Syntheses, NMR study and stereochemistry of new P-H tricyclophosphoranes

Four new tricyclophosphoranes 1b-4b were synthesized derived from N,N'-bis[2-hydroxy phenyl]ethylenediamine (1a), N,N'-bis[2-hydroxyphenyl]oxamide (2a), N,N'-bis[(-)-norephedrine]ethylene (3a), N,N'-bis[(-)-norpseudoephedrine]oxalyl (4a), The syntheses of compounds 3b and 4b were completely stereoselective giving only one epimer in each case (epimer helix Delta, 3b; helix Lambda, 4b). For both the phosphorus configuration was established. The phosphorus atoms in 1b-4b adopt a trigonal bipyramid geometry with an oxygen and a nitrogen atoms in apical positions as deduced from P-31, C-13, H-1 and H-1/C-13 HETCOR NMR studies and confirmed by the x-ray diffraction structure of 3b, Compound 1b reacts with BH3-THF giving the N-->BH3 adduct 1c; borane coordinates to the tetrahedral apical nitrogen atom. Compound 3b reacted with BH3 giving two isomeric adducts, 3c and 3c'. The main product 3c came from attack of BH3 on the equatorial nitrogen followed by epimerization of the phosphorus atom giving the molecule with the N-->BH3 group in an apical position and helicoidal structure Lambda. The minor isomer 3c' came directly from addition of BH3 on an apical nitrogen atom giving the Delta helix.