3 research outputs found
LightâPromoted Organocatalysis with NâHeterocyclic Carbenes
The combination of Nâheterocyclic carbene (NHC)âorganocatalysis with photochemical activation can lead to unprecedented reaction pathways not observed without light. A selection of dual catalytic systems merging NHCs with photoredox catalysis, for example, have leveraged the ability of NHCs to stabilize intermediate radical species and facilitate single electron transfer events. Furthermore, NHCs can transiently modulate the photochemical properties of a substrate and enable direct absorption of otherwise photoinactive organic compounds. In this Concept article, the recent advances in this rapidly developing field are summarized. For each transformation, a particular focus is placed on the mechanistic aspects of the process while key features that highlight the synthetic potential of the light mediated NHC organocatalysis are presented
Nâheterocyclic carbene catalyzed photoenolization/DielsâAlder reaction of acid fluorides
The combination of light activation and Nâheterocyclic carbene (NHC) organocatalysis has enabled the use of acid fluorides as substrates in a UVAâlightâmediated photochemical transformation previously observed only with aromatic aldehydes and ketones. Stoichiometric studies and TDâDFT calculations support a mechanism involving the photoactivation of an orthoâtoluoyl azolium intermediate, which exhibits âketoneâlikeâ photochemical reactivity under UVA irradiation. Using this photoâNHC catalysis approach, a novel photoenolization/DielsâAlder (PEDA) process was developed that leads to diverse isochromanâ1âone derivatives
A (TD-)DFT study on photo-NHC catalysis: photoenolization/DielsâAlder reaction of acid fluorides catalyzed by N-heterocyclic carbenes
A comprehensive mechanistic study on the N-heterocyclic carbene (NHC) catalyzed photoenolization/DielsâAlder (PEDA) reaction of acid fluorides was performed in the framework of (time-dependent) density functional theory ((TD)-DFT). The 1,5-hydrogen atom transfer (1,5-HAT) during photoenolization of an ortho-toluoyl azolium salt was found to be feasible via, first, singlet excitation and photoenolization, and then, after crossing to the triplet manifold, populating a biradical dienol which allows for the formation of two ortho-quinodimethane (o-QDM) isomers due to a low rotational barrier. The (Z)-isomer is mostly unproductive through sigmatropic rearrangement back to the starting material while the (E)-isomer reacts in a subsequent concerted DielsâAlder reaction likely as the deprotonated dienolate. The experimentally observed diastereoselectivity is correctly predicted by theory and is determined by a more favorable endo trajectory in the cycloaddition step. These findings demonstrate that ortho-toluoyl azolium species exhibit similar photophysical properties as structurally related benzophenones, highlighting the unique ability of the NHC organocatalyst to transiently alter the excited state properties of an otherwise photoinactive carboxylic acid derivative, thereby expanding the scope of classical carbonyl photochemistry