Modeling solvent effects and convergence of 31P-NMR shielding calculations with COBRAMM

Solvent effects on 31P-NMR parameters for triphenylphosphine oxide and triphenylphosphine in chloroform have been extensively investigated by testing different solvation models. The solvent is described implicitly, mixed implicitly/explicitly, and using full explicit models. Polarizable continuum model (PCM), molecular dynamic (MD) simulations, and hybrid quantum mechanics/molecular mechanics (QM/MM) calculations are used to disclose the effects of solute/solvent interactions and, more generally, the role of the embedding in NMR simulations. The results show the beneficial effect of carrying out QM/MM optimizations on top of geometries directly extracted from classical MD simulations, used to ensure representative conformational sampling. The nuclear shielding convergence has been tested against a different number of snapshots and with the inclusion of solvent shells into the QM region. An automated MD//QM/MM//GIAO protocol, implemented in the COBRAMM package, is here proposed and tested on trimethyl phosphite showing that our approach boosts the convergence of nuclear shielding satisfactorily. The present work aims to be a stepping-stone to assess proper QM/MM computational strategies in simulating chemical shifts in non-homogeneous systems like supramolecular and biological systems.

Stereodivergent Synthesis of 1,4-Dicarbonyl Compounds through Sulfonium Rearrangement: Mechanistic Investigation, Stereocontrolled Access to γ-Lactones and γ-Lactams, and Total Synthesis of Paraconic Acids

Although simple γ-lactones and γ-lactams have received considerable attention from the synthetic community, particularly due to their relevance in biological and medicinal contexts, stereoselective synthetic approaches to more densely substituted derivatives remain scarce. The in-depth study presented herein, showcasing a straightforward method for the stereocontrolled synthesis of γ-lactones and γ-lactams, builds on and considerably expands the stereodivergent synthesis of 1,4-dicarbonyl compounds by a ynamide/vinyl sulfoxide coupling. A full mechanistic and computational study of the rearrangement was conducted, uncovering the role of all of the reaction components and providing a rationale for stereoselection. The broad applicability of the developed tools to streamlining synthesis is demonstrated by concise enantioselective total syntheses of (+)-nephrosteranic acid, (+)-rocellaric acid, and (+)-nephromopsinic acid.

Reactions of Heteroallenes with Salan-based Ti(IV) Complexes: A Joint Experimental and Computational Study

The reaction of Ti(NMe2)4 with the salan ligand precursor H2N2O2H2 led to the formation of [(L*)Ti(NHMe2)2] (L*=N2O2 4-) that forms [(H2N2O2)TiCl2] upon reaction with two equiv. of Me3SiCl. [(L*)Ti(py)2] was obtained from the reaction of [Ti(NtBu)Cl2(py)3] with the sodium salt H2N2O2Na2. Treatment of [(L*)Ti(NHMe2)2] with two equiv. of tBuNCO led to the insertion of the isocyanate molecules into the Ti-Nsalan bonds with the formation of [{L*(N(tBu)CO)2}Ti]. Conversely, the reaction of [(H2N2O2)Ti(OiPr)2] with two equiv. of tBuNCO led to the insertion of one isocyanate molecule into a Ti-Nsalan bond with the formation of [{(HN2O2)(N(tBu)CO)}Ti(OiPr)]. Computational studies were performed to gain insight into the reactivity of isocyanates with salan-based Ti(IV) complexes.

Photoredox-Catalyzed α-C-H Monoalkylation of Symmetric Polyols in the Presence of CO2

Achieving the selective modification of symmetric poly-hydroxylated compounds presents a significant challenge due to the presence of identical active sites. Herein, we address this challenge through the design of a ternary catalytic system that includes a photoredox catalyst, a hydrogen atom transfer promotor and a carbonation catalyst. This catalytic system enables the reversible carbonation of acyclic polyols under CO2 atmosphere, which modulates the reactivity of its distinct C-H bonds toward hydrogen atom transfers. An exquisite selectivity for the monoalkylation is achieved in a variety of unprotected light polyols, yielding valuable building blocks in short reaction times. Mechanistic and computational studies demonstrate that the formation of an intramolecular hydrogen bond between the transient carbonate and the free alcohol is pivotal for the kinetic and thermodynamic activation of a specific alcohol.

