Patnaik Smita, Kanbur Uddhav, Ellern Arkady, Sadow Aaron D
Department of Chemistry, Iowa State University, Ames, IA 50011, USA.
US Department of Energy Ames Laboratory, Iowa State University, Ames, IA 50011, USA.
Chemistry. 2021 Jul 16;27(40):10428-10436. doi: 10.1002/chem.202101146. Epub 2021 May 21.
Three-coordinate BOX ZnR ( BOX =phenyl-(4,4-dimethyl-oxazolinato; R=Me: 2 a, Et: 2 b) catalyzes the dehydrocoupling of primary or secondary silanes and alcohols to give silyl ethers and hydrogen, with high turnover numbers (TON; up to 10 ) under solvent-free conditions. Primary and secondary silanes react with small, medium, and large alcohols to give various degrees of substitution, from mono- to tri-alkoxylation, whereas tri-substituted silanes do not react with MeOH under these conditions. The effect of coordinative unsaturation on the behavior of the Zn catalyst is revealed through a dramatic variation of both rate law and experimental rate constants, which depend on the concentrations of both the alcohol and hydrosilane reactants. That is, the catalyst adapts its mechanism to access the most facile and efficient conversion. In particular, either alcohol or hydrosilane binds to the open coordination site on the BOX ZnOR catalyst to form a BOX ZnOR(HOR) complex under one set of conditions or an unprecedented σ-adduct BOX ZnOR(H-SiR' ) under other conditions. Saturation kinetics provide evidence for the latter species, in support of the hypothesis that σ-bond metathesis reactions involving four-centered electrocyclic 2σ-2σ transition states are preceded by σ-adducts.
三配位的BOX ZnR（BOX =苯基 - （4,4 - 二甲基 - 恶唑啉基）；R = Me：2 a，Et：2 b）催化伯硅烷或仲硅烷与醇的脱氢偶联反应，生成硅醚和氢气，在无溶剂条件下具有高周转数（TON；高达10）。伯硅烷和仲硅烷与小分子、中等大小和大分子醇反应，可得到从单烷氧基化到三烷氧基化的不同取代程度，而三取代硅烷在这些条件下不与甲醇反应。通过速率定律和实验速率常数的显著变化揭示了配位不饱和对锌催化剂行为的影响，这取决于醇和硅烷反应物的浓度。也就是说，催化剂调整其反应机理以实现最简便和高效的转化。具体而言，在一组条件下，醇或硅烷会与BOX ZnOR催化剂上的开放配位位点结合，形成BOX ZnOR（HOR）络合物，而在其他条件下则会形成前所未有的σ - 加合物BOX ZnOR（H - SiR'）。饱和动力学为后一种物种提供了证据，支持了涉及四中心电环化2σ - 2σ过渡态的σ键复分解反应之前存在σ - 加合物的假设。
