The Photooxygenation of Benzyl, Heteroarylmethyl, and Allyl Sulfides

Abstract

The photosensitised oxidation of benzyl ethyl sulfides in aprotic solvents (benzene or acetonitrile) gives the corresponding aldehydes under mild conditions. This is a general reaction which applies to benzyl derivatives containing either electron‐donating or electron‐withdrawing substituents and furthermore to hetero analogues such as 2‐pyridinylmethyl sulfide (not to the 3‐indolylmethyl sulfide, since reaction at the heterocycle moiety competes) as well as to allyl sulfides. In a protic solvent (methanol) these sulfides give the sulfoxides instead (except for the nitrobenzyl derivatives, where the aldehyde remains the major product). Among the α‐substituted sulfides tested, the α‐phenylbenzyl and the 3‐cyclohexenyl sulfide give the corresponding ketone (the latter in a low yield), but the α‐methylbenzyl sulfide gives the sulfoxide as the main product. The rate for singlet oxygen quenching and for chemical reaction have been measured for representative benzyl sulfides. The reaction is discussed in the frame of the currently accepted mechanism for sulfide photooxygenation. The key step for oxidative C–S bond cleavage appears to be hydrogen transfer from the activated α position in the first formed intermediate, the persulfoxide. This reaction is inhibited in methanol where the persulfoxide is hydrogen‐bonded.

Efficient Metal‐Free Aerobic Photooxidation of Sulfides to Sulfoxides Mediated by a Vitamin B2 Derivative and Visible Light

“…Our method may also be used for other substrates that hardly undergo oxidation, such as tert ‐butyl phenyl sulfide 2i (entry 9) and diphenyl sulfide 2l (entry 12), which is relatively resistant to ET photooxidation and is unreactive towards singlet oxygen 33. Oxidation of benzylic substrates 2k and 2m took place without side oxidation to aldehyde (entries 11 and 13), which is often observed in photooxidations 14a,20a,d,34. We also did not observe side oxidation of 2j (entry 10) and substrates of high practical importance, e.g., sulfide 2o , smoothly afford the sulfoxidation product modafinil ( 3o , Provigil; see entry 15), a psychostimulant used for the treatment of narcolepsy, extensive daytime sleepiness and hypersomnia 35 .…”

Aerobic Photocatalysis: Oxidation of Sulfides to Sulfoxides

“…Albini and coworkers pioneered the field of sulfide photooxidation and significantly contributed to the understanding of the reaction mechanism through seminal publications in the 2000s, [23] using a vast variety of organic catalysts, such as Rose Bengal ( 5 , Scheme 5), [23a–d] tetraphenylporphyrine, [23a,c,d] DCA ( 4 , Scheme 5), [23e,f] NMQ + ( 6 , Scheme 5), [23e] and triphenylpyrylium salt ( 7 , Scheme 5). [23e,f] A variety of experimental, monitoring and quenching methods were employed, to study the behavior of aromatic, benzylic and tert ‐butyl sulfides towards oxidation and photodecomposition (cleavage).…”

“…Initially, Albini and coworkers studied the photooxidation of substituted benzyl, heteroaryl methyl and allyl phenyl sulfides, using Rose Bengal ( 5 ) as the photocatalyst, either in MeCN or MeOH or triphenylporphyrin as the catalyst in benzene and differences in the reaction outcome were revealed [23a] . More specifically, in aprotic solvents, such as acetonitrile or benzene, the main product after irradiation were aryl carbonylic compounds, whereas in methanol, sulfoxides became the main products.…”

Flavin Dibromide as an Efficient Sensitizer for Photooxidation of Sulfides

“…However, the poor absorption of these inorganic photocatalysts in the visible light region has limited their practical applications. − Different from inorganic materials, the photoredox properties of organic sensitizers may be precisely tailored by rational design and synthetic approach . Recently, several organic sensitizers, such as N -methylquinolinium tetrafluoroborate, 9,10-dicyanoanthracene, rose bengal, , 2,4,6-triphenylpyrylium tetrafluoroborate, 4,4-difluoro-4-bora-3a,4a-diaza- s -indancene (BODIPY), methyl blue, Eosin Y, , etc., have been investigated for photocatalytic oxidation of sulfides. The major drawback of these organic sensitizer is the high oxidation potential of ROS, leading to overoxidation and competitive carbon oxidation, making the product containing not only sulfoxides but also sulfones and aldehydes, etc. , Therefore, it is still demanding for efficient and selective production of sulfoxides in mild reaction condition via sulfide oxidation …”