Mechanistic Study on Selenium- and Sulfur-Mediated Isomerization of Hydroxamic Acids

An in-depth computational study reveals the intriguing mechanism of the recently reported isomerization of hydroxamic acids into para-aminophenols catalyzed by phenylselenyl bromide under mild conditions. The computations not only align with the reported experimental data, effectively explaining observed phenomena such as para-selectivity but also shed light on crucial aspects of the reaction mechanism that establish limitations on the scope of the studied rearrangement. Additionally, a joint theoretical/experimental study was performed to examine the potency of the phenylsulfenyl bromide to mediate the reaction under the same conditions.

Biomimetic Cationic Cyclopropanation Enables an Efficient Chemoenzymatic Synthesis of 6,8-Cycloeudesmanes

Cationic cyclopropanation involves the γ-elimination at carbocations to form a new σ-C-C bond through proton loss. While exceedingly rare in bulk solution, it is recognized as one of the main biosynthetic cyclopropanation pathways. Despite the rich history of bioinspired synthetic chemistry, cationic cyclopropanation has not been appropriated for the synthetic toolbox, likely due to the preference of carbocations to undergo competing E1 β-elimination pathways. Here, we present an in-depth synthetic and computational study of cationic cyclopropanation, focusing on the 6,8-cycloeudesmanes as a platform for this investigation. We were able to apply biomimetic cationic cyclopropanation to the synthesis of several 6,8-cycloeudesmanes and non-natural analogues─in doing so, we showcase the power of this transformation in the preparation of complex cyclopropanes.

Direct Synthesis of α-Amino Acid Derivatives by Hydrative Amination of Alkynes

α-Amino acid derivatives are key components of the molecules of life. The synthesis of α-amino carbonyl/carboxyl compounds is a contemporary challenge in organic synthesis. Herein, we report a practical method for the preparation of α-amino acid derivatives via direct hydrative amination of activated alkynes under mild conditions, relying on sulfinamides as the nitrogen source. Computational studies suggest that the reaction is enabled by a new type of sulfonium [2,3]-sigmatropic rearrangement.

Synthese von α-Aminosäurederivaten durch hydrative Aminierung von Alkinen

α‐Aminosäurederivate sind Kernbestandteile jeglichen Lebens. Die Synthese von α‐Aminocarbonyl/carboxyl Verbindungen ist jedoch weiterhin eine Herausforderung für die organische Synthese. In dieser Arbeit berichten wir von einer praktischen Herstellungsmethode für α‐Aminosäurederivate durch direkte hydrative Aminierung von aktivierten Alkinen mit Sulfinamiden unter milden Bedingungen. Computergestützte Untersuchungen legen nahe, dass eine [2,3]‐sigmatrope Sulfoniumumlagerung der zentrale Schritt der Reaktion ist.

Diastereo- and Enantioselective Inverse-Electron-Demand Diels-Alder Cycloaddition between 2-Pyrones and Acyclic Enol Ethers

A broadly applicable diastereo- and enantioselective inverse-electron-demand Diels-Alder reaction of 2-pyrones and acyclic enol ethers is reported herein. Using a copper(II)-BOX catalytic system, bridged bicyclic lactones are obtained in very high yields (up to 99 % yield) and enantioselectivities (up to 99 % ee) from diversely substituted 2-pyrones and acyclic enol ethers. Mechanistic experiments as well as DFT calculations indicate the occurrence of a stepwise mechanism. The synthetic potential of the bridged bicyclic lactones is showcased by the enantioselective synthesis of polyfunctional cyclohexenes and cyclohexadienes, as well as a carbasugar unit.

