N-coordinated Ru(ii) catalyzed solvent free N-alkylation of primary amines with alcohols through borrowing hydrogen strategy

Abstract

Borrowing hydrogenation (BH) is a powerful methodology, which is widely adopted for the 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 the 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 benign and solvent-free conditions. 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 conditions. This drastic variation in catalytic performance was observed due to the co-operative effect of –NH₂ in [Ru-1] and [Ru-3]. Metal–ligand cooperation (MLC) in both the complexes stabilizes the intermediate transition state and also the electron deficient Ru(II) center in [Ru-1] has very good binding affinity towards the substrate, which leads to superior catalytic conversion with [Ru-1] compared to [Ru-2] and [Ru-3].