Polymer-Supported 2,2′-Bis(oxazolin-2-yl)-1,1′-binaphthyls (Boxax): Immobilized Chiral Ligands for Asymmetric Wacker-Type Cyclizations

Abstract

Homochiral 2,2′-bis(oxazolin-2-yl)-1,1′-binaphthyl (boxax) ligands were anchored on various polymer supports including PS-PEG, PS, PEGA, and MeO-PEG via selective monofunctionalization at the 6-position of the binaphthyl backbone. Palladium(II) complexes of the supported boxax ligands catalyzed Wacker-type cyclization of 2-(2,3-dimethyl-2-butenyl)phenol to give 2-methyl-2-isopropenyl-2,3-dihydrobenzofuran with up to 96% ee.

Key words

boxax - immobilized chiral catalysts - asymmetric catalysis - Wacker-type cyclization

References

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• 19a

  Immobilization of Boxax Ligands: A typical procedure is given for the preparation of the PS-PEG resin-supported ligand 4. Boxax methyl ester 18 and LiOH·H₂O (1.05 equiv to methyl ester) was stirred in a mixture of THF/H₂O at 25 °C for 24 h. After removal of the solvent in vacuo a white precipitate 19 was obtained. A mixture of 19 (1.1 equiv to an amino residue of resin), HOBt (1.0 equiv) and EDCI (2.5 equiv) and PS-PEG amino resin in DMF was shaken at 25 °C for 12 h. To a reaction mixture was added EDCI (5 equiv) and the mixture was continuously shaken for an additional 24 h during which the progress of the reaction was monitored by a Kaiser test. [19b] The reaction mixture was filtered and the resin beads were rinsed with DMF, MeOH and DCM to give a polymer resin supported boxax ligand. A negative Kaiser test and ¹³C-MASNMR studies indicated the complete consumption of the terminal amino residue of the starting resin and the exclusive formation of the desired PS-PEG supported boxax ligands. The loading value of the polymer-supported boxax ligands 4a-d were estimated to be 0.35 mmol/g based on the initial loading value of the amino residue. ¹³C-MASNMR data of 4a-d: 4a: δ = 17.92, 18.25, 31.35, 32.36, 37.33, 39.01, 69.64, 72.08, 124.86, 125.39, 131.29, 132.62, 133.79, 134.04, 134.12, 137.33, 137.63, 139.31, 163.31, 171.63. 4b: δ = 31.26, 37.12, 38.94, 74.10, 131.23, 132.58, 133.97, 134.27, 137.46, 137.81, 139.70, 142.05, 164.68, 171.68. 4c: δ = 25.21, 31.34, 33.27, 37.43, 39.00, 67.74, 75.75, 124.66, 125.18, 131.39, 132.69, 133.95, 134.18, 137.74, 137.81, 138.02, 139.28, 162.75, 171.68. 4d: δ = 27.67, 31.30, 37.29, 39.00, 66.53, 78.96, 125.40, 131.17, 132.50, 133.60, 133.88, 136.54, 136.82, 139.35, 163.06, 171.59.

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10

The enantiomeric purity of (S)-12 and (S)-14 were determined to be both >98% by HPLC analysis using chiral stationary phase column (Daicel Chiracel OD-H, eluent:
n-hexane-isopropanol = 10:1).

12

Compounds 17a-c were obtained as single diastereomers.

13

Specific rotation data of 18a-d: 18a: [α]_D ²⁵ -97.75 (c 0.75, THF). 18b: [α]_D ²⁵ -16.93 (c 0.48, THF). 18c: [α]_D ²⁵ -54.95 (c 0.58, THF). 18d: [α]_D ²⁵ -7.28 (c 0.49, THF).

14

ArgoGel amino resin (1% DVB cross-linked) purchased from Argonaut Technologies Inc. was used.

15

Aminomethylated polystyrene (1% DVB cross-linked) was purchased from Novabiochem.

17

MeO-PEG5000 amino resin was purchased from Fluka.

20

General Procedure for the Asymmetric Wacker-Type Cyclization of 1: A mixture of immobilized catalyst (0.02 mmol Pd), 2-(2,3-dimethyl-2-butenyl)-phenol(1) (0.2 mmol, 35.2 mg), benzoquinone (0.8 mmol, 86.4 mg) and MeOH (1 mL) was agitated at 60 °C with shaking for 20 h. After being cooled to r.t., the resin was filtered off and washed twice with MeOH (1 mL). The solvent was removed in vacuo and the residue was purified by column chromatography using silica gel and n-hexane-EtOAc (95:5) giving (S)-2-iso-propenyl-2-methyl-2,3-dihydrobenzofuran(2) as colorless oil. The ee was determined by GC (chiral stationary phase capillary column Cyclodex CB).