Deployment of Sulfinimines in Charge-Accelerated Sulfonium Rearrangement Enables a Surrogate Asymmetric Mannich Reaction

β-Amino acid derivatives are key structural elements in synthetic and biological chemistry. Despite being a hallmark method for their preparation, the direct Mannich reaction encounters significant challenges when carboxylic acid derivatives are employed. Indeed, not only is chemoselective enolate formation a pitfall (particularly with carboxamides), but most importantly the inability to reliably access α-tertiary amines through an enolate/ketimine coupling is an unsolved problem of this century-old reaction. Herein, we report a strategy enabling the first direct coupling of carboxamides with ketimines for the diastereo- and enantioselective synthesis of β-amino amides. This conceptually novel approach hinges on the innovative deployment of enantiopure sulfinimines in sulfonium rearrangements, and at once solves the problems of chemoselectivity, reactivity, and (relative and absolute) stereoselectivity of the Mannich process. In-depth computational studies explain the observed, unexpected (dia)stereoselectivity and showcase the key role of intramolecular interactions, including London dispersion, for the accurate description of the reaction mechanism.

A disulfide-based linker for thiol–norbornene conjugation: formation and cleavage of hydrogels by the use of light

A photolinker for formation of disulfide-crosslinked hydrogels via light-triggered thiol–ene conjugation has been developed. Hydrogels can be patterned via two-photon cleavage of disulfide-linkages in the presence of a two-photon active compound.

HFIP Mediates a Direct C-C Coupling between Michael Acceptors and Eschenmoser's salt

A direct C-C coupling process merging Michael acceptors and Eschenmoser's salt is presented. Albeit reminiscent of the Morita-Baylis-Hillman reaction, this process requires no Lewis base catalyst. The underlying mechanism could be unveiled by a combination of kinetic, isotopic labelling experiments as well as computational investigations, showcasing the critical role of HFIP as a superior mediator for proton transfer events as well as the decisive role of the halide counterion.

Strong Ligand Stabilization Based on π-Extension in a Series of Ruthenium Terpyridine Water Oxidation Catalysts

The substitution behavior of the monodentate Cl ligand of a series of ruthenium(II) terpyridine complexes (terpyridine (tpy)=2,2′:6′,2′′‐terpyridine) has been investigated. ¹H NMR kinetic experiments of the dissociation of the chloro ligand in D₂O for the complexes [Ru(tpy)(bpy)Cl]Cl (1, bpy=2,2’‐bipyridine) and [Ru(tpy)(dppz)Cl]Cl (2, dppz=dipyrido[3,2‐a:2′,3′‐c]phenazine) as well as the binuclear complex [Ru(bpy)₂(tpphz)Ru(tpy)Cl]Cl₃ (3  b, tpphz=tetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐h:2′′′,3′′′‐j]phenazine) were conducted, showing increased stability of the chloride ligand for compounds 2 and 3 due to the extended π‐system. Compounds 1–5 (4=[Ru(tbbpy)₂(tpphz)Ru(tpy)Cl](PF₆)₃, 5=[Ru(bpy)₂(tpphz)Ru(tpy)(C₃H₈OS)/(H₂O)](PF₆)₃, tbbpy=4,4′‐di‐tert‐butyl‐2,2′‐bipyridine) are tested for their ability to run water oxidation catalysis (WOC) using cerium(IV) as sacrificial oxidant. The WOC experiments suggest that the stability of monodentate (chloride) ligand strongly correlates to catalytic performance, which follows the trend 1>2>5≥3>4. This is also substantiated by quantum chemical calculations, which indicate a stronger binding for the chloride ligand based on the extended π‐systems in compounds 2 and 3. Additionally, a theoretical model of the mechanism of the oxygen evolution of compounds 1 and 2 is presented; this suggests no differences in the elementary steps of the catalytic cycle within the bpy to the dppz complex, thus suggesting that differences in the catalytic performance are indeed based on ligand stability. Due to the presence of a photosensitizer and a catalytic unit, binuclear complexes 3 and 4 were tested for photocatalytic water oxidation. The bridging ligand architecture, however, inhibits the effective electron‐transfer cascade that would allow photocatalysis to run efficiently. The findings of this study can elucidate critical factors in catalyst design.

