Iminol-amide

In the third transition state (TS3), the neutral catalyst is recovered by transferring the proton back from the catalyst to the substrate. In other words, the (former) azlactone ether oxygen atom deprotonates the tertiary ammonium ion. For proton transfer, again an LBHB is formed (N-0 distance 2.479 A, <(0,H,N)=166.2°). In the product complex, the catayst is neutral and the A-acylamino acid ester is bound in its iminol form to the catalyst (Product(iininol)). Finally, an additional 66.6 kJ moF are gained by the subsequent iminol-amide tautomerization (Product(ainide)) (Fig. 1). 

These compounds exempHfy a type of vinylogous iminol-amide tautomerism in which both tautomers, except for 23, are conjugated chelates (Figure 11.8). Compounds 19-23 have been examined by both experimental and computational means [32], the LSER study being pubhshed separately [33]. Dudek and Dudek [34] studied tautomerism in some Schiff bases, one of which is 24, but while these... 

The cationic species 4 thus formed reacts with water to give the iminol 5, which tautomerizes to a more stable amide tautomer, the N-substituted carboxylic amide 2. Those steps correspond to the formation of amides by the Schmidt reaction. A side reaction can give rise to the formation of nitriles. 

Since the second solvent pair fall within the poor hydrogen bonding group of solvents, increased basicity of the organic base in these solvents would be consistent with the observed behavior. Based on the model compound studies, indications are that the base-catalyzed imidization process may involve a two-step mechanism, Jee Scheme 23. The first step corresponds to the complete or partial proton abstraction from the amide group with the formation of an iminolate anion. Since this iminolate anion has two possible tautomers, the reaction can proceed in a split reaction path to either an isoimide- or imide-type intermediate. Although isoimide model reactions indicate an extremely fast isomerization to the imide under the conditions employed for base-catalysis, all indications to date are that it is not an intermediate in the base-catalyzed imidization of amic alkyl esters. 

The [(dien)Pt(OH2)] complex reacts with urea or 1,1-dimethylurea (dmu) in acetone to initially give the O-bound compound (39) as the kinetically preferred product (Scheme 6). This compound then rearranges (t j 5h for urea and 18 min for dmu at 22°C) to the slightly more thermodynamically favored N-bound linkage isomer (Kj o = [N isomer]/[0 isomer] 10 for urea and 3 for dmu) (87, 88). An X-ray crystal structure confirms the presence of the amide tautomer 40b, not the iminol tautomer 40b (Fig. 7). Apparently, in the urea complex the O atom is less basic than the N atom. 

H NMR spectroscopy. Despite the disadvantage of a slow time scale which, in contrast to vibrational spectroscopy does not permit a direct observation of individual tautomers (at least not of tautomers with amide, iminol structures), NMR spectroscopy proved to be a powerful technique for the investigation of kinetically inert metal complexes of tautomers. Its usefulness is based on the following arguments ... 

The search for a compound RjSiX strongly electrophilic (but yielding a nonacidic HX) led to the silyl iminol ethers, the prototype of which is A, 0-bis-trimethylsilyla-cetamide (BSA) [186]. Reactions with proton donors such as alcohols are thermodynamically driven by the formation of the carbonyl group of the corresponding amide ... 

Reaction between two amide groups in the tautomeric iminole form. 

Fig. 9 IRMPD spectra black) compared with calculated CS spectra red) and calculated Iminol spectra blue) for several M PhePhe complexes. The red bar marks the expected position of the Amide II peak of the CS complex, while the blue bar marks that of the iminol COH bending mode. Reproduced with permission from [109]...

The NR-CAD and CAD spectra of formamide and the carbene are essentially indistinguishable, providing strong evidence that the amide and carbene tautomers retain their structures upon NR and do not undergo any rearrangements. In sharp contrast, NR-CAD and CAD spectra of the iminol ion differ markedly from each other the NR-CAD spectrum can be interpreted as a combination of the CAD spectra of genuine amide and iminol ions, in agreement with a partial tautomerization of the neutral iminol to the amide. 

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