Synthesis and dynamic properties of cycloheptatrienyl(dipropyl)borane. Equilibrium with 7-dipropylborylnorcaradiene

Ilya D. Gridnev

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26 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)1034-1043
Number of pages10
JournalJournal of the American Chemical Society
Issue number5
Publication statusPublished - 1998 Feb 11
Externally publishedYes

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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