N O N O N O R R R 1: R = i Pr 2: R = t Bu 3: R = Ph CN N O N O i Pr i Pr (S,S,S )-BTO (S,S )-BBO, 4 TETRAHEDRON LETTERS Tetrahedron Letters 42 (2001) 4175–4177 Pergamon Enantioselective Michael addition catalyzed by chiral tripodal oxazoline–t BuOK complexes Sung-Gon Kim and Kyo Han Ahn* Department of Chemistry and Center for Integrated Molecular Systems, Division of Molecular and Life Sciences, Pohang University of Science and Technology, San 31 Hyoja -dong, Pohang 790 -784, Republic of Korea Received 21 March 2001; revised 16 April 2001; accepted 20 April 2001 Abstract—Benzene-based tripodal oxazolines are found to be novel chiral ligands for the catalytic enantioselective Michael addition via potassium enolates. Thus, methyl phenylacetate undergoes 1,4-addition to methyl acrylate using a catalytic amount of a t BuOK–oxazoline complex in toluene at -78°C, and up to 82% ee is obtained. © 2001 Elsevier Science Ltd. All rights reserved. Conjugate addition reactions are some of the funda- mental organic reactions, which are widely used for carboncarbon bond forming reactions. Their asym- metric versions have been intensively investigated in recent years catalyzed by various organometallic spe- cies. 1,2 Among them, the conjugate additions of eno- lates generated from carbonyl or nitro compounds, usually called Michael addition reactions, have been widely studied with catalysts such as alkali metal and transition metal complexes. For the alkali metal com- plexes, chiral ligands such as Cinchona alkaloids, alkox- ides, phenoxides, crown ethers, and their derivatives have been used as the ligands. 2 For the title reaction, several crown ether derivatives 3,4 have been studied since the early report of Cram. 5 Herein, we wish to report that a new class of ligands, the benzene-based tripodal oxazoline (BTO) system, is potentially useful chiral ligands for the potassium complexes, 6 which is demonstrated for the title reaction. In the course of our study on the selective molecular recognition of NH 4 + over K + using BTOs as artificial receptors, 7 we have found that they have significant affinities toward K + ion. This finding led us to evaluate the compounds as chiral ligands in the catalytic asym- metric reactions that involve complexes of K + such as potassium enolates. An initial experiment, the Michael reaction between methyl phenylacetate and methyl acrylate, with 10 mol% of t BuOK–valinol-derived i Pr-BTO 1 7 in toluene at -78°C yielded the addition product in 75% yield after 3 h and with an enantioselectivity of 35% ee. 8 To find optimal conditions with the BTO we varied the reaction conditions including solvents and the molar ratio of the base to the ligand, the results of which are listed in Table 1. Toluene and dichloromethane are found to be good solvents, but the former is favored over the latter in terms of enantioselectivity. As Keywords : Michael addition; tripodal oxazoline; asymmetric catalyst; potassium enolate. * Corresponding author. Tel.: +82 54-279-2105; fax: +82 54-279-3399; e-mail: ahn@postech.ac.kr 0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII:S0040-4039(01)00681-5