Origin of the Bürgi-Dunitz Angle

Ring-Opening Competes with Peroxidation in Fenchone Low-Temperature Autoignition

We report an atypical competition between fenchyl radical β-scission and peroxidation at low temperatures and unravel the impacts of strain energy and ring substituent location on their respective contributions. Our RRKM modeling reveals that radicals positioned on secondary carbons are the fastest-scission ones, exhibiting maximum local ring relief. Dimethyl substituents contribute to increased local strain compared to norbornane, hindering bridge scission and leading to cyclopentene and isoprene products. The dimethyl corset generates extra torsional strain during HO2 elimination from QOOH, while ether formation is favored by electron donation from the carbonyl group. The falloff extent is also affected by steric hindrance, insofar as it increases bridge stiffness, leading to a lower vibrational partition function and low-pressure rate constant. Furthermore, methyl-induced restrictions on reactant reorganization are found to modulate an enthalpy-entropy compensation in the Korcek reaction of fenchyl hydroperoxide. Unlike in our previous stirred reactor experiments, the impact of fenchyl peroxidation on reactivity is notable under our new rapid compression machine (RCM) experiments. The present model predicts contrasted fenchyl selectivities with radical position, β-scission and peroxidation prevailing respectively for F1/F2/F3/F4 and F5/F6 radicals. The kinetic mechanism accurately predicts the experimental IDT but indicates a slight first-stage pressure inflection point at the lower experimental temperature, which could not be confirmed experimentally. This new insight into fenchone ring-opening and -closing mechanisms under high-pressure oxidation can be useful for other polycyclic ketones.

Crystal structure and Hirshfeld surface analyses, inter-molecular inter-action energies and energy frameworks of methyl 6-amino-5-cyano-2-(2-meth-oxy-2-oxoeth-yl)-4-(4-nitro-phen-yl)-4H-pyran-3-carboxyl-ate

The title compound, C17H15N3O7, contains pyran and phenyl rings, with the pyran ring exhibiting a flattened-boat conformation. In the crystal, inter-molecular N-H⋯N hydrogen bonds link the mol-ecules into centrosymmetric dimers, forming R 2 2(12) ring motifs. These dimers are linked through N-H⋯O hydrogen bonds into a three-dimensional architecture. A Hirshfeld surface analysis indicates that the most important contributions for the crystal packing are from H⋯O/O⋯H (29.7%), H⋯H (28.7%), H⋯C/C⋯H (16.0%) and H⋯N/N⋯H (12.9%) inter-actions. In addition to van der Waals inter-actions and N-H⋯N and N-H⋯O hydrogen bonds, halogen bonds, tetrel bonds and pnictogen bonds also play an important role in the cohesion of the crystal structure.

Exploring chemical reactivity through a combined conceptual DFT and ELF topology approach

CONTEXT

In a proof-of-concept study, we explore how a combined approach using the topology of the electron localization function (ELF) and the condensed dual descriptor (DD) function can guide the optimal orientation between reactants and mimic the potential energy surfaces of molecular systems at the beginning of the chemical pathway. The DD has been chosen for its ability to evaluate the regioselectivity of neutral and soft species and to potentially mimic the interaction energy obtained from the mutual interactions between nucleophilic and electrophilic regions of the building blocks under perturbative theory.

METHOD

Our method has been illustrated with examples in which the optimal orientation of several systems can be successfully identified. The limitations of the presented model in predicting chemical reactivity are outlined in particular the influence of the selected condensation scheme.

Origin of the Felkin-Anh(-Eisenstein) model: a quantitative rationalization of a seminal concept

Quantum chemical calculations were carried out to quantitatively understand the origin of the Felkin-Anh(-Eisenstein) model, widely used to rationalize the π-facial stereoselectivity in the nucleophilic addition reaction to carbonyl groups directly attached to a stereogenic center. To this end, the possible approaches of cyanide to both (S)-2-phenylpropanal and (S)-3-phenylbutan-2-one have been explored in detail. With the help of the activation strain model of reactivity and the energy decomposition analysis method, it is found that the preference for the Felkin-Anh addition is mainly dictated by steric factors which manifest in a less destabilizing strain-energy rather than, as traditionally considered, in a lower Pauli repulsion. In addition, other factors such as the more favorable electrostatic interactions also contribute to the preferred approach of the nucleophile. Our work, therefore, provides a different, more complete rationalization, based on quantitative analyses, of the origin of this seminal and highly useful concept in organic chemistry.

What defines electrophilicity in carbonyl compounds

The origin of the electrophilicity of a series of cyclohexanones and benzaldehydes is investigated using the activation strain model and quantitative Kohn-Sham molecular orbital (MO) theory. We find that this electrophilicity is mainly determined by the electrostatic attractions between the carbonyl compound and the nucleophile (cyanide) along the entire reaction coordinate. Donor-acceptor frontier molecular orbital interactions, on which the current rationale behind electrophilicity trends is based, appear to have little or no significant influence on the reactivity of these carbonyl compounds.

