Stereochemistry of the formation and cleavage of silicon–platinum bonds

Abstract

The complex (+)-trans-[PtCl(*SiR₃)(PMe₂Ph)₂], [α]_D²⁵+ 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]_D²⁵+ 70·0°, and (+)-trans-[Ptl(*SiR₃)(PMe₂Ph)₂], [α]_D²⁵+ 54·6°, on treatment with lithium bromide or sodium iodide in acetone.

The complex (–)-cis-[PtH(*SiR₃)(PPh₃)₂], [α]_D²⁵– 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]_D²⁵+12·9°, and with [Pt(PPh₃)₂C₂H₄] to give a complex believed to be (–)-cis-[PtH(*GeR₃′)(PPh₃)₂], [α]_D²⁵–6·0°, neither optically pure. Both products are thought to be formed with predominant retention of configuration at germanium.