Manganese-Catalyzed Environmentally Benign Dehydrogenative Coupling of Alcohols and Amines to Form Aldimines and H₂: A Catalytic and Mechanistic Study

Abstract

The catalytic dehydrogenative coupling of alcohols and amines to form aldimines represents an environmentally benign methodology in organic chemistry. This has been accomplished in recent years mainly with precious-metal-based catalysts. We present the dehydrogenative coupling of alcohols and amines to form imines and H2 that is catalyzed, for the first time, by a complex of the earth-abundant Mn. Detailed mechanistic study was carried out with the aid of NMR spectroscopy, intermediate isolation, and X-ray analysis.

Direct Synthesis of Amides by Dehydrogenative Coupling of Amines with either Alcohols or Esters: Manganese Pincer Complex as Catalyst

“…[8,18] Theintermediate II then reacts with the amine to form am anganese complex bound to the hemiaminal III,which can form the amide and 5 through asimilar b-hydride elimination step.The complex 5,upon dehydrogenation, regenerates I.W ebelieve that amechanism, where the alkoxide moiety dissociates from 4,f ollowed by hydride abstraction to form af ree aldehyde,i su nlikely, [19] since aldehyde coordination seems important for the amidation. [8,18] Theintermediate II then reacts with the amine to form am anganese complex bound to the hemiaminal III,which can form the amide and 5 through asimilar b-hydride elimination step.The complex 5,upon dehydrogenation, regenerates I.W ebelieve that amechanism, where the alkoxide moiety dissociates from 4,f ollowed by hydride abstraction to form af ree aldehyde,i su nlikely, [19] since aldehyde coordination seems important for the amidation.…”

“…[6] Thedevelopment of synthetic methods that avoid the use of noble metals is in high demand. [8,9] Beller and co-workers have reported the synthesis of various lactams from amino alcohols using an iron pincer catalyst. [8,9] Beller and co-workers have reported the synthesis of various lactams from amino alcohols using an iron pincer catalyst.…”

Catalytic Hydrogenation of Cyclic Carbonates using Manganese Complexes

“…The potential of hydrogenation catalysts based on abundant and cheap third‐row metal Manganese and its pincer complexes12 has been pointed out by the groups of Milstein13 and Beller14 in 2016. In particular, complexes of this type have been successfully employed in the hydrogenation of CO 2 to formic acid derivatives 5, 15.…”

Manganese‐Catalyzed Direct Olefination of Methyl‐Substituted Heteroarenes with Primary Alcohols

“…[15] In this regard, manganese,a st he third most abundant transition metal, has appeared as an attractive alternative. [17] Subsequently,m anganese complexes were utilized for catalytic dehydrogenations of methanol to CO 2 and H 2 , [18] dehydrogenative Nformylations of amines, [19] couplings of alcohols to esters, [20] syntheses of imines, [21] quinolines, [22] pyrimidines, [22,23] pyrroles, [24] cyclic imides, [25] amides, [26] and a,bunsaturated nitriles, [27] as well as hydrogenations of polar double bonds. [17] Subsequently,m anganese complexes were utilized for catalytic dehydrogenations of methanol to CO 2 and H 2 , [18] dehydrogenative Nformylations of amines, [19] couplings of alcohols to esters, [20] syntheses of imines, [21] quinolines, [22] pyrimidines, [22,23] pyrroles, [24] cyclic imides, [25] amides, [26] and a,bunsaturated nitriles, [27] as well as hydrogenations of polar double bonds.…”