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.