Ligand‐assisted Hydride Transfer: A Pivotal Step for CO 2 Hydroboration Catalyzed by a Mononuclear Mn(I) PNP Complex

• Published in: Chemistry, an Asian journal Vol. 16; no. 17; pp. 2529 - 2537
• Main Authors: Wan, Xiang, Li, Man, Liao, Rong‐Zhen
• Format: Journal Article
• Language: English
• Published: 01.09.2021
• ISSN: 1861-4728; 1861-471X
• DOI: 10.1002/asia.202100582

Abstract A mononuclear Mn(I) pincer complex [Mn(Ph 2 PCH 2 SiMe 2 ) 2 NH(CO) 2 Br] was disclosed to catalyze the pinacolborane (HBpin)‐based CO 2 hydroboration reaction. Density functional calculations were conducted to reveal the reaction mechanism. The calculations showed that the reaction mechanism could be divided into four stages: (1) the addition of HBpin to the unsaturated catalyst C1 ; (2) the reduction of CO 2 to HCOOBpin; (3) the reduction of HCOOBpin to HCHO; (4) the reduction of HCHO to CH 3 OBpin. The activation of HBpin is the ligand‐assisted addition of HBpin to the unsaturated Mn(I)‐N complex C1 generated by the elimination of HBr from the Mn(I) pincer catalyst. The sequential substrate reductions share a common mechanism, and every hydroboration commences with the nucleophilic attack of the Mn(I)‐H to the electron‐deficient carbon centers. The hydride transfer from Mn(I) to HCOOBpin was found to be the rate‐limiting step for the whole catalytic reaction, with a total barrier of 27.0 kcal/mol, which fits well with the experimental observations at 90 °C. The reactivity trend of CO 2 , HCOOBpin, HCHO, and CH 3 OBpin was analyzed through both thermodynamic and kinetic analysis, in the following order, namely HCHO>CO 2 >HCOOBpin≫CH 3 OBpin. Importantly, the very high barrier for the reduction of CH 3 OBpin to form CH 4 reconciles with the fact that methane was not observed in this catalytic reaction.