Autotandem Aromatization–Dearomatization Pathways for PNP-RuII-Catalyzed Formation of Imine and Hydrogen from Alcohol and Amine

Abstract

Mechanism of autotandem catalytic reactions involving EtNH2 and EtOH promoted by the complex (PNP)Ru(CO)H (1) has been investigated at the TPSS level of DFT (PNP = C5H3N(CH2PtBu2)(CHPtBu2), C5H3N(CH2PisoPr2)(CHPisoPr2)). The reaction is identified by three mechanically distinct catalytic cycles. In cycle I, the EtOH adduct of 1 undergoes O–H proton transfer to the PNP ligand to yield the aromatic complex (PN′P)Ru(CO)(OEt)H (3). Subsequent dearomatization of PN′P leads to the formation of dihydrogen and the complex (PNP)Ru(CO)(OEt) (5). However, an outer-sphere mechanism involving the formation of a dihydrogen intermediate complex between 1 and EtOH is more facile for dihydrogen elimination (ΔG⧧ = 18.4 kcal/mol) than the ligand aromatization–dearomatization pathway (ΔG⧧ = 23.9 kcal/mol). In 5, the migration of a β-hydrogen from OEt to the unsaturated P arm of PNP forces removal of MeCHO and formation of the aromatic complex (PN′P)Ru(CO). Regeneration of the catalyst occurs by dearomatization of the PN′P ligand through proton migration from the P arm to the metal. In cycle II, aromatization of the amine adduct (PNP)Ru(CO)(EtNH2)H leads to (PN′P)Ru(CO)(EtNH)H, and subsequent reaction with MeCHO (formed in cycle I) yields the hemiaminal EtNHCH(CH3)OH. The direct reaction between aldehyde and amine can also yield the hemiaminal. In cycle III, the hemiaminal adduct of the catalyst undergoes aromatization at the PNP ligand via N–H proton migration, which simultaneously activates C–OH bond to produce the imine (EtNCHMe) and (PN′P)Ru(CO)(OH)H (17). Removal of water from 17 leads to dearomatization of PN′P and regeneration of the catalyst. Aromatization steps are characterized by proton migration from the coordinated ligands, viz. the O–H bond of the alcohol, the C–H bond of the alkoxide, the N–H bond of the amine, and the N–H bond of the hemiaminal, to the unsaturated P arm of PNP, and the respective ΔG⧧ values are 2.4, 15.7, 18.6, and 13.8 kcal/mol. The efficiency of this autotandem catalytic reaction is mainly attributed to metal–ligand cooperativity operating through several facile “aromatization–dearomatization” steps.