Dehydrogenative Oxidation of Cyclic Amines on a Diruthenium Complex

Abstract

The dehydrogenative oxidation of cyclic amines catalyzed by a diruthenium complex and mechanistic studies are described. Cyclic amines and water reacted in the presence of Cp ‡ Ru(μ-H) 4 RuCp ‡ (1) (Cp ‡ = 1,2,4tri-tert-butylcyclopentadienyl) to afford lactams accompanied by elimination of the hydrogen gas. The reaction of hexamethylenimine with 1 at 160 °C afforded Cp ‡ Ru(μ-H) 2 (μ-C 6 H 11 N)RuCp ‡ (2), having a novel μ-cyclic imine ligand through N−H and C−H bond cleavages. Further C−H bond cleavage of 2 proceeded at 180 °C to afford Cp ‡ Ru(μ-H)(μ-η 2 :η 2 -C 6 H 10 N)RuCp ‡ (4), having a perpendicularly coordinated imidoyl ligand. Complex 4 readily reacted with water and liberated ε-caprolactam. The cooperative interaction of the two ruthenium atoms leading to N−H and double C−H bond cleavages was the key to the dehydrogenative oxidation of cyclic amines.

Synthesis of a diruthenium μ-η4-α-diimine complex via dehydrogenative coupling of cyclic amines and its role in dehydrogenative oxidation of pyrrolidine

“…The unsaturated μ-imidoyl complex 3c was shown to be an active species for the dehydrogenative oxidation of cyclic amines. 22 Thus, we attempted to synthesize the corresponding μ-imidoyl complex 3a by the thermolysis of 2a . Since 3c is highly unstable and readily reacts with H 2 to regenerate 2c , the thermolysis had to be conducted in an evacuated vessel to suppress the re-hydrogenation of 3c .…”

“…aDetermined by GC analysis using biphenyl as an external standard.bRef. 22.cAddition of 100 equiv. of methanol for 5a .…”

“…C 6 D 6 and toluene- d 8 were dried over sodium–benzophenone ketyl and stored under an argon atmosphere. [Cp ‡ Ru(μ-H) 4 RuCp ‡ ] ( 1 ), 74 [(Cp ‡ Ru) 2 (μ-H)(μ-η 2 :η 2 -C 6 H 10 N)] ( 3c ), and [Cp ‡ Ru(μ-OH)] 2 ( 4 ) 22 were prepared according to the published methods. Other reagents were used as received.…”

“…21 Complex 1 also reacts with hexamethyleneimine at 160 °C to yield μ-cyclic imine complex 2c , which equilibrates with μ-imidoyl complex 3c via hydrogen elimination at 180 °C (Scheme 1). 22…”

“…25 We reported that complex 3c reacts with H 2 O to yield bis(μ-hydroxo) complex 4 with the liberation of ε-caprolactam. 22 In terms of related bridging pyridyl complexes, Moore et al . reported catalytic acylation of pyridine via the formation of a triruthenium μ-pyridyl complex.…”