PdCl2 and NiCl2-catalyzed hydrogen–halogen exchange for the convenient preparation of bromo- and iodosilanes and germanes
Treatment of hydrosilanes with an excess of alkyl bromides in the presence of a PdCl2 or NiCl2 catalyst gave bromosilanes in good to high yield. Using propyl iodide as the iodine source, similar iodination of hydrosilanes occurred. Halogenation of hydrogermanes also proceeded by similar treatment with PdCl2 as the catalyst.
Introduction
Halosilanes and germanes are of important reagents as the building units for a variety of organosilicon and germanium compounds. In addition, bromo- and iodosilanes exhibit high Lewis-acidity to permit the interaction with CO and CO bonds, unlike fluoro- and chlorosilanes [1], [1](a), [1](b), [1](c), [1](d). Examples include the reactions of ethers and esters with iodo- and bromosilanes, which afford silyl ethers and esters, respectively, together with halides arising from CO bond cleavage, in good yield [2], [2](a), [2](b), [2](c), [2](d), [2](e), [2](f), [3]. It is also known that aldehydes react with iodosilanes to give silyl α-iodoalkyl ether, by iodosilylation of the carbonyl bond [4], [5].
Conventional methods to prepare bromo- and iodosilanes [1], [1](a), [1](b), [1](c), [1](d) and germanes [6], [6](a), [6](b) involve the cleavage of MM, MH, M–allyl, and M–aryl bonds (M=Si, Ge) with X2 (X=Br, I), reactions of MH with HX (X=Br, I) and NBS, and halogen exchange of MCl with inorganic halides. Reactions of an MOM bond with HX in the presence (M=Si) or absence (M=Ge) of a Lewis-acid also produce an MX bond. Cleavage of an alkyl-Ge bond with X2 and Lewis-acid-catalyzed redistribution reactions of R4Ge and GeBr4 are also often used for the formation of a GeX bond. However, most of these methods involve the use or by-production of unpleasant HX or X2. Furthermore, traces of HX and X2 may contaminate the resulting halosilanes and germanes.
Previously, we have found that the reactions of mono- and dihydrosilanes with alkyl iodides in the presence of a catalytic amount of PdCl2 afford mono- and diiodosilanes in high yield [7]. We have also reported the CuI-catalyzed bromination of hydrosilanes with CuBr2 [8], in which an SiH bond is replaced with an SiBr group with the use of two equivalents of CuBr2. However, there may be a limitation that this method can not be applied to the preparation of tribromosilanes, and treatment of trihydrosilanes with six equivalents of the reagent gives dibromohydrosilanes as the sole isolable products. In this paper, we report a versatile method to prepare bromo- and iodosilanes, including tribromo- and triiodosilanes from PdCl2- or NiCl2-catalyzed hydrogen–halogen exchange reactions of hydrosilanes with alkyl bromides and iodides, respectively. Similar halogenation of hydrogermanes is also described.
Section snippets
Bromination of hydrosilanes
The reactions of hydrosilanes with alkyl or allyl bromides in the presence of a catalytic amount of PdCl2 or NiCl2 proceeded with the liberation of alkanes to give brominated products in good to high yields (Table 1 and Scheme 1). As can be seen in Table 1, the reactions proceeded more rapidly when allyl bromide, rather than ethyl and propyl bromide, was used as the halide. Typically, PdCl2-catalyzed bromination of PhMe2SiH with EtBr or PrBr completed by refluxing the mixture without solvent
Conclusions
In conclusion, we studied the PdCl2- and NiCl2-catalyzed hydrogen–halogen exchange of hydrosilanes and germanes with alkyl halides. The reactions proceeded more readily with the PdCl2 catalyst, but more economic NiCl2 also can be used as the catalyst for the preparation of halosilanes. This provides a versatile synthetic method leading to a variety of bromo- and iodosilanes and germanes, which would be expected as useful synthetic tools for various organosilicon and germanium compounds.
General
All reactions were carried out under an inert atmosphere. NMR spectra were recorded on JEOL Model JNM-EX 270 and JEOL Model JNM-LA 400 spectrometers. Mass spectra were measured with a Hitachi M80B spectrometer.
Materials
Alkyl halides and allyl halides were distilled from P2O5 and stored over molecular sieves in dark until use. The starting hydrosilanes [7], [11] and germanes [7], [10] were prepared by the reactions of the respective chlorides with lithium aluminum hydride.
Halogenation of hydrosilanes and germanes
An illustrative procedure is as
Acknowledgements
We thank Sankyo Kasei Co. Ltd and Sumitomo Electric Industry for financial support, and Shin-Etsu Chemical Co. Ltd for gifts of chlorosilanes.
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