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 ± O.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-(dimethylboryl)norcaradiene (2c) is 7.6 kcal mol^-1 less stable than the exo-isomer 2b due to more favorable overlap of the unoccupied boron 2p AO with the Walsh orbital of the three-membered ring moiety in 2b. Line shape analyses together with 2D ¹H EXSY data for the equilibrating system of 2a and 2b allowed the detection of a [1,7] sigmatropic shift in 2a at temperatures above 293 K. This is confirmed by the computations identifying the [1,7] B shift to have the lowest activation enthalpy (2a, ΔH@? = 18.4 kcal mol^-1). In the unsymmetrical deuteriopyridine complex 9, the empty boron 2p AO interacts strongly with the nitrogen lone pair. This reduces the stabilization of the exo-norcaradiene skeleton, yielding only the 'pure' cycloheptatrienyl form 9 in the NMR spectra. Both NMR data acid the computations show that the rotation about the B-C bond in 9 is hindered.