Synlett 2004(9): 1557-1560  
DOI: 10.1055/s-2004-829057
LETTER
© Georg Thieme Verlag Stuttgart · New York

Zn(Salen)-Catalyzed Asymmetric Alkynylation of Ketones

Bunnai Saito, Tsutomu Katsuki*
Department of Chemistry, Faculty of Science, Graduate School, Kyushu University, 33, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
Fax: +81(92)6422607; e-Mail: katsuscc@mbox.nc.kyushu-u.ac.jp;
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Publikationsverlauf

Received 7 April 2004
Publikationsdatum:
01. Juli 2004 (online)

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.

12

Zn(salen) complexes bearing a Lewis basic substituent at C3 and C3′ has been prepared and used as the catalysts for addition of alkynylzinc to carbonyl compounds (ref. [7b] [9d] ).

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).