Photocatalytic Dehydrogenative Lactonization of 2-Arylbenzoic Acids

Abstract

A metal-free dehydrogenative lactonization of 2-arylbenzoic acids at room temperature was developed. This work illustrates the first application of visible-light photoredox catalysis in the preparation of benzo-3,4-coumarins, an important structural motif in bioactive molecules. The combination of photocatalyst [Acr + -Mes] with (NH 4 ) 2 S 2 O 8 as terminal oxidant provides an economical and environmentally benign entry to different substituted benzocoumarins. Preliminary mechanistic studies suggest that this reaction most likely occurs through a homolytic aromatic substitution pathway.

Organic Photoredox Catalysis

“…349 A considerable number of benzo-3,4-coumarins 84.2 were synthesized by this protocol which employs ammonium persulfate (NH 4 ) 2 S 2 O 8 as a terminal oxidant, many in excellent yields (e.g., 84.3−84.5). Varying degrees of regioselectivity were observed when the 2-aryl group possessed a substituent at the 3′ position (e.g., lactone 84.4).…”

Novel Oxidative Ugi Reaction for the Synthesis of Highly Active, Visible‐Light, Imide‐Acridinium Organophotocatalysts

“…The ground state reduction of catalysts 4 occurs at low potentials (around −0.3 V vs. SCE) with a relative excited state reduction potential around 2.4 V (showing slightly variations depending on the substituents), which are above the values of the Fukuzumi's photocatalyst III ( E 1/2 = +2.18 V vs. SCE) . After obtaining these appealing results, the catalytic activity of 4 was then investigated in the dehydrogenative lactonization of 2‐phenylbenzoic acid ( 5 ) as model reaction (see the optimization Table S2) . This transformation is known to proceed through a reductive quenching cycle and was already reported with the Fukuzumi's commercial catalyst III .…”

Mild, visible light-mediated decarboxylation of aryl carboxylic acids to access aryl radicals

“…5b,c Though generation of I via photoredox catalysis has been described, either via single electron reduction of benzoyl peroxide 6 a or single electron oxidation of benzoates, 6b,7 decarboxylation was not reported in these cases. Instead, I has been reported to add to arenes to provide benzoate esters (Scheme 1a).…”

“…Instead, I has been reported to add to arenes to provide benzoate esters (Scheme 1a). 6 Additionally, our laboratory recently disclosed the use of I as a hydrogen atom transfer (HAT) catalyst for the functionalisation of unactivated C(sp 3 )–H bonds. 7 While the activation energy for the decarboxylation of I is only 8–9 kcal mol –1 , 8 a the rate of decarboxylation ( k = 1.4 × 10 6 s –1 ) is not competitive with nondecarboxylative pathways including addition to arenes ( k = 2.2 × 10 8 M –1 s –1 ) 8 a and HAT ( k = 1.2 × 10 7 M –1 s –1 ).…”