Highly Efficient Silylcyanation of Ketones by a Catalytic Double Activation Method Using Lewis Acid and N-Oxide Catalysts

Abstract

An efficient protocol for addition of TMSCN to ketones by employing 3 mol% achiral Schiff base-Ti(IV) complex as the Lewis acid and 3 mol% N-oxide as the Lewis base in a manner of double activation was described. Aromatic, aliphatic, cyclic and heterocyclic ketones all gave the racemic O-TMS cyanohydrins in good to excellent yields (up to 99%) under mild conditions.

Key words

cyanohydrins - double activation catalysis - ketones - N-oxides - Schiff bases

References

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  When the reaction was catalyzed by (1R,2R)-N,N′-bis(3,5-di-tert-butyl-salicylidene)-1,2-diphenyl-1,2-diamine-Ti(O-i-Pr)₄ complex (20 mol%) and achiral N-oxide 2 (20 mol%), the product was obtained in 95% yield with 67% ee at 0 °C for 84 h {GC [Varian, Chirasil DEX CB, column temperature = 100 °C (isothermal), injector temperature = 200 °C, detector temperature = 250 °C]: t _r(major) = 24.5 min, t _r(minor) = 25.4 min}.

8

When NMO (N-methylmorpholine N-oxide) and PyNO (pyridine N-oxide) were used as the Lewis bases under the optimized conditions, the yield was reduced to 42% and 52%, respectively.

9

The standard procedure is given: Ti(i-PrO)₄ (96 µL, 1 M in toluene) was stirred with ligand 1 (47.4 mg, 0.096 mmol) in anhyd CH₂Cl₂ (1 mL) at 35 °C for 1 h under N₂ atmosphere. The solvents were removed in vacuo. After redissolved in CH₂Cl₂ (0.5 mL), the mixture was added, in sequence, acetophenone (3.2 mmol) and a solution of N-oxide 2 (13.2 mg, 0.096 mmol) and TMSCN (2 equiv) in CH₂Cl₂ (1.0 mL), which was also stirred at 35 °C for 1 h. The reaction was performed at 23 °C. At completion, the reaction mixture was concentrated and put on a silica gel column to give the O-TMS cyanohydrin as clear colorless oil (680 mg, 97%).