Stereochemically non-rigid silanes, germanes, and stannanes. Part 11. Mechanistic implications of diastereotopic effects induced in methylcyclopentadienyl, indenyl, and pentamethylcyclopentadienyl ring systems by chiral tin substituents

Abstract

The synthesis and characterization by ¹H and ¹³C n.m.r. spectroscopy of the racemic, chiral cyclopentadienyl-related compounds Sn*MePrⁱPh(R)[R = C₅H₄Me (2); R = C₉H₇(3); R = C₅Me₅(4)] is reported. For (2) and (4) the n.m.r. data are exclusively fast-limiting; compound (3) exhibits temperature-dependent behaviour. Diastereotopic effects in the isopropyl substituent identified by ¹H [(2) and (3)] or ^l3C n.m.r. [compound (4)] establish that for each compound metallotropic rearrangement occurs with retention of configuration at tin. The dynamic process in (2) is discussed in terms of facile epimerization; slow-limiting data for (3) show the presence of two diastereoisomers which stereomutate at elevated temperature; the degenerate rearrangement of (4) is related to the behaviour of unsubstituted analogues and the Woodward–Hoffmann [1,5] sigmatropic shift.