Studies on the Himbert intramolecular arene/allene Diels-Alder cycloaddition. Mechanistic studies and expansion of scope to all-carbon tethers

A Dual Cobalt-Photoredox Catalytic Approach for Asymmetric Dearomatization of Indoles with Aryl Amides via C-H Activation

In this study, we unveil a novel method for the asymmetric dearomatization of indoles under cobalt/photoredox catalysis. By strategically activating C-H bonds of amides and subsequent migratory insertion of π-bonds present in indole as reactive partner, we achieve syn-selective tetrahydro-5H-indolo[2,3-c]isoquinolin-5-one derivatives with excellent yields and enantiomeric excesses of up to >99 %. The developed method operates without a metal oxidant, relying solely on oxygen as the oxidant and employing an organic dye as a photocatalyst under irradiation. Control experiments and stoichiometric studies elucidate the reversible nature of the enantiodetermining C-H activation step, albeit not being rate-determining. This study not only expands the horizon of cobalt-catalyzed asymmetric C-H bond functionalization, but also showcases the potential synergy between cobalt and photoredox catalysis in enabling asymmetric synthesis of complex molecules.

2H-Phosphindole-Enabled Dearomatization and [4+2] Cycloaddition of (Hetero)Arenes

The heavier main group multiple bonds offer an effective tool for small molecule activation. Transient 2H-phosphinidole working as a reactive phosphadiene system undergoes phospha-Diels-Alder reaction with a wide range of non-activated aromatic carbocycles and heterocycles, including naphthalene, anthracene, phenanthrene, furan, thiophene, pyrrole, pyridine, and benzo-fused heterocycles, affording concise access to a range of polycyclic fused rings feature with phosphorus at the bridgehead. These results demonstrate that non-activated (hetero)arenes are capable of acting as 2π systems in [4+2] cycloaddition with highly reactive 2H-phosphindole complex.

Pd-Catalyzed Intermolecular Carbonylative Dearomatization of Arylamines with Propargylic Acetates for Synthesis of Bridged Polycyclic Lactams

A new palladium-catalyzed multicomponent dearomatization of arylamines with CO and propargylic acetates for the synthesis of bridged polycyclic lactams is described. This method allows double annulation at the ipso and para positions of the amino group to form four new bonds, three C-C bonds and one C-N bond. DFT calculations and experimental studies indicate that the efficient formation of the allenecarboxanilide intermediate is the key step to achieve the dearomative transformation.

Stereospecific [3+2] Cycloaddition of Chiral Arylallenes with C,N-Cyclic Azomethine Imines

A novel α,β-regioselective [3+2] cycloaddition reaction of arylallene with C,N-cyclic azomethine imine is reported. The axial-to-central chirality transfer phenomenon has been disclosed with chiral allenes in the reaction. The wide substrate scope, including different functional groups and natural products, reveals the generality of the methodology. Both experiments and density functional theory calculations have been used to elucidate a plausible mechanism.

Visible-Light Promoted Intramolecular para-Cycloadditions on Simple Aromatics

Dearomative cycloadditions are a powerful tool to access a large chemical space exploiting simple and ubiquitous building blocks. The energetic burden due to the loss of aromaticity has however greatly limited their synthetic potential. We devised a general intramolecular method that overcomes these limitations thanks to the photosensitization of allenamides. The visible-light-promoted process gives complex [2.2.2]-(hetero)-bicyclooctadienes at room temperature, likely through the stabilization of transient (bi)radicals by naphthalene. The reaction tolerates several valuable functionalities, offering a convenient handle for a myriad of applications, including original isoindoles and metal complexes.

Dearomatization [4+2] Cycloaddition of Nonactivated Benzene Derivatives

Dearomatization reactions have recently emerged as a powerful tool for the rapid buildup of molecular complexity. Here, an unparalleled thermal dearomatization [4+2] cycloaddition reaction between benzene derivatives and a 2H-phosphindole tungsten complex was reported. The unique reactivity of the in situ-generated 2H-phosphindole complex toward benzene was revealed by density functional theory calculations. We thus provide new insights into the dearomatization of nonactivated arenes and pave the way for the manipulation of the dearomatization for further applications.

