Borane‐Catalyzed Hydrogenation of Tertiary Amides Activated by Oxalyl Chloride: DFT Mechanistic Insights

Abstract

For the first time, extensive DFT calculations provide complete mechanistic insights into the electrophilic activation of tertiary amides with oxalyl chloride (COCl)2 and the subsequent borane‐catalyzed reduction using dihydrogen H2. The amide carbonyl group is activated via a fast, electrophilic attack by (COCl)2 at oxygen, followed by a slower nucleophilic attack by in‐situ formed chloride Cl– at carbon, which is thermodynamically favoured by exergonic CO2 and CO release to form the separately solvated chloroiminium and Cl– ions. The borane BAr3 (Ar = 2,6‐F2C6H3) catalyst and solvated Cl– act as a frustrated Lewis pair (FLP) to cleave H2 over a barrier of about 24 kcal/mol, affording the transient borate HBAr3– (and the HCl by‐product) that rapidly reduce the chloroiminium cation into tertiary amine in the HCl salt form. However, the recently adopted one‐pot amide reduction is suffering the BAr3 catalyst trapping by amide substrates that effectively increases the rate‐limiting H2‐cleavage barrier by about 3–5 kcal/mol. Stepwise amide activation and reduction is thus suggested for milder reaction conditions and higher product yields.