N-coordinated Ru(II) Catalyzed Solvent Free N-alkylation of Primary Amines with Alcohols through Borrowing Hydrogen Strategy

Borrowing Hydrogenation (BH) is a powerful methodology which is widely adopted for N-alkylation of amines with alcohols using homogeneous molecular catalysts. To explore the chemistry, herein two novel monomeric Ru(II) complexes [(η6-p-cymene)Ru(L1)Cl2] [Ru-1] bearing 4-trifluromethoxyaniline (L1) and [(η6-p-cymene)Ru(L2)Cl] [Ru-2] bearing quinaldic acid (L2) were synthesized and their structural features were revealed by utilizing analytical and spectroscopic methods like FTIR, 1H and 13C NMR, ESI-MS, and elemental analysis. The lattice structures of both [Ru-1] and [Ru-2] were elucidated by Single Crystal XRD (SC-XRD). Also, aniline based complex [Ru-3] was synthesized according to literature and used for comparison. The catalytic efficacy of [Ru-(1-3)] was examined for N-alkylation of amines utilizing alcohols as alkylating agents in a benign and solvent-free condition. The catalytic performance was monitored by GC-MS and HPLC analyses. The primary amine-based(L1) complex [Ru-1] showed a very impressive and efficient conversion of 96%, whereas -N, O donor ligand based(L2) complex [Ru-2] and the unsubstituted aniline based [Ru-3] were able to achieve 25% and 90% conversion, respectively in similar reaction condition. This drastic variation in catalytic performance was observed due to co-operative effect of -NH2 in [Ru-1] and [Ru-3]. Metal-Ligand Cooperation (MLC) in both the complexes stabilize the intermediate transition state and also electron deficient Ru(II) center in [Ru-1] has very good binding affinity towards substrate which leads to superior catalytic conversion with [Ru-1] compared to [Ru-2] and [Ru-3].

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