Photocatalytic oxidation of dihydropyrimidinones using titanium dioxide suspension

Photocatalytic oxidation has been used for the oxidation of some ethyl 3,4-dihydropyrimidin-2(1 H )-one-5-carboxylates to their corresponding ethyl pyrimidin-2(1 H )-one-5-carboxylates using a TiO 2 /O 2 system under UV irradiation by a 400 W high pressure mercury lamp in acetonitrile. The results revealed that the order of photocatalytic activity for photooxidation was TiO 2 (anatase) > TiO 2 (rutile) . The effects of some other physicochemical parameters such as amount of photocatalyst, pH, solvent and time of irradiation were studied. The pyrimidinones were attained from the related dihydropyrimidinones after 2-4.5 h. The results showed the photo stability of this type of compound.

The potential of heterogeneous photocatalysis in chemistry is now a well-established procedure. 23The strong oxidizing power of the photogenerated holes of semiconductors (large band gap material), the chemical inertness and resistance to both photocorrosion and decomposition reactions, which plague other band gap materials (e.g., Si, GaAs, GaP, Inp, CdS, etc.), low cost and wide availability in addition to the nontioxicity of TiO 2 (anatase and rutile) and zinc oxide have made them superior photocatalysts. 24][25][26][27][28][29][30] With illumination of a semiconductor photocatalyst such as TiO 2 by photons whose energy is equal to or higher than their band-gap energy (for anatase, Eg = 3.23 eV), absorption of these photons occurs and the bulk of electron-hole pairs generate.These electron-hole pairs dissociate into free photoelectrons in the conduction band e - CB and photoholes in the valence band h P + VB (equation 1).(1) Some of the photoelectrons and photoholes can reach the surface of the photocatalyst and then an electron transfer proceeds towards adsorbed acceptor molecules and positive photoholes are transferred to adsorbed donor molecules.The photohole transfer corresponds to the cession of an electron by donor molecules to the photocatalyst.A chemical acceptor species can be photocatalytically reduced by e CB only if the conduction band potential of the photocatalyst is more negative than the redox potential of the acceptor species.In the same way, a chemical donor species can be photocatalytically oxidized by h B VB only if the valence band potential of the photocatalyst is more positive than the redox potential of the donor species.Both reactions should occur simultaneously because electroneutrality has to be maintained. 31,32n continuation of our previous studies on photochemical reactions, [33][34][35][36][37] we report here the photocatalytic oxidation of DHPMs.To the best of our knowledge this is the first report of the oxidation of 3,4-dihydropyrimidin-2(1H)-ones using photocatalytic system.

Results and Discussion
Our investigation showed us that UV irradiation is nesessary for the effective progress of the oxidation reactions and, without the selected oxidant and oxygen, oxidation did not occur.With this preliminary result, the optimization of important operational parameters was performed in the photooxidation reaction of 3,4-dihydropyrimidin-2(1H)-ones (Scheme 1).

Isolated yields
The amount of photocatalyst Using the better photocatalyst, the optimum amount of it required for photocatalytic oxidation was investigated.Thus, oxidations were run using various amounts of TiO 2 .As shown in Table 2, oxidation times were decreased by increasing the photocatalyst quantity, then reached the lowest time of completion and finally remained constant at a value of 40 mg.][40][41] This can be rationalized in terms of availability of active sites on the TiO 2 surface and the poor penetration of photoactivating light into the suspension.The availability of active sites increases with the suspension of photocatalyst loading, but the light penetration and hence the photoactivated volume of the suspension shrinks.Moreover, the increase in the time of oxidation at higher photocatalyst loading may be due to deactivation of activated molecules by collision with ground state molecules.Shielding by TiO 2 may also take place (equation 2).
Where TiO 2 * is the TiO 2 with active species adsorbed on its surface and TiO 2 is the deactivated form.

Effect of pH
The potentials of both valence and conduction bands of TiO 2 follow a pH dependence, according to equations 3 and 4, that show decreasing 59 mV per pH unit and consequently, the ability of electrons and holes to participate in redox processes is determined by the pH of the medium. 42hese equations are associated with TiO 2 in anatase form at 25 °C.
E CB = -0.05-0.059 pH (3) E VB = 3.15 -0.059 pH (4) The effects of varying of pH from 3-11 are summarized in Table 3. From the table we see that pH 7 was optimum in the presence of TiO 2 (anatase).It seems that, since TiO 2 usually has an isoelectric point of charge at a pH about ~7, its surface will gain a positive charge at pHs lower than ~7 via protonation (equation 5) and a negative charge when the material is suspended in a solution with pHs higher than ~7 via deprotonation (equation 6), respectively.At pHs lower than ~7, both the titled compounds and TiO 2 surface are present mostly in positively charged and protonated form and therefore repel each other.At pH 7, both the DHPMs and photocatalyst surface are mostly in neutral and in an un-protonated form and therefore the substrate molecules are more readily adsorbed on to the photocatalyst surface and the DHPMs oxidation reaction is favored. 43OH + H + ⇆ TiOH 2 + ( 5)

Photocatalytic oxidation of dihydropyrimidinones
In the optimum conditions, the photocatalytic reactions (Scheme 1) proceed efficiently in high yields.The results are summarized in Table 4. DHPMs with electron withdrawing substituents take longer reaction times than those with electron donor substituents. Yields refer to isolated and purified products.

Comparative results
To demonstrate the potential of our new approach, the presently obtained experimental results and the data acquired with the other methodologies are compared in Table 5.The yield/time ratios of the present method are better or comparable with others.

Conclusions
The reported work demonstrates that using the TiO 2 /O 2 photocatalytic system for dehydrogenation of 3,4-dihydropyrimidin-2(1H)-ones enhances the reaction rate compared to thermal oxidation.The results also show that TiO 2 in anatase form is more effective than its rutile form in the oxidation.Easy reaction progress, moderate reaction times and good to excellent yields are some advantages of this oxidative method.

Experimental Section
General Procedures.Chemicals were purchased from Merck, Fluka and Aldrich chemical companies.The commercially available TiO 2 powders were anatase in crystalline form with a surface area about 50 m 2 /g and primary particle size of 30 nm and rutile with approximate 0.2 micron in size and surface area of about 14.747 m 2 /g.3,4-Dihydropyrimidin-2(1H)-ones were prepared according to the reported procedures. 45Reactions were monitored by TLC.The products were isolated and identified by comparison of their physical and spectral data with authentic samples.IR spectra were recorded on FT-IR 680-Jasco-instrument model.

Table 1 .
The effect of photocatalyst type on the photocatalytic oxidation of DHPMs

Table 3 .
Effect of pH on the photocatalytic oxidation of DHPMs a

Table 5 .
Comparison of some our results with those reported in the litrature a