C-H Halogenation of Pyridyl Sulfides Avoiding the Sulfur Oxidation: A Direct Catalytic Access to Sulfanyl Polyhalides and Polyaromatics

Alkali halides as nucleophilic reagent sources for N-directed palladium-catalysed ortho-C-H halogenation of s-tetrazines and other heteroaromatics

A general palladium-catalysed selective C-H halogenation reaction is reported, which was successfully achieved for a large variety of functionalized aromatic rings incorporating diverse N-directing groups. By using simple alkali halides of MX type as the nucleophilic reagent source (M = Li, Na, K, Cs and X = I, Br and Cl), and phenyliodanediacetate oxidant, clean C-H-iodination, bromination and chlorination reactions were performed. This general protocol of selective ortho-monohalogenation, which complements but contrasts with the classical methods using electrophilic reagents, is achievable in a short time (30 min) with microwave irradiation assistance. The reaction was extended to substrates bearing N-directing pyridine, pyrimidine, pyrazole, oxazoline, naphtho[1,2-d]thiazole, and azobenzene groups. Notably, the topical and selectivity-challenging s-tetrazine, as a nitrogen-rich heteroaromatic, was successfully halogenated by this protocol.

Nanocatalysts for High Selectivity Enyne Cyclization: Oxidative Surface Reorganization of Gold Sub-2-nm Nanoparticle Networks

Ultrasmall gold nanoparticles (NPs) stabilized in networks by polymantane