Redox-Neutral Selenium-Catalysed Isomerisation of para-Hydroxamic Acids into para-Aminophenols

A selenium-catalysed para-hydroxylation of N-aryl-hydroxamic acids is reported. Mechanistically, the reaction comprises an N-O bond cleavage and consecutive selenium-induced [2,3]-rearrangement to deliver para-hydroxyaniline derivatives. The mechanism is studied through both 18 O-crossover experiments as well as quantum chemical calculations. This redox-neutral transformation provides an unconventional synthetic approach to para-aminophenols.

Redox-Neutrale Selen-katalysierte Isomerisierung von para-Hydroxamsäuren zu para-Aminophenolen

Über eine Selen‐katalysierte para‐Hydroxylierung von N‐Arylhydroxamsäuren wird berichtet. Mechanistisch verläuft diese über N‐O‐Bindungsbruch und nachfolgende Selen‐induzierte [2,3]‐Umlagerungen um para‐Hydroxylanilinderivate zu erzeugen. Der Mechanismus wurde sowohl mittels 18O‐Überkreuzungsexperimenten als auch quantenchemischen Berechnungen untersucht. Diese redox‐neutrale Transformation ermöglicht einen ungewöhnlichen synthetischen Zugang zu para‐Aminophenolen.

Meyer–Schuster-type rearrangement for the synthesis of α-selanyl-α,β-unsaturated thioesters

A new approach to prepare α-selanyl-α,β-unsaturated thioesters from propargylthioalkynes and an electrophilic selenium species is reported.

α-Functionalisation of Ketones Through Metal-Free Electrophilic Activation

Triflic anhydride mediated activation of acetophenones leads to highly electrophilic intermediates that can undergo a variety of transformations when treated with nucleophiles. This electrophilic ketone activation gives access to α‐arylated and α‐oxyaminated acetophenones under metal‐free conditions in moderate to excellent yields and enables extension to the synthesis of arylated morpholines via generation of vinylsulfonium salts. Computational investigations confirmed the transient existence of intermediates derived from vinyl triflates and the role of the oxygen atoms at the para position of aromatic ring in facilitating their stabilisation.

Unified Approach to the Chemoselective α-Functionalization of Amides with Heteroatom Nucleophiles

Functionalization at the α-position of carbonyl compounds has classically relied on enolate chemistry. As a result, the generation of a new C–X bond, where X is more electronegative than carbon requires an oxidation event. Herein we show that, by rendering the α-position of amides electrophilic through a mild and chemoselective umpolung transformation, a broad range of widely available oxygen, nitrogen, sulfur, and halogen nucleophiles can be used to generate α-functionalized amides. More than 60 examples are presented to establish the generality of this process, and calculations of the mechanistic aspects underline a fragmentation pathway that accounts for the broadness of this methodology.

A Domino 10-Step Total Synthesis of FR252921 and Its Analogues, Complex Macrocyclic Immunosuppressants

FR252921, FR252922, and FR256523 are a family of potent macrocyclic polyene immunosuppressive agents with a novel mode of action. However, the lack of an efficient and flexible synthesis has hindered further biological studies, mostly due to the fact that the natural products appear to be kinetic isomers regarding the triene moiety. Herein, we report the development and application of an unprecedented, unique domino Suzuki–Miyaura/4π-electrocyclic ring-opening macrocyclization, resulting in a concise, unified, and stereoselective synthetic route to these complex targets in only 10 steps. This in turn enables ready access to a range of unnatural analogues, among which several compounds showed inhibition of T-lymphocyte proliferation at levels equal or superior to those of the natural products themselves.

A redox-neutral synthesis of ketones by coupling of alkenes and amides

The direct synthesis of ketones via carbon-carbon bond formation represents one of the most important challenges in organic synthesis. Hydroacylation of alkenes offers perhaps the most efficient and atom-economical approach for the preparation of ketones employing carbonyl compounds and alkenes as feedstocks. State-of-the-art hydroacylation is typically achieved by a transition metal-catalysed coupling of an aldehyde and an alkene but is plagued by competing decarbonylation, requiring the installation of directing groups in the aldehyde reactant. Herein, we present a method for the hydroacylation of alkenes employing amides in a metal-free regime, proceeding by a new mechanism and offering orthogonal reactivity to the conventional, metal-catalysed alternatives.

