Axial chirality in 1,4-disubstituted (ZZ)-1,3-dienes. Surprisingly low energies of activation for the enantiomerization in synthetically useful fluxional molecules.

Trialkylsilyltrialkylstannes (R(3)Si-SnR'(3)) add to 1,6-diynes in the presence of Pd(0) and tris-pentaflurophenylphosphine to give 1,2-dialkylidenecyclopentanes with terminal silicon and tin substituents. The (ZZ)-geometry of these s-cis-1,3-dienes, resulting from the organometallic reaction mechanisms involved, forces the silicon and tin groups to be nonplanar, thus making the molecules axially chiral. There is rapid equilibration between the two helical forms at room-temperature irrespective of the size of the Si and Sn substituents. However, the two forms can be observed by 1H, 13C, and 119Sn NMR spectroscopy at low temperature. The rates of enantiomerization, which depend on the Si and Sn substituents, and the substitution pattern of the cylopentane ring can be studied by dynamic NMR spectroscopy using line shape analysis. The surprisingly low energies of activation (DeltaG++ = 52-57 kJ mol(-1)) for even the bulky Si and Sn derivatives may be attributed to a widening of the exo-cyclic bond-angles of the diene carbons.