Bond-Forming Processes Enabled by Silicon-Masked Aryl- and Alkyl-Substituted Diazenes

Aryl- and alkyldiimides (R-N═NH with R = aryl or alkyl) are elusive intermediates of valuable synthetic use, as they are assumed to be transient species in processes involving both carbon (with concomitant loss of N2) and nitrogen nucleophiles (with conservation of the N═N moiety). The actual compounds are fragile and as such not bench stable which is why they have not yet found the attention they deserve. Conversely, early contributions showed that the stability of the parent diimide is significantly increased by replacing the hydrogen atom by a silyl group, but the synthetic applicability of these silicon-protected aryl- and alkyldiazenes has been far less explored, in part due to the absence of general procedures for their preparation. This Perspective provides an overview of the underexplored diazene chemistry that has witnessed considerable progress in recent years and highlights the potential of this motif in a range of synthetically useful (catalytic) transformations. The rediscovered silicon-masked diazenes constitute a versatile platform possessing enhanced stability and tamed reactivity in comparison to the parent hydrogen-substituted diimides. Aryl, diazenyl, and alkyl anionic key intermediates can be selectively generated in situ under Lewis base or transition metal catalysis, giving rise to novel synthetic approaches as viable alternatives to the already existing methodologies.

Palladium-Catalyzed Cross-Electrophile Coupling between Aryl Diazonium Salt and Aryl Iodide/Diaryliodonium Salt in H2O-EtOH

We report herein a mild highly chemoselective palladium-catalyzed cross-electrophile coupling between readily accessible aromatic diazonium salt and aryl iodide or diaryliodonium salt in water-ethanol (2:1) medium. Mechanistic studies revealed that ethanol is crucial to generate an active Pd(0) catalyst, and the counterion of the diazonium salt renders a cationic Pd(II) species that facilitates subsequent oxidative addition to aryl iodide/diaryliodonium salt. Silver(I) salt was crucial to retain the catalytic activity of palladium, removing the iodide ion as a precipitate.

FeO(OH)@C-Catalyzed Selective Hydrazine Substitution of p-Nitro-Aryl Fluorides and their Application for the Synthesis of Phthalazinones

An efficient hydrazine substitution of p-nitro-aryl fluorides with hydrazine hydrates catalyzed by FeO(OH)@C nanoparticles is described. This hydrazine substitutions of p-nitro-aryl fluorides bearing electron-withdrawing groups proceeded efficiently with high yield and selectivity. Similarly, hydrogenations of p-nitro-aryl fluorides containing electron-donating groups also smoothly proceeded under mild conditions. Furthermore, with these prepared aryl hydrazines, some phthalazinones, interesting as potential structures for pharmaceuticals, have successfully been synthesized in high yields.

2-Thiazolines: an update on synthetic methods and catalysis

2-Thiazolines are important building blocks in organic synthesis and are of great importance in many areas of chemistry. At the end of the last century, the use of 2-thiazolines increased in a significant way, especially in synthesis and catalysis. This review highlights the synthetic and catalytic value of 2-thiazolines in the last two decades. We will discuss the new synthetic methodologies for obtaining these heterocycles including new schemes for accessing their asymmetric versions. Most of the new catalytic applications include a variety of 2-thiazoline ligands containing diverse donor atoms, which in combination with metals like Pd, Ir, and Cu, among others, exhibit remarkable catalytic performances.

Room-Temperature Palladium(II)-Catalyzed Direct 2-Arylation of Indoles with Tetraarylstannanes

A palladium(II)-catalyzed direct 2-arylation of indoles by tetraarylstannanes with oxygen (balloon) as the oxidant at room temperature has been developed. Various tetraarylstannanes can be employed as aryl sources for 2-arylation of indoles in up to 89% yield, providing a practical and efficient catalytic protocol for accessing 2-arylindoles.

The synthesis of quinolines via denitrogenative palladium-catalyzed cascade reaction of o-aminocinnamonitriles with arylhydrazines

The first example of the palladium-catalyzed cascade reaction of o-aminocinnamonitriles with arylhydrazines has been achieved, providing an efficient synthetic pathway to access quinolines with moderate to good yields. Preliminary mechanistic experiments indicate that this cascade process involves sequential denitrogenative addition followed by an intramolecular cyclization.

Pd-catalyzed cascade denitrogenative addition and intramolecular cyclization of o-aminocinnamonitriles with arylhydrazine for the synthesis of quinolines.

Pd-Catalyzed decarboxylative alkynylation of alkynyl carboxylic acids with arylsulfonyl hydrazides via a desulfinative process

The decarboxylative alkynylation of alkynyl carboxylic acids and arylsulfonyl hydrazides by desulfinative coupling could provide aryl alkynes in satisfactory yields by either judiciously selecting palladium catalysts or modulating phosphine ligands under mild conditions.