Theoretical and experimental investigation of the formation of E- and Z-Aldimines from the reaction of methylamine with acetaldehyde

Abstract

A reação entre metilamina e acetaldeído em pentano a 230 K leva à formação das aldiminas Ee Z-, sendo o isômero Z-o produto principal e gerado sob controle cinético. O isômero E-é mais estável do que o Z-por 3.3 kcal/mol (∆G*). Os cálculos ab initio mostraram que a etapa bimolecular de adição ocorre por um processo concertado, com formação da ligação C-N e transferência de próton através de um estado de transição de quatro centros. O intermediário zwiteriônico, frequentemente invocado neste tipo de reação, não existe como um mínimo na superfície de energia potencial. Entretanto, a energia livre de ativação para o mecanismo concertado é muito alta, e não é capaz de explicar a cinética deste processo. O mesmo acorre com a etapa de eliminação de água para formar a imina, ou seja, possui uma barreira de ativação muito alta. Estes resultados implicam que tanto a etapa de adição, como a de eliminação, devem ocorrer por um mecanismo de catálise.O isômero Z-pode se interconverter para a forma E-através de uma inversão sobre o nitrogênio sp 2 . A 298.15 K, a energia livre de ativação para este processo foi calculada como sendo 27.0 kcal/mol, o que leva a um tempo de vida de quatro meses para este isômero.The reaction of methylamine with acetaldehyde in pentane at 230 K leads to formation of both E-and Z-aldimines, the Z-isomer being the main product and generated upon kinetic control. The E-form is more stable than the Z-form by 3.3 kcal/mol (∆G*). Ab initio calculations have indicated that a tetrahedral zwitterion intermediate is not formed, and the bimolecular process, forming the aminoalcohol intermediate, occurs through a four center transition state, with concerted formation of C-N bond and proton transfer. However, the predicted ∆G* ≠ is very high, indicating that the true mechanism for this step is not bimolecular. A catalyzed pathway must be actuating. The following step, unimolecular elimination of water, also presents a very high activation free energy. Similarly, a catalyzed mechanism is needed. The isomerization from the Z-form to the E-form through inversion of the nitrogen atom has a predicted ∆G* ≠ of 27.0 kcal/mol at 298.15 K, resulting in a lifetime of four months for the Z-isomer.

Reactions of the activated, rigid, α‐diazomethine group of 1,2,5‐thiadiazole 1,1‐dioxides with nitrogenated nucleophiles. Part III: aliphatic monoamines and phenylhydrazine

“…The E,E configuration is proposed as the most stable for benzilosazones according to published studies, 21 while for monoimines the relative stability of the E or Z configuration depends on the substituent groups. 22 As reported in the experimental part and discussed below, a Z,Z configuration was found for benzil bis(ethanolimine) (3a) and benzilosazone (4a).…”

Mechanisms of the formation of imines in aqueous solution and the effect of the pH: a theoretical analysis

“…Previous theoretical studies have not considered the solvent effect in the geometry optimizations. 13,34 However, the electronic energies are in agreement. Our Eel barrier (M06-2X) is 22.9 kcal mol -1 while Pliego et al have reported a value of 26.7 kcal mol -1 using single point MP2 calculations on the gas phase Hartree-Fock geometries.…”

“…Our Eel barrier (M06-2X) is 22.9 kcal mol -1 while Pliego et al have reported a value of 26.7 kcal mol -1 using single point MP2 calculations on the gas phase Hartree-Fock geometries. 34 The solvent effect is very important to stabilize this TS, because the gas phase G ‡ is high, 34.4 kcal mol -1 . Nevertheless, we have found that in aqueous solution TS1 does not connect methylamine and acetaldehyde reactants to MS3 (carbinolamine).…”

Unusual Reactivity Patterns of 1,3,6,8-Tetraazatricyclo-[4.4.1.13,8]-dodecane (TATD) Towards Some Reducing Agents: Synthesis of TMEDA

“…The presence of an excess of formic acid suggests that both ionic species can be present in the reaction mixture. However, although it is known that the formation of open species is favoured in the ring-chain tautomeric equilibrium of aminals under acid conditions [29][30], the lifetime of aliphatic iminium ions in an aqueous solution is quite short [31].…”