Li Longfei, Zhu Huajie, Liu Li, Song Datong, Lei Ming
State Key Laboratory of Chemical Resource Engineering, College of Science , Beijing University of Chemical Technology , Beijing 100029 , People's Republic of China.
Chinese Center for Chirality, College of Pharmacy , Hebei University , Baoding 071002 , Hebei , People's Republic of China.
Inorg Chem. 2018 Mar 19;57(6):3054-3060. doi: 10.1021/acs.inorgchem.7b02887. Epub 2018 Mar 1.
Herein we report our investigation into the mechanism of CO reduction by HBpin catalyzed by [Ru(CO)H(L)(PPh)] (2; L is the 4,5-diazafluorenyl ligand with a Bpin functional group at the 9-position) through computational studies using the model complex [Ru(CO)H(L)(PMe)] (A1). The reaction consists of four stages: (1) CO insertion into the C-B bond of A1 to form A4, (2) the reduction of A4 by HBpin to afford HCOOBpin (P2) and regenerate A1, (3) the reduction of P2 by HBpin to HCHO (P5), and (4) the reduction of P5 to CHOBpin (P6). We found that Lewis adduct formation plays a key role in all stages of the mechanism, in that it forms more relaxed rings in the key transition states and makes the hydride more hydridic. Oftentimes, the hydride and Bpin moieties can transfer within the Lewis adducts in a concerted manner in our proposed hydride-shuttle mechanism. The energy spans for all stages of our proposed mechanism are within the range of 15.7-22.6 kcal/mol in terms of Gibbs free energy. In contrast, the direct hydroboration and σ-bond metathesis mechanisms proposed in the literature have extremely high energy barriers because of the highly strained four-membered rings in the transition states and the unactivated hydride in HBpin.
在此，我们通过使用模型配合物[Ru(CO)H(L)(PMe)] (A1)进行计算研究，报告了我们对[Ru(CO)H(L)(PPh)] (2；L为9位带有Bpin官能团的4,5 - 二氮杂芴基配体)催化HBpin还原CO的机理的研究。该反应包括四个阶段：(1) CO插入A1的C - B键形成A4，(2) HBpin将A4还原生成HCOOBpin (P2)并再生A1，(3) HBpin将P2还原为HCHO (P5)，以及(4) 将P5还原为CHOBpin (P6)。我们发现路易斯加合物的形成在该机理的所有阶段都起着关键作用，因为它在关键过渡态形成了更松弛的环，并使氢化物更具氢化物性质。在我们提出的氢化物穿梭机理中，氢化物和Bpin部分常常能以协同方式在路易斯加合物内转移。就吉布斯自由能而言，我们提出的机理所有阶段的能量跨度在15.7 - 22.6 kcal/mol范围内。相比之下，文献中提出的直接硼氢化和σ键复分解机理由于过渡态中高度紧张的四元环以及HBpin中未活化的氢化物而具有极高的能垒。
