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Dynamic NMR and X-ray diffraction study of (N-B)-diphenyl(2-aminoethoxy) borane derivatives of ephedrines and pseudoephedrines

1997; Elsevier BV; Volume: 544; Issue: 2 Linguagem: Inglês

10.1016/s0022-328x(97)00193-9

1872-8561

Herbert Höpfl, Norberto Farfán, Dolores Castillo, Rosa Santillán, Rosalinda Contreras, Francisco J. Martı́nez-Martı́nez, Marcelo Galván, Rodolfo Alvarez, Lilia Fernández, Sabine Halut, Jean‐Claude Daran,

Advanced Synthetic Organic Chemistry

The preparation and characterization of various (N-B)-diphenyl-(2-aminoethoxy)boranes derived from ephedrine and pseudoephedrine derivatives (1b–6b) are reported: (N-B)-diphenyl(1-(R)-phenyl-2-(S)-methyl-2-aminoethoxy)borane (1b), (N-B)-diphenyl(1-(R)-phenyl-2-(R)-methyl-2-aminoethoxy)borane (2b), (N-B)-diphenyl[N-(R)-methyl-(1-(R)-phenyl-2-(S)-methyl-2-aminoethoxy)]borane (3b-trans), (N-B)-diphenyl[N-(S)-methyl-(1-(R)-phenyl-2-(S)-methyl-2-aminoethoxy)]borane (3b-cis), (N-B)-diphenyl[N-(S)-methyl-(1-(R)-phenyl-2-(R)-methyl-2-aminoethoxy)]borane (4b-trans). (N-B)-diphenyl[N,N-dimethyl-(1-(R)-phenyl-2-(S)-methyl-2-aminoethoxy)]borane (5b) and (N-B)-diphenyl[N,N-dimethyl-(1-(R)-phenyl-2-(R)-methyl-2-aminoethoxy)]borane (6b). The five membered N → B cyclic structures 1b–6b were assigned based on 1H-. 13C-, 11B- and 15N-NMR data and all compounds except for 3b-trans were subjected to X-ray diffraction analysis showing N → B bond lengths of 1.66(2) and 1.64(2)Å for 1b, 1.657(9) and 1.664(9))Å for 2b, 1.68(2) Å for 3b-cis, 1.66(1)Å for 4b-trans, 1.744(8)Å for 5b and 1.74(1)Å for 6b. The study of the intramolecular N → B coordination by means of dynamic NMR spectroscopy afforded ΔG‡ values of 67.9, 70.9, 64.8, 68.2, 49.7 and 52.7 kJ mol−1 for the dissociation of the N → B bond in compounds 1b–6b respectively. The results show that steric interactions between the substituents at the (2-aminoethoxy)borane ring determine the stability of the N → B bond as well as the nitrogen configuration. Theoretical calculations of the electrostatic charges for the boron and nitrogen atoms in 1b, 2b, 3b-cis, 3b-trans, 4b-cis, 5b and 6b show that the increase of positive charge on the nitrogen atom causes a shift to lower frequencies in the 15N NMR spectra.