Journal article

Authors list: Gridnev, ID; Tok, OL; Gridneva, NA; Bubnov, YN; Schreiner, PR

Publication year: 1998

Pages: 1034-1043

Journal: Journal of the American Chemical Society

Volume number: 120

Issue number: 5

ISSN: 0002-7863

DOI Link: https://doi.org/10.1021/ja9724699

Publisher: American Chemical Society

Abstract: 
The synthesis of cycloheptatrienyl(dipropyl)borane (2a) was accomplished via the exchange reaction of trimethyl(cycloheptatrienyl)tin (6) and dipropylchloroboran. 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 (Delta H = 2.4 +/- 0.1 kcal mol(-1); Delta S = -5.5 +/- 0.3 cal mol(-1) K-1) and the activation barriers at 228 K (Delta G(228)double dagger(2a-->2b) = 8.2 +/- 0.1 kcal mol(-1), Delta G(228)double dagger(2b-->2a) = 9.4 +/- 0.1 kcal mol(-1)) are in reasonable agreement with the computed results (Delta H = 2.0 kcal mol(-1), Delta S = -3.7 cal mol(-1) K-1; Delta G(228)double dagger(2a-->2b) = 3.2 kcal mol(-1) and Delta G(228)double dagger(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 H-1 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, Delta H double dagger = 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 and the computations show that the rotation about the B-C bond in 9 is hindered.

Harvard Citation style: Gridnev, I., Tok, O., Gridneva, N., Bubnov, Y. and Schreiner, P. (1998) Synthesis and dynamic properties of cycloheptatrienyl(dipropyl)borane. Equilibrium with 7-dipropylborylnorcaradiene, Journal of the American Chemical Society, 120(5), pp. 1034-1043. https://doi.org/10.1021/ja9724699

APA Citation style: Gridnev, I., Tok, O., Gridneva, N., Bubnov, Y., & Schreiner, P. (1998). Synthesis and dynamic properties of cycloheptatrienyl(dipropyl)borane. Equilibrium with 7-dipropylborylnorcaradiene. Journal of the American Chemical Society. 120(5), 1034-1043. https://doi.org/10.1021/ja9724699