Title : Sustainable heterogeneous catalysts for arylation of arenes via C-H and C-O bond activation
Selective cleaving and functionalizing C-H, C-O bond in organic chemistry still remains a great challenge due to its weakest reactivity. Transition metal catalysts played a significant role in constructing various organic molecules through C-C coupling by activating the C-H and C-O bond. However, the need for ligands and additives are inevitable for homogeneous catalysts to accomplish the good yield and selectivity. On the other hand, heterogeneous catalysts do not find widespread application in organic transformations involving C-H and C-O bond activation due to its poor catalytic activity. Thus, an effective heterogeneous catalyst for challenging organic transformations besides possessing its advantages must have distinct active sites like its homogeneous counterpart. Here we show Fe and Al grafted over mesoporous silica SBA-15 as sustainable, efficient heterogeneous catalysts towards arylation of arenes. 8wt%Fe/SBA-15 has proven as efficient heterogeneous iron catalyst for direct arylation of biphenyl methane C(sp3)-H and benzene C(sp2)-H. Selective extraction deposition (SED) technique was adopted to graft iron easily over the mesoporous support SBA-15 using LaFeO3 as an iron precursor. The adopted SED technique is highly successful to graft 8wt% iron without forming iron oxide particle into the mesopores of SBA-15. It could be successfully recycled for 5 cycles. In the same way, alumina grafted SBA-15 catalyst demonstrated excellent catalytic activity towards the construction of diarylmethane using aryl boronic acid and 2-naphthalene methanol as reactants. It was observed that the presence of Brønsted acid sites (originates by incorporation of Al in the silica framework) and Lewis acid-base sites together over the support is critical to catalyzing the above reaction with 79% isolated yield. The catalyst was successfully recycled 5 times. In another study, iron grafted over 10wt% alumina coated SBA-15 (AlSBA-15) through SED technique demonstrated excellent redox catalytic activity like homogeneous iron catalyst in arylation of benzene with 74% biphenyl yield without additives. Alumina act as a solid-state ligand to polarize the electrons of Fe(III) grafted over them through its Lewis acid property. The catalyst performed successfully for 7 cycles.
- Fabrication of heterogeneous catalysts for challenging organic transformations involving C-H and C-O activation.
- Nature of the catalytic sites required at the surface of the heterogeneous catalysts to accomplish high catalytic activity like homogeneous catalysts.
- Immobilization of two or more functionalities over mesoporous supports at the appropriate distance, which is difficult or impossible to achieve in solution, to catalyze consecutive reactions by cooperative effects.