Selective Debromination and a-Hydroxylation of a-Bromoketones Using Hantzsch Esters as
Recent advances in synthetic chemistry benefit from one-electron processes mediated by photoredox catalysts. The use of photoredox catalysts enables reactions to proceed under mild conditions, as photon absorption provides large redox power to execute reactions that are thermodynamically challenging. Indeed, a variety of photoredox reactions have been devised to promote diverse organic transformations with synthetic and industrial utility, thus replacing the traditional approaches, such as those relying on organotin hydrides. In particular, major advances have been made by employing transition metal complexes of iridium or ruthenium as visible-light photocatalysts. The visible light-driven photoredox catalysis involves photoinduced one-electron transfer of the metal complexes, which produces reactive radical intermediates along with radical cationic or radical anionic species of the catalysts. Completion of the catalytic cycle necessitates restoration of the radical ion species of the catalysts to the ground state by dumping or scavenging one electron from sacrificial electron acceptors and donors, respectively. 1,4-Dihydropyridine-3,5-di-carboxylate esters [Hantzsch esters (HEs)] have been widely employed as the sacrificial electron donor for this purpose.
In our continuing efforts on the development of photoredox catalysis, we serendipitously found that HEs could act as photoreductants. We report herein a simple metal-free protocol for transformations of α-bromo ketones through the efficient gene ration of radical intermediates under photoexcitation of HEs. HE and its analogues have been used as both electron and hydrogen-atom donors to promote selective debromination and α-hydroxylation reactions in the absence of any additional reagents (Figure 1). There-action pathways depended strongly on the conditions, such as solvents, presence of O2, and the concentration of HEs. The synthetic protocol exhibited high functional group compatibility under mild reaction conditions. Studies based on the Stern–Volmer analysis and photoluminescence lifetime measurements using time-correlated single-photon-counting techniques were performed to elucidate the role of HEs. The mechanistic studies revealed that HEs were versatile photoreductants.
* Related article
“Selective Debromination and a-Hydroxylation of a-Bromoketones Using Hantzsch Esters as Photoreductants” Advanced Synthesis & Catalysis, 13 January 2016, 358, 74-80