Hydride Source in Ethers Hydrosilylation Reaction Catalyzed by Brookhart's Ir(Ⅲ) Pincer Complex

Abstract

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.

Light-Promoted Transfer of an Iridium Hydride in Alkyl Ether Cleavage

“…This proposal is inspired by work on related systems that operate without light, [33][34][35][36][37]43,44 as well as the reported photochemical behavior of the bipyridine-supported iridium hydride reported by Miller. 24 Complex 2 likely reacts with NaBAr F 4 and triethylsilane to form the electrophilic σ-silane complex 4 that has been previously characterized by Djukic and co-workers under relevant alcohol dehydrosilylation conditions.…”

Mechanism of Silyl Enol Ethers Hydrogenation Catalysed by Frustrated Lewis Pairs: A Theoretical Study

“…不饱和有机化合物的催化加氢反应是化学中应用 最广泛的反应之一 [1][2][3][4][5][6][7][8] . 长期以来, 各国的研究者致力于 开发不同类型的催化氢化反应体系, 过渡金属催化剂在 该领域一直占主导地位, 并已经取得了很多辉煌的成 就 [9][10][11][12][13][14][15][16][17] . 直到 2006 年, Stephan 小组 [18] 发现"受阻路易斯 酸碱对" (frustrated Lewis pairs, 简称为 FLPs)在 298.15 K 时可以使氢气发生异裂, 其还原形式在 373.15 K 时可释 放氢气, 首次实现了非金属催化剂对氢气的可逆活化.…”

Mechanism of rhodium catalyzed desymmetrization by hydroacylation: a DFT study

“…有机金属催化氢官能团化反应构建新手性中 心的方法在学术界和工业界都得到了广泛的应用. 其 中, Rh [12,14] 或Ir催化剂 [15] 在很多氢官能团化反应中表 现出独特的反应活性, 包括二氢化反应 [16] 、氢化酰 化 [17~19] 、氢化胺化 [12,13,20] 和氢化硅基化 [21] 等(图1). 在 这些反应中, 氢源的活化以及接下来亲电试剂或亲核 试剂的转化都会导致复杂的反应机理 [11,22] .…”