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

Abstract

The mechanistic understanding of catalytic radical reactions currently lags behind the flourishing development of new types of catalytic activation. Herein, an innovative single electron transfer (SET) model has been expanded by using the nonadiabatic crossing integrated with the rate-determining step of 1,5-hydrogen atom transfer (HAT) reaction to provide the control mechanism of radical decay dynamics through calculating excited-state relaxation paths of a paradigm example of the amide-directed distal sp 3 C−H bond alkylation mediated by Ir-complex-based photocatalysts. The stability of carbon radical intermediates, the functional hindrance associated with the back SET, and the energy inversion between the reactive triplet and closed-shell ground states were verified to be key factors in improving catalytic efficiency via blocking radical inhibition. The expanded SET model associated with the dynamic behaviors and kinetic data could guide the design and manipulation of visible-light-driven inert bond activation by the utilization of photocatalysts bearing more or less electronwithdrawing groups and the comprehensive considerations of kinetic solvent effects and electron-withdrawing effects of substrates.

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

“…The predicted λ max is closely correlated to experimentally measured values for 2 a and 2 b (452–457 nm) , as well as the irradiation wavelength (456 nm) used for the photosensitized nitrene generation in Chang’s study . Similar to our previous computations, ,, MLCT absorption bands of Ru-based complexes studied here also originate from the strong coupling among all possible electron transitions from metal to various ligands, which provides a potential way to tune the intensity of the maximum MLCT absorption of photocatalysts.…”

“…The predicted λ max is closely correlated to experimentally measured values for 2 a and 2 b (452−457 nm) 46,47 as well as the irradiation wavelength (456 nm) used for the photosensitized nitrene generation in Chang's study. 67 Similar to our previous computations, 68,69,72 MLCT absorption bands of Rubased complexes studied here also originate from the strong coupling among all possible electron transitions from metal to various ligands, which provides a potential way to tune the intensity of the maximum MLCT absorption of photocatalysts. It is worth noting that previous experiments have revealed that this photosensitized C−H amidation reaction takes place in high yields both in the gas phase (70%) and in a weakly polar environment (1,1,2,2-tetrachloroethane: 75%; dichloromethane: 70%), while it is less effective in polar media (acetonitrile: 9%; N,N-dimethylformamide: <5%).…”

“…The photocatalyst 2 a is promoted by blue light irradiation (414 nm) at first, populating in the FC of S MLCT , and then speedily relaxing to its minimum. As observed in our previous studies, − the STCs between singlet and triplet MLCT states ( 1 dπ*/ 3 dπ*) are ubiquitous when the visible-light-initiated photosensitizers just evolve out of the FC region. The strong relativistic effect of transition metal ions leads to the ultrafast ISC in the timescale of subpicoseconds (see Table S17), thereby improving the quantum yield of the T MLCT state.…”

Diradical Generation via Relayed Proton-Coupled Electron Transfer

“…Once dismissed as unselective and suitable only for initiating polymerization, in recent years there has been enormous progress toward taming free radicals for applications in target synthesis. − Critical to their success has been the development of powerful but highly selective methods for radical generation, particularly using photochemistry, − electrochemistry, − organic superelectron donors, and transition metal catalysis. − Nonetheless, the discovery of new strategies of radical generation, − particularly under light-free, electricity-free, metal-free conditions, remains crucial. Organocatalysis, particularly involving N-heterocyclic carbenes (NHCs), shows promise approach for realizing radical generation and radical–radical cross-coupling. − However, to develop this chemistry, a greater understanding of how they mediate electron transfer is critical.…”

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

“…However, the HMO calculations are not accurate due to the nature of semi-empirical quantum chemical calculations. To solve this issue and promoted by our interest in organocatalysis 46,[60][61][62][63][64] and theoretical research experiences, [65][66][67][68][69] we believe that the DFT version of the HFV index should be more suitable to predict the actual active site of the organocatalytic reaction systems, in which the chemoselectivity is generally associated with the different active sites of the conjugated structures of the reactants or intermediates. As depicted in Scheme 1, the HFV index (denoted as F i ) represents the activity of the i atom obtained by the maximum π bond order of benchmark trimethylenemethane minus its related π bond orders, and the HFV index of the active carbon atom is obtained using F i = P max − P P ij .…”