Selective oxidation of benzylic alcohols via synergistic bisphosphonium and cobalt catalysis

Photoinduced Vicinal Difunctionalization of Diaryliodonium Salts To Access Bis(tetraphenylphosphonium) Salts

Vicinal bis(tetraarylphosphonium) salts have scarcely been reported in the literature. In this study, we demonstrate that visible-light-induced difunctionalization of ortho-trifluoromethylsulfonylated diaryliodonium salts conveniently furnishes bis(phosphonium) salts without additional catalysts or photoinitiators. The methodology establishes a practical platform for the preparation of bis(phosphonium) salts using readily available tertiary phosphines. The bis(tetraarylphosphonium) salts are anticipated to garner a great deal of interest in catalytic and medicinal chemistry.

[(NHC)Pd(OAc)2]: Highly Active Carboxylate Pd(II)-NHC (NHC = N-Heterocyclic Carbene) Precatalysts for Suzuki-Miyaura and Buchwald-Hartwig Cross-Coupling of Amides by N-C(O) Activation

In the past eight years, the selective cross-coupling of amides by N-C(O) bond activation has emerged as a highly attractive manifold for the manipulation of traditionally unreactive amide bonds. In this Special Issue on Next-Generation Cross-Coupling Chemistry, we report the Suzuki-Miyaura and Buchwald-Hartwig cross-coupling of amides by selective N-C(O) cleavage catalyzed by bench-stable, well-defined carboxylate Pd(II)-NHC (NHC = N-heterocyclic carbene) catalysts {[(NHC)Pd(O2CR)2]}. This class of Pd(II)-NHCs promotes cross-coupling under exceedingly mild room-temperature conditions owing to the facile dissociation of the carboxylate ligands to form the active complex. These readily accessible Pd(II)-NHC precatalysts show excellent functional group tolerance and are compatible with a broad range of amide activating groups. Considering the mild conditions for the cross-coupling and the facile access to carboxylate Pd(II)-NHC complexes, we anticipate that this class of bench-stable complexes will find wide application in the activation of amide N-C(O) and related acyl X-C(O) bonds.

Selective monodeuteration enabled by bisphosphonium catalyzed ring opening processes

The selective incorporation of a deuterium atom into small molecules with high selectivity is highly valuable for medical and chemical research. Unfortunately, this remains challenging due to the complete deuteration caused by commonly used hydrogen isotope exchange strategies. We report the development of a photocatalytic selective monodeuteration protocol utilizing C-C bond as the unconventional functional handle. The synergistic combination of radical-mediated C-C bond scission and deuterium atom transfer processes enables the effective constructions of benzylic CDH moieties with high selectivity for monodeuteration. The combinational use of a bisphosphonium photocatalyst, thiol catalyst, and CH3OD deuteration agent provides operationally simple conditions for photocatalytic monodeuteration. Moreover, the photoinduced electron transfer process of the bisphosphonium photocatalyst is elucidated through a series of spectroscopy experiments, identifying a peculiar back electron transfer process that can be regulated by subsequent nucleophilic additions.

Strategies and Mechanisms of First-Row Transition Metal-Regulated Radical C-H Functionalization

Radical C-H functionalization represents a useful means of streamlining synthetic routes by avoiding substrate preactivation and allowing access to target molecules in fewer steps. The first-row transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are Earth-abundant and can be employed to regulate radical C-H functionalization. The use of such metals is desirable because of the diverse interaction modes between first-row transition metal complexes and radical species including radical addition to the metal center, radical addition to the ligand of metal complexes, radical substitution of the metal complexes, single-electron transfer between radicals and metal complexes, hydrogen atom transfer between radicals and metal complexes, and noncovalent interaction between the radicals and metal complexes. Such interactions could improve the reactivity, diversity, and selectivity of radical transformations to allow for more challenging radical C-H functionalization reactions. This review examines the achievements in this promising area over the past decade, with a focus on the state-of-the-art while also discussing existing limitations and the enormous potential of high-value radical C-H functionalization regulated by these metals. The aim is to provide the reader with a detailed account of the strategies and mechanisms associated with such functionalization.

Continuous Flow Approach for Benzylic Photo-oxidations Using Compressed Air

A continuous flow approach for the aerobic photo-oxidation of benzylic substrates to ketone and aldehyde products is presented. The resulting process exploits UV-A LEDs (375 nm) in combination with a Corning AFR reactor that ensures effective gas-liquid mixing and leads to short residence times of 1 min. A variety of benzylic substrates are converted to their corresponding carbonyl products, and scalability is demonstrated to produce multigram quantities of products within a few hours. Overall, this continuous flow approach offers several improvements over alternative oxidation methods due to the combined use of air as an oxidant and SAS (sodium anthraquinone-2 sulfonate) as a water-soluble photocatalyst. The use of greener and safer conditions together with process intensification principles renders this flow approach attractive for further industrial applications.

Visible Light-Mediated Selective Aerobic Oxidation of Alcohols Catalyzed by Disulfide in Batch and Flow

Selective aerobic oxidation of alcohols in batch and flow can be realized under light irradiation, utilizing disulfide as the photocatalyst, and a variety of primary and secondary alcohols were converted to the corresponding aldehydes or ketones in up to 99% yield and high selectivity. The reaction efficiency could be increased even further by combining a continuous-flow strategy. Detailed mechanistic studies have also been achieved to determine the role of oxygen and disulfides in this oxidation.