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Computational Comparison of the Stability of Iminium Ions and Salts from Enals and Pyrrolidine Derivatives (Aminocatalysts)

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Formal exchange reactions between chiral secondary amines and iminium ions, as predicted by DFT calculations at the M06-2X level, are compared for the first time. The order of stability for these iminium species is thus established. The relative concentrations of the possible iminium ions in different media, if an enal is treated with two or more aminocatalysts, may be anticipated.


Abstract

The energies of CH2=CH−CH=N+R2+HNR*2→CH2=CH−CH=N+R*2+HNR2 reactions (exchange of propenal between two secondary amines) and of similar equilibria with cinnamaldehyde have been calculated and compared. Iminium ions from pyrrolidines with substituents that can help stabilize the positive charge are especially stable in the gas phase, as expected, whereas in very polar solvents the predicted order of stability (of their iminium ions) is: O-tert-butyldiphenylsilylprolinol>pyrrolidine>O-methylprolinol>2-tert-butylpyrrolidine>Jørgensen-Hayashi catalyst>2-tritylpyrrolidine>N,N-dimethylprolinamide>trimethylsilyl prolinate>3-triflamidopyrrolidine>methyl prolinate≫MacMillan-1 catalyst>MacMillan-2 catalyst. When ion pairs such as iminium tetrafluoroborates, in CHCl3, are compared, the order is similar. These data can be used to predict which iminium salts may predominate when two or more secondary amines and appropriate acids are added to conjugated carbonyl compounds.

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