Preparations, Crystal Structures, and Unusual Proton NMR Characteristics of Some Phthalimides

Article abstract of DOI:10.1021/jo00123a035

Protection of NH2 groups as phthalimides using a mixture of acetic acid, phthalic anhydride,and the relevant polyamine can take unusual routes.For diethylenetriamine the major product is diphthalimidodiethylammonium-hydrogen phthalate (DPDAH-HP).For triethylenetetraamine, ethylene migration products N,N-bis(2-phthalimidoethyl)piperazine (2) and N,N',N''-(nitrilotriethylene)trisphthalimide (3) were obtained from room temperature and refluxing reaction mixtures, respectively.The crystal structures of 2, 3, and DPDAH-HP were determined and reveal a series of stabilizing complementary interactions for these favored products (i.e., offset ?-? stacking and C-H...O and possibly C-H...N hydrogen bonds (2), electrostatic interactions between the amine and pyrrolic functionalities (3), and offset ?-? stacking and N-H...O hydrogen bonding (DPDAH-HP).The DPDAH-HP units stack along a screw axis parallel to the b-direction to yield a striking _n motif and are linked in the a and c directions by N-H...O bonds.Solution 1H NMR studies of 3 reveal unusual temperature (178-420 K) evolution of sharp aromatic proton resonances in various solvents.Typically, the spectra exhibit a spectacular temperature evolution from a sharp doublet of quartets at high temperature to complex second-order behavior and then a singlet.As the temperature is further lowered, a complex second order spectrum reappears followed by a doublet of quartets.The phthalimide-amine interaction energy, determined from temperature dependent NMR studies, is ca. 20 kJ/mol.Solid state 13C aromatic resonances are broadened by ?-? interactions in 2 and DPDAH-HP while two sets of 13C resonances are found for 3 as expected from the solid state structures.The hydrogen phthalate anion in DPDAH-HP exchanges rapidly with free phthalic acid (equilibrium constant, K ca. 4E2 L/mol); this phthalate exchange exhibits almost temperature independent behavior indicating that it is predominantly entropy driven.