Archetypal Reactions Research Articles

Boryl Ancillary Ligands: Influencing Stability and Reactivity of Amidinato‐Silanone and Germanone Systems in Ammonia Activation

AbstractWhile the nucleophilic addition of ammonia to ketones is an archetypal reaction in classical organic chemistry, the reactivity of heavier group 14 carbonyl analogues (R2E=O; E=Si, Ge, Sn, or Pb) with NH3 remains sparsely investigated, primarily due to the synthetic difficulties in accessing heavier ketone congeners. Herein, we present a room‐temperature stable boryl‐substituted amidinato‐silanone {(HCDippN)2B}{PhC(tBuN)2}Si=O (Dipp=2,6‐iPr2C6H3) (together with its germanone analogue), formed from the corresponding silylene under a N2O atmosphere. This system reacts cleanly with ammonia in 1,2‐fashion to give an isolable sila‐hemiaminal complex {(HCDippN)2B}{PhC(tBuN)2}Si(OH)(NH2). Quantum chemical calculations reveal that the formation of this sila‐hemiaminal is crucially dependent on the nature of the ancillary ligand scaffold. It is facilitated thermodynamically by the hemi‐lability of the amidinate ligand (which allows for the formation of an energetically critical intramolecular N⋅⋅⋅HO hydrogen bond within the product) and is enabled mech‐anistically by a process in which the silanone initially acts in umpolung fashion as a base (rather than an acid), due to the strongly electron‐releasing and sterically bulky nature of the ancillary boryl ligand.

Boryl Ancillary Ligands: Influencing Stability and Reactivity of Amidinato-Silanone and Germanone Systems in Ammonia Activation.

While the nucleophilic addition of ammonia to ketones is an archetypal reaction in classical organic chemistry, the reactivity of heavier group 14 carbonyl analogues (R2E=O; E=Si, Ge, Sn, or Pb) with NH3 remains sparsely investigated, primarily due to the synthetic difficulties in accessing heavier ketone congeners. Herein, we present a room-temperature stable boryl-substituted amidinato-silanone {(HCDippN)2B}{PhC(tBuN)2}Si=O (Dipp=2,6-iPr2C6H3) (together with its germanone analogue), formed from the corresponding silylene under a N2O atmosphere. This system reacts cleanly with ammonia in 1,2-fashion to give an isolable sila-hemiaminal complex {(HCDippN)2B}{PhC(tBuN)2}Si(OH)(NH2). Quantum chemical calculations reveal that the formation of this sila-hemiaminal is crucially dependent on the nature of the ancillary ligand scaffold. It is facilitated thermodynamically by the hemi-lability of the amidinate ligand (which allows for the formation of an energetically critical intramolecular N⋅⋅⋅HO hydrogen bond within the product) and is enabled mech-anistically by a process in which the silanone initially acts in umpolung fashion as a base (rather than an acid), due to the strongly electron-releasing and sterically bulky nature of the ancillary boryl ligand.

Microkinetic modelling of heterogeneous catalysis revisited: Adsorption energies can triumph over activation barriers

In mechanistic studies of heterogeneous catalysis, emphasis is usually placed on the reaction barriers of individual reaction steps despite a long-standing empirical understanding of the importance of adsorption interactions. We construct a few archetypal reaction mechanisms and show the contribution of the adsorption interactions to the overall reaction rate. We investigated the degree of rate control and the reaction rate as the function of the adsorption interactions. The results show that even when leaving the reaction barriers intact, changes in the adsorption interaction have profound effects on the overall turn-over frequencies. This was also shown on a real-life example of ethane dehydrogenation. We re-confirm that although finding the rate-determining step is important in catalyst optimisation, improving the adsorption is at least equally important.

Charge-Tagged N-Heterocyclic Carbenes (NHCs): Revealing the Hidden Side of NHC-Catalysed Reactions through Electrospray Ionization Mass Spectrometry.

N-heterocyclic carbenes (NHCs) are key intermediates in a variety of chemical reactions. Owing to their transient nature, the interception and characterization of these reactive species have always been challenging. Similarly, the study of reaction mechanisms in which carbenes act as catalysts is still an active research field. This Minireview describes the contribution of electrospray ionization mass spectrometry (ESI-MS) to the detection of charge-tagged NHCs resulting from the insertion of an ionic group into the molecular scaffold. The use of different mass spectrometric techniques, combined with the charge-tagging strategy, allowed clarification of the involvement of NHCs in archetypal reactions and the study of their intrinsic chemistry.

