Mechanistic Insights for Nitromethane Activation into Reactive Nitrogenating Reagents

• Published in: ChemCatChem Vol. 13; no. 9; pp. 2132 - 2137
• Main Authors: Qu, Zheng‐Wang, Zhu, Hui, Grimme, Stefan
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 07.05.2021
• Subjects: Acetic acid; acid catalysis; Additives; hydrolysis; nitrogenation; Nitromethane; reaction mechanism; Reagents; Triflic acid
• ISSN: 1867-3880; 1867-3899
• DOI: 10.1002/cctc.202100086

Recently, it was found that nitromethane (CH3NO2) can be activated into a useful nitrogenating reagent for the synthesis of amide and nitrile compounds. In this work, the mechanisms of CH3NO2 activation are explored in detail by extensive DFT calculations. In aqueous triflic acid (HOTf) solution, the formal 1,3‐H‐shift of CH3NO2 into transient CH2=NO2H is identified as the rate‐limiting step over a barrier of 29.6 kcal/mol, followed by multistep acid hydrolysis to cleave the C=N bond to form NH3OH+ (and HCOOH) as final product. In contrast, in non‐aqueous acetic acid (AcOH) solution with electrophilic Tf2O and nucleophilic HCOOH additives, the C=N bond is cleaved by sequential steps of H2O elimination, electrophilic Tf2O activation, nucleophilic HCOOH addition and acetate/TfO− exchange, affording AcONH3+ as more reactive nitrogenating reagent for Beckmann‐type reactions. Nitromethane activation: DFT calculations show that acid hydrolysis of CH3NO2 is rate‐limited by acid‐catalyzed conversion to CH2=NO2H, followed by protic activation and nucleophilic H2O addition to form NH3OH+. In non‐aqueous solution, acid‐catalyzed H2O‐elimination of CH2=NO2H is followed by electrophilic Tf2O activation, nucleophilic HCOOH addition and final AcOH stabilization to form AcONH3+ as more reactive nitrogenating reagent.