The dynamic behavior of cy clononatetraenyl(trimethyl)tin (3) was analyzed
in detail by a combination of dynamic NMR techniques and high-level, ab ini
tio, density functional calculations (Becke3 - Lee - Yang - Parr (B3LYP) in
conjunction with 6-31G* (C and H) and 3-21G* (Sn) basis sets for optimizat
ions as well as 6-311G* (C and H) and 3-21G* (Sn) basis sets for single-poi
nt energy evaluations on the optimized geometries). Complete H-1 and C-13 N
MR spectra of 3 were assigned at 173 K; a comparison of computed and measur
ed NMR data was used to elucidate the peak assignments of the endo gound-st
ate structure of 3. 2D C-13, C-13-EXSY experiments in the temperature range
171-195 K provide strong evidence for [1,9]-SnMe3 migrations in 3, The exp
erimental activation energy for this process (25.1 +/- 2.5 kJ mol(-1)), obt
ained from a series of 2D EXSY spectra, is in excellent agreement with the
computed value (26.4 kJ mol(-1)). The analysis of the selectivities of sigm
atropic migrations in a series of cycloheptatrienyl and cyclononatetraenyl
derivatives of boron and tin suggests that orbital control is the dominant
factor governing the selectivities and mechanisms of these rearrangements.
If several nearly degenerate migrations are possible, the least-motion prin
ciple favors the rearrangement which involves minimal motion of the migrati
ng group. Hence, the barrier of a particular migration is determined by the
properties of the carbon cycle rather than by the nature of migrating grou
p.