Hantzsch Ester Anion as Electron Donor in Transition-Metal-Free Photoinducided N-Desulfonylation Reactions

Viña del Mar

Congreso; XIV Encuentro LAtinoamericano de Fotoquímica y Fotobiología; 2020

ELAFOT

Hantzsch esters (HEs) have been used as reductants in catalytic hydrogenation reactions.Recently, with the advances in the visible light photoredox catalysis, it has been grown theuse of HEs as electron donors and proton sources in many photoredox reaction types. 1 Forexample, in hydrodifluoroalkylation of alkenes, 2 reductive couplings of carbonyls withalkenes 3 or carboxylation of aromatic and aliphatic bromides and triflates. 4Sulfonyl group is useful for protecting nitrogen containing compounds. However, due toits great stability, requires rigorous deprotection conditions. 5 Recently, it has been developeda photocatalytic N-S cleavage using an iridium complex and the HE to give amides asproducts in excellent yields. 6Here, we report an efficient and transition-metal-free photoinducided desulfonylationmethod using the Hantzsch ester anion as promotor. The photostimulated reaction (visiblelight) of 1 (R=H), in the presence of HE anion (2 equiv.) in DMSO for 1 h afforded thedeprotected product 2 (R=H) in 92 % yield at room temperature (eq 1). There was notreaction under dark conditions, which excludes a polar mechanism. The effect of the basewas evaluated by carrying out the photostimulated reaction in the absence of KO t Bu. Underthis condition the product 2 was not obtained indicating that the base is necessary. Also, thereaction without HE only in presence of KO t Bu (or NaH) gave the product in 15% yield. Thisresult indicates that HE anion is responsible to initiate the ET reaction.Our proposal is to investigate the mechanism of these visible-light-induced photo-removable sulfonyl group reactions supplying information from the study of experimentalreaction conditions and photophysical properties.