Synthesis and Dynamic Properties of Cycloheptatrienyl(dipropyl)borane. Equilibrium with 7-Dipropylborylnorcaradiene

The synthesis of cycloheptatrienyl(dipropyl)borane (2a) was accomplished via the exchange reaction of trimethyl(cycloheptatrienyl)tin (6) and dipropylchloroborane. Compound 2a was found by NMR spectroscopy to equilibrate with its valence tautomer 7-exo-(dipropylboryl)norcaradiene (2b). The equilibrium between 2a and 2b was studied in detail experimentally by variable temperature NMR and theoretically by ab initio calculations of size-reduced molecular systems (2a and 2b, with methyl instead of n-propyl groups) at the B3LYP/6-311+G*//B3LYP/6-31G* + ZPVE level. Experimentally determined thermodynamic parameters of the equilibrium (ΔH = 2.4 ± 0.1 kcal mol-1; ΔS = −5.5 ± 0.3 cal mol-1 K-1) and the activation barriers at 228 K (ΔG⧧228(2a→2b) = 8.2 ± 0.1 kcal mol-1, ΔG⧧228(2b→2a) = 9.4 ± 0.1 kcal mol-1) are in reasonable agreement with the computed results (ΔH = 2.0 kcal mol-1, ΔS = −3.7 cal mol-1 K-1; ΔG⧧228(2a→2b) = 3.2 kcal mol-1 and ΔG⧧228(2b→2a) = 6.7 kcal mol-1). The computations also indicate that 7-endo...