Enantioselective Hydrolytic Kinetic Resolution of 1,2-epoxy-3-phenoxy Propane Derivatives by New Chiral (Salen) Cobalt C
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Catalysis Letters Vol. 104, Nos. 3–4, November 2005 ( 2005) DOI: 10.1007/s10562-005-7944-x
Enantioselective hydrolytic kinetic resolution of 1,2-epoxy-3-phenoxy propane derivatives by new chiral (salen) cobalt complexes Santosh Singh Thakur, Wenji Li, Chang-Kyo Shin, and Geon-Joong Kim* Department of Chemical Engineering, Inha University, Incheon, 402751, South Korea
Received 10 May 2005; accepted 6 July 2005
The enantioselective hydrolytic kinetic resolution (HKR) of racemic terminal epoxides catalyzed by new chiral (Salen) Co complex linked with Al or Ga provides a practical access of the enantiomerically enriched terminal epoxides (>98% ee) and 1,2diols simultaneously. In the present method, bimetallic catalysts show dramatically increased reaction rates with high enantioselectivity relative to the monomeric salen catalyst. KEY WORDS: hydrolytic kinetic resolution; chiral cobalt salen catalyst; terminal epoxides.
1. Introduction Terminal epoxides are very important subclass of epoxides for organic synthesis [1]. As a consequence, the preparation of optically pure terminal epoxides has long stood as a most significant target for asymmetric, chiral building blocks [2]. Hydrolytic kinetic resolution (HKR) provides most efficient, scalable and practical manner to prepare stereochemically enriched terminal epoxides [3–7]. Pursuant to our own efforts directed toward the designing of the catalyst system for the HKR [8–11], we have synthesized new dimeric chiral cobalt salen catalyst containing anhydrous aluminium halide salts attached to oxygen of salen complex as a Lewis acid, and it could be applied successfully in the HKR of various terminal epoxides [8]. The mechanistic study for HKR of epoxides, as reported by Annis and Jacobsen [12], indicates that the epoxide ring opening reactions proceed through the mechanism involving cooperative interaction between salen catalyst units, so linking the salen catalysts as dimers or to dendrimeric frameworks leads to catalytic systems with similar high enantioselectivity and substantially enhanced reactivity relative to the monomeric salen catalyst. The nonlinear effects in kinetic resolution and second-order kinetic dependence on catalysts also suggest a mechanism wherein two discrete catalyst molecules cooperate to activate both epoxides and water. Herein we report the synthesis of new chiral salen complexes having Al or Ga halide compounds and demonstrate that these catalysts exhibit substantially enhanced catalytic activity in the HKR of terminal epoxides as a dimeric form. The easily synthesized new bimetallic (salen) Co complex 2 catalyzes hydrolytic epoxide ring opening reactions with improved enanti* To whom correspondence should be addressed. E-mail: [email protected]
oselectivities and dramatically increased reaction rates relative to the monomeric catalyst 1.
2. Experimental 2.1. Synthesis of the catalyst 1 and 2 (R,R)-cobalt salen (structure A procured from Aldrich; 5.0 g, 8.28 mmol, 1.0 equiv.) was charged in the 100 mL flask equipped with a magnetic stir bar and w
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