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Nitrogen-doped graphene as metal free basic catalyst for coupling reactions

Candu, Natalia, Man, Isabela, Simion, Andrada, Cojocaru, Bogdan, Coman, Simona M., Bucur, Cristina, Primo, Ana, Garcia, Hermenegildo, Parvulescu, Vasile I.
Journal of catalysis 2019 v.376 pp. 238-247
Lewis bases, active sites, adsorption, catalysts, catalytic activity, chitosan, graphene, models, organic chemistry, protonation, pyrolysis, solvents
N-doped defective graphene [(N)G] obtained by pyrolysis at 900 °C of chitosan contains about 3.7% of residual N atoms, distributed as pyridinic, pyrrolic and graphitic N atoms. It has been found that (N)G acts as basic catalyst promoting two classical CC bond forming nucleophilic additions in organic chemistry, such as the Michael and the Henry additions. Computational calculations at DFT level of models corresponding to the various N atoms leads to the conclusion that N atoms are more stable at the periphery of the graphene sheets and that H adsorption on these sites is a suitable descriptor to correlate with the catalytic activity of the various sites. According to these calculations the most active sites are pyridinic N atoms at zig-zag edges of the sheets. In addition, N as dopant changes the reactivity of the neighbour C atoms. Water was found a suitable solvent to achieve high conversions in both reactions. In this solvent the initial distribution of N atoms is affected due to the easy protonation of the NPy to NPyH sites. As an effect, C edge sites adjacent at NPyH with an appropriate reactivity towards the α-C-H bond breaking are formed. The present results show the general activity of N-doped graphene as base catalysts and illustrate the potential of carbocatalysis to promote reactions of general interest in organic synthesis.