Concerted [4 + 2] and Stepwise (2 + 2) Cycloadditions of Tetrafluoroethylene with Butadiene: DFT and DLPNO-UCCSD(T) Explorations

Cycloaddition of Butadiene with Perfluoroethylene: Prediction of a Periselectivity Switch under Pressure

Defying the common Diels-Alder reactivity, the thermal cycloaddition between butadiene and tetrafluoroethylene (TFE) yields exclusively a [2 + 2] cycloadduct via a stepwise diradical mechanism. Here, we study the possibility of reverting to the normal [4 + 2] reactivity in this reaction under high pressure. DFT calculations using the eXtreme Pressure Polarizable Continuum Model (XP-PCM) suggest a more negative activation volume for the concerted [4 + 2] mechanism than the stepwise [2 + 2] mechanism and predict a switch in periselectivity at 1.4 gigapascal (GPa).

A new chapter in the never ending story of cycloadditions: The puzzling case of SO2 and acetylene

A comprehensive study of the different classes of cycloaddition reactions ([3+2], [2+2], and [2+1]) of SO2 to acetylene and ethylene has been performed using density functional theory (DFT) and composite wavefunction methods. The [3+2] cycloaddition reaction, that was previously explored in the context of the cycloaddition of thioformaldehyde S-methylide (TSM) to ethylene and acetylene, proceeds in a concerted way to the formation of stable heterocycles. In this paper, we extend our study to the [2+2] and [2+1] cycloadditions of SO2 to acetylene, which would produce 1,1-oxathiete-2-oxide and thiirene-1,1-dioxide, respectively. One of the main conclusions is that cyclic 1,1-oxathiete-2-oxide can open through a relatively easy breaking of the SO single bond and rearrange toward sulfinyl acetaldehyde (SA). The SA molecule can easily undergo several internal rearrangements, which eventually lead to sulfenic acid and sulfoxide derivatives of ethenone, 1,2,3-dioxathiole, and CO plus sulfinylmethane. The most probable path, however, produces 2-thioxoacetic acid, whose derivatives (or those of the corresponding acetate) are usually obtained by Willgerodt-Kindler-type sulfuration of acetates. This product can in turn decompose, leading to the final products CO2 and H2CS. Comparison of this decomposition path with that of 2-amino-2-thioxoacetic acid shows that the process occurs through different H-transfer processes.

Desulfurdioxidative N-N Coupling of N-Arylhydroxylamines and N-Sulfinylanilines: Reaction Development and Mechanism

A highly efficient and chemoselective approach for the divergent assembling of unsymmetrical hydrazines through an unprecedented intermolecular desulfurdioxidative N-N coupling is developed. This metal free protocol employs readily accessible N-arylhydroxylamines and N-sulfinylanilines to provide highly valuable hydrazine products with good reaction yields and excellent functional group tolerance under simple conditions. Computational studies suggest that the in situ generated O-sulfenylated arylhydroxylamine intermediate undergoes a retro-[2π+2σ] cycloaddition via a stepwise diradical mechanism to form the N-N bond and release SO2.

Can Twisted Double Bonds Facilitate Stepwise [2 + 2] Cycloadditions?

Computational studies for a series of low to high strain anti-Bredt alkenes suggest that those with highly twisted bridgehead double bonds and a small singlet-triplet energy gap may undergo facile stepwise [2 + 2] cycloadditions to furnish four membered rings. A selection of reaction substrates, including ethylene, acetylene, perfluoroethylene, and cyclooctyne are considered.

Computational Organic Chemistry: The Frontier for Understanding and Designing Bioorthogonal Cycloadditions

Computational organic chemistry has become a valuable tool in the field of bioorthogonal chemistry, offering insights and aiding in the progression of this branch of chemistry. In this review, I present an overview of computational work in this field, including an exploration of both the primary computational analysis methods used and their application in the main areas of bioorthogonal chemistry: (3 + 2) and [4 + 2] cycloadditions. In the context of (3 + 2) cycloadditions, detailed studies of electronic effects have informed the evolution of cycloalkyne/1,3-dipole cycloadditions. Through computational techniques, researchers have found ways to adjust the electronic structure via hyperconjugation to enhance reactions without compromising stability. For [4 + 2] cycloadditions, methods such as distortion/interaction analysis and energy decomposition analysis have been beneficial, leading to the development of bioorthogonal reactants with improved reactivity and the creation of orthogonal reaction pairs. To conclude, I touch upon the emerging fields of cheminformatics and machine learning, which promise to play a role in future reaction discovery and optimization.

