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

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 (Ib), (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 H-1-, C-13-, B-15- and N-15-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)Angstrom for 1b, 1.657(9) and 1.664(9)Angstrom for 2b, 1.68(2)Angstrom for 3b-cis, 1.66(1)Angstrom for 4b-trans, 1.744(8)Angstrom for 5b and 1.74(1)Angstrom for 6b. The study of the intramolecular N --> B coordination by means of dynamic NMR spectroscopy afforded Delta G(double dagger) 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 Ib, 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 N-15 NMR spectra. (C) 1997 Elsevier Science S.A.