Multifaceted catalytic hydrogenation of amides via diverse activation of a sterically confined bipyridine–ruthenium framework

Amides are ubiquitous and abundant in nature and our society, but are very stable and reluctant to salt-free, catalytic chemical transformations. Through the activation of a “sterically confined bipyridine–ruthenium (Ru) framework (molecularly well-designed site to confine adsorbed H2 in)” of a precatalyst, catalytic hydrogenation of formamides through polyamide is achieved under a wide range of reaction conditions. Both C=O bond and C–N bond cleavage of a lactam became also possible using a single precatalyst. That is, catalyst diversity is induced by activation and stepwise multiple hydrogenation of a single precatalyst when the conditions are varied. The versatile catalysts have different structures and different resting states for multifaceted amide hydrogenation, but the common structure produced upon reaction with H2, which catalyzes hydrogenation, seems to be “H–Ru–N–H.”

and

Experimental procedures
Preparation of ligands and base
The reaction mixture was quenched by adding a small portion of water (ca. <2 mL) at 0 °C, and the organic phase was removed in vacuo (ca. 1 mmHg, room temperature).

trans-5-Decen
were calculated based on the integral ratio among the signals of these compounds with respected to an internal standard (mesitylene).

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Representative procedure for hydrogenation of cyclic amides with RuPIP2 (1a), sodium tetraphenylborate and L-Selectride: The reaction of ε-caprolactam (3h) (Fig. 3b) RuPIP2 (1a) (2.9 mg, 0.005 mmol), sodium tetraphenylborate (17.1 mg, 0.05 mmol), ε-caprolactam (3h) (56.6 mg, 0.5 mmol) and a magnetic stirring bar were placed in a glass tube (21 mL capacity). The glass tube was inserted into an autoclave, and anhydrous toluene (1.5 mL) and L-Selectride ® solution (1 M in THF, 25 µL, 0.025 mmol) were added to the mixture under argon atmosphere. The autoclave was purged several times with hydrogen gas (1 MPa). The autoclave was pressurized with 2 MPa of hydrogen gas at 25 °C, and heated for 24 hours at 160 °C under stirring (1000 rpm). The autoclave was cooled to room temperature in an ice-water (0 °C) bath. The reaction mixture was diluted with CDCl 3 and analyzed by 1 H NMR. The yield of azepane (97%) was calculated based on the integral ratio among the signals of these compounds with respected to an internal standard (mesitylene).

X-ray single crystal structure analysis of RUPCY2 (1b).
Single crystals of RUPCY2 suitable for X-ray crystal analysis were obtained by slow diffusion of hexane into a chloroform solution of RUPCY2. Intensity data were collected at 103 K on a Rigaku Single Crystal CCD X-ray Diffractometer (Saturn 70 with MicroMax-007) with Mo Kα radiation (λ = 0.71075 Å) and graphite monochromater. A total of 21375 reflections were measured at a maximum 2θ angle of 49.96°, of which 6042 were independent reflections (R int = 0.1018). The structure was solved by direct methods (SHELXS-97) and refined by the full-matrix least-squares on F 2 (SHELXL-97). All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed using AFIX instructions. The following crystal structure has been