Phenyl Sulfones: A Route to a Diverse Family of Trisubstituted Cyclohexenes from Three Independent Nucleophilic Additions

A novel process is described for the synthesis of di- and trisubstituted cyclohexenes from an arene. These compounds are prepared from three independent nucleophilic addition reactions to a phenyl sulfone (PhSO₂R; R = Me, Ph, and NC₄H₈) dihapto-coordinated to the tungsten complex {WTp(NO)(PMe₃)}(Tp = trispyrazolylborate). Such a coordination renders the dearomatized aryl ring susceptible to protonation at a carbon ortho to the sulfone group. The resulting arenium species readily reacts with the first nucleophile to form a dihapto-coordinated sulfonylated diene complex. This complex can again be protonated, and the subsequent nucleophilic addition forms a trisubstituted cyclohexene species bearing a sulfonyl group at an allylic position. Loss of the sulfinate anion forms a π-allyl species, to which a third nucleophile can be added. The trisubstituted cyclohexene can then be oxidatively decomplexed, either before or after substitution of the sulfonyl group. Nucleophiles employed include masked enolates, cyanide, amines, amides, and hydride, with all three additions occurring to the same face of the ring, anti to the metal. Of the 12 novel functionalized cyclohexenes prepared as examples of this methodology, nine compounds meet five independent criteria for evaluating drug likeliness. Structural assignments are supported with nine crystal structures, density functional theory studies, and full 2D NMR analysis.

BH9, a New Comprehensive Benchmark Data Set for Barrier Heights and Reaction Energies: Assessment of Density Functional Approximations and Basis Set Incompleteness Potentials

The calculation of accurate reaction energies and barrier heights is essential in computational studies of reaction mechanisms and thermochemistry. To assess methods regarding their ability to predict these two properties, high-quality benchmark sets are required that comprise a reasonably large and diverse set of organic reactions. Due to the time-consuming nature of both locating transition states and computing accurate reference energies for reactions involving large molecules, previous benchmark sets have been limited in scope, the number of reactions considered, and the size of the reactant and product molecules. Recent advances in coupled-cluster theory, in particular local correlation methods like DLPNO-CCSD(T), now allow the calculation of reaction energies and barrier heights for relatively large systems. In this work, we present a comprehensive and diverse benchmark set of barrier heights and reaction energies based on DLPNO-CCSD(T)/CBS called BH9. BH9 comprises 449 chemical reactions belonging to nine types common in organic chemistry and biochemistry. We examine the accuracy of DLPNO-CCSD(T) vis-a-vis canonical CCSD(T) for a subset of BH9 and conclude that, although there is a penalty in using the DLPNO approximation, the reference data are accurate enough to serve as a benchmark for density functional theory (DFT) methods. We then present two applications of the BH9 set. First, we examine the performance of several density functional approximations commonly used in thermochemical and mechanistic studies. Second, we assess our basis set incompleteness potentials regarding their ability to mitigate basis set incompleteness errors. The number of data points, the diversity of the reactions considered, and the relatively large size of the reactant molecules make BH9 the most comprehensive thermochemical benchmark set to date and a useful tool for the development and assessment of computational methods.

Heck Reaction Boosted Heterocycle Ring-Closing and Ring-Opening Rearrangement: A Strategy for the Synthesis of Indolyl-Type Ligands

A novel method for P-involved heterocycle ring-closing-ring-opening rearrangement (HRR) via the Heck reaction is disclosed. The approach enables direct installation of a phosphorus-containing aryl group onto the C2 position of indole. This new rearrangement directly transforms easily prepared indole derivatives into indolyl-derived phosphonates and phosphinic acids with high yields, and many of the products are difficult to obtain by using established methods. This new HRR reaction provides an extremely simple and step-economic method to induce C-C bond formation and P-N bond cleavage for the synthesis of a variety of indolyl-type ligands.

Trends in computational molecular catalyst design

Computational methods have emerged as a powerful tool to augment traditional experimental molecular catalyst design by providing useful predictions of catalyst performance and decreasing the time needed for catalyst screening. In this perspective, we discuss three approaches for computational molecular catalyst design: (i) the reaction mechanism-based approach that calculates all relevant elementary steps, finds the rate and selectivity determining steps, and ultimately makes predictions on catalyst performance based on kinetic analysis, (ii) the descriptor-based approach where physical/chemical considerations are used to find molecular properties as predictors of catalyst performance, and (iii) the data-driven approach where statistical analysis as well as machine learning (ML) methods are used to obtain relationships between available data/features and catalyst performance. Following an introduction to these approaches, we cover their strengths and weaknesses and highlight some recent key applications. Furthermore, we present an outlook on how the currently applied approaches may evolve in the near future by addressing how recent developments in building automated computational workflows and implementing advanced ML models hold promise for reducing human workload, eliminating human bias, and speeding up computational catalyst design at the same time. Finally, we provide our viewpoint on how some of the challenges associated with the up-and-coming approaches driven by automation and ML may be resolved.

