Reusable Ni(II) Catalyst Embedded in a Porous Organic Polymer

Author: ChemCatChem
Published On: February 9, 2024
Copyright: Wiley-VCH GmbH

N-Alkylation is a very common type of reaction in organic synthesis. Environmentally friendly and efficient methods for the formation of C–N bonds are, thus, interesting research targets. The direct N-alkylation using alcohols is interesting in this context because it can use renewable, easy-to-obtain feedstocks. This type of reaction can proceed via the conversion of the alcohol to a more reactive carbonyl compound, followed by the condensation with an amine to give an imine intermediate, and completed by a reduction of the C=N unit. This can also be achieved by a catalytic protocol in which a transition metal catalyst “borrows” H₂ from the alcohol and transfers it back during the reduction of the imine intermediate. This is known as a borrowing hydrogen strategy.

Bipul Sarma, Tezpur University, Assam, India, and colleagues have developed a reusable catalyst for the N-alkylation of amines via a borrowing hydrogen strategy. The team synthesized a nitrogen-rich porous organic polymer (POP) with embedded Ni(II) catalytic centers for this purpose. They used an amide-functionalized, triazine-based POP that was prepared from 2,4,6-tris-(4-aminophenyl)-1,3,5-triazine (TAPT) and 1,3,5-benzenetricarbonyl chloride (BzCl). The nickel was then incorporated by adding NiCl₂·6H₂O.

The resulting solid catalyst was used for the N-alkylation of amines using alcohols. The researchers found that the catalyst was suitable for the alkylation of a wide range of anilines/amines with different alcohols. The catalyst system reached turnover numbers (TON) of up to 932 and turnover frequencies (TOF) of up to 51 h⁻¹. The catalyst is easily recovered and can be reused for multiple reaction cycles.