INTRODUCTION

Alkynylsilanes are very common reagents in organic synthesis. These derivatives of alkynes were mainly used as protected equivalents of 1-alkynes¹, for Lewis acid catalyzed alkylation of different electrophiles² and for the Pd catalyzed alkynylation of halides and triflates³.

Usually trimethylsilyl (TMS) derivatives of 1-alkynes have been synthesized from the trimethylsilyl chloride and corresponding alkynylmetal derivatives. According to classical two-step silylation procedure the organolithium⁴ or organomagnesium⁵ derivatives of 1-alkynes were used as intermediates.

Recently H. Sugita et al.⁶ described the one-pot synthesis where the trimethylsilylation of 1-alkynes was performed through alkynyl zinc compound, prepared in situ.

H. Sugita et al.⁷ used also an other modified procedure where Zn was prepared by reduction of ZnCl₂ with samarium.

It was also published a copper mediated silylation of 1-alkynes by using triphenyl phosphine, TMEDA or metallic zinc as promoters⁸.

Most of all these methods afford good to excellent yields, but are often difficult to proceed i. e. sealed reaction flask is needed. There is not described the behaviour of the tetrahydropyranyl(THP) protected alkynols in zinc mediated one-pot synthesis.

RESULTS AND DISCUSSION

In the present investigation we have studied the trimethylsilylation of some derivatives of 1-alkynes with trimethylsilyl chloride in presence of Zn dust or Zn-Cu couple in acetonitrile according to following scheme:

R = -Ph; -(CH₂)₇-CH₃; -CH₂-OH; -CH₂-O-THP.

M = Zn, Zn-Cu

Results are given in following Table.

It has been established that the trimethylsilylation of 1-alkynes proceeds smoothly in refluxing acetonitrile using Zn dust or Zn-Cu couple. In the presence of Zn-Cu couple the reaction was substantially faster and completed already by the temperature of 80°C with the same reaction time as with Zn by 100°C, and sometimes even earlier. The conversation of 1-alkynes was always complete.

By the reaction with 2-propyn-1-ol and 2-methyl-3-butyn-2-ol as well only trimethylsilylation on carbon was obtained. The OTMS-group, if formed, probably cleaved during the reaction or by isolation of products. The similar problem was happened with 1-tetrahydropyranyloxy-3-propyne. In the present reaction conditions the THPO-group was partly cleaved and the yield was decreased (57-61 %). Using Zn-Cu couple, prepared according to H. E. Simmons et al.⁹, the THP-protecting group was cleaved completely during 2h by 73°C.

Zn-Cu couple and sometimes Zn are also good agents for the hydrogenation of triple bond, particularly in protic solvents¹⁰. In our case only by using Zn-Cu couple the partial hydrogenation (ca 30%) of triple bond in 1-tetrahydropyranyloxy-3-propyne was detected. The trimethylsilylation of 1-alkynes seems to be faster than reduction and when already inserted the TMS-group is a good protecting group against the hydrogenation as described by Collins et al.¹¹ .

Here we have demonstrated the usefulness of the trimethylsilylation of 1-alkynes by refluxing in acetonitrile in open flask. The work-up is not yet completely optimized and therefore yields of TMS derivatives could be improved.

EXPERIMENTAL SECTION

¹H and ¹³C NMR spectra were measured with Bruker AC200P (Spectrospin AG) spectrometer at 200 MHz and 50 MHz respectively. Chemical shifts are reported relative to TMS in CDCl₃.

IR spectra from neat liquids were recorded on Specord M82 (Carl Zeiss, Jena) using KBr cell.

GLC analysis were performed on Fractovap 4160 series (Carlo Erba Strumentazione) capillary gas chromatograph and Chrom 5 (Laboratorni Przistroje, Praha) equipped with FID, using fused silica capillary columns OV-101 25 m x 0.2 mm, Nordibond NB 20M 25 m x 0.32 mm and glass column 2.5 m x 3 mm packed with 5% Carbowax 20M on Chromosorb W AW-DMCS 80-100 mesh.

All starting compounds were purified by fractionating distillation and their purity (GLC) was find to be excellent (>99%).

Typical procedure

To a Zn dust or Zn-Cu couple (40 mmol) in acetonitrile (10 mL) in a round bottomed flask equipped with reflux condensor , protected with drying tube and magnetic stirrer first 1-alkyne (10 mmol) was added and then the trimethylsilyl chloride (20 mmol) was added by thoroughly stirring and cooling with ice water. The reaction mixture was boiled under reflux until the 1-alkyne was consumed. The mixture was filtered, poured into water, extracted with diethyl ether, combined ether extracts were dried on MgSO₄ and evaporated. The products, so prepared were pure for synthetical purpose (>95-98%) as confirmed by distillation, GLC and structure analysis.

Preparation of Zn-Cu couple

To a suspension of 4g Zn powder in 5 ml distilled water 1g of CuCl2 2H2O in 5 ml of distilled water was added by shaking or vigorous stirring by temperature of 80-85°C. The black precipitate prepared was washed twice with 10 ml of distilled water ,five times with ethanol and five times with acetonitrille.

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