Photoinduced C(sp³)–H Chalcogenation of Amide Derivatives and Ethers via Ligand-to-Metal Charge-Transfer

Abstract

A photoinduced, iron(III) chloride-catalyzed C−H activation of Nmethyl amides and ethers leads to the formation of C−S and C−Se bonds via a ligand-to-metal charge transfer (LMCT) process. This methodology converts secondary and tertiary amides, sulfonamides, and carbamates into the corresponding amido-N,S-acetal derivatives in good yields. Mechanistic work revealed that this transformation proceeds through a hydrogen atom transfer (HAT) involving chlorine radical intermediates.

Ligand‐to‐Metal Charge Transfer (LMCT) Photochemistry at 3d‐Metal Complexes: An Emerging Tool for Sustainable Organic Synthesis

“…Interestingly, steric hindrance seems to affect more to this reaction in comparison with the previously discussed Giese additions, resulting in lower propensity to undergo functionalization in C−H tertiary centers. Very recently, Laulhé and co‐workers also used this activation mode for the formation of C−S and C−Se bonds (Scheme 18B) [113] . Upon light irradiation in presence of catalytic FeCl 3 , C−H chalcogenation of linear and cyclic amides was reported exclusively at the position in α to nitrogen, enabling access to amido‐N,S‐acetals moieties that are found in several natural products and antibacterials.…”

“…Very recently, Laulhé and co-workers also used this activation mode for the formation of CÀ S and CÀ Se bonds (Scheme 18B). [113] Upon light irradiation in presence of catalytic FeCl 3 , CÀ H chalcogenation of linear and cyclic amides was reported exclusively at the position in α to nitrogen, enabling access to amido-N,S-acetals moieties that are found in several natural products and antibacterials. The α-N-alkyl radical accessed via HAT with Cl * attacks to a disulfide/diselenide compound, leading to the desired product and a chalcogen radical (e. g. PhS * ) that undergoes SET oxidation of Fe(II) completing the catalytic cycle.…”

Iron Photoredox Catalysis–Past, Present, and Future

“…The activated substrate can oxidize the iron species, regenerate the catalyst, and form a substrate anion that reacts further. Although this reaction mode is of use in the catalysis of a wide variety of organic transformations, − the exact photoreactive species are in many cases not unambiguously assigned by thorough mechanistic investigations. The traditional reaction systems utilizing visible light (Scheme A–D) − suffer from impaired reactivity in solvents other than acetonitrile and show a general necessity for irradiation with higher-energy light (390 nm).…”

“…Although this reaction mode is of use in the catalysis of a wide variety of organic transformations, − the exact photoreactive species are in many cases not unambiguously assigned by thorough mechanistic investigations. The traditional reaction systems utilizing visible light (Scheme A–D) − suffer from impaired reactivity in solvents other than acetonitrile and show a general necessity for irradiation with higher-energy light (390 nm). However, the adaption of the reaction system through the addition of additives, such as TRIP 2 S 2 (1,2-bis­(2,4,6-triisopropylphenyl) disulfane) and 2,4,6-collidine (Scheme A) or the introduction of pyridine-diimine (PDI)-based ligands (Scheme E) increases the absorption wavelength to 440–450 nm, resulting in more benign reaction conditions, thereby potentially diminishing undesirable side-reactions.…”

Recent Advances in Visible‐Light‐Driven C−S Bond Formation

“…A broad range of aryl alkyl sulfides ( 20.3 a – c , 46 examples, some of them are depicted in Scheme 20) was obtained in moderate to good yields via photoredox catalyzed coupling of redox‐active N‐hydroxyphthalimide esters and disulfides, providing a broad range of aryl alkyl sulfides in moderate (when alkyl disulfides are employed) to good yields under mild conditions (Scheme 20). [61] The preparation of aryl alkyl sulfides was also achieved starting from aryl halides as the arylating agents under UV (254 nm) irradiation [62] …”