The structure of electronically excited α,β‐unsaturated lactones

Abstract

A better knowledge of the structure of the electronically excited state of substrates is indispensable for the understanding and optimization of photochemical reactions. For this study, triplet energies of a variety of α,β-unsaturated γ-lactones (furanones) as well as the structures of the vibrationally relaxed triplet state (T 1 ) have been determined using ab initio coupled-cluster (CCSD) method and/or density functional theory (DFT) calculation. A twist of the original planar structure around C = C bond is found in the relaxed triplet state, π-π*. In the 5-membered ring of furanones the contribution of this mode is limited and the pyramidalization in the C 4 position also contributes to the stabilization. The contribution of each stabilization mode is characterized by the dihedral angles and the Mulliken atomic spin densities. The substituent effect on the pyramidalization and the spin density distribution in the C 4 and in the C 5 position are reported. Depending on the substitution in the C 4 position, the orientation of the pyramidalization is either favored syn or anti with respect of the hydroxyl substituent in the C 5 position. Figure 3. Energies of different electronic states for furanone 1. Structures were optimized in the ground state (left-hand side) and in the T 1 state (right-hand side) at the B3LYP/6-31G(d,p) level. Energies for the higher excited states, T 2 , S 1 , and S 2 , were calculated using TD-DFT Chart 1. Model compounds chosen for the computational study. M. FRÉNEAU ET AL. wileyonlinelibrary.com/journal/poc

Photosensitization and Photocatalysis—Perspectives in Organic Synthesis

“…After intersystem crossing (ISC) to the corresponding triplet state, efficient triplet energy transfer to the furanones 13 or 14 occurs, leading to the ππ* triplet state 17 . The relaxed structure is characterized by a high spin density in the β-position . For this reason, a hydrogen atom is transferred from 1′ or the 5′ position of the glucosyl moiety to the β-position of the furanone moiety, leading to diradical intermediates 18 and 19 depending on the relative configuration of the substrate.…”

Photocatalytic Transformation of Biomass and Biomass Derived Compounds—Application to Organic Synthesis

“…The thesis also provides a very convenient access to 5-hydroxymethylfuranones Using conventional methods of carbohydrate chemistry, these compounds a Chiral synthons can be obtained from this compound and many efficient applications to asymmetric synthesis have been reported [237][238][239][240]. At the electronically excited state, reactions are generally less stereoselective due to the suppression of the C-C π-bond [141] Hydorxyfuranone 72 was also used for the preparation of biodegradable surfactants (Figure 3) [241]. A photoredox catalytic process was also applied for the preparation of such compounds [242,243].…”

“…The electronic excitation occurs via triplet energy transfer from acetone to 42 . The 3 (ππ)* state ( 43 ) is characterized by the suppression of the C=C π-bond and an increased spin density in the β-position [ 135 , 141 ]. For this reason, hydrogen abstraction occurs from the anomeric center of the sugar moiety into this position and the intermediates 46 and 47 are formed, both in conformational equilibrium.…”

Photochemically Induced Hydrogen Atom Transfer and Intramolecular Radical Cyclization Reactions with Oxazolones

“…These mechanisms have a significant influence on the stereo- and regioselectivity of such hydrogen atom transfer reactions. Some time ago, we have performed investigation on a photosensitized intramolecular hydrogen abstraction–radical combination sequence with furanone derivatives in which the stereo- and regioselectivity was studied in detail. , Similar reactions have been carried out by direct light absorption by the substrate . Recently, we studied photochemically induced intramolecular hydrogen abstraction reactions with similar heterocyclic compounds, oxazolones carrying an imine function .…”