P(V)-bis(amidophenolate) ligand cooperation: stoichiometric CO-bond cleavage in aldehydes and ketones.

The cooperation between a geometrically constrained, highly electrophilic phosphorus(V) center, and an electronically rich tetradentate bis(amidophenolate) ligand enables the cleavage of the CO bond from typical aldehydes and ketones delivering iminio phosphoramidate species. The amphiphilic nature of these products, which is demonstrated through their reaction with typical Lewis acids and bases, enables their use as a mild source of silylium cations from silanes, allowing the selective reductive coupling of aldehydes to ethers under catalytic conditions.

Cooperation between p-Block Elements and Redox-Active Ligands: Stoichiometric and Catalytic Transformations.

The relevance acquired by redox-active ligands in modern catalysis stems from their facile delivery and acceptance of electrons, either to the metal they coordinate to, or directly to an incoming substrate that also binds the central metal. Doing that, they generate coordinated radicals and provide access to more than one spin state during the catalytic cycle. As a consequence, the new reaction barriers are reduced when compared to similar processes that are restricted to a single spin surface. The principles that govern this genuine approach to catalyst design are well-established, and their implementation has allowed the development of synthetically useful catalytic transformations; however, the extension of the concept to species in which p-block elements take the role of central atoms remains largely underdeveloped. Through discussion of the key achievements and recent progress, this Concept Article highlights this original approach to designing (organo)catalysts, discloses the progress achieved, and also reveals the many shortcomings that still exist in the field.

Ligand-Enabled Disproportionation of 1,2-Diphenylhydrazine at a PV -Center.

We present herein the synthesis of a nearly square-pyramidal chlorophosphorane supported by the tetradentate bis(amidophenolate) ligand, N,N'-bis(3,5-di-tert-butyl-2-phenoxy)-1,2-phenylenediamide. After chloride abstraction the resulting phosphonium cation efficiently promotes the disproportionation of 1,2-diphenylhydrazine to aniline and azobenzene. Mechanistic studies, spectroscopic analyses and theoretical calculations suggest that this unprecedented reactivity mode for PV -centres is induced by the high electrophilicity at the cationic PV -center, which originates from the geometry constraints imposed by the rigid pincer ligand, combined with the ability of the o-amidophenolate moieties to act as electron reservoir. This study illustrates the promising role of cooperativity between redox-active ligands and phosphorus for the design of organocatalysts able to promote redox processes.

Synthesis of 6H-Benzo[c]chromene Scaffolds from O-Benzylated Phenols through a C-H Sulfenylation/Radical Cyclization Sequence.

S-Aryl dibenzothiophenium salts, obtained through a highly regioselective C-H sulfenylation of o-benzyl-protected phenols, are used as precursors of 6H-benzo[c]chromenes. The reaction starts with a photocatalytically triggered single-electron transfer to the sulfonium salt, which promotes the formation of an aryl radical via selective mesolitic cleavage of the S-Arexo bond. Mechanistic studies reveal that this initial radical species cyclizes following a kinetically favored 5-exo-trig pathway. Subsequent ring expansion, favored by rearomatization, delivers the desired tricyclic systems.