Photocatalytic Aerobic Oxidative Ring Expansion of Cyclic Ketones to Macrolactones by Cerium and Cyanoanthracene Catalysis

Abstract

We describe ac erium-catalyzed aerobic oxidative ring expansion for the expedient construction of synthetically challenging macrolactones under visible-light conditions.C yanoanthracene has been employed as co-catalyst to accelerate the turnover of the cerium cycle leading to af ast conversion within 20 min of irradiation. Taking advantage of the high efficiency and operationally simple conditions,acollection of over 100 macrolactones equipped with ring systems ranging from 9-to 19-membered macrocycles have been prepared from simple building blocks.Moreover,the enabling potential of this strategy to simplify the generation of molecular complexity has been demonstrated through the concise synthesis of sonnerlactone.

Visible‐Light‐Induced Homolysis of Earth‐Abundant Metal‐Substrate Complexes: A Complementary Activation Strategy in Photoredox Catalysis

“…The same group has developed a straightforward ring‐expansion strategy of cyclic alkoxyketones to synthesize 9‐ to 19‐membered macrolactones in the presence of a cerium salt and cyanoanthracene under aerobic conditions through irradiation with visible light [Scheme 6 C, Eq. b‐vi] [69] . The successful development of the strategy relied upon the capability of the [Cl n Ce IV ‐OR] intermediate to undergo VLIH to form [Cl n Ce III ] and an alkoxy radical species for the subsequent β‐scission step.…”

“…b‐vi]. [69] The successful development of the strategy relied upon the capability of the [Cl n Ce IV ‐OR] intermediate to undergo VLIH to form [Cl n Ce III ] and an alkoxy radical species for the subsequent β‐scission step. Of note, the mild Lewis acidic nature of the cerium salt was also conducive to enhance the formation of the lactol intermediate from the ketone/lactol tautomeric equilibrium.…”

Cerium-photocatalyzed aerobic oxidation of benzylic alcohols to aldehydes and ketones

“…An operationally simple method avoiding waste and potentially toxic transition-metal catalysts that is able to convert any benzylic alcohol selectively to the aldehyde or ketone is still desirable. Recently, cerium photocatalysis was introduced as a robust alternative to generate oxygen or carbon-centered radicals under mild reaction conditions [57][58][59][60][61][62][63][64]. CeCl 3 reacts via ligand-to-metal charge transfer generating oxygen-centered radicals, that lead to carbon-centered radicals through intra/intermolecular hydrogen atom transfer (HAT) processes, radical decarboxylative or radical deformylation [57][58][59].…”

Durch sichtbares Licht induzierte Homolyse unedler, gut verfügbarer Metallsubstratkomplexe: Eine komplementäre Aktivierungsstrategie in der Photoredoxkatalyse

“…[65] Basierend auf diesen mechanistischen Te ilschritten entwickelten Zuo ). [69] Die erfolgreiche Entwicklung der Strategie beruhte auf der Fähigkeit des [Cl n Ce IV -OR]-Zwischenprodukts,V LIH zu [Cl n Ce III ]-und einer Alkoxylradikalspezies fürd en anschließenden b-Spaltungsschritt einzugehen. Bemerkenswerterweise war die milde Lewis-Acidität des Cersalzes auch fçrderlich, um die Bildung des Lactol-Zwischenprodukts aus dem tautomeren Keton/Lactol-Gleichgewicht zu bewirken.…”

“…Dieselbe Gruppe hat eine einfache ringerweiternde Strategie von cyclischen Alkoxyketonen entwickelt, um 9‐ bis 19‐gliedrige Makrolactone in Gegenwart von einem Cersalz und Cyananthracen unter Bestrahlung mit sichtbarem Licht und aeroben Bedingungen zu synthetisieren (Schema 6 C, Gl. [b‐vi]) [69] . Die erfolgreiche Entwicklung der Strategie beruhte auf der Fähigkeit des [Cl n Ce IV ‐OR]‐Zwischenprodukts, VLIH zu [Cl n Ce III ]‐ und einer Alkoxylradikalspezies für den anschließenden β‐Spaltungsschritt einzugehen.…”