Use of Graphite Oxide and Graphene Oxide as Catalysts in the Synthesis of Dipyrromethane and Calix[4]pyrrole

Graphite oxide and graphene oxides have been used as solid catalysts for the synthesis of 5,5-dialkyldipyrromethanes and calix[4]pyrroles in organic and aqueous solutions at room temperature.


OPEN ACCESS
One of the most important methods developed for the mass production of graphene is exfoliation of graphite oxide in aqueous solution to single layers of graphene oxide. This exfoliation can be performed in aqueous solution alone or in presence of surfactants [30], polymers [31,32], ionic liquids [33] and polar solvents [34,35]. Aqueous suspensions of graphene oxide have been used in the oxidation of alcohols and cis-stilbene and the hydration of various alkynes to the corresponding aldehydes, acids and ketones under mild conditions [36,37]. The electrical conductivity of the graphene oxide differs from that of pristine graphene, hence various attempts have been made to deoxygenate graphene oxide to the maximum extent to regain the aromaticity and electrical conductivity. The hydroxyl and the epoxy groups and some carboxylic groups can be removed by reduction of graphite oxide by hydrazine, ascorbic acid and other reducing agents [38][39][40].
Porphyrinogens and more stable calix [4]pyrroles are important tetrapyrrolic macrocycles used in biosynthesis of porphyrinoids [41], supramolecular chemistry [42], anion receptor [43,44] and material chemistry [45][46][47]. They are synthesized by condensation of pyrroles with dialkylketones in the presence of aqueous acids [48,49], Lewis acids [50,51] and solid acids including zeolites [52], molecular sieves [53,54] and Amberlyst-15 [55] under different reaction conditions. Although various solid acids have been used in the synthesis of different calix [4]pyrroles, to the best of our knowledge graphite oxide and graphene oxides have not been used for this purpose. Hence, we report the synthesis of selected calix [4]pyrroles in aqueous suspensions of graphite oxide and graphene oxides under different reaction conditions (Scheme 1).

Results and Discussion
The reaction of acetone (2a) with pyrrole (1) in a suspension of graphite oxide (GO) in dichloromethane at room temperature gave dipyrromethane (3a) as the major product. The structure of 3a was confirmed by spectroscopic data, mixed melting point [56] and comparison of HPLC retention times with an authentic sample ( Table 1). The reaction of 1 with cyclohexanone (2b) gave the corresponding dipyrromethane 3b in 70% yield. An analogous condensation of pyrrole and 2b in dichloromethane in the presence of the solid acid zeolite HY gave dipyrromethane 3b in 63% yield, along with other products, whereas the same reaction in the presence of HZSM in dichloromethane gave dipyrromethane 3b and calix [4]pyrrole 4b in 53 and 11% yield, respectively [52]. The reaction of 1 with 2a in the presence of Al-MCM-41 zeolite gave 3a and 4a in 12 and 70% yield, respectively [52]. The reaction of the above in the presence of Amberlyst TM -15 gave 4a and N-confused calix [4]pyrrole in 83 and 14% yield, respectively [55]. The high yield of 4a in acetone may be explained by initial formation of 3a which on subsequent reaction with acetone leads to calix [4]pyrrole. This was also confirmed by reaction of 3a with 2b with the formation of 5 in 17% yield. (Table 1) [52]. The reaction of 1 and 2a was performed in other polar solvents and the results are reported in Table 1. A moderate yield of calix [4]pyrrole 4 was obtained when the reaction was performed in methanol or acetonitrile ( Table 1). The graphite oxide is stable in organic solvents and it may be used several times without much loss of catalytic activity (Table 1, entry 14).  [4]pyrrole; d reaction were performed in 10h in order to compare the results; e N-confused calix [4]pyrrole; f Isolated yield; g Catalyst was reused over three runs.
The formation of calix [4]pyrrole 4a in excess acetone may be explained by initial formation of 3a which on subsequent reaction with acetone leads to 4a. The reaction of 3a with cyclohexanone lead to cailx [4]pyrrole 5 (Scheme 1).
The reaction of 1 and 2a in the presence of graphite oxide suspended in aqueous solution gave 3a as major product in 92% yield ( Table 2). The reaction of pyrrole and ketones regardless of the ratio of starting compounds, forms the dialkyldipyrromethanes in the presence of weak acid in aqueous solutions [48]. The increase in the ratio of acetone decreases the yield of 3a and a minor amount of 4a was observed (Table 2). The reaction of 1 and 2a in the presence of graphite oxide and SDS gave dipyrromethane (48%), calix [4]pyrrole (35%) and N-confused calix [4]pyrrole in 10% yield at room temperature in 1.5 to 3 h (Table 3). The addition of SDS to graphite oxide in aqueous solution forms the corresponding acid which catalyzes the formation of dipyrromethane in low and calix [4]pyrrole in high yields. The graphite oxides have been dispersed in SDS and poly(sodium 4-styrenesulphonate) in aqueous solution [32]. The reactions of 1 with 2a in the presence other surfactants and salts are given in Table 3. The reaction of 1 and 2a in the presence of reduced graphene oxide [57,58] in aqueous solution gave 3a as major product and 4a as minor product (Table 4). The reduced graphene oxide agglomerizes in aqueous solution, but in the presence of organic solvents [59] and ionic liquids [60] it may be dispersed in aqueous solution. Suitable chemical modifications of reduced graphene oxides have been used to solubilize the graphene oxides in aqueous [60] and organic solvents [61]. The above results indicate that graphite and graphene oxide may be used as solid acids in various organic transformations.

General
The infrared spectra (IR) were recorded on a Perkin-Elmer FT-1710 spectrophotometer. 1 H-NMR spectra were recorded in CDCl 3 on a Bruker Avance 400 MHz spectrophotometer with TMS as internal standard. UV-Vis spectra were recorded on Perkin-Elmer Lambda 35 spectrophotometer. Raman spectra were recorded on inVia Renishaw Raman spectrophotometer using a green (514 nm) laser. Powder XRD were recorded on a Bruker Discover 8 X-ray diffractometer. HPLC analysis was performed on a Waters 2998 using a Waters PAH C 18 HPLC column (4.6 × 250 mm) and methanol as the eluent. Starting materials such as pyrrole (1) and acetone (2) were obtained from Acros USA and distilled immediately prior to use. The experimental operations were performed under ambient conditions. Neutral alumina was used for all the chromatographic purifications. Graphite powder was obtained from Alfa Aesar, USA.

Graphite Oxide
KMnO 4 (9 g) was added in portions to a cooled (0 °C) solution of conc. H 2 SO 4 (69 mL) containing graphite (3 g) and NaNO 3 (1.5 g). The mixture was stirred at room temperature for 5 days. Distilled water (138 mL) was added slowly to the reaction mixture while the temperature was kept well below 98 °C for 3 h. The resultant bright-yellow suspension was diluted and a solution of H 2 O 2 (6 mL, 30%) was added dropwise. The reaction mixture was centrifuged and washed to remove the remaining salts. The wet graphite oxide was dewatered by vacuum drying (50 °C

Graphene Oxide
Aqueous colloids of single layer graphene oxide nanosheets were produced by exfoliation of graphite oxide dispersed in deionized water with ultrasonication [32].

Preparation of Reduced Graphene Oxide
Graphite oxide (75 mg) was dispersed in water (75 mL) with sonication. Sodium borohydride (600 mg) was added to the GO dispersion after the pH being adjusted to 9-10 with 5 wt% sodium carbonate solution. The mixture was then kept at 80 °C for 1 h under constant stirring. During reduction, the dispersion turned from dark brown to black accompanied by outgassing. UV-Vis

The Reaction of Pyrrole and Acetone in Organic Solvents
Equimolar amounts of pyrrole (14.4 mmol) and acetone (14.4 mmol) were taken up in dichloromethane (20 mL). Graphite oxide (10% w/w) was added in portions to the reaction mixture, which was stirred at ambient temperature for the appropriate time (as indicated in Table 1). The reaction progress was monitored by thin layer chromatography (TLC) with petroleum ether-chloroform. After the completion of reaction, the catalyst was removed by filtration and washed thoroughly with CH 2 Cl 2 to dissolve all the contents. The filtrate was concentrated to give the crude product, which was subjected to column chromatography over neutral alumina eluting with petroleum ether-chloroform to afford pure calix [4]pyrrole and further elution of column gave dipyrromethane. The reaction mixture was analysed by HPLC. The HPLC yields are given in Table 1.

The Reaction of Pyrrole and Acetone in Aqueous Solution
Equimolar amounts of pyrrole (14.4 mmol) and acetone (14.4 mmol) were taken up in water (20 mL). Graphite oxide (10% w/w) was added in portions to the above reaction mixture that was stirred at ambient temperature for the appropriate time ( Table 2). The reaction progress was monitored by thin layer chromatography (TLC) with petroleum ether-chloroform. After completion of reaction, the catalyst was removed by extraction with CH 2 Cl 2 . The organic layer was separated dried over sodium sulfate, filtered and the filtrate was concentrated to give the crude product, which was subjected to column chromatography over neutral alumina eluting with petroleum ether-chloroform to afford pure calix [4]pyrrole and further elution of column gave dipyrromethane. The reaction mixture was analysed by HPLC. The HPLC yields are given in Table 2.

The Reaction of Pyrrole and Acetone in Aqueous Solution in Presence of Suphonate Salts
Pyrrole (14.4 mmol), acetone (14.4 mmol) and surfactant/salt (0.034 mmol) were taken up in water (20 mL). Graphite oxide (10% w/w) was added in portions to the reaction mixture, which was stirred at ambient temperature for the appropriate time ( Table 3). The reaction progress was monitored by thin layer chromatography (TLC) with petroleum ether-chloroform. After the completion of reaction, the catalyst was removed by extraction with CH 2 Cl 2 . The organic layer was separated dried over sodium sulfate, filtered and the filtrate was concentrated to give the crude product, which was subjected to column chromatography over neutral alumina eluting with petroleum ether-chloroform to afford pure calix [4]pyrrole and further elution of column gave dipyrromethane.The reaction mixture wasanalysed by HPLC. The HPLC yields are given in Table 3.

The Reaction of Pyrrole and Acetone Catalyzed by Reduced Graphene Oxide under Different Conditions
Equimolar amounts of pyrrole (14.4 mmol) and acetone (14.4 mmol) were taken up in different solvents (20 mL). Graphene oxide (10% w/w) was added in portions to the above reaction mixture that was stirred at ambient temperature for the appropriate time ( Table 4). The reaction progress was monitored by thin layer chromatography (TLC) with petroleum ether-chloroform. After completion of reaction, the catalyst was removed by filtration or extraction. Filtrate was concentrated to give the crude product, which was subjected to column chromatography over neutral alumina eluting with petroleum ether-chloroform to afford pure calix [4]pyrrole and further elution of column gave dipyrromethane.The reaction mixture was analysed by HPLC. The HPLC yields are given in Table 4.

Conclusions
Graphite oxide and reduced graphene oxides have been used as solid acid catalysts for the room temperature preparation of dipyrromethanes and calix [4]pyrroles in organic and aqueous solutions.