Theoretical Study of the Proton Transfer in Enaminones

Problem statement: Hydrogen bonding has a vital rule to unrevealed the nature of many different interactions both in gas phase and conden sed media thus it is one of the most important concepts in chemistry. Enaminoes with their ability to form intrahydrogen bonded cheated rings represent one of the suitable compounds to study su ch concept. Approach: The three possible tautomers of eneminones were fully optimized at sev eral theoretical levels including B3LYP with the 6-31+G(d,p), 6-311++G(d,p) and aug-cc-pVDZ basis se t as well as MP2/6-311++G(d,p) level of theory. Then a search for the possible transition s tate between the two most stable tautomers; the ketamine and the enolimine was done at the B3LYP/6311++G(d,p) level of theory. Results: The ketamine is the most stable while the diketo is the less stable within the tautomeric mixture and the proton transfer occurs through a transition state t hat is energetically resemble the enolimine tautome r. Conclusion: The interconversion of the tautomers is preferable in the enolimine → ketamine direction.


INTRODUCTION
The term enaminone is used to indicate any compound containing the conjugated system N-C=C-C=O. It may be mono-enamine of 1,3-diketone or 3keto-ester: (Saeed et al., 2011). They are versatile synthetic intermediates that combine the ambident electrophilicity of enones with the ambident nucleophilicity of enamines. They are typical push-pull systems in which the amine group pushes and the carbonyl pulls electron density and have the ability to function both as ambident nucleophiles and as ambident electrophiles (Michael et al., 1999;Almazroa1 et al., 2004;Ruffino and Zanetti, 2009). The carbonyl group, conjugated with the enamine moiety, gives this system enough stability to be easily prepared, isolated and stored under atmospheric conditions at room temperature (Junior et al., 1999;Shawali, 2010). Enaminones are known to possess a variety of medicinal properties including anticonvulsant, antimalarial, antiinflamatory, antitumor and cardiovascular effects (Chaaban et al., 1979;Purnami et al., 2009). Enaminones are an important class of organic synthetic intermediates for the synthesis of a variety of heterocycles and pharmaceutical compounds. Their basic structural units, N-C=C-C=O, are responsible for the synthesis of many therapeutic agents of both natural and synthetic sources, including taxol, anticonvulsants, anti-inflammatory agents and ducarmycin classes of anti-tumor agents, as well as quinoline antibacterial and quinoline antimalarial agents (Hogenkamp et al., 2007;Idafiogho et al., 2006;Ghandi and Jamea, 2011).

MATERIALS AND METHODS
Full geometry optimizations were carried out using Density Functional Theory (DFT) method with functional hybrid B3LYP with the 6-31+G(d,p), 6-311++G(d,p) and aug-cc-pVDZ basis sets. Post Hatree-Fock MP2 method was also used with 6-311++G(d,p) basis set. The search for imaginary frequencies that indicates the transition states was done by calculating the frequencies of the geometry optimized structures at the same level of theory used to the geometry optimization. No imaginary frequencies was found in the calculated spectra of the studied tautomers indicating a that the structures at a global minima, while the spectra of the transition states were characterized by a single imaginary frequency.

