Asymmetric Cyanosilylation of Ketones Catalyzed by Chiral N-Oxide-titanium (IV) Complex

Abstract

A new approach for the asymmetric addition of trime­thylsilylcyanide to ketones using chiral N-oxide-Ti(iPrO)₄ complex as catalyst is reported. The screening of a number of chiral N-oxides has resulted in a system that gives the O-TMS ethers of cyanohydrins in good isolated yields with enantiomeric excesses of up to 69%.

Key words

asymmetric synthesis - catalysis - cyanohydrins - chiral N-oxides - bifunctional catalyst

References

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11

When S-N-oxide 2 coordinating with Sm(iPrO)₃, Zr(tBuO)₄ and ZnI₂ in 1:1.2 molar ratio was used as catalyst, product was obtained in ee of 0%,1% and 0%, respectively.

12

CHCl₃ used as solvent, product was obtained in 47% ee, but in toluene and diethyl ether, product was given in 14% ee and 4% ee, respectively.

13

Cyanosilylation adduct was found to be trace in 57% ee.

14

Benzylacetone and trans-4-phenyl-3-buten-one as substrate gave the products in 85% ee and 91% ee, respectively, reported by Shibasaki. [7] Catalyst showed better asymmetric induction for trans-4-phenyl-3-buten-one than benzylacetone. But our results showed that there existed great difference in asymmetric induction for between benzylacetone and trans-4-phenyl-3-buten-one.

15

A representative procedure: To a solution of S -2 (12.2 mg, 0.034 mmol) in CH₂Cl₂ (1 mL) was added Ti(iPrO)₄ (1 M in toluene, 41 µL, 0.041 mmol) at room temperature, and the mixture was stirred for 1 h, CH₂Cl₂ was evaporated under reduced pressure. The resulting residue was further dried in vacuo for 30 min. The residue was dissolved in CH₂Cl₂ (0.5 mL). To this solution, the ketone (0.17 mmol) was added under ice-water bath, followed by the addition of TMSCN (45 µL, 0.34 mmol) as shown in Table [2] . The reaction was monitored by TLC, and after the reaction period described in Table [2] , the solution was concentrated, usual workup and purification by silica gel chromatography gave the product.