Thermodynamic and Kinetic Activity Descriptors for the Catalytic Hydrogenation of Ketones

Activity descriptors are a powerful tool for the design of catalysts that can efficiently utilize H2 with minimal energy losses. In this study, we develop the use of hydricity and H- self-exchange rates as thermodynamic and kinetic descriptors for the hydrogenation of ketones by molecular catalysts. Two complexes with known hydricity, HRh(dmpe)2 and HCo(dmpe)2, were investigated for the catalytic hydrogenation of ketones under mild conditions (1.5 atm and 25 °C). The rhodium catalyst proved to be an efficient catalyst for a wide range of ketones, whereas the cobalt catalyst could only hydrogenate electron-deficient ketones. Using a combination of experiment and electronic structure theory, thermodynamic hydricity values were established for 46 alkoxide/ketone pairs in both acetonitrile and tetrahydrofuran solvents. Through comparison of the hydricities of the catalysts and substrates, it was determined that catalysis was observed only for catalyst/ketone pairs with an exergonic H- transfer step. Mechanistic studies revealed that H- transfer was the rate-limiting step for catalysis, allowing for the experimental and computation construction of linear free-energy relationships (LFERs) for H- transfer. Further analysis revealed that the LFERs could be reproduced using Marcus theory, in which the H- self-exchange rates for the HRh/Rh+ and ketone/alkoxide pairs were used to predict the experimentally measured catalytic barriers within 2 kcal mol-1. These studies significantly expand the scope of catalytic reactions that can be analyzed with a thermodynamic hydricity descriptor and firmly establish Marcus theory as a valid approach to develop kinetic descriptors for designing catalysts for H- transfer reactions.

The Cambridge Structural Database and structural dynamics

With the availability of the computer readable information in the Cambridge Structural Database (CSD), wide ranging, largely automated comparisons of fragment, molecular, and crystal structures have become possible. They show that the distributions of interatomic distances, angles, and torsion angles for a given structural fragment occurring in different environments are highly correlated among themselves and with other observables such as spectroscopic signals, reaction and activation energies. The correlations often extend continuously over large ranges of parameter values. They are reminiscent of bond breaking and forming reactions, polyhedral rearrangements, and conformational changes. They map-qualitatively-the regions of the structural parameter space in which molecular dynamics take place, namely, the low energy regions of the respective (free) energy surfaces. The extension and continuous nature of the correlations provides an organizing principle of large groups of structural data and suggests a reconsideration of traditional definitions and descriptions of bonds, "nonbonded" and "noncovalent" interactions in terms of Lewis acids interacting with Lewis bases. These aspects are illustrated with selected examples of historic importance and with some later developments. It seems that the amount of information in the CSD (and other structural databases) and the knowledge on the nature of, and the correlations within, this body of information should allow one-in the near future-to make credible interpolations and possibly predictions of structures and their properties with machine learning methods.

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.

Crystal Clear: Decoding Isocyanide Intermolecular Interactions through Crystallography

The isocyanide group is the chameleon among the functional groups in organic chemistry. Unlike other multiatom functional groups, where the electrophilic and nucleophilic moieties are typically separated, isocyanides combine both functionalities in the terminal carbon. This unique feature can be rationalized using the frontier orbital concept and has significant implications for its intermolecular interactions and the reactivity of the functional group. In this study, we perform a Cambridge Crystallographic Database-supported analysis of isocyanide intramolecular interactions to investigate the intramolecular interactions of isocyanides in the solid state, excluding isocyanide-metal complexes. We discuss examples of different interaction classes, including the isocyanide as a hydrogen bond acceptor (RNC···HX), halogen bonding (RNC···X), and interactions involving the isocyanide and carbon atoms (RNC···C). The latter interaction serves as an intriguing illustration of a Bürgi-Dunitz trajectory and represents a crucial experimental detail in the well-known multicomponent reactions such as the Ugi- and Passerini-type mechanisms. Understanding the spectrum of intramolecular interactions that isocyanides can undergo holds significant implications in fields such as medicinal chemistry, materials science, and asymmetric catalysis.

Unraveling the Bürgi-Dunitz Angle with Precision: The Power of a Two-Dimensional Energy Decomposition Analysis

Understanding the geometrical preferences in chemical reactions is crucial for advancing the field of organic chemistry and improving synthetic strategies. One such preference, the Bürgi-Dunitz angle, is central to nucleophilic addition reactions involving carbonyl groups. This study successfully employs a novel two-dimensional Distortion-Interaction/Activation-Strain Model in combination with a two-dimensional Energy Decomposition Analysis to investigate the origins of the Bürgi-Dunitz angle in the addition reaction of CN- to (CH3)2C═O. We constructed a 2D potential energy surface defined by the distance between the nucleophile and carbonylic carbon atom and by the attack angle, followed by an in-depth exploration of energy components, including strain and interaction energy. Our analysis reveals that the Bürgi-Dunitz angle emerges from a delicate balance between two key factors: strain energy and interaction energy. High strain energy, as a result of the carbonyl compound distorting to avoid Pauli repulsion, is encountered at high angles, thus setting the upper bound. On the other hand, interaction energy is shaped by a dominant Pauli repulsion when the angles are lower. This work emphasizes the value of the 2D Energy Decomposition Analysis as a refined tool, offering both quantitative and qualitative insights into chemical reactivity and selectivity.