RESULTS AND DISCUSSION
Enaminones could exist as an equilibrium mixture of three tautomers namely ketoamine, enolimine and diketo. The optimized structures of the three tautomers at the B3LYP/aug-cc-pVDZ level of theory are shown in Fig. 1.
Due to the lack of crystallographic data about the structure of the ketamine tautomer its structure is calculated in this study at several levels of theory for comparison ( Table 1).
The predicted bond angles and bond length at the B3LYP/6-31+G(d,p) level of theory are identical to those calculated at the same level in a previous study (Gilli et al., 2000;Eberlin et al., 1990). Table 1 shows that the calculated properties depend on the theoretical level used for optimization but in general the results are almost indicate a strong conjugation within the sexmembered chelated ring of the molecule. This is obvious from the deviation of the calculated bond lengths from the values of the pure single or double bonds. The calculated C=O and C=C bonds at the B3LYP/6-311++G(d,p) level are 1.235 and 1.371 Å and are longer than their values in aliphatic ketones and ethylene which are while the bond C-C and C-N are shorter than their value in alkanes and aliphatic amines.
With exception to the MP2 case which predicted the ebolimine and the dike to tautomers as degenerate species with total energies -245.8598864 and -45.8608857 a.u. respectively (Table 2), the predicted total energies suggest that the embolic forms (the ketamine and enolimine) are more stable than the diketo. This is in agreement with the experimental studies. This could be explained on the basis that the enolic forms are stabilized by the conjugated intrahydrogen bonded chelated ring. Tabel 2 shows that within the enolic forms the hetamine is the most stable.
The total energies calculated for the ketamine, enolimine and diketo tautomers at the CBS-QB3 method are -247.3616027, -247.3493333 and -247.3365632 respectively. Accordingly the proton transfer calculations in this study are confined to the ketamine and enolimine tautomers only.
The infrared spectra of the ketamine are shoen and interpreted in Table 3. The number of frequency modes (24) is in consistent with that predicted by the rule 3N-6. The N-H stretching frequencies are calculated to be 3704 and 3471 cm −1 for the free and the intrahydrogen bonded N-H groups respectively. This in agreement with finding that hydrogen bonded groups have frequencies at lower wavenumbers compared to those of free groups (Kiss et al., 2000). With exception of the amni group frequencies which are over istimated at the B3LYP/6-311++G(d,p) level, the predicted other frequencies are within the experimental ranges.
Proton transfer: Figure 2 traces the gradual structural changes as the ketamine tautomer transferred to the inolimine by a proton transfer through the Transition State (TS) as follows.
The length of the carbonyl group (calculated at B3LYP/6-311++G(d,p) level of theory) changes from 1.235 Å in the ketamine through 1.292 Å in the TS to 1.322 Å in the enolimine. The length of the imino group changes from 1.343 Å in the ketamine through 1.310 Å in the TS to 1.294 Å in the enolimine. In the same time the bonds C1-C2 and C2-C3 changes from approximately single and double bonds respectively to approximately double and single bond through intermediate values on going from the ketamine to the enolimine. The bonds between transferred proton and the oxygen and nitrogen atoms changed in the direction of the reaction coordinates. In the ketamine it is bonded to the nitrogen atom through a 1.017 Å bond, while in the transition state this linkage becomes 1.310 Å and the proton being in the mid distance between the nitrogen and the oxygen atoms. Finally, in the enolimine the N-H bond break and O-H bond with a length of 1.009 Å is formed. It worthy to note from Fig. 2 that the bond lengths of C-O and C-N in the TS are almost similar with values 1.292 and 1.310 Å. This is also true for the bonds C1-C2 and C2-C3.
To ascertain that the intermediate structure (TS) is a transition state the search for the single imaginary frequency is done by calculating the frequencies at the same level of theory and found that the calculated frequencies of the TS species contains a signal at -1196 cm −1 which indicates the presence of a transition state. From Figure 2 it clear that the transition state is near the enolimine as the stabilization energy between this tautomer and the transition state is 7.3 kJ.mol −1 only. In contrast the stabilization energy with respect to the ketoamine is 41.4 kJ.mol −1 . The calculated total energies of the species involved in the proton transfer at different levels of theory are gathered in Table 3.
From Table 4 it is clear that the calculated energies depend on the theoretical level used. Figure 3 sketches the transition process showing the relative energies of the different species involved in the process.      3125329 -247.2997625 -247.2977806 B3LYP/6-311G (d,p -247.3616027 -247.3493332 -247.3464642 B3LYP/6-311+G (d,p -247.3700201 -247.3570366 -245.8502458 B3LYP/6-311++G (d,p -247.3701168 -247.3571141 -247.3543292 The same trends are found when the study of the proton transfer is conducted on a phenyl substituted enaminone. The introducing a phenyl ring to the enaminone chelated ring reduces the C=O bond length from 1.292-1.250 Å due to the conjugation between the two groups. The other effect of introducing the phenyl ring is that it reduces the relative energies between the ketoamine and the enolimine with respect to the transition satate. The stabilizing energies with respect to the ketamine and the enolimine are 38.9 and 5.1 kJ.mol −1 compared to 41.4 and 7.3 kJ.mol −1 for the case of the substituted enaminone. This implies that the proton transfer in aromatic enaminones is more feasible than non substituted ones.
We try to study the proton transfer process in a methyl substituted enaminone at DFT and MP2 methods with several basis sets but in all cases the transition state converged to the enolimine.

CONCLUSION
The ketamine is the most stable tautomer in the equilibrium mixture of the enaminones while the diketo is the less stable. The interconversion between theketamine and the enolime tautomers is preferable in the enolimine → ketamine direction. In addition the presence of an aromatic substituent makes the proton transfer in these compounds mare feasible by reduction of the stabilization energy between the tautomers and the transition state.