An Expanded SET Model Associated with the Functional Hindrance Dominates the Amide-Directed Distal sp3 C-H Functionalization

Unveiling Mechanistic Insights and Photocatalytic Advancements in Intramolecular Photo-(3 + 2)-Cycloaddition: A Comparative Assessment of Two Paradigmatic Single-Electron-Transfer Models

The synthesis of 1-aminonorbornane (1-aminoNB), a potential aniline bioisostere, through photochemistry or photoredox catalysis signifies a remarkable breakthrough with implications in organic chemistry, pharmaceutical chemistry, and sustainable chemistry. However, an understanding of the underlying mechanisms involved in these reactions remains limited and ambiguous. Herein, we employ high-precision CASPT2//CASSCF calculations to elucidate the intricate mechanisms regulating the intramolecular photo-(3 + 2)-cycloaddition reactions for the synthesis of 1-aminoNB in the presence or absence of the Ir-complex-based photocatalyst. Our investigations delve into radical cascades, stereoselectivity, particularly single-electron-transfer (SET) events, etc. Furthermore, we innovatively introduce and compare two SET models integrating Marcus electron-transfer theory and transition-state theory. These models combined with kinetic data contribute to recognizing the critical control factors in diverse photocatalysis, thereby guiding the design and manipulation of photoredox catalysis as well as the improvement and modification of photocatalysts.

Theoretical Understanding on the Facilitated Photoisomerization of a Carbonyl Supported Borane System

Boron compound BOMes2 containing an internal B-O bond undergoes highly efficient photoisomerization, followed by sequential structural transformations, resulting in a rare eight-membered B, O-heterocycle (S. Wang, et al. Org. Lett. 2019, 21, 5285-5289). In this work, the detailed reaction mechanisms of such a unique carbonyl-supported tetracoordinate boron system in the first excited singlet (S1 ) state and the ground (S0 ) state were investigated by using the complete active space self-consistent field and its second-order perturbation (MS-CASPT2//CASSCF) method combined with time-dependent density functional theory (TD-DFT). Moreover, an imine-substituted tetracoordinated organic boron system (BNMes2 ) was selected for comparative study to explore the intrinsic reasons for the difference in reactivity between the two types of compounds. Steric factor was found to influence the photoisomerization activity of BNMes2 and BOMes2 . These results rationalize the experimental observations and can provide helpful insights into understanding the excited-state dynamics of heteroatom-doped tetracoordinate organoboron compounds, which facilitates the rational design of boron-based materials with superior photoresponsive performances.

Co-function Mechanisms of Chlorine and Alkoxy Radicals in Cerium-Catalyzed C-H Functionalization of Alkane Mediated by Visible Light

Identification of radical intermediates for the catalytic functionalization of alkanes offers a number of unique challenges and has recently raised a controversial issue concerning the subtle role of chlorine versus alkoxy radicals in cerium photocatalysis. This study is an attempt to settle the controversy within the theoretical frameworks of Marcus electron transfer and transition state theory. Co-function mechanisms were proposed together with a scheme of kinetic evaluations to account for ternary dynamic competition among photolysis, back electron transfer, and hydrogen atom transfer (HAT). Cl•-based HAT has been proven to initially control the early dynamics of the photocatalytic transformation on the picosecond to nanosecond time scale, which is subsequently taken over by a postnanosecond event of alkoxy radical-mediated HAT. The theoretical models developed herein provide a uniform understanding of the continuous time dynamics of photogenerated radicals to address some paradoxical arguments in lanthanide photocatalysis.

Extended Single-Electron Transfer Model and Dynamically Associated Energy Transfer Event in a Dual-Functional Catalyst System

Organic photocatalysis has been developed flourishingly to rely on bimolecular energy transfer (EnT) or oxidative/reductive electron transfer (ET), promoting a variety of synthetic transformations. However, there are rare examples to merge EnT and ET processes rationally within one chemical system, of which the mechanistic investigation still remains in its infancy. Herein, the first mechanistic illustration and kinetic assessments of the dynamically associated EnT and ET paths were conducted for realizing the C-H functionalization in a cascade photochemical transformation of isomerization and cyclization by using the dual-functional organic photocatalyst of riboflavin. An extended single-electron transfer model of transition-state-coupled dual-nonadiabatic crossings was explored to analyze the dynamic behaviors in the proton transfer-coupled cyclization. This can also be used to clarify the dynamic correlation with the EnT-driven E → Z photoisomerization that has been kinetically evaluated by using Fermi's golden rule with the Dexter model. The present computational results of electron structures and kinetic data contribute to a fundamental basis for understanding the photocatalytic mechanism of the combined operation of EnT and ET strategies, which will guide the design and manipulation for the implementation of multiple activation modes based on a single photosensitizer.