Low-Temperature Intramolecular [4+2] Cycloaddition of Allenes with Arenes for the Synthesis of Diene Ligands

The intramolecular [4+2] cycloaddition between arenes and allenes first reported by Himbert gives rapid access to rigid polycyclic scaffolds. Herein, we report a one-pot oxyalkynylation/cycloaddition reaction proceeding under mild conditions (23-90 °C) and providing complex polycyclic architectures with high efficiency, and atom and step economy. The bicyclo[2.2.2]octadiene products were obtained with a wide variety of useful functional groups and were successfully applied as chiral ligands for metal catalysis. Computational studies gave a first rationalization of the low activation energy for the cycloaddition based on counter-intuitive favorable dispersive interactions in the transition state.

Rapid Access to Dispirocyclic Scaffolds Enabled by Diastereoselective Intramolecular Double Functionalization of Benzene Rings

Here we describe the diastereoselective synthesis of (5r,8r)-1,9-diazadispiro[4.2.4⁸ .2⁵ ]tetradecatrienes via domino double spirocyclization of N-arylamide derivatives. This reaction can serve as a fast way to synthesize diazadispirocycles, which are found in the core structures of bioactive natural products. Product diversification via Suzuki-Miyaura cross coupling and application to the synthesis of 1-oxa-9-azadispiro[4.2.4⁸ .2⁵ ]tetradecatrienes were also conducted.

Understanding the 1,3-Dipolar Cycloadditions of Allenes

We have quantum chemically studied the reactivity, site-, and regioselectivity of the 1,3-dipolar cycloaddition between methyl azide and various allenes, including the archetypal allene propadiene, heteroallenes, and cyclic allenes, by using density functional theory (DFT). The 1,3-dipolar cycloaddition reactivity of linear (hetero)allenes decreases as the number of heteroatoms in the allene increases, and formation of the 1,5-adduct is, in all cases, favored over the 1,4-adduct. Both effects find their origin in the strength of the primary orbital interactions. The cycloaddition reactivity of cyclic allenes was also investigated, and the increased predistortion of allenes, that results upon cyclization, leads to systematically lower activation barriers not due to the expected variations in the strain energy, but instead from the differences in the interaction energy. The geometric predistortion of cyclic allenes enhances the reactivity compared to linear allenes through a unique mechanism that involves a smaller HOMO-LUMO gap, which manifests as more stabilizing orbital interactions.

Shaping Molecular Landscapes: Recent Advances, Opportunities, and Challenges in Dearomatization

Dearomatization is a fundamental chemical transformation, and it underlies some of the most efficient tactics for generating three-dimensional complexity from basic two-dimensional precursors. The dearomative toolbox, once restricted to only a handful of reactions, has begun to grow more sophisticated as novel methods are added, introducing more functionality under milder conditions and with more control over chemo-, regio-, and stereoselectivity than ever before. Over the past two decades, major developments in dearomative processes have bolstered significant total-synthesis endeavors and greatly expanded the scope and complexity of chemical building blocks accessible from feedstock arenes. In this Perspective, we highlight some of the recent advances and key challenges that remain in this vibrant area of organic chemistry.

