Synthesis, structure, and stereodynamics of an N,N-chiral "proton sponge"

1,8-Bis(N-benzyl-N-methylamino)naphthalene (2) and its hydrogen iodide salt [2H](+)[I](-) were synthesised from 1,8-diamino-naphthalene. The thermodynamic diastereomer ratios of (2) and [2H](+) in [D-7]DMF are 73.4:26.6 and 89.7:10.3 respectively at 293 K. The solid state structures of the major dl-(RNRN/SNSN) diastereomers were determined by single crystal X-ray diffraction. The minor diastereomers were shown to be the meso-(RNSN) forms by performing H-1-NMR NOE studies on isotopically desymmetrized 1-(N-benzyl-N-[C-13]-methylamino)-8-(N'-benzyl-N'-methylamino)naphthalene [C-13](-2) and the salt [C-13]-[2H](+). In [D-7]DMF at 298 K, the meso form of the free base 2 is 0.6 (+/- 0.07) kcal.mol(-1) less stable than the dl form (Delta H degrees = -0.64 (+/-0.03) kcal.mol(-1); Delta S degrees = -0.18 (+/- 0.13) cal.K(-1)mol(-1)) and the activation barriers for interconversion are ca. 14.2 (+/-0.4) and 14.8 (+/-0.4) kcal mol-l respectively. The effect of solvent polarity on the entropy and enthalpy change on approach to the transition state was studied. An approximate correlation with the solvent dielectric constant was found for both Delta H-double dagger and Delta S-double dagger and this may be ascribed to the development of a net dipole in the transition state due to substantially different hybridisation and geometry at the two nitrogen centres.