Directionality of Halogen-Bonds: Insights from 2D Energy Decomposition Analysis

Halogen bonds are typically observed to have a linear arrangement with a 180° angle between the nucleophile and the halogen bond acceptor X-R. This linearity is commonly explained using the σ-hole model, although there have been alternative explanations involving exchange repulsion forces. We employ two-dimensional Distortion/Interaction and Energy Decomposition Analysis to examine the archetypal H3 N⋯X2 halogen bond systems. Our results indicate that although halogen bonds are predominantly electrostatic, their directionality is largely due to decreased Pauli repulsion in linear configurations as opposed to angled ones in the I2 and Br2 systems. As we move to the smaller halogens, Cl2 and F2 , the influence of Pauli repulsion diminishes, and the energy surface is shaped by orbital interactions and electrostatic forces. These results support the role of exchange repulsion forces in influencing the directionality of strong halogen bonds. Additionally, we demonstrate that the 2D Energy Decomposition Analysis is a useful tool for enhancing our understanding of the nature of potential energy surfaces in noncovalent interactions.

Computational Exploration of the Thermal Rearrangement of Basketene: One Forbidden versus Two Allowed Pericyclic Reactions

The thermal rearrangement of basketene to Nenitzescu's hydrocarbon has been explored using density functional theory (M06-2X and ωB97X-D) and DLPNO-CCSD(T) quantum mechanics. Both the sequential thermally allowed retro Diels-Alder followed by Cope rearrangement and the thermally forbidden retro-[2 + 2] cycloaddition were studied. The controlling role of orbital symmetry rather than reaction thermodynamics is demonstrated.

Topology of electrostatic potential and electron density reveals a covalent to non-covalent carbon-carbon bond continuum

The covalent and non-covalent nature of carbon-carbon (CC) interactions in a wide range of molecular systems can be characterized using various methods, including the analysis of molecular electrostatic potential (MESP), represented as V(r), and the molecular electron density (MED), represented as ρ(r). These techniques provide valuable insights into the bonding between carbon atoms in different molecular environments. By uncovering a fundamental exponential relationship between the distance of the CC bond and the highest eigenvalue (λv1) of V(r) at the bond critical point (BCP), this study establishes the continuum model for all types of CC interactions, including transition states. The continuum model is further delineated into three distinct regions, namely covalent, borderline cases, and non-covalent, based on the gradient, , with the bond distance of the CC interaction. For covalent interactions, this parameter exhibits a more negative value than -5.0 a.u. Å-1, while for non-covalent interactions, it is less negative than -1.0 a.u. Å-1. Borderline cases, which encompass transition state structures, fall within the range of -1.0 to -5.0 a.u. Å-1. Furthermore, this study expands upon Popelier's analysis of the Laplacian of the MED, denoted as ∇2ρ, to encompass the entire spectrum of covalent, non-covalent, and borderline cases of CC interactions. Therefore, the present study presents compelling evidence supporting the concept of a continuum model for CC bonds in chemistry. Additionally, this continuum model is further explored within the context of C-N, C-O, C-S, N-N, O-O, and S-S interactions, albeit with a limited dataset.

A cyclase that catalyses competing 2 + 2 and 4 + 2 cycloadditions

Cycloaddition reactions are among the most widely used reactions in chemical synthesis. Nature achieves these cyclization reactions with a variety of enzymes, including Diels-Alderases that catalyse concerted 4 + 2 cycloadditions, but biosynthetic enzymes with 2 + 2 cyclase activity have yet to be discovered. Here we report that PloI4, a β-barrel-fold protein homologous to the exo-selective 4 + 2 cyclase that functions in the biosynthesis of pyrroindomycins, catalyses competitive 2 + 2 and 4 + 2 cycloaddition reactions. PloI4 is believed to catalyse an endo-4 + 2 cycloaddition in the biosynthesis of pyrrolosporin A; however, when the substrate precursor of pyrroindomycins was treated with PloI4, an exo-2 + 2 adduct was produced in addition to the exo- and endo-4 + 2 adducts. Biochemical characterizations, computational analyses, (co)crystal structures and mutagenesis outcomes have allowed the catalytic versatility of PloI4 to be rationalized. Mechanistic studies involved the directed engineering of PloI4 to variants that produced the exo-4 + 2, endo-4 + 2 or exo-2 + 2 product preferentially. This work illustrates an enzymatic thermal 2 + 2 cycloaddition and provides evidence of a process through which an enzyme evolves along with its substrate for specialization and activity improvement.