Substituent Effect on the Himbert Intramolecular Arene/Allene Diels-Alder Reaction: NBO Analysis and State Specific Dual Descriptors

Conceptual density functional theory has been applied to study the Himbert intramolecular arene/allene Diels-Alder reaction. The effect of substitutions at different positions on the kinetics of these reactions has been analyzed. Therefore, from the calculation of the activation energies of more than 27 reactions involving concerted mechanisms, the selectivity of these reactions can be predicted and rationalized with the aid of conceptual DFT descriptors. An application of the two concepts, natural population analysis (NBO) and the state-specific dual descriptor (SSDD) for evaluating substituent effects, allows the investigation of the different interactions that promote a reaction compared to another. The SSDDs computed for the transition state structures provide important information about charge transfer interactions during the chemical reaction. In our case, the SSDD results show that the substituents promoting Himbert reaction have the lowest excitation energies, a fact which facilitates the allene/arene interaction. The NBO results show that according to the nature of the substituent, the Himbert reaction stands as a normal-electron demand or reverse. Thus, the interactions favoring each reaction are mentioned. The geometric deformation observed in the case of OCH3 is at the origin to the emergence of other low interactions between diene and dienophile as well as a strong electronic delocalization stabilizing the arene moiety. The calculated synchronicity indexes show that the Himbert intramolecular Diels-Alder reactions are very synchronous.

Catalytic asymmetric multiple dearomatizations of phenols enabled by a cascade 1,8-addition and Diels-Alder reaction

A direct catalytic asymmetric multiple dearomatization reaction of phenols was disclosed, which provides expedient access to a series of architecturally complex polycyclic compounds bearing four stereogenic centers in high enantiopurity. The key to achieve such a transformation is the combination of a dearomative 1,8-addition of β-naphthols to para-quinone methides generated in situ from propargylic alcohols and a subsequent intramolecular dearomative Diels–Alder reaction. Noteworthily, this protocol enrichs not only the diversity of dearomatized products but also the toolbox of dearomatization strategies.

The first chiral phosphoric acid catalyzed asymmetric multiple dearomatizations of phenols for the synthesis of bridged polycyclic compounds are reported.

Benzoazepine-Fused Isoindolines via Intramolecular (3 + 2)-Cycloadditions of Azomethine Ylides with Dinitroarenes

Aminobenzaldehydes bearing a pendant 3,5-dinitrophenyl group react thermally with N-substituted α-amino acids to form unprecedented benzoazepine-fused isoindolines. The reaction proceeds via a dearomatization/rearomatization sequence involving an intramolecular (3 + 2)-cycloaddition between the in situ formed azomethine ylide and the dinitroarene. Various glycine derivatives are tolerated as well as branched substrates based on cyclic, α-mono-, and α,α-disubstituted amino acids, giving single diastereomers in many cases. The method is scalable and gives products with a nitro group ready for further manipulation.

Recent advances in chemical dearomatization of nonactivated arenes

Dearomatization reactions provide a synthetic connection between readily available, simple aromatic starting materials and more saturated intermediates of greater molecular complexity and synthetic utility. The last decade has witnessed a steady increase in the development of dearomative methods, providing new synthetic approaches to high-value building blocks and natural products. This review highlights advances both in the area of dearomatization methodologies for the most chemically inert arenes and in synthetic applications of such strategies.

Lewis Acid-Promoted Enantioselective Dearomative Spirocyclizations of Allenes

A chiral oxazaborolidine combined with SnCl₄ has been found to promote the dearomative spirocyclization of electron-rich benzyl allenyl ketones. The reaction outcome is sensitive to the nature of activating acid, which was rationalized using hard-soft acid-base (HSAB) theory. The spirocyclic product was obtained with up to 72% ee, which is the best result reported to date for these substrates. The formation of cross-conjugated or conjugated products is readily controlled by changing the oxygen-protecting groups.

Additive Tuned Selective Synthesis of Bicyclo[3.3.0]octan-1-ols and Bicyclo[3.1.0]hexan-1-ols Mediated by AllylSmBr

The selective construction of bicyclo[3.3.0]octan-1-ols and bicyclo[3.1.0]hexan-1-ols was achieved by using an allylSmBr/additive(s) system. By employing HMPA as the only additive, the momoallylation/ketone-alkene coupling occurred preferably and afforded bicyclo[3.3.0]octan-1-ols in good yields with high diastereoselectivities. While the ester-alkene coupling predominated to generate bicyclo[3.1.0]hexan-1-ols in moderate yields with excellent diastereoselectivities in the presence of a proton source, such as pyrrole as the coadditive with HMPA. The tunable reactivity of allylSmBr by additive(s) would make it a versatile reagent in organic synthesis.

