N-[5,5-dimethyl-2(5H)-thiophenyliden]amines and N-(1-thiaspiro[4.5]dec-3-en-2-yliden)amines: Synthesis and isomerism

A number of N-(5,5-dimethyl-2(5H)-thiophenyliden)amines and N-(1-thiaspiro(4.5)dec-3-en-2- yliden)amines, existing exclusively in one isomeric form, have been obtained in good to excellent yields by reaction of lithiated 1,1-disubstituted 1,2-dienes with isothiocyanates followed by treatment with t-BuOH and t-BuOK in DMSO. Investigation of the isomeric structures using 1H, 13C, 15N, 2D NOESY NMR spectroscopy and quantum-chemical calculation (ab initio HF/6-31G*) of the total energies of the fully optimized geometries indicated that for both N-(5,5-dimethyl-2(5H)-thiophenyliden)methanamine and N-(1- thiaspiro(4.5)dec-3-en-2-yliden)methanamine the Z-isomers are more stable, the energy differences with respect to E-isomers being 3.37-3.87 kcal/mol. It was established that the chair conformer with axial orientation of sulfur of (Z)-N-(1-thiaspiro(4.5)dec-3-en-2- yliden)methanamine the has the lowest energy of all possible conformers.