2.1 Synthesis of pyrazolines

We have also investigated the cycloaddition reaction of several pyridazine-3,6-diones 1a–d [18] with 2-diazopropane 2 [19]. The 1,3-dipolar cycloaddition of 2-diazopropane is, in each case, regiospecific. Unambiguous proofs for the obtained cycloadducts regiochemistry arised from their spectral data. However, regiochemical assignments of all adduct were deduced from their ¹³C-NMR spectra. Particularly the chemical shifts of C-6a (97.01–97.21 ppm) are in excellent agreement with those usually obtained when this quaternary carbon is attached to an oxygen atom (Table 1) [20].

2.2 Formation of cyclopropanes

The photolysis of an ethereal solution of the pyrazolines 3a–d through pyrex with a high pressure mercury arc lamp at 0–5 °C led to exclusive formation of bicyclo-cyclopropanes 4a–d.

As shown in Table 2, pyrazoline derivatives 3a–d were sonicated in EtOH in an ultrasonic cleaning bath afforded gem-dimeylcyclopropanes 4a–d.

The results were summarized in Table 2. Firstly, it can easily be seen that the irradiation of pyrazolines was carried out in good yield in ethanol under ultrasound irradiation (30 kHz, 150 W) within 15 min. The results show that the method to obtain bicyclo-cyclopropanes 4a–d under ultrasonic irradiation from pyrazoline derivatives 3a–d offers several significant advantages including faster reaction rates, higher purity, and higher yields. In comparison with conventional methods, the main advantage of ultrasound application is the significant decrease in the reaction times and milder experimental conditions. Thus, while the conventional method requires 35–45 min, ultrasonic irradiation affords the respective products in only 15–20 min (Table 2). These results support the idea that the energy provided by ultrasound significantly accelerates these reactions. The difference in yields and reaction times may be a consequence of the specific effects of ultrasound, in particular, cavitation, a physical process that creates, enlarges, and implodes gaseous and vaporous cavities in an irradiated liquid, thus enhancing the mass transfer [21] and allowing chemical reactions to occur. The creation of the so-called hot spots in the reaction mixture produces intense local temperatures and high pressures generated inside the cavitation bubble and at its interfaces when it collapses. In order to gauge the effect of different irradiation frequencies, the model reaction was performed under three different frequencies of 30, 40 and 50 kHz. The cyclopropane 4a yield for these frequencies was 95, 80 and 65% respectively. It seems that the lower frequency of ultrasound irradiation can improve the yield of cyclopropane derivatives.

Secondly, using the formation of 1,7,7-trimethyl-3-phenyl-3,4-diazabicyclo[4.1.0]heptane-2,5-dione 4a as a standard reaction, pyrazolines was sonicated under various sets of conditions in order to obtain optimal irradiation power conditions at a constant room temperature of 25 ± 1 °C and constant frequency 30 kHz (Table 3). By increasing the irradiation power from 100 to 200 W, the reaction time of 4a decreased from 30 to 10 min and the yield increased from 70 to 95%. The reaction time and yield of 4a did not change from 200 to 250 W, therefore, 200 W of ultrasonic irradiation was sufficient to push the reaction forward. The best yield for 4a was obtained by ultrasonic irradiation for 10 min at room temperature and 200 W.

Under stationary irradiations, pyrazolines 3a–d afforded the corresponding cyclopropanes 4a–d with total stereospecificity [22]. The stereochemistry of this ultrasonic product was determined from a NOESY spectrum. The cis relation ship existing between the Me groups and the proton H-6 in compounds 4a could be deduced from observation of an NOE effect between H-6 and the methyl protons (Fig. 1).

Whereas a diradical mechanistic pathway has been proposed by several groups [23] to explain the highly stereoselective evolution of pyrazolines under photolytic conditions, the course of the thermal decomposition of pyrazolines into cyclopropanes still remains controversial. Indeed, the hitherto reported stereochemical results from thermal decomposition of 1-pyrazolines are quite divergent and strongly dependent on the nature of the substituents at the ring. Concerted [24] or diradical [25] mechanisms have been postulated to explain the most usual pyrolytic processes proceeding without stereoselectivity. Bearing in mind the electron-withdrawing groups existing at the pyrazoline rings reported herein, our stereochemical results are consistent with the concerted thermal decomposition proposed by José et al. [26] of pyrazoline derivatives, which afford cyclopropanes by extrusion of nitrogen through a polar transition state, where the degree of bond breaking of the C(3a)-N bond is advanced over the bond breaking of the C(7)-N bond (Fig. 2).

4.1 Apparatus and analysis

IR spectra were recorded on a Perkin-Elmer IR-197 spectrometer. NMR spectra were obtained on a Bruker AC 300 spectrometer operating at 300 MHz for ¹H and at 75.64 MHz for ¹³C. Melting points were determined on a Buchi-510 capillary melting point apparatus. Coupling constants are given in Hz. Elemental analyses were performed on a PERKIN-ELMER 240B microanalyzer. Ultrasonication was performed in a GEX750-5 C ultrasonic processor equipped with a 3 mm wide and 140 mm long probe that was immersed directly into the reaction mixture. The operating frequency was 30–50 kHz and the output power was 0–750 W through manual adjustment. The temperature was controlled by a Buchi B-491 water bath at 25 ± 1 °C. All reagents were of commercial quality or purified by standard procedures.

4.2 General procedure for trapping of 2-diazopropane 2 with pyridazine-3,6-diones 1a–d