Nitrone Cycloadditions of 1,2-Cyclohexadiene

We report the first 1,3-dipolar cycloadditions of 1,2-cyclohexadiene, a rarely exploited strained allene. 1,2-Cyclohexadiene is generated in situ under mild conditions and trapped with nitrones to give isoxazolidine products in synthetically useful yields. The reactions occur regioselectively and exhibit a notable endo preference, thus resulting in the controlled formation of two new bonds and two stereogenic centers. DFT calculations of stepwise and concerted reaction pathways are used to rationalize the observed selectivities. Moreover, the strategic manipulation of nitrone cycloadducts demonstrates the utility of this methodology for the assembly of compounds bearing multiple heterocyclic units. These studies showcase the exploitation of a traditionally avoided reactive intermediate in chemical synthesis.

Synthesis of 2,3-allenylamides utilizing [1,2]-phospha-Brook rearrangement and their application to gold-catalyzed cycloisomerization providing 2-aminofuran derivatives

An efficient synthetic method for 2,3-allenylamides having an oxygen functionality at the 2-position was newly developed by utilizing the [1,2]-phospha-Brook rearrangement under Brønsted base catalysis.

Design and synthesis of fused polycycles via Diels-Alder reaction and ring-rearrangement metathesis as key steps

Atom efficient processes such as the Diels–Alder reaction (DA) and the ring-rearrangement metathesis (RRM) have been used to design new polycycles. In this regard, ruthenium alkylidene catalysts are effective in realizing the RRM of bis-norbornene derivatives prepared by DA reaction and Grignard addition. Here, fused polycycles are assembled which are difficult to produce by conventional synthetic routes.

Mechanistic studies on the Rh(III)-mediated amido transfer process leading to robust C-H amination with a new type of amidating reagent

Mechanistic investigations on the Cp*Rh(III)-catalyzed direct C-H amination reaction led us to reveal the new utility of 1,4,2-dioxazol-5-one and its derivatives as highly efficient amino sources. Stepwise analysis on the C-N bond-forming process showed that competitive binding of rhodium metal center to amidating reagent or substrate is closely related to the reaction efficiency. In this line, 1,4,2-dioxazol-5-ones were observed to have a strong affinity to the cationic Rh(III) giving rise to dramatically improved amidation efficiency when compared to azides. Kinetics and computational studies suggested that the high amidating reactivity of 1,4,2-dioxazol-5-one can also be attributed to the low activation energy of an imido-insertion process in addition to the high coordination ability. While the characterization of a cationic Cp*Rh(III) complex bearing an amidating reagent was achieved, its facile conversion to an amido-inserted rhodacycle allowed for a clear picture on the C-H amidation process. The newly developed amidating reagent of 1,4,2-dioxazol-5-ones was applicable to a broad range of substrates with high functional group tolerance, releasing carbon dioxide as a single byproduct. Additional attractive features of this amino source, such as they are more convenient to prepare, store, and use when compared to the corresponding azides, take a step closer toward an ideal C-H amination protocol.

Diels-Alder reactions of allene with benzene and butadiene: concerted, stepwise, and ambimodal transition states

Multiconfigurational complete active space methods (CASSCF and CASPT2) have been used to investigate the (4 + 2) cycloadditions of allene with butadiene and with benzene. Both concerted and stepwise radical pathways were examined to determine the mechanism of the Diels–Alder reactions with an allene dienophile. Reaction with butadiene occurs via a single ambimodal transition state that can lead to either the concerted or stepwise trajectories along the potential energy surface, while reaction with benzene involves two separate transition states and favors the concerted mechanism relative to the stepwise mechanism via a diradical intermediate.

Computation and experiment reveal that the ring-rearrangement metathesis of Himbert cycloadducts can be subject to kinetic or thermodynamic control

Unusual observations in the ring-rearrangement metathesis of Himbert arene/allene cycloadducts to form fused polycylic lactams led to a more in-depth experimental study that yielded conflicting results. Differences in reactivity within related systems and unexpected changes in diastereoselectivity among other similar substrates were not readily explained on the basis of the experimental results. Computational investigations demonstrated substrate-dependent changes in reaction pathways (ring-opening metathesis/ring-closing metathesis [ROM/RCM] cascade vs ring-closing metathesis/ring-opening metathesis [RCM/ROM] cascade). Furthermore, some reactions were judged to be under thermodynamic control and others under kinetic control. The greater understanding of the most likely reaction pathways and their energetics provides a reasonable explanation for the previously irreconcilable results.