A Unified Mechanism for the PhCOCOOH‐mediated Photochemical Reactions: Revisiting its Action and Comparison to Known Photoinitiators

Abstract

Since 2014, we have introduced in literature the use of phenylglyoxylic acid (PhCOCOOH), a small and commercially available organic molecule, as a potent promoter in a variety of photochemical processes. Although PhCOCOOH has a broad scope of photochemical reactions that can promote, the understanding of its mode of action in our early contributions was moderate. Herein, we are restudying and revisiting the mechanism of action of PhCOCOOH in most of these early contributions, providing a unified mechanism of action. Furthermore, the understanding of its action as a photoinitiator opened a new comparison study with known and commercially available photoinitiators.

Light-promoted oxidation of aldehydes to carboxylic acids under aerobic and photocatalyst-free conditions

“…Again, using a simple work-up procedure, the peracid could be transformed into the desired acid. 25 Thus, we can generalise, that in the cases of 2i–k , the slightly lower yields can be due to the above-mentioned fragmentation, 26 while a mixture of acid : peracid was observed and it can be transformed to the desired acid, upon the appropriate treatment. Aromatic aldehydes were found to require longer reaction times, usually 5–8 hours.…”

“…When α,α-disubstituted aldehydes are employed under photochemical conditions or radical chemistry conditions, in general, the produced acyl radical can be fragmented to the corresponding secondary aliphatic radical with CO extrusion, which usually leads to lower yields. 7,26 Herein, when the aldehyde group was at a secondary carbon atom, products 2i–k were produced in slightly lower yields (53–75%) under either LED 370 nm or sunlight. It is interesting to note that 2-ethyl-hexanoic acid ( 2j ), which is industrially synthesised employing an aerobic oxidation process, 3 b can be produced in good yield (66%) at 370 nm.…”

Photochemical aerobic upcycling of polystyrene plastics to commodity chemicals using anthraquinone as the photocatalyst

“…At the beginning, we employed aromatic ketones that are known to perform HAT reactions, like thioxanthone ( 2a ) 41 or benzophenone ( 2g ), which was presented in literature to work (under mercury lamp irradiation though) 31 or acenaphthoquinone ( 2h ), but in all cases, the yield of benzoic acid was 1.9–6.0% (Scheme 2). Aryl ketones that perform as photoinitiators, such as phenylglyoxylic acid ( 2e ) 42 or 2,2-dimethoxy-2-phenylacetophenone ( 2f ) 43 led to a slight increase in yield (Scheme 2). Organic dyes that are known as singlet oxygen generators or can perform HAT processes were also tested, 44 however similar low yields were obtained (Scheme 2, 2i or 2j ).…”

Functionalisation of ethereal-based saturated heterocycles with concomitant aerobic C–H activation and C–C bond formation

“…17 Radical-based C-H bond activation is a potentially powerful and versatile strategy for the functionalisation of saturated heterocycles. [18][19][20][21] However, existing approaches, whilst effective, typically employ specialised initiators and/or careful management of reaction conditions to achieve this in a controlled fashion. [22][23][24][25][26] Aerobic C-H activation, utilising oxygen in air to promote radical-based C-H bond cleavage, represents a potentially ideal approach, [26][27][28][29][30] but to date has not been exploited for the formation of C-C bonds on saturated heterocycles.…”