Determining the Factors Accounting for Reaction Selectivity: A Relative Energy Gradient - Interacting Quantum Atoms and Natural Bonding Orbitals Study

Identifying the main physicochemical properties accounting for the course of a reaction is of utmost importance to rationalize chemical syntheses. To this aim, the relative energy gradient (REG) method is an appealing approach because it is an unbiased and automatic process to extract the most relevant pieces of energy information. Initially formulated within the interacting quantum atoms (IQA) framework for a single reaction, here we extend the REG method to natural bond orbitals (NBO) analysis and to the case of two competitive processes. This development enables the determination of the driving forces of any chemical selectivity. We illustrate the extended REG method on the case study of ring opening in cyclobutenes, which is an important instance of the so-called torquoselectivity.

New insights about electronic mechanism of electrocyclic reactions: theoretical study about stereoselectivity in cyclobutenes

This work presents the study of a series of electrocyclic reactions with the main aim of obtaining new insights into the reaction process along IRCs. The energy variation of the different reaction paths as well as the different transition states have been calculated. These trends are according to the experimental data. The natural bond orbitals have been obtained and the second order perturbational theory analysis has been carried out to determine the main charge transfers due to delocalization. Bond reactivity indexes have been used to describe the reactivity mechanism in a local way. These reactivity indexes are also based on NBOs and this has made it possible to connect the results of the indexes with the previous analysis. To determine quantitatively the bond structure, we used the quantum theory of atoms in molecules and we have hereby completed the information obtained from the NBO analysis. Finally, we used the Hirshfeld population analysis as an approximation to understand how the load density changes in the different reaction pathways, and we have connected these variations with the information obtained from the bond structure.

The results has found that the reaction path with the lowest energy barrier Transition State Inward Conrotatory (TSIC) or Transition State Outward Conrotatory (TSOC) is determined by two magnitudes: the charge donations by delocalisation of the substituents (which we obtained from the Second Order Perturbational Theory Analysis of the NBOs) and in the case that these donations were very similar, the non-covalent interactions dominated (which we studied by means of the interaction energies of the Hirshfeld charges). Additionality, the most important factor influencing the lower energy reaction path was the interaction of lone pairs of the substituents with the σ∗(C–C) bond that is broken at the opening of the cycle. The alignment of these lone pairs with the C–C bond favours charge donation between them and, as can be seen in the discussion, this alignment varies depending on whether the structure is TSIC and TSOC.

Discerning the thermal cyclotrimerizations of fluoro- and chloroacetylenes through ELF, NBO descriptors and QTAIM analysis: pseudodiradical character

In this study the thermal cyclotrimerization reactions of fluoro- and chloroacetylenes involving regioselectively stepwise {2 + 2} and stepwise {4 + 2} cycloadditions were studied using the topological analysis of the electron localization function (ELF), the quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. These methodologies have shown that the electronic reorganization in the regioselectively stepwise {2 + 2} and stepwise {4 + 2} cycloadditions may be considered as {2n+2n} and {2π+2n} pseudodiradical process, respectively. Finally, the last phase of this thermal reaction can be understood as an electronic migration process under the pseudodiradical character in the thermal ring-opening reaction, with the subsequent formation of reaction products. In this sense, new insights are reported on the electronic behavior in the bond formation in the thermal cyclotrimerization of fluoroacetylene.

Torquoselective Mechanochemical Activation of the Staudinger Reaction To Form β-Lactams

The Staudinger reaction yielding β-lactam rings via [2 + 2] cycloaddition is a torquoselective reaction where the stereochemistry of the product can be steered by suitable substituents. Although the mechanochemical ring-opening of β-lactams has been investigated recently, the force-assisted synthesis of this important functional four-ring motif remains unexplored. As it will be computationally demonstrated, mechanochemical activation greatly reduces the barrier of the rate-limiting ring-closure step while, at the same time, preserves its torquoselectivity. This finding strongly suggests that strained four-membered rings can be readily incorporated in chain molecules using sonication techniques that greatly enhance reactivity while conserving selectivity.

MRMP2, CCSD(T), and DFT Calculations of the Isomerization Barriers for the Disrotatory and Conrotatory Isomerizations of 3-Aza-3-ium-dihydrobenzvalene, 3,4-Diaza-3-ium-dihydrobenzvalene, and 3,4-Diaza-diium-dihydrobenzvalene

The isomerizations of 3-aza-3-ium-dihydrobenzvalene, 3,4-diaza-3-ium-dihydrobenzvalene, and 3,4-diaza-diium-dihydrobenzvalene to their respective cyclic-diene products were studied using electronic structure methods with a multiconfigurational wave function and several single reference methods. Transition states for both the allowed (conrotatory) and forbidden (disrotatory) pathways were located. The conrotatory pathways of each structure all proceed through a cyclic intermediate with a trans double bond in the ring: this trans double bond destroys the aromatic stabilization of the π electrons due to poor orbital overlap between the cis and trans π bonds. The 3,4-diaza-3-ium-dihydrobenzvalene structure has C₁ symmetry, and there are four separate allowed and forbidden pathways for this structure. The 3-aza-3-ium-dihydrobenzvalene structure is C_s symmetric, and there are two separate allowed and forbidden pathways for this structure. For 3,4-diaza-3,4-diium-dihydrobenzvalene, there was a single allowed and single forbidden pathway due to the C_2v symmetry. The separation of the barrier heights for all three molecules was studied, and we found the difference in activation barriers for the lowest allowed and lowest forbidden pathways in 3,4-diaza-3-ium-dihydrobenzvalene, 3-aza-3-ium-dihydrobenzvalene, and 3,4-diaza-diium-dihydrobenzvalene to be 9.1, 7.4, and 3.7 kcal/mol, respectively. The allowed and forbidden barriers of 3,4-diaza-diium-dihydrobenzvalene were separated by 3.7 kcal/mol, which is considerably less than the 12-15 kcal/mol expected based on the orbital symmetry rules. The addition of the secondary ammonium group tends to shift the conrotatory and disrotatory barriers up in energy (∼12-14 kcal/mol (conrotatory) and 5-10 kcal/mol (disrotatory) per secondary NH₂ group) relative to the barriers of dihydrobenzvalene, but there is negligible effect on E,Z to Z,Z isomerization barriers, which remain in the expected range of greater than 4 kcal/mol.

Torquoselectivity in Cyclobutene Ring Openings and the Interatomic Interactions That Control Them

Torquoselectivity has explained diasteromeric preferences of a number electrocyclic ring openings. The quantum theory of atoms in molecules (QTAIM), the electron localizability indicator (ELI-D), and the interacting quantum atoms (IQA) energy partition method are used to evaluate qualitatively and quantitatively the atomic interactions behind the torquoselectivity of a series of 3-substituted cyclobutenes. ELI-D topology and IQA energies show that the interaction between the distal terminus carbon atom of cyclobutene (C4) with the substituent at C3 (R5) in the transition state governs torquoselectivities. In the case of 3-borylcyclobutene, this interaction is so strong that a protocovalent bond is actually formed between B5 and C4. The evaluation of the interatomic energies allowed us to identify an additional interaction that contribute to a minor extent to the stabilization of the TS. Despite the fact that C4,R5 interaction is the main cause of the torquoselectivity, a bonding path (BP) between these two atoms was not observed. However, the lack of a BP between C4 and R5 does not mean that the topology of the electron density was not affected by the interaction of these two atoms. Surprisingly, we found a strong correlation between the density at the bond critical point (BCP) and the BP shape of C3-C4 breaking bond with the observed activation energies and torquoselectivities.