1,3 Dipolar Cycloaddition of Münchnones: Factors behind the Regioselectivity

The 1,3 dipolar cycloaddition reactions of münchnones and alkenes provide an expedite synthetic way to substituted pyrroles, an exceedingly important structural motif in the pharmaceutical and material science fields of research. The factors governing their regioselectivity rationalization are not well understood. Using several approaches, we investigate a set of 14 reactions (featuring two münchnones, 12 different alkenes, and two alkynes). The Natural Bond Theory and the Non-Covalent Interaction Index analyses of the noncovalent interaction energies fail to predict the experimental major regioisomer. Employing global cDFT descriptors or local ones such as the Fukui function and dual descriptor yields similarly inaccurate predictions. Only the local softness pairing, within Pearson's Hard and Soft Acids and Bases principle, constitutes a reliable predictor for the major reaction product. By taking into account an estimator for the steric effects, the correct regioisomer is predicted. Steric effects play a major role in driving the regioselectivity, as was corroborated by energy decomposition analysis of the transition states.

Mechanism and the Origins of Periselectivity in Cycloaddition Reactions of Benzyne with Dienes

Density functional theory calculations have been used to explore the reaction mechanism of (4 + 2) and (2 + 2) cycloadditions of benzyne with classical dienes. The results indicate the following: (1) (4 + 2) products arise via concerted pathways, (2) (2 + 2) products arise via stepwise pathways with diradical intermediates, and (3) these diradical intermediates are formed via isomerization of carbene intermediates. The origins of periselectivity in these reactions are analyzed using distortion/interaction analysis for the key steps, and they indicate that the tiny distortion in the very early [4 + 2] transition structure, coupled with an entropic favorability, controls selective (4 + 2) cycloaddition.

Theoretical investigation of the mechanism of DMAP-promoted [4 + 2]-annulation of prop-2-ynylsulfonium with isatoic anhydride

The mechanism for DMAP-promoted [4 + 2]-annulation of prop-2-ynylsulfonium with isatoic anhydride is investigated using the M06-2X functional. The reaction comprises isomerization of prop-2-ynylsulfonium in stage 1. Stage 2 includes DMAP-promoted deprotonation, nucleophilic addition, ring opening, and decarboxylation. Three steps of intramolecular cycloaddition, DMAP-promoted protonation, and dealkylation occur in stage 3, generating methylated DMAP and neutral thioether, which undergo double-bond isomerization to yield 3-methylthio-4-quinolone. The ability of DMAP to promote the reaction lies in the barrier decrease for alkyne isomerization, deprotonation/protonation of allenes, and dealkylation as effective bases for transferring protons and methyl groups. The roles of prop-2-ynylsulfonium and isatoic anhydride were demonstrated to be C2 and C4 synthons via Multiwfn analysis on the frontier molecular orbital. An alternative path was also confirmed by the Mayer bond order of the vital transition states.

Redirecting a Diels-Alder Reaction toward (2 + 2)-Cycloaddition

Recently, reactions of allylidenhydrazones with tetracyanoethylene were found to lead to cyclobutanes-products of usually unfavorable (2 + 2) cycloaddition. Herein we computationally demonstrate that the (4 + 2) product of this reaction is severely destabilized by incomplete C-N bond formation, arising from a complex interplay of substituent electronic effects. We show how destabilization of a single bond in the front-runner product averts its formation and redirects chemical reaction toward an uncharacteristic pathway.

Bootstrap Embedding For Large Molecular Systems

Recent developments in quantum embedding theories have provided attractive approaches to correlated calculations for large systems. In this work, we extend our previous work [J. Chem. Theory Comput. 2019, 15, 4497-4506; J. Phys. Chem. Lett. 2019, 10, 6368-6374] on bootstrap embedding (BE) to enable correlated ab initio calculations at the coupled cluster with singles and doubles (CCSD) level for large molecules. We introduce several new algorithmic developments that significantly reduce the computational cost of BE, while maintaining its accuracy. The resulting implementation scales as O(N³) for the integral transform and O(N) for the CCSD calculation. Numerical results on a series of conjugated molecules suggest that BE with reasonably sized fragments can recover more than 99.5% of the total correlation energy of a full CCSD calculation, while the required computational resources (time and storage) compare favorably to one popular local correlation scheme: domain localized pair natural orbital (DLPNO). The largest BE calculation in this work involves ∼2900 basis functions and can be performed on a single node with 16 CPU cores and 64 GB of memory in a few days. We anticipate that these developments represent an important step toward the application of BE to solve practical problems.