Synthesis of 3,4‐Dihydro‐2H‐Pyrroles from Ketones, Aldehydes, and Nitro Alkanes via Hydrogenative Cyclization

Abstract

Syntheses of N‐heterocyclic compounds that permit a flexible introduction of various substitution patterns by using inexpensive and diversely available starting materials are highly desirable. Easy to handle and reusable catalysts based on earth‐abundant metals are especially attractive for these syntheses. We report here on the synthesis of 3,4‐dihydro‐2H‐pyrroles via the hydrogenation and cyclization of nitro ketones. The latter are easily accessible from three components: a ketone, an aldehyde and a nitroalkane. Our reaction has a broad scope and 23 of the 33 products synthesized are compounds which have not yet been reported. The key to the general hydrogenation/cyclization reaction is a highly active, selective and reusable nickel catalyst, which was identified from a library of 24 earth‐abundant metal catalysts.

A Highly Active Nickel Catalyst for the Selective Hydrogenation of Functionalized Nitroarenes

“…For the synthesis of the Ni/SiO 2 catalyst, a well-established and simple synthesis concept, introduced by our group, was used, employing metal salen complexes as precursors and commercially available support materials. [16][17][18][19][20] At first, the SiO 2 support material is wet impregnated with a solution of the nickel salen complex (NiÀ sal) in acetonitrile, followed by a pyrolysis (700 °C, N 2 ) and a reduction step (500 °C, N 2 /H 2 ; Figure 1). This catalyst has already been fully characterized.…”

“…This catalyst has already been fully characterized. [16] Decomposition of the salen complex during synthesis results in the formation of nickel nanoparticles with a small diameter of 4.7 nm and an Ndoped carbon shell around the particles. [16] Such N-doped carbon shells were also found earlier for nickel catalysts synthesized according to this concept.…”

“…[16] Decomposition of the salen complex during synthesis results in the formation of nickel nanoparticles with a small diameter of 4.7 nm and an Ndoped carbon shell around the particles. [16] Such N-doped carbon shells were also found earlier for nickel catalysts synthesized according to this concept. [17,18] It is also assumed that the sublimation behavior of the salen complexes leads to an atomic dispersion of the complex prior to decomposition.…”

“…[13] We reported an early example of a nanostructured and selective nickel nitroarene hydrogenation catalyst [15] and recently, our group introduced a new synthesis route for 3,4dihydro-2H-pyroles from ketones, aldehydes and nitro alkanes via hydrogenative cyclization. [16] The key to this novel reaction was a Ni/SiO 2 catalyst capable of hydrogenating aliphatic nitro compounds and tolerating ketones. [16] These results motivated us to investigate the nitroarene hydrogenation capability of our Ni/SiO 2 catalyst.…”

“…[16] The key to this novel reaction was a Ni/SiO 2 catalyst capable of hydrogenating aliphatic nitro compounds and tolerating ketones. [16] These results motivated us to investigate the nitroarene hydrogenation capability of our Ni/SiO 2 catalyst. Herein, we report on the ability of a reusable and nanostructured nickel catalyst for the hydrogenation of functionalized nitroarenes.…”