[16966-40-8] · C4H9NSeSi · Isoselenocyanatotrimethylsilane · (MW 178.19)
Alternate Name: trimethylsilyl isoselenocyanate.
Physical Data: mp 5 °C; bp 175-177 °C, 65 °C/20 mmHg, 75-76 °C/30 mmHg; n
Solubility: sol most aprotic solvents: hexane, benzene, CH2Cl2, THF, MeCN, etc.
Preparative Methods: by reactions of Me3SiCl or Me3SiI with KSeCN or AgSeCN. The following simple method is recommended: a mixture of Chlorotrimethylsilane (10 mmol), Potassium Selenocyanate (18 mmol; dried at 90 °C under vacuum (0.05 mmHg) for 4 h), and hexane or CH2Cl2 (20 mL) is stirred for 5 h at rt. After filtration and removal of the solvent, the residue is distilled under vacuum.
Handling, Storage, and Precautions: strongly sensitive to moisture and air; its preparation should be carried out under a nitrogen atmosphere; a one-pot procedure is recommended in which Me3SiNCSe is not isolated and the substrate is added to the isoselenocyanate reaction mixture.
There is virtually no reaction between the title reagent and aldehydes in the absence of catalyst. Zinc Chloride and KSeCN increase the reaction rate, whereas KSeCN/18-crown-6 has a slight rate-diminishing effect. The reaction proceeds smoothly only in nonpolar solvents such as hexane and CH2Cl2, giving the corresponding O-silylated cyanohydrins (eq 1).
No reaction takes place with ketones and the reaction is very slow with aromatic aldehydes and a,b-unsaturated aldehydes. Consequently, highly chemoselective cyanosilylation of aldehydes is achieved: the results of competition experiments between hexanal and 2-heptanone (1:1 mixture, eq 2), between hexanal and benzaldehyde (1:1 mixture, eq 3), and between propionaldehyde and crotonaldehyde (1:1 mixture, eq 4) are summarized. The use of Cyanotrimethylsilane in place of the title reagent leads to lower chemoselectivities. Unfortunately, only slight chemoselectivity is achieved between hexanal and isobutyraldehyde (eq 5).
The reaction with oxiranes gives trimethylsiloxyalkyl selenocyanates (eq 6) in good to excellent yields. KSeCN, KSeCN/18-crown-6, KSeCN/XAD-4 (KSeCN impregnated on Amberlite XAD-4 resin), ZnCl2, and Ti(O-i-Pr)4 promote the reaction.
In general, the reaction proceeds smoothly in polar solvents such as MeCN. The reaction of 1,2-epoxyalkanes having a substituent at the C-3 position gives products in which the epoxide is opened selectively at the less-hindered carbon (product 1 in eq 6). On the other hand, the reaction of 2-phenyloxirane or 2,2-dimethyloxirane gives a mixture of regioisomers, the ratio of which depends on the catalyst and the solvent (eqs 7 and 8). The reaction of oxetane produces 3-trimethylsiloxypropyl selenocyanate in excellent yield.
Orient Chemical Industries, Osaka, Japan