Does Silica Surface Catalyse Peptide Bond Formation? New Insights from First‐Principles Calculations

The role that silica surface could have played in prebiotic chemistry as a catalyst for peptide bond formation has been addressed at the B3LYP/6-31+G(d,p) level for a model reaction involving glycine and ammonia on a silica cluster mimicking an isolated terminal silanol group present at the silica surface. Hydrogen-bond complexation between glycine and the silanol is followed by the formation of the mixed surface anhydride Si(surf)-O-C(=O)-R, which has been suggested in the literature to activate the C=O bond towards nucleophilic attack by a second glycine molecule, here simulated by the simpler NH3 molecule. However, B3LYP/6-31+G(d,p) calculations show that formation of the surface mixed anhydride Si(surf)-O-C(=O)-R is disfavoured (delta(r)G298 approximately 6 kcal mol(-1)), and that the surface bond only moderately lowers the free-energy barrier of the nucleophilic attack responsible for peptide bond formation (deltaG298(double dagger) approximately 48 kcal mol(-1)) in comparison with the uncatalysed reaction (deltaG298(double dagger) approximately 52 kcal mol(-1)). A further decrease of the free-energy barrier of peptide bond formation (deltaG298(double dagger) approximately 41 kcal mol(-1)) is achieved by a single water molecule close to the reaction centre acting as a proton-transfer helper in the activated complex. A possible role of strained silica surface defects on the formation of the surface mixed anhydride Si(surf)-O-C(=O)-R has also been addressed.     

季铵盐C—N键断裂构建C—X键的研究进展

胺类种类繁多, 原料易得. 胺类的C—N键键能较大, 一般需要通过活化再进行断裂. 近些年发展了多种氨基的活化方法, 其中把胺类转化为季铵盐的活化方法, 制备容易、存放稳定, 具有一定优势. 最近十年左右, 芳香胺和苄胺衍生的季铵盐通过C—N键断裂、构建各种C—X键的研究取得了巨大的研究进展. 本综述主要论述了最近几年需要和不需要过渡金属催化的季铵盐通过C—N键断裂构建C—X键的反应. 通过C—N键断裂, 季铵盐可以构建C—B键、C—C键、C—N键、C—O键、C—Si键、C—P键、C—S键、C—Se键等, 合成硼酸酯、芳烃、烷烃、醚类、胺类、硅烷、膦、硫醚、二硫化物、硒醚、二硒化物等化合物. 而且, 如果是采用手性苄胺衍生的季铵盐, 还可以得到多种高对映体纯的手性有机化合物; 季铵盐的手性在产物中保持良好, 并且, 对所有反应都发生S_N2型的构型翻转.     

Site‐Selective Tertiary Alkyl–Fluorine Bond Formation from α‐Bromoamides Using a Copper/CsF Catalyst System

A copper‐catalyzed site‐selective fluorination of α‐bromoamides possessing multiple reaction sites, such as primary and secondary alkyl?Br bonds, using inexpensive CsF is reported. Tertiary alkyl?F bonds, which are very difficult to synthesize, can be formed by this fluorination reaction with the aid of an amide group. Control experiments revealed that in situ generated CuF₂ is a key fluorinating reagent that reacts with the tertiary alkyl radicals generated by the reaction between an α‐bromocarbonyl compound and a copper(I) salt.