Stereochemically non-rigid silanes, germanes, and stannanes. Part 3. Intramolecular rearrangements in silyl-, germyl-, and trimethylstannyl-cyclononatetraenes

Abstract

Reaction of SnBrH₃, GeBrH₃, or SnMe₃Cl with the cyclononatetraenyl anion in the range –78 to –45 °C affords the 9-metallo-all-cis-cyclonona-1,3,5,7-tetraenes C₉H₉Y [Y = SiH₃(1), GeH₃(2), or SnMe₃(3)] characterized by ¹H and ¹³C n.m.r. spectroscopy. On warming to ambient temperature, compounds (1)–(3) undergo electrocyclic ring closure to the isomeric 1-metallo-8,9-dihydroindenes (5)–(7), for which i.r., mass, and ¹H and ¹³C n.m.r. spectral parameters have been recorded. N.m.r. data show that compounds (5)–(7) are formed as mixtures of exo and endo isomers, the former predominating in each case. The ¹H and ¹³C n.m.r. spectra of cis-cyclononatetraene (4) and 8,9-dihydroindene (8) have also been obtained. At temperatures between –90 and 5 °C compound (3) is fluxional; comparisons with n.m.r. data for SnMe₃(C₅H₅) and SnPh₃(C₇H₇) indicate that successive 1,9 (‘least-motion’) sigmatropic shifts rather than the alternative symmetry-allowed thermal migration (1,5; ‘molecular broad-jump’) are responsible for metallotropic character. In contrast, compounds (1) and (2) are stereochemically rigid on the n.m.r. time scale at –45 °C and isomerization to compounds (5) and (6) at higher temperatures prevents observation of fluxional behaviour.