Selective Construction of a Biomass-Based Secondary Amine by Hydrogenative Homocoupling of Nitrile Using an Iridium Complex in a Metal–Organic Framework

Abstract

Reductive amination of furfural was recently investigated as a straightforward method for the construction of biomass-based primary amine (furfurylamine) and tertiary amine [tris(2-furanylmethyl)amine] with, however, secondary amine [bis(2-furanylmethyl)amine] as a problem due to a selectivity issue. In this research, we demonstrated a highly selective and efficient strategy for the construction of bis(2-furanylmethyl)amine in 99% yield by Ir-catalyzed hydrogenative homocoupling of biomass-based 2-furanacarbonitrile in one pot. The Ir catalyst was prepared by immobilization of the [Cp*Ir(bpy)Cl]Cl complex in a 2,2′-bipyridine-functionalized UiO-67. Both furfurylamine and furfurylamine-derived secondary imine were successively detected as intermediates. Detailed kinetic analysis suggested the secondary imine hydrogenation as the rate-determining step instead of 2-furanacarbonitrile hydrogenation. A variety of symmetry secondary amines (18 examples) were selectively prepared in excellent to moderate yields from the corresponding nitriles with the Ir catalyst. This research thus built a bridge between a well-defined single-site catalyst with a metal–organic framework as ligand/support and its homogeneous counterpart to understand kinetic details in the biomass-based amine formation.

Construction of biomass-based amines via Ir-mediated N-alkylation: kinetic analysis

“…Previously, cationic CTF of CTF-3 (Scheme S1, ESI †) was reported by ionothermal self-polymerization of 1,3-bis(4-cyanophenyl)imidazolium chloride (12) with anhydrous ZnCl 2 as both solvent and Lewis acid catalyst at 350-500 °C under vacuum for 40 h. To introduce N,N-bidentate ligands in CTF for the purpose of subsequent metalation, cationic CTF of CTF-1 was developed in this research by ionothermal copolymerization of 12 with 2,6-pyridinedicarbonitrile (14) in the presence of ZnCl 2 under microwave-irradiation (175 W) conditions for 3 h (Fig. 1a).…”

“…40,[61][62][63] In contrast, Path b describes a homocoupling route following a successive fourstep process: (i) Ir-promoted oxidative dehydrogenation of 1a ( primary amine) to 6a (methanimine), (ii) nucleophilic attack of 1a to 6a to yield 7v (gem-diamine), (iii) NH 3 elimination from 7v to produce 4v (secondary imine), (iv) Ir-promoted transfer hydrogenation of 4v to give 5v (secondary amine). 12 Evidently, the N-alkylation reaction leads to the formation of the asymmetrical secondary amine 5a. Meanwhile, the homocoupling reaction affords the symmetrical secondary amine 5v.…”

“…S3c and S3f, ESI †). Based on the above discussions, the kinetic analysis on Scheme 5a leads to the ordinary differential equations (eqn ( 8)- (12), see Scheme S3 for the kinetic equations in the ESI †). During the reaction, the Ir catalyst is 1.4 mol% relative to 2a and it is always true for eqn (13) (Scheme S3 †) with [Cat] 0 as the initially loaded catalyst amount.…”

“…To address this issue, the hydrogenative homocoupling of 2-furonitrile (11) for 5m synthesis (Scheme 1b) was examined by a variety of catalysts, such as Ir, Cu, and PdNi. [12][13][14] However, the hydrogenative homocoupling method generally suffers from a very limited substrate scope. Moreover, only symmetrical secondary amines can be obtained using this strategy.…”

“…In contrast, hydrogenative crosscoupling of 11 usually resulted in serious selectivity issues for the desired asymmetrical secondary amines. 12 Secondary amines are important in the fields of pharmaceuticals, medicinal chemistry, corrosion inhibitors of carbon steel, and extractants of rare-earth elements and noble metals. [15][16][17][18][19][20][21][22] Thus, various secondary amines were developed herein employing the catalytic N-alkylation of biomass-based primary amines with biomass-based primary alcohols under environmentally friendly conditions.…”