Stereochemistry of the formation and cleavage of silicon–platinum bonds

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Abstract
The complex (+)-trans-[PtCl(*SiR3)(PMe2Ph)2], [α]D 25+ 72°(I), is formed with a high degree of retention of configuration at silicon from (+)-R3Si*H [R3Si*= Me(1-C10H7)PhSi] and cis-[PtCl2(PMe2Ph)2] in the presence of triethylamine. The (+)-R3Si*H is regenerated with 93% overall retention of configuration when (I) is treated with lithium aluminium hydride, and with rather smaller degrees of retention when (I) is treated with benzenethiol or triethylsilane. Bromine liberates (–)-R3Si*Cl with much racemisation, while iodine gives the racemic chloride. Complex (I) is converted into (+)-trans-[PtBr(*SiR3)(PMe2Ph)2], [α]D 25+ 70·0°, and (+)-trans-[Ptl(*SiR3)(PMe2Ph)2], [α]D 25+ 54·6°, on treatment with lithium bromide or sodium iodide in acetone. The complex (–)-cis-[PtH(*SiR3)(PPh3)2], [α]D 25– 18·5°(II), is formed from (+)-R3Si*H and [Pt(PPh3)2C2H4], with little loss of optical activity, and probably with retention of configuration at silicon; the (+)-R3Si*H is regenerated with 97% overall retention on treatment with lithium aluminium hydride, and with a slightly smaller degree of overall retention on treatment with phenylacetylene, benzenethiol, or benzoyl chloride. It is suggested that the cleavages of the Si–Pt bonds in (I) and (II), and especially those involving reaction or formation of R3Si*H, may occur via oxidative-addition–reductive-elimination sequences, with complete or almost complete retention of configuration at silicon, with the observed losses of activity arising from secondary racemisation; it was shown that (+)-R3Si*H and (more readily)(–)-R3Si*Cl do undergo racemisation in the presence of platinum complexes. Triphenylsilane and cis-[PtCl2(PMe2Ph)2] have been found to react to give trans-[PtH(Cl)(PMe2Ph)2], which then reacts with additional triphenylsilane to give trans-[PtCl(SiPh3)(PMe2Ph)2]. (+)-Et(1-C10H7)PhGeH (denoted R′3Ge*H), [α]D25+ 15·0°, reacts with trans-[PtH(Cl)(PMe2Ph)2] to give a complex believed to be (+)-trans-[PtCl*(GeR′3)(PMe2Ph)2], [α]D 25+12·9°, and with [Pt(PPh3)2C2H4] to give a complex believed to be (–)-cis-[PtH(*GeR3′)(PPh3)2], [α]D 25–6·0°, neither optically pure. Both products are thought to be formed with predominant retention of configuration at germanium.

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