Stereoselective synthesis of (-)-tetrahydropyrenophorol

Tetrahydropyrenophorol, an interesting macrodiolide, was isolated from the plant Fagonia cretica. The total synthesis of (?)-1-tetrahydropyrenophorol was achieved in an elegant and linear manner from readily an accessible racemic epoxide. The archetypal reactions include regioselective opening of the epoxide, Sharpless asymmetric dihydroxylation, and Mitsunobu cyclodimerization to construct the requisite 16-membered bis-lactone. The synthetic approach demonstrated here is very simple and could be used for the syntheses of related compounds in an economic and highly stereoselective way.

Scaling Relations and Kinetic Monte Carlo Simulations To Bridge the Materials Gap in Heterogeneous Catalysis

Scaling relations combined with kinetic Monte Carlo simulations are used to study catalytic reactions on extended metal surfaces and nanoparticles. The reaction energies are obtained by density functional theory calculations, where the site-specific values are derived using generalized coordination numbers. This approach provides a way to handle the materials gap in heterogeneous catalysis. CO oxidation on platinum is investigated as an archetypical reaction. The kinetic simulations reveal clear differences between extended surfaces and nanoparticles in the size range of 1–5 nm. The presence of different types of sites on nanoparticles results in a turnover frequency that is orders of magnitude larger than on extended surfaces. For nanoparticles, the reaction conditions determine which sites dominate the overall activity. At low pressures and high temperatures, edge and corner sites determine the catalytic activity, whereas facet sites dominate the activity at high pressures and low temperatures. Furtherm...

One-step preparation of hierarchically porous polyureas: Simultaneous foaming and hyper-crosslinking

A series of hierarchically structured porous polyureas are prepared through the polymerization of diisocyanate monomers and H2O via a one-step and template-free strategy. This simple archetypal reaction combines carbon dioxide foaming process and hyper-crosslinking reaction. The obtained hierarchically porous polyureas (HPUs) consist of micro- and mesopores created by hyper-crosslinking reaction, and macropores generated by carbon dioxide foaming process. The HPUs have a high surface area up to 483.3 m2/g, and large pore volume (0.76 cm3/g). Besides, the nitrogen-doped carbon materials prepared by pyrolyzing the HPUs show a high specific capacitance (209 F/g at 1.0 A/g) and good cycling stability (99.8% capacitance retention after 5000 cycles), demonstrating the advantages of the hierarchically porous structure and nitrogen-rich chemical composition of HPUs.

Patched bimetallic surfaces are active catalysts for ammonia decomposition.

Ammonia decomposition is often used as an archetypical reaction for predicting new catalytic materials and understanding the very reason of why some reactions are sensitive on material's structure. Core–shell or surface-segregated bimetallic nanoparticles expose outstanding activity for many heterogeneously catalysed reactions but the reasons remain elusive owing to the difficulties in experimentally characterizing active sites. Here by performing multiscale simulations in ammonia decomposition on various nickel loadings on platinum (111), we show that the very high activity of core–shell structures requires patches of the guest metal to create and sustain dual active sites: nickel terraces catalyse N−H bond breaking and nickel edge sites drive atomic nitrogen association. The structure sensitivity on these active catalysts depends profoundly on reaction conditions due to kinetically competing relevant elementary reaction steps. We expose a remarkable difference in active sites between transient and steady-state studies and provide insights into optimal material design.

The Valence Bond Way: Reactivity Patterns of Cytochrome P450 Enzymes and Synthetic Analogs

