Phosphaketenes as Building Blocks for the Synthesis of Triphospha Heterocycles

Unsaturated phosphorus compounds, such as phosphaalkenes and phosphaalkynes, show a versatile reactivity in cycloadditions. Although phosphaketenes (R?P?C?O) have been known for three decades, their chemistry has remained limited. Herein, we show that heteroatom‐substituted phosphaketenes, R₃E?P?C?O (E=Si, Sn), are building blocks for silyl‐ and stannyl‐substituted five‐membered heterocycles containing three phosphorous atoms. The structure of the heterocyclic anion depends on the nature of the tetrel atom involved. Although the silyl analogue [P₃C₂(OSiR₃)₂]^? is an aromatic 1,2,4‐triphospholide, the stannyl compound [P(CO)₂(PSnR₃)₂]^? is a 1,2,4‐triphosphacyclopenta‐3,5‐dionate with a delocalized OCPCO fragment. Because of their anionic character, these compounds can easily be used as building blocks, for example, in the preparation of a silyl‐functionalized hexaphosphaferrocene or the parent 1,2,4‐triphosphacyclopenta‐3,5‐dionate [P(CO)₂(PH)₂]^?. NMR spectroscopic investigations and computations have shown that the heterocycle‐formation reactions presented herein are remarkably complex.     

Combining Experimental and Computational Studies to Understand and Predict Reactivities of Relevance to Homogeneous Catalysis

This article showcases three major uses of computational chemistry in reactivity studies: the application after, in combination with, and before experiment. Following a brief introduction of suitable computational tools, challenges and opportunities in the implementation of computational chemistry in reactivity studies are discussed, exemplified with selected case studies from our and other laboratories.     

Base‐Catalyzed Hydrosilylation of Ketones and Esters and Insight into the Mechanism

Simple bases (KOtBu, KOH) catalyze the silane‐promoted reduction of ketones and esters to alcohols and of aldimines to amines. The inexpensive silane PMHS (polymethylhydrosiloxane) can be used as the reducing reagent. Double and triple bonds, as well as nitro‐ and cyano‐groups are tolerated. Careful dosing of the silane allows for chemoselective reduction of a more reactive group in the presence of a less reactive group (for example, aldehyde reduction in the presence of ketone/ketone reduction in the presence of ester group). Mechanistic studies showed that addition of base to silanes leads to silicate species, which are the acting reducing agents. Under basic conditions, hydrosiloxanes (tetramethyldisiloxane, TMDS; PMHS) convert into simple silanes (H₂SiMe₂, H₃SiMe), making this a practical method to generate these challenging silanes.     

Aryl‐Copper(III)‐Acetylides as Key Intermediates in CCsp Model Couplings under Mild Conditions

The mechanism of copper‐mediated Sonogashira couplings (so‐called Stephens–Castro and Miura couplings) is not well understood and lacks clear comprehension. In this work, the reactivity of a well‐defined aryl‐Cu^III species ( 1 ) with p‐R‐phenylacetylenes (R=NO₂, CF₃, H) is reported and it is found that facile reductive elimination from a putative aryl‐Cu^III‐acetylide species occurs at room temperature to afford the C_aryl?C_sp coupling species ( I_R ), which in turn undergo an intramolecular reorganisation to afford final heterocyclic products containing 2H‐isoindole ( P , P , P_Ha ) or 1,2‐dihydroisoquinoline ( P_Hb ) substructures. Density Functional Theory (DFT) studies support the postulated reductive elimination pathway that leads to the formation of C?C_sp bonds and provide the clue to understand the divergent intramolecular reorganisation when p‐H‐phenylacetylene is used. Mechanistic insights and the very mild experimental conditions to effect C_aryl?C_sp coupling in these model systems provide important insights for developing milder copper‐catalysed C_aryl?C_sp coupling reactions with standard substrates in the future.     

Aryl–Phenyl Scrambling in Intermediate Organopalladium Complexes: A Gas‐Phase Study of the Mizoroki–Heck Reaction

The intramolecular aryl–phenyl scrambling reaction within palladium–DPPP–aryl complex (DPPP=1,3‐bis(diphenylphosphino)propane) ions was analyzed by state‐of‐the‐art tandem MS, including gas‐phase ion/molecule reactions. The Mizoroki–Heck cross‐coupling reaction was performed in the gas phase, and the intrinsic reactivity of important intermediates could be examined. Moreover, linear free‐energy correlations were applied, and a mechanism for the scrambling reaction proceeding via phosphonium cations was assumed.     

