79, benzene); lit. [8c] [α]_D +4.04 (c 1.93, benzene) for 77% ee (R).