Note
Synthesis of (Z)-1-aryl-2-(germyl)-1-(stannyl)ethenes and the related ethenes, precursors to stereodefined germylethenes, via Pd(dba)2–P(OCH2)3CEt-catalyzed germastannation of acetylenes in THF

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Abstract

(Z)-1-Aryl-2-(germyl)-1-(stannyl)ethenes are synthesized in high yields by the addition of tributyl(triethylgermyl)stannane to arylacetylenes catalyzed by a specific combination catalyst, Pd(dba)2 and 4-ethyl-1-phospha-2,6,7-trioxabicyclo[2.2.2]octane, in tetrahydrofuran. Ethynylthiophene and 2-methyl-3-butyn-2-ol are also subject to the germastannation to afford the respective adducts in high yields. In addition, the JSn–H and 13C-NMR data for their adducts are presented.

Introduction

The bis-silylation [1], bis-germylation [2], silastannation [3] or bis-stannation [4] of acetylenes forming new sp2carbon–silicon, sp2carbon–germanium or sp2carbon–tin bonds has been reported. The importance of their products is the possibility of transforming them to other new derivatives through the reactions such as Kosugi Migita-Stille coupling [5]. For example, the demetallations such as the desilylation of silylethenes [3f], the destannylation of stannylethenes [3g] and carbon–carbon formation by the cross couplings using silylethenes [6] or stannylethenes [5] have been reported. In all these cases, the reaction proceeds with the retention of the configuration, except for an example using vicinal di(silyl)ethenes [6f]. Recently, much attention has been focused on the germylethene derivatives, because pyridyl(germyl)ethene was reported to undergo [2+3] cycloaddition with nitrile oxide to produce (germyl)isoxazolines possessing vasodilating, antithrombotic and cardioprotective activity [7]. To synthesize such isoxazolines with a stereodefined structure, the use of stereodefined germylethene derivatives is essential. The germastannation of triple bonds can provide precursors to a variety of stereodefined vinylgermane derivatives through the destannylation [3g] or the Kosugi Migita-Stille coupling [5]. For the germastannation of acetylenes, Piers et al. [8] previously reported that addition of tributyl(trimethylgermyl)stannane to nonterminal α,β-acetylenic esters in the presence of Pd(PPh3)4 gave a mixture of vicinal (germyl)stannylethenes with (E) and (Z)-configurations. Mitchell et al. investigated the addition of the (germyl)stannane to terminal alkynes including phenylacetylene catalyzed by Pd(PPh3)4 and reported that the reaction formed vicinal (germyl)stannylethenes with the Z-configuration [9]. However the product yields did not exceed 50%.

As mentioned above, in view of the importance for the (Z)-(germyl)stannylethenes as key compounds to synthesize stereodefined germylethene derivatives, we examined the addition of tributyl(triethylgermyl)stannane 1 to phenylacetylene in the presence of a transition metal complex catalyst and found that a specific palladium combination catalyst, Pd(dba)2–P(OCH2)3CEt (dba=dibenzylideneacetone, P(OCH2)3CEt=4-ethyl-1-phospha-2,6,7-trioxabicyclo[2.2.2]octane2), very effectively accelerated the germastannation in tetrahydrofuran (THF) to afford (Z)-2-(germyl)-1-phenyl-1-(stannyl)ethenes in a high-isolated yield. We now report an alternative synthesis of (Z)-1-aryl-2-(germyl)-1-(stannyl)ethenes as precursors to a variety of germylethene derivatives and related (Z)-(germyl)stannylethenes by the reaction shown in Scheme 1.

Section snippets

Results and discussion

The Pd(PPh3)4-catalyzed addition of 1 to phenylacetylene was carried out at 80°C for 19 h in a sealed glass ampoule tube. However, the reaction produced (Z)-1-(tributylstannyl)-2-(triethylgermyl)-1-phenylethene (2a) in only 5% (entry 1 in Table 1). These results suggest that the reactivity of 1 is much lower than that of the tributyl(trimethylgermyl)stannane previously used by Mitchell et al. The reaction conditions are almost the same as those reported by Mitchell et al. [9] except for the

Measurements

GLC analyses were performed using an Ohkura Model 103 instrument equipped with a thermal conductivity detector and a stainless column packed with 20% or 10% Silicone KF-96/Celite 545 SK (60–80 mesh, 2 m×3 mm). The IR spectra were measured using a JASCO A-102 spectrophotometer. Mass spectra were obtained using a JEOL JMSAX-500 spectrometer with the DA7000 data system. 1H-NMR spectra and 13C-NMR spectra were recorded at 400 MHz and 100 MHz, respectively, on a Varian UNITY-400 spectrometer in

Acknowledgements

Partial financial support from the Shizuoka Shougou Kenkyu Kikoh Foundation of Japan is gratefully acknowledged by one of the authors (T.N.). The authors are indebted to Ms. Fumiyo O-ikawa (Tokai University) for the mass measurements, especially the high resolution ones on the (Z)-2-(germyl)-1-(stannyl)-1-(substituted)ethenes.

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