Mechanism and kinetics of sigmatropic rearrangements in cyclononatetraenyl(trimethyl)tin

The dynamic behavior of cy clononatetraenyl(trimethyl)tin (3) was analyzed in detail by a combination of dynamic NMR techniques and high-level, ab initio, density functional calculations (Becke3 - Lee - Yang - Parr (B3LYP) in conjunction with 6-31G* (C and H) and 3-21G* (Sn) basis sets for optimizations as well as 6-311G* (C and H) and 3-21G* (Sn) basis sets for single-point energy evaluations on the optimized geometries). Complete H-1 and C-13 NMR spectra of 3 were assigned at 173 K; a comparison of computed and measured NMR data was used to elucidate the peak assignments of the endo gound-state 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 experimental activation energy for this process (25.1 +/- 2.5 kJ mol(-1)), obtained 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 sigmatropic 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 principle favors the rearrangement which involves minimal motion of the migrating group. Hence, the barrier of a particular migration is determined by the properties of the carbon cycle rather than by the nature of migrating group.