Imines protonation

Step 5 The nitrogen stabilized carbocation is the conjugate acid of the imine Proton transfer to water gives the imine... 

Derivatization of the optically active aldehydes to imines has been used for determination of their enantiomeric excess. Chi et al.3 have examined a series of chiral primary amines as a derivatizing agent in determination of the enantiomeric purity of the a-substituted 8-keto-aldehydes obtained from catalysed Michael additions. The imine proton signals were well resolved even if the reaction was not completed. The best results were obtained when chiral amines with —OMe or —COOMe groups were used [2], The differences in chemical shifts of diastereo-meric imine proton were ca. 0.02-0.08 ppm depending on amine. This method has been also used for identification of isomers of self-aldol condensation of hydrocinnamaldehyde. 

The values of 3/(NH,H) coupling constant observed for imine proton can be helpful in detection of the proton transfer processes and determination of mole fractions of tautomers in equilibrium. For NH-form, this value is close to 13 Hz, lower values usually indicate the presence of tautomeric equilibrium. It should be mentioned that the values below 2.4 Hz have not been reported. The chemical shift of C—OH (C-2 for imines, derivatives of aromatic ortho-hydroxyaldehydes or C-7 for gossypol derivatives) carbon to some extent can be informative, however, this value depends on type of substituents and should be interpreted with caution. 

Because an FI catalyst has a pair of non-symmetric phenoxy-imine ligands, it potentially possesses five isomers stemming from the coordination modes of ligands. Zr-, Ti-, and Hf-FI catalysts 1-3 display three sets of signals in XH NMR, attributed to the imine proton, suggesting that these FI catalysts exist as isomeric mixtures in solution, which is probably an intrinsic feature of FI catalysts. On the basis of the symmetry of the possible isomers A-E (Fig. 10), as well as the relative formation... 

The excited-state deprotonation of the hydroxyl group of 6HQ(C) in acid aqueous solution is notably faster (2.2 ps) than the excited-state protonation of the nitrogen atom of 6HQ(A) in alkaline solution. It suggests that, in neutral solution, the excited state of the neutral form 6HQ(N) should undergo hydroxyl deprotonation prior to imine protonation. Preliminary results at pH 7 confirm indeed [6] that the excited tautomer is produced from 6HQ(N ) via two consecutive proton transfers in the order 6HQ(N )->6HQ(A )- 6HQ(T ). 

HNMR spectra of azirines which are unsubstituted at C-2 (69TL4073). The imine protons are commonly observed at ca. 10 p.p.m. whereas those of normal acyclic aldimines are observed in the vicinity of 8 p.p.m. imine (12), for example, displays a singlet at 7.63 p.p.m. (62CJC882). A similar but less pronounced shielding effect of ca. 0.4 p.p.m. hat been observed for the protons at C-3 compare, for example, the resonances assigned to the azirine (13)... 

Mechanistically we assume that the reaction proceeds via initial imine protonation... 

The rates of the various steps are a function of the pH of the solution, the basicity of the imine, and the reactivity of the aldehyde. Imine protonation enhances reactivity toward either water or hydroxide ion as nucleophiles. Ai-Protonation in the tetrahedral intermediate makes the amine a better leaving group. The zwitterionic intermediate... 

---