The analogy between CO and C(CN)2. Part 2.1 Structural properties of N,N-dialkylaminobenzamides and the analogously substituted 2-(phenylmethylene)propanedinitriles

The barrier to rotation around the carbon amino nitrogen bond in 2-[N,N-dimethylamino(phenyl)methylene]propanedinitrile (4a), 2-[N,N-diethylamino(phenyl)methylene]propanedinitrile (4b), 2-[(phenyl)pyrrolidin-1-ylmethylene]propanedinitrile (4c), N,N-dimethylbenzamide (5a), N,N-diethylbenzamide (5b) and (phenyl)pyrrolidin-1-ylmethanone (5c) were determined using dynamic 1H NMR spectroscopy. X-Ray crystal structures of 4a, 4b, 4c and 5a were determined, and quantum chemical calculations were carried out for 4a and 5a and for the transition structures for the rotation process of these two compounds. The barriers were generally lower than for the vinyl analogues 1 and 2. Although the calculated transition structures for 4a and 5a indicate some steric strain, the reduction of the barriers, especially for series 4, indicates that steric strain is more important in the ground state, thus raising its energy relatively more than for the transition state.