The preceding decade has witnessed an immense surge of activity in the bioinorganic chemistry of transition metal enzymes and synthetic analogs that model their operation. The wide range of research covers both experimental and theoretical investigations of structure and reactivity patterns. Theory, and especially density functional theory (DFT), has become a very useful tool, an important partner of experiment in resolving structural and mechanistic issues. This flare of activity has generated a great deal of knowledge on intermediates, transition states, barriers, rate constants, rate-equilibrium relationships, stereoselectivity, and so forth. This abundance of acquired knowledge has created the need for establishing order, namely, the outlining of broad generalizations, as well as the creation of a more-intuitive interface between experimental and theoretical data. The valence bond (VB) diagram model, originally developed for organic reactions, is such a theoretical framework that has the potential to guide the requisite generalizations in the field of bioinorganic chemical reactivity. In this Account, we briefly describe the principles of construction of VB diagrams for bioinorganic reactions, detailing applications in the booming research area of heme enzyme (specifically cytochrome P450) reactivity, and particularly two archetypal reactions of these enzymes, alkane hydroxylation and thioether sulfoxidation. For congruence with the lingua franca of bioinorganic chemistry, the VB model is formulated to create bridges to (i) the molecular orbital (MO) description, (ii) the oxidation state formulation of transition metal complexes, and (iii) widely used concepts such as the Bell-Evans-Polanyi (BEP) principle. The VB diagram model reveals the origins of the barrier, describes the formation of transition states and reaction intermediates, and allows the prediction of barrier heights and structure-reactivity relationships. Thus, from the VB diagram model, we can rationalize the mechanistic selection during alkane hydroxylation compared with thioether sulfoxidation, as well as the different behaviors of the spin states during the reactions with the active species of P450, the high-valent iron oxo species called compound I (Cpd I). Furthermore, the VB model leads to expressions that enable us to estimate barrier heights from easily accessible reactant properties, such as bond energies, ionization potential, and electron affinities. We further show that the model is not limited to these archetypal processes: its applicability is wider and more general. Accordingly, we outline the potential applications of these principles to other reactions of P450 (such as olefin epoxidation and arene hydroxylation) and to similar reactions of nonheme enzymes and synthetic models. The VB diagram model leads to a unified understanding of complex bioinorganic transformations, creates order in the data, and provides an important framework for making useful predictions.

Energetics, transition states, and intrinsic reaction coordinates for reactions associated with O(3P) processing of hydrocarbon materials.

Electronic structure calculations based on multiconfiguration wave functions are used to investigate a set of archetypal reactions relevant to O(3P) processing of hydrocarbon molecules and surfaces. These include O(3P) reactions with methane and ethane to give OH plus methyl or ethyl radicals, O(3P) + ethane to give CH3O + CH3, and secondary reactions of the OH product radical with ethane and the ethyl radical. Geometry optimization is carried out with CASSCF/cc-pVTZ for all reactions, and with CASPT2/cc-pVTZ for O(3P) + methane/ethane. Single-point energy corrections are applied with CASPT2, CASPT3, and MRCI + Q with the cc-pVTZ and cc-pVQZ basis sets, and the energies extrapolated to the complete basis set limit (CBL). Where comparison of computed barriers and energies of reaction with experiment is possible, the agreement is good to excellent. The best agreement (within experimental error) is found for MRCI + Q/CBL applied to O(3P) + methane. For the other reactions, CASPT2/CBL and MRCI + Q/CBL predictions differ from experiment by 1-5 kcal/mol for 0 K enthalpies of reaction, and are within 1 kcal/mol of the best-estimate experimental range of 0 K barriers for O(3P) + ethane and OH + ethane. The accuracy of MRCI + Q/CBL is limited mainly by the quality of the active space. CASPT2/CBL barriers are consistently lower than MRCI + Q/CBL barriers with identical reference spaces.

STM ELUCIDATES MECHANISM

WANT TO VALIDATE A REACtion mechanism? Why not prepare the reaction intermediate, trap it, take a picture of it, and then watch it react—one molecule at time? Researchers in California have taken this approach to study the oxidation of carbon monoxide to carbon dioxide. One of the simplest and most well-studied reactions, oxidation of CO to CO 2 stands as an archetypal reaction in heterogeneous catalysis. But more than a model system, the elementary reaction is central to automobile emissions control, air purification, and chemical sensing. For decades, surface scientists have used nearly every tool in their bags to get a handle on the reaction mechanism. Yet a clear picture of the fundamental steps remained elusive. Now, using a scanning tunneling microscope (STM) with unique capabilities, University of California, Irvine, physics and chemistry professor Wilson Ho and postdoctoral associate Jae Ryang Hahn have synthesized and studied an O-CO-O complex. And they've identified ...

Measure for measure: Patterns of reciprocity in militarized disputes, 1816–1975

Pruitt's theory of reciprocity is the starting point for this study of patterns of response in militarized disputes. The theory gives a geometrical interpretation to the phenomenon of reciprocity by postulating archetypal reaction functions, and suggests that stability and sudden change in international relationships may be understood as movement between upper and lower equilibria in a reaction system. The purpose of this study is to determine whether the joint outcomes of interaction between actors involved in a confrontation exemplify the archetypes postulated by Pruitt. Data from the Behavioral Correlates of War project furnished information on levels of provocation and response between actors on a weekly basis. Plots of joint outcomes for a series of disputes were constructed and compared to Pruitt's archetypes. The results indicate that patterns of reciprocity in actual disputes correspond to several of the reaction functions in Pruitt's original theory. In addition, it was found that reaction functi...