The chemistry of N,N ′-dimethylformamidine. III. Tautomerism and hindered rotation

Abstract

Proton nmr and infrared studies of neat N,N′-dimethylformamidine (DMFA) have shown that the cis isomer 1 is present exclusively and that it undergoes tautomerism. At 25 °C, k taut = 8.2 × 10 2 s −1 , E a = 41.5 ± 1.5 kJ mol −1 , ΔH ≠ = 39.1 ± 1.5 kJ mol −1 , and ΔS ≠ =−57.5 ± 5.0 J mol −1 K −1 . The tautomerism is proposed to occur through a hydrogen-bonded cyclic dimer. The effects of solvent on the tautomeric rate in DMFA have also been investigated for CD 3 NH 2 and CDCl 3 solutions.Potassium N,N′-dimethylformamidide (PDMFA) in CD 3 NH 2 does not exhibit hindered rotation about the single C—N bond on the nmr lime scale over the temperature range −56 to 25 °C. However, protonated DMFA undergoes hindered rotation about this bond in highly acidic media (97 wt.% H 2 SO 4 at 25 °C, k rot = 0.16 × 10 2 s −1 , E a = 35.4 ± 0.8 kJ mol −1 ). The acid catalysis is interpreted as occurring through formation of the diprotonated species. The two nitrogens in protonated DMFA have different base strengths.