Bench‐Stable Cobalt Pre‐Catalysts for Mild Hydrosilative Reduction of Tertiary Amides to Amines and Beyond

Abstract

The readily synthesized and bench‐stable cobalt dichloride complex (dpephos)CoCl2 is employed as a pre‐catalyst for a diversity of silane additions to unsaturated organic molecules, including the normally challenging reduction of amides to amines. With regard to hydrosilative reduction of amides even more effective and activator free catalytic systems can be generated from the bench‐stable, commercially available Co(acac)2 and Co(OAc)2 with dpephos and PPh3 ligands. These systems operate under mild conditions (<100 °C), with many examples of room temperature transformations, presenting a first example of mild cobalt‐catalyzed hydrosilylation of amides.

Efficient Co‐Catalyzed Double Hydroboration of Nitriles: Application to One‐Pot Conversion of Nitriles to Aldimines

“…We have recently reported a diversity of Co(acac) 2 /dpephos‐catalyzed deoxygenative hydrosilylation of carboxamides, including rare examples of reduction of secondary and primary amides to the corresponding amines . Although deoxygenative hydrosilylation of primary amides is generally thought to proceed via silane‐assisted dehydration of the amides to nitriles, followed by their hydrosilylation to N,N‐disilylamines, Co(acac) 2 was found to be inactive in hydrosilylation of nitriles .…”

“…We have recently reported a diversity of Co(acac) 2 /dpephos‐catalyzed deoxygenative hydrosilylation of carboxamides, including rare examples of reduction of secondary and primary amides to the corresponding amines . Although deoxygenative hydrosilylation of primary amides is generally thought to proceed via silane‐assisted dehydration of the amides to nitriles, followed by their hydrosilylation to N,N‐disilylamines, Co(acac) 2 was found to be inactive in hydrosilylation of nitriles . Believing that compared to hydrosilanes hydroboranes, such as HBCat and HBPin, may exhibit superior reactivity patterns due to more hydridic nature of the B−H bond vs. the Si−H bond and the enhanced Lewis acidity of the boron center vs. the silicon center, we elected to study the hydroboration of nitriles.…”

“…First, hydroboration of PhCN with HBPin (2.2 equiv) was tested on the NMR scale (0.27 m ) by using a series of bench‐stable commercially available or easily prepared Co II precatalysts: (dpephos)CoCl 2 , Co(OAc) 2 /dpephos and Co(acac) 2 /dpephos (5 mol % of Co). The choice of dpephos ligand was determined by our previous success of its application in Co‐catalyzed hydrosilylation reactions . For (dpephos)CoCl 2 the hydroboration reactions were activated with 2 equiv of LiBHEt 3 to generate the reactive Co I −H species, whereas in the case of the Co II acetate and acetylacetonate precatalysts the reactions were initiated by HBPin .…”

Hydrosilylative Reduction of Tertiary Amides to Amines Catalyzed by N‐(Phosphinoaryl)anilido Complexes of Iron and Cobalt

“…PhSiH 3 , 18 h) to those reported herein. Impressive Co‐catalyzed room temperature amide hydrosilylation has also recently been reported by Khalimon and co‐workers, who achieved high conversion for the reduction of substrates such as N,N ‐dimethylacetamide, N,N ‐dibenzylacetamide, and N ‐methyl‐2‐pyrrolidinone using 5 mol % Co(acac) 2 in combination with 5.5 mol % dpephos (bis[(2‐diphenylphosphino)phenyl] ether) and 1.5 equiv. PhSiH 3 .…”

“…In an effort to further probe the efficacy of pre‐catalyst 2 , we also evaluated its reactivity under room temperature conditions. Such room temperature reactivity is exceedingly rare for the reduction of tertiary amides by use of 3 d transition‐metal catalysts . Indeed, at a loading of 5 mol % 2 , the efficient reduction of a number of amide substrates was observed over the course of 24 h at room temperature (Table , Entries 1–4; Table , Entries 1, 2, and 7).…”