On the Stability of Disubstituted Cyclobutenes - A Computational Study

A computational study of the electrocyclic ring‐opening of 2‐substituted cyclobutenecarboxylic acids is presented. Detailed calculations suggest a model to predict whether the product of nucleophilic alkylation of a bicyclic lactone electrophile will be a cyclobutenecarboxylic acid or its dienoic acid isomer, based on the used nucleophile.

Redox-Neutral Synthesis of Selenoesters by Oxyarylation of Selenoalkynes under Mild Conditions

An approach for the mild synthesis of selenoesters starting from selenoalkynes through an acid-catalyzed, redox-neutral oxyarylation reaction is reported. Brønsted acid activation of a selenoalkyne leads to a selenium-stabilized vinyl cation, which is captured by an aryl sulfoxide and undergoes sigmatropic rearrangement to deliver the final α-arylated selenoester product. Computational studies have been carried out to elucidate the nature of the Se-stabilized carbocation.

Machine Learning for Organic Synthesis: Are Robots Replacing Chemists?

Machines learn chemistry: An artificial intelligence algorithm has learned to predict the outcomes of C-N coupling reactions from a few thousand nanomole-scale experiments. This Highlight discusses this work in the context of other state-of-the-art approaches for predicting the yields of organic reactions and explains the significance of the results.

Unusual mechanisms in Claisen rearrangements: an ionic fragmentation leading to a meta-selective rearrangement

A mechanistic investigation of the acid-catalysed redox-neutral oxoarylation reaction of ynamides using electrospray ionisation mass-spectrometry (ESI-MS) and quantum chemical calculations (DFT and MP2) is presented. This study reveals the diversity of pathways and products available from an otherwise deceptively simple-looking, classical transformation: fragmentation, an unusual meta-arylation and competing α-carbonyl cation pathways are some of the alternatives unveiled by ESI-MS and mechanistic experiments. Detailed calculations explain the observed trends and rationalise the results.

Hydrative Aminoxylation of Ynamides: One Reaction, Two Mechanisms

Organic synthesis boasts a wide array of reactions involving either radical species or ionic intermediates. The combination of radical and polar species, however, has not been explored to a comparable extent. Herein we present the hydrative aminoxylation of ynamides, a reaction which can proceed by either a polar-radical crossover mechanism or through a rare cationic activation. Common to both processes is the versatility of the persistent radical TEMPO and its oxidised oxoammonium derivative TEMPO⁺ . The unique mechanisms of these processes are elucidated experimentally and by in-depth DFT-calculations.

Mechanistic Pathways in Amide Activation: Flexible Synthesis of Oxazoles and Imidazoles

The preparation of substituted aminooxazoles and aminoimidazoles from α-arylamides and α-aminoamides through triflic anhydride-mediated amide activation is reported. These reactions proceed via the intermediacy of nitrilium adducts and feature N-oxide-promoted umpolung of the α-position of amides as well as a mechanistically intriguing sequence that results in sulfonyl migration from nitrogen to carbon. Quantum-chemical mechanistic analysis sheds light on the intricacies of the process.

Direct Regioselective Synthesis of Tetrazolium Salts by Activation of Secondary Amides under Mild Conditions

Tetrazolium salts are biologically active molecules that have found broad applications in biochemical assays. A regioselective synthesis of tetrazolium salts is described through a formal (3 + 2) cycloaddition. The possibility of employing simple amides and azides as starting material and the mild conditions allow a broad functional group tolerance.

An Asymmetric Redox Arylation: Chirality Transfer from Sulfur to Carbon through a Sulfonium [3,3]-Sigmatropic Rearrangement

A general, asymmetric redox arylation of ynamides and thioalkynes with chiral sulfoxides is reported. This is the first example of a general 1,4-chirality transfer from sulfur to a carbon stereocenter through a sulfonium [3,3]-sigmatropic rearrangement. This reaction delivers α-arylated thioesters and amides under mild conditions in an atom-economical manner. The products are formed in high yields with enantiomeric ratios up to 99.5:0.5. Quantum chemical calculations suggest a mechanism for the chirality transfer from sulfur to carbon and explain the experimentally observed correlation of the enantioselectivity with both the catalyst and the substrate.