Constrained‐Geometry Bisphosphazides Derived from 1,8‐Diazidonaphthalene: Synthesis,Spectroscopic Characteristics,Structural Features,and Theoretical Investigations

Investigations on the Staudinger reaction between 1,8‐diazidonaphthalene and phosphorous(III) building blocks, a key step in the synthesis of superbasic bisphosphazene proton sponges, yielded a set of bisphosphazides with a constrained geometry 1,8‐disubstituted naphthalene backbone. This compound class has attracted our interest not only due to their surprisingly high stability, but in particular because of their theoretically predicted basicity in the range of their bisphosphazene analogues that can be referred to the constrained geometry interaction of two highly basic nitrogen atoms. Eleven new bisphosphazides bearing simple P‐amino groups as well as P‐guanidino substituents, azaphosphatrane moieties, P₂ building blocks, or chiral P‐amino substituents derived from L ‐proline are presented. They were studied concerning their spectroscopic properties and partly also their chromophoric and structural features. In the case of the pyrrolidino‐substituted TPPN(2N₂) (TPPN=1,8‐bis(trispyrrolidinophosphazenyl)naphthalene), the stepwise nitrogen elimination is investigated theoretically and experimentally, which led to the isolation and structural characterization of TPPN(1N₂) bearing a phosphazide and a phosphazene functionality in one molecule. Attempts to protonate the obtained bisphosphazides and to prove the computationally predicted pK_BH⁺ values through NMR titration reactions resulted in their decay, which again was rationalized by theoretical calculations. Altogether we present the so far most extensive spectroscopic, structural and theoretical investigation of constrained geometry bisphosphazides and their Brønsted and Lewis basic properties.     

A Facile Asymmetric Approach to the Multicyclic Core Structure of Mangicol A

Chiral propargylic ether‐based triene–ynes are synthesized with high enantiomeric purity by employing an asymmetric enyne addition to aldehydes catalyzed by 1,1′‐bi‐2‐naphthol in combination with ZnEt₂, Ti(OiPr)₄ and dicyclohexylamine at room temperature. These substrates are found to undergo a one‐pot domino Pauson–Khand and Diels–Alder cycloaddition catalyzed by [RhCl(CO)₂]₂ under CO to generate a series of multicyclic products with high chemoselectivity and stereoselectivity. These products contain the multicyclic core structure of mangicol A which could facilitate the synthesis and study of this class of natural products.     

金红石相TiO_2纳米棒的氢化处理及其光催化活性

以钛酸四丁酯为钛源,通过盐酸调制的水热法制备出了具有棒状结构的金红石相纳米TiO2,并进一步进行高温氢化处理.采用X射线衍射(XRD),透射电镜(TEM),紫外-可见-近红外漫反射(UV-Vis-NIR DRS),电子顺磁共振(EPR)和表面光伏(SPS)等测试手段对样品进行表征,以气相乙醛和液相苯酚为目标污染物考察催化剂的光催化活性.结果表明:随着高温氢化处理时间的延长,TiO2样品的可见光吸收逐渐增强,其颜色逐渐由白色转变成灰色,这主要与引入的Ti3+/氧空位缺陷有关.表面光电压谱和羟基自由基测试表明,适当时间的氢化处理有利于光生电荷的分离.在光催化氧化降解气相乙醛和液相苯酚过程中,经适当时间氢化处理的样品表现出高的可见光催化活性.并且可见光催化活性的规律与紫外光下的是一致的.这是因为氢化处理后在导带底下方引入了缺陷能级,拓展了可见光响应.过度的氢化处理会在TiO2导带下方引入较低的缺陷能级,使光生电荷的复合加剧,导致光催化活性降低.     

Insights into the Decomposition of Olefin Metathesis Precatalysts

The decomposition of a series of benzylidene, methylidene, and 3‐phenylindenylidene complexes has been probed in alcohol solution in the presence of base. Tricyclohexylphosphane‐containing precatalysts are shown to yield [RuCl(H)(H₂)(PCy₃)₂] in isopropyl alcohol solutions, while 3‐phenylindenylidene complexes lead to η⁵‐(3‐phenyl)indenyl products. The potential‐energy surfaces for the formation of the latter species have been probed using density functional theory studies.     

Exceedingly Fast Copper(II)‐Promoted ortho CH Trifluoromethylation of Arenes using TMSCF3

The direct ortho‐trifluoromethylation of arenes, including heteroarenes, with TMSCF₃ has been accomplished by a copper(II)‐promoted C? H activation reaction which completes within 30 minutes. Mechanistic investigations are consistent with the involvement of C? H activation, rather than a simple electrophilic aromatic substitution (S_EAr), as the key step.