Stereochemically non-rigid silanes, germanes, and stannanes. Part 5. Characterization of non-degenerate rearrangements in methylcyclopentadienyl derivatives using carbon-13 nuclear magnetic resonance spectroscopy

Abstract

The intramolecular rearrangement behaviour of silyl(methyl)cyclopentadiene, (1), germyl(methyl)cyclopentadiene, (2), trimethylgermyl(methyl)cyclopentadiene, (3), and trimethylstannyl(methyl)cyclopentadiene, (4), has been investigated using variable-temperature ¹³C Fourier-transform n.m.r. spectroscopy. The exchange process is discussed in terms of (a) vanishingly small equilibrium concentration for the geminal isomer compared with those of the two possible non-geminal isomers, (b) predominance of the non-geminal isomer with the substituents on non-adjacent carbon atoms of the C₅ ring over that with substituents on adjacent carbon atoms where one of these configurations is preferred. From the temperature dependence of the equilibrium constant for compound (3) the enthalpy difference between the non-geminal isomers is estimated to be ca. 2 kJ mol^–1 and may be even smaller for compounds (1) and (2). The frequency separation between the ¹³C resonances in fast-limiting spectra due to the two pairs of equivalent carbon atoms is correlated with the equilibrium ratio of the species participating in the non-degenerate exchange.