Importance of dipole moments and ambient polarity for the conformation of Xaa-Pro moieties - a combined experimental and theoretical study

NMR spectroscopic studies with a series of proline derivatives revealed that the polarity of the environment has a significant effect on the trans : cis isomer ratio of Xaa-Pro bonds. Computational studies showed that this effect is due to differences in the overall dipole moments of trans and cis conformers. Comparisons between the conformational properties of amide and ester derivatives revealed an intricate balance between polarity effects and n → π* interactions of adjacent carbonyl groups. The findings have important implications for protein folding and signaling as well as the performance of proline-based stereoselective catalysts.

[P3Se4](+): A Binary Phosphorus-Selenium Cation

Although a fairly large number of binary group 15/16 element cations have been reported, no example involving phosphorus in combination with a group 16 element has been synthesized and characterized to date. In this contribution is reported the synthesis and structural characterization of the first example of such a cation, namely a nortricyclane-type [P3Se4](+). This cation has been independently discovered by three groups through three different synthetic routes, as described herein. The molecular and electronic structure of the [P3Se4](+) cage and its crystal properties in the solid state have been characterized comprehensively by using X-ray diffraction, Raman, and nuclear magnetic resonance spectroscopies, as well as quantum chemical calculations.

The aza-Morita-Baylis-Hillman reaction: a mechanistic and kinetic study

The aza-Morita-Baylis-Hillman (aza-MBH) reaction has been studied in a variety of solvents, a selection of imine substrates and with various combinations of PPh3 and para-nitrophenol as the catalyst system. The measured kinetic data indicates that the effects of solvent and protic co-catalyst are strongly interdependent. These results are most easily reconciled with a mechanistic model involving the reversible protonation of zwitterionic intermediates in the catalytic cycle, which is also supported by (31)P NMR spectroscopy and quantum chemical studies.

Cation affinity numbers of Lewis bases

Using selected theoretical methods the affinity of a large range of Lewis bases towards model cations has been quantified. The range of model cations includes the methyl cation as the smallest carbon-centered electrophile, the benzhydryl and trityl cations as models for electrophilic substrates encountered in Lewis base-catalyzed synthetic procedures, and the acetyl cation as a substrate model for acyl-transfer reactions. Affinities towards these cationic electrophiles are complemented by data for Lewis-base addition to Michael acceptors as prototypical neutral electrophiles.

The aza-Morita-Baylis-Hillman reaction of electronically and sterically deactivated substrates

The aza-Morita-Baylis-Hillman (azaMBH) reaction has been studied for electronically and sterically deactivated Michael acceptors. It is found that electronically deactivated systems can be converted with electron-rich phosphanes and pyridines as catalysts equally well. For sterically deactivated systems clearly better catalytic turnover can be achieved with pyridine catalysts. This is in accordance with the calculated affinities of the catalysts towards different Michael-acceptors.

The performance of computational techniques in locating the charge separated intermediates in organocatalytic transformations

The formation of zwitterionic adducts between neutral nucleophiles such as NMe(3) and PMe(3) with neutral electrophiles such as methyl vinyl ketone (MVK) has been studied with a wide variety of theoretical methods. It has been found that hybrid density functional methods such as B3LYP are not capable of describing these zwitterionic structures as minima on the potential energy surface. This is also true for combinations of MP2 theory with basis sets lacking diffuse basis functions. The mPW1K hybrid functional, in contrast, correctly describes zwitterionic adducts as true intermediates on the PES. On the basis of this insight, a new version of the G3 compound energy scheme has been developed for the accurate description of zwitterionic structures. It has also been verified that modifications of the B2-PLYP double-hybrid functional are equally capable of the proper description of zwitterionic adducts. The applicability of this latter class of methods to a larger dataset involving combinations of different nucleophiles and electrophiles has been documented.