| Budget Amount *help |
¥4,120,000 (Direct Cost: ¥3,700,000、Indirect Cost: ¥420,000)
Fiscal Year 2007: ¥1,820,000 (Direct Cost: ¥1,400,000、Indirect Cost: ¥420,000)
Fiscal Year 2006: ¥2,300,000 (Direct Cost: ¥2,300,000)
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| Research Abstract |
The possibility that silicon might serve as a useful Lewis acid catalyst for asymmetric synthesis has long captured the imagination of organic chemist. We have studied asymmetric Diels-Alder reactions catalyzed by a chiral silicon Lewis acid having (R)-(+)-binaphthol (R-BINOL). In the presence of chiral silicon catalyst having unsabstituted R-BINOL, 3-acyl-1,3-oxazolidin-2-one smoothly reacted with cyclopentadiene to afford the endo Diels-Alder adduct in high yield with moderate enantiomeric excess (at-78℃ for 6h, 87% yield, endo/exo=98/2, (2S)-Diels-Alder adduct=69% ee/endo). Enantioselectivity was found to depend strongly on the size of both the substituents on 3,3'-position of the BINOL ligands and the substituents on the silicon atom. Thus, when the silicon catalyst having (R)-3,3'-dibromo-binaphthol was used, enantioselectivity was reversed and endo adduct with opposite absolute configuration (2R) was obtained (61% ee, endo/exo=98/2). Furthermore, enantioselectivities increased wi
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th the steric bulk of the aryl substituents on 3,3'-position of the BINOL ligands. When the reaction of 3-acyl-1,3-oxazolidin-2-one with cyclopentadiene was conducted by using silicon Lewis acid catalyst bearing (R)-3,3'-bis(4-(2-naphthyl)phenyl)binaphthol, the corresponding Diels-Alder adduct was obtained with 91% ee (2R). The size of substituents on the silicon atom also exerts a strong influence to the enantioselectivity. For example, enantiomeric excess of the product was greatly improved to 99% ee when silicon catalyst having phenyl group on the silicon and (R)-3,3'-bis(4-(2-naphthyl)phenyl)binaphthol ligand was used as the catalyst.
Under the optimized reaction conditions, chiral silyltriflimides (Si-NTf2) bearing (R)-BINOL ligands also effectively catalyzed the enantioselective Diels-Alder reaction of alkenoyl-1,3-oxazolidin-2-ones with cyclohexabutadiene and linear 1,3-dienes to give corresponding Diels-Alder adducts in high yields with high enantioselctivity (up to 98% ee). Catalyst loadings as low as 2.5mol% were enough to complete the reaction. Si29-NMR studies revealed that five-coordinate silicon species formed by the coordination with dienophiles are involved in transition state of the reaction. Less
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