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
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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 CH2 Cl2 (1 mL) at
35 °C for 1 h under N2 atmosphere. The
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mL), the mixture was added, in sequence, acetophenone (3.2 mmol)
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0.096 mmol) and TMSCN (2 equiv) in CH2 Cl2 (1.0
mL), which was also stirred at 35 °C for 1 h.
The reaction was performed at 23 °C. At completion,
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to give the O -TMS cyanohydrin as clear
colorless oil (680 mg, 97%).
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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}.