Non-Covalent Supported of l-Proline on Graphene Oxide/Fe3O4 Nanocomposite: A Novel, Highly Efficient and Superparamagnetically Separable Catalyst for the Synthesis of Bis-Pyrazole Derivatives
A superparamagnetic graphene oxide/Fe 3 O 4 /L-proline nano hybrid that was obtained from the non-covalent immobilization of L-proline on graphene oxide/Fe 3 O 4 nanocomposite was used as a new magnetically separable catalyst for the efficient synthesis of 4,4 -(arylmethylene)bis(1H-pyrazol-5-ol) derivatives. The prepared heterogeneous catalyst was characterized using FTIR, TGA, DTG, XRD, TEM, SEM, and elemental analysis techniques. Short reaction times (5-15 min), excellent yields (87-98%),
... yields (87-98%), and simple experimental procedure with an easy work-up are some of the advantages of the introduced catalyst. Molecules 2018, 23, 330 2 of 16 metal or metal oxide nanoparticles/nanocrystals on GO to fabricate composites or hybrids is an area of great potential     . Moreover, integrating GO with inorganic nanoparticles allows for the properties of the nanocomposite to be engineered for specific applications. In addition, the existence of the abundant oxygen functional groups affords GO sheets with great ability for the loading of organic molecules via covalent or non-covalent methods, simplifying the development of a broad new class of materials with improved properties, and even introducing new functionalities to GO sheet    . These catalysts have showed high catalytic activity and could be reused for several cycles. Metal NPs can endow their unique properties to the resulting nanoassemblies derived from hybrid systems composed of polymer-nanoparticle or inorganic material-nanoparticle arrangements. The structure and functionalities of the host support can control the spatial distribution of nanoparticles. Therefore, the most prominent problem, which is the tendency of nanoparticles to agglomerate due to their high surface energy, can be avoided  . For example, various organic materials, such as dendrimers  , polymers brushes , graphene sheets , as well as inorganic materials      , have been utilized as carriers for the immobilization of metal NPs. In recent years, amino acids, especially L-proline and proline-derivatives, have attracted much attention as promising catalysts for essential transformations in the fine chemical and pharmaceutical industries    . L-proline amino acid has been used more and more as catalyst, and is commercially available at low cost, but often employed at high catalyst loading as high as 30 mol %  . This unpleasant point is the main reason to take efforts for improving or modifying its catalytic activity through its immobilization on suitable supports, if and only the immobilization approach does not require the use of synthetic methodologies more expensive than proline itself  . When considering the above mentioned argument, we recently introduced a new facile and cheap methodology for the preparation of L-prolinate-amberlite adduct by using ion-pair immobilization of L-proline on the surface of commercially available amberlite IRA-900OH as a heterogeneous and recoverable organocatalyst for the efficient synthesis of 2-amino-4H-chromenes , spiroindoles , 3,3 -diaryloxindoles , bis-pyrazoles , 4H-pyrano[2,3-c] pyrazole derivatives , and biscoumarin derivatives . Herein, we wish to introduce the L-proline on the GO/Fe 3 O 4 nanocomposite through the non-covalent immobilization via hydrogen bonding interaction between the L-proline and the GO/Fe 3 O 4 to prepare the GO/Fe 3 O 4 /L-proline nano hybrid. This technique gives robustness to the catalytic system, and, on the other hand, lets the L-proline organocatalyst to be flexible, mobile, and free on the surface of the GO/Fe 3 O 4 support at the same time. Moreover, this method can enhance the thermal stability of the L-proline organocatalyst. Finally, this catalyst can be recovered simply by applying an external magnet. Such advantages are characteristic properties of homogeneous and heterogeneous catalysts, which have been included in GO/Fe 3 O 4 /L-proline hybrid. The prepared heterogeneous catalyst was used as an efficient and magnetically recoverable organocatalyst for the one-pot pseudo three-component synthesis of bis-pyrazoles in ethanol. Results and Discussion Catalyst Preparation The concise method for the preparation of GO/Fe 3 O 4 /L-proline is illustrated in Scheme 1. GO and GO/Fe 3 O 4 were prepared according to the modified Hummer's method [33, 34] and Song's method  , respectively. Then, a mixture of the achieved GO/Fe 3 O 4 nano hybrid and pristine L-proline was sonicated in deionized water for 0.5 h and further stirred at room temperature for 24 h. Hydrogen-bonding interaction between hydroxyl, epoxy and carboxyl groups on the GO sheet in GO/Fe 3 O 4 nano hybrid with carboxyl and secondary amine groups of L-proline is the driving force for L-proline binding to the GO/Fe 3 O 4 nano hybrid, which has been presented in Scheme 1. The catalyst has been characterized by Fourier transform infrared (FT-IR), XRD, TGA, DTG, TEM, SEM, and elemental analysis. Molecules 2018, 23, 330 3 of 16 Molecules 2018, 23, x 3 of 16 Scheme 1. Preparation pathway of graphene oxide (GO)/Fe3O4/L-proline nano hybrid.