Abstract
An in situ generated Zn complex of salen ligand 4 was found to serve as an efficient catalyst for asymmetric addition reaction of an alkynylzinc reagent to various ketones. For example, addition of phenylacetylene to 3,3-dimethyl-2-butanone using the Zn complex as catalyst showed enantioselectivity as high as 91% ee.
Key words
Zn(salen) complex - asymmetric catalysis - asymmetric alkynylation - ketone - tertiary propargylic alcohol
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15 Typical experimental procedure is exemplified with the reaction of phenylacetylene and 3,3-dimethyl-2-butanone using 4 as the chiral ligand: Dimethylzinc (2.0 M solution in toluene, 150 µL) and phenylacetylene (32.9 µL, 0.3 mmol) were dissolved in a mixture of CH2Cl2 and toluene (300 µL/150 µL) under nitrogen atmosphere, and the solution was stirred for 1 h at r.t. Then, ligand 4 was added to the mixture and stirred for another 1 h. To this mixture was added 3,3-dimethyl-2-butanone (12.5 µL, 0.1 mmol) and the mixture was stirred for 2 d. The reaction mixture was quenched by addition of water (2 drops), diluted with Et2O, and passed through a pad of Celite and Na2SO4. After removal of the solvent under reduced pressure, the residue was chromatographed on silica gel (hexane-EtOAc = 19:1) to give the corresponding alcohol (15.2 mg, 75%). The enantiomeric excess was determined to be 91% ee by HPLC analysis (Daicel Chiralcel OD-H; hexane-i-PrOH = 49:1).
Enantiomeric excesses of the products were determined by HPLC analysis under the following conditions:
16a Daicel Chiralcel OD-H, hexane-i-PrOH = 24:1.
16b Daicel Chiralcel OD-H, hexane-i-PrOH = 19:1.
16c Daicel Chiralpak AD-H, hexane-i-PrOH = 19:1.
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