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The hydrosilylative reduction with silane is a popular defunctionalization strategy to convert biomass into chemicals and energies because of the mild reaction conditions. Among these, the reduction of C-O bond is particularly important because of its application in sugar biomass reduction. The (C 6 F 5 ) 3 B/silane catalytic system has been frequently used in the reduction of C-O bonds in the past years. However, Brookhart et al. reported alkyl ethers reduction by using Ir(III) pincer catalyst and reductant HSiEt 3 . This work provides a novel hydrosilylation catalyst for C-O reduction and an effective method for sugar biomass deoxygenation. According to the previous mechanistic proposals on similar Ir catalysed hydrosilylation reactions, the iridium dihydride complex, iridium silyl hydride complex, silane adduct iridium complex and iridium silyl trihydride complex might possibly act as the hydride source. We carried out the theoretical study on Brookhart's Ir(III) Pincer Complex/HSiEt 3 catalyzed hydrosilylation reaction of EtOEt yielding ethane and EtOSiEt 3 . The density functional theory (DFT) calculations in our study indicate that the iridium dihydride complex is the best hydride source. Our calculation result is consistent well with experimental observations in Brookhart's experiment. For example, the phenomenon that adding iridium dihydride complex into the reaction system increases the reaction rate is understandable because the complex is involved in the rate-determining step. From the Distortion/Interaction analysis, we found that hydride transfer steps on the other three possible hydride sources are disfavoured by the HSiEt 3 /-SiEt 3 group (derived from HSiEt 3 ) bonded with Ir center. The iridium silyl hydride complex is unfavourable because the Ir-H bond is strengthened and the pincer ligand is distorted. For the silane adduct iridium complex, the coordination of HSiEt 3 destabilizes iridium complex intermediate for entropy increases and trans effect, and destabilizes the related transition state by damaging its pincer ligand. Further, the corresponding hydride transfer transition state from iridium silyl trihydride is highly unstable and Si-H bond always reform automatically. What's more important, the moderate bond dissociation energy of Ir-hydride, small steric hindrance and the promotion effect of SiEt 3 group coordination with ether all facilitate the hydride transfer on the iridium dihydride complex.

With the development and widespread use of transition metal catalysts, C-H activation has become a hot topic in organic synthesis, especially in the construction of CC bond of organic compounds. As an important and cheap catalyst, manganese complex has shown great potential for catalyzing C-H activation both in academic and industrial applications. In this paper, the mechanism of manganese-catalyzed dehydrogenative [4＋2] annulation by C-H/N-H activation was investigated systematically with the aid of density functional theory (DFT) calculations in 1,4-dioxane solvent. In detail, we use M06-L/[SDD:6-311＋G(d,p)(SMD)]//M06-L/[LANL2DZ:6-31G(d)] to examine the Gibbs free energy, structure and other properties of possible intermediates and transition states in this catalytic cycle. By comprehensive comparison and discussion, we obtained a favorable pathway consisting of five steps: (1) catalyst initiation occurred with the assistance of bromine anion rather than imide to form active catalyst; (2) alkyne inserted into the active catalyst to generate a seven-membered manganacycle after dissociation of a carbon monoxide; (3) double bond migration happened in this seven-membered manganacycle to form a product precursor; (4) the product precursor would dissociate by β-H elimination and generated product isoquinoline and active Mn-H complex; (5) the active Mn-H complex was subsequently combined with an imine followed by dehydrogenative C-H activation to complete the whole catalytic cycle. In this context, the reason for the highly atom-economical C-H activation by direct dehydrogenation (eliminates the necessity for oxidants or additives) has been clarified by this mechanism. The present study was aimed at further understanding of Mn(I)-catalyzed dehydrogenative C-H activation, and provided more theoretical basis for future more Mn-catalyzed C-H activation.

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Lai( ), Xin Xiang-Jun( ) † , Ma Jian-Xin( ), Zhang Qi( ), Wang Kui-Ru( ), Yu Chong-Xiu( ), and Liu Bo( )