Mechanistic Understanding and Reactivity Analyses for the Photochemistry of Disubstituted Tetrazoles

The photolysis of tetrazoles has undergone extensive research. However, there are still some problems to be solved in terms of mechanistic understanding and reactivity analyses, which leaves room for theoretical calculations. Herein, multiconfiguration perturbation theory at the CASPT2//CASSCF level was employed to account for electron correction effects involved in the photolysis of four disubstituted tetrazoles. Based on calculations of vertical excitation properties and evaluations of intersystem crossing (ISC) efficiencies in the Frank-Condon region, the combination of space and electronic effects is found in maximum-absorption excitation. Two types of ISC (¹ππ* → ³nπ*, ¹ππ* → ³ππ*) are determined in disubstituted tetrazoles, and the obtained rates follow the El-Sayed rule. Through mapping three representative types of minimum energy profiles for the photolysis of 1,5-, and 2,5-disubstituted tetrazoles, a conclusion can be drawn that the photolysis of tetrazoles exhibits reactivity characteristic of bond-breaking selectivity. Kinetic evaluations show that the photogeneration of singlet imidoylnitrene operates predominately over that in the triplet state, which can be confirmed by a double-well model in the triplet potential energy surface of 1,5-disubstituted tetrazole. Similar mechanistic explorations and reactivity analyses were also applied to the photolysis of 2,5-disubstituted tetrazole to unveil fragmentation patterns of nitrile imine generation. All computational efforts allow us to better understand the photoreactions of disubstituted tetrazoles and to provide useful strategies for regulating their unique reactivity.

Synergetic argentophilic and through space electronic interactions in a single-crystal-to-single-crystal photocycloaddition reaction: a mechanistic study

Ag(I) is able to mediate single-crystal-to-single-crystal transformation through [2+2] photocycloaddition to prepare high-conductivity materials. However, the intrinsic mechanism of Ag(I) mediation, the detailed photophysical and photochemical processes as well as the origin of the enhanced conductivity of nanocrystals are still unclear. In this work, the comprehensive kinetic scheme and regulation mechanism are established by the accurate QM/MM calculations at the CASPT2//CASSCF/AMBER level of theory with consideration of the crystal environment. We find that the argentophilic interaction and through space electronic interaction are the key factors that promote Ag(I)-mediated [2+2] PCA reactions and may account for the enhancement of conductivity. These mechanistic insights into the Ag(I)-regulated photo-dimerization in the crystal surrounding are beneficial for the design of the structurally and electrically favorable skeletons of a metal-organic coordination polymer.

Theoretical Exploration of Energy Transfer and Single Electron Transfer Mechanisms to Understand the Generation of Triplet Nitrene and the C(sp3)-H Amidation with Photocatalysts

Mechanistic explorations and kinetic evaluations were performed based on electronic structure calculations at the CASPT2//CASSCF level of theory, the Fermi's golden rule combined with the Dexter model, and the Marcus theory to unveil the key factors regulating the processes of photocatalytic C(sp³)-H amidation starting from the newly emerged nitrene precursor of hydroxamates. The highly reactive nitrene was found to be generated efficiently via a triplet-triplet energy transfer process and to be benefited from the advantages of hydroxamates with long-range charge-transfer (CT) excitation from the N-centered lone pair to the 3,5-bis(trifluoromethyl)benzoyl group. The properties of the metal-to-ligand charge-transfer (MLCT) state of photocatalysts, the functionalization of chemical moieties for substrates involved in the charge-transfer (CT) excitation, such as the electron-withdrawing trifluoromethyl group, and the energetic levels of singlet and triplet reaction pathways may regulate the reaction yield of C(sp³)-H amidation. Kinetic evaluations show that the triplet-triplet energy transfer is the main driving force of the reaction rather than the single electron transfer process. The effects of electronic coupling, molecular rigidity, and excitation energies on the energy transfer efficiency were further discussed. Finally, we investigated the inverted behavior of single-electron transfer, which is correlated unfavorably to the catalytic efficiency and amidation reaction. All theoretical explorations allow us to better understand the generation of nitrene with visible-light photocatalysts, to expand highly efficient substrate sources, and to broaden our scope of available photosensitizers for various cross-coupling reactions and the construction of N-heterocycles.

Diradical Generation via Relayed Proton-Coupled Electron Transfer

Diradical generation followed by radical-radical cross-coupling is a powerful synthetic tool, but its detailed mechanism has yet to be established. Herein, we proposed and confirmed a new model named relayed proton-coupled electron transfer (relayed-PCET) for diradical generation, which could open a door for new radical-radical cross-coupling reactions. Quantum mechanics calculations were performed on a selected carbene-mediated diradical cross-coupling reaction model and a designed model, and the exact electronic structural changes during the radical processes have been observed for the first time.

Prediction on chemoselectivity for selected organocatalytic reactions by the DFT version of the Hückel-defined free valence index

The DFT version of the Hückel-defined free valence (HFV) index has been suggested and successfully used for predicting the origin of chemoselectivity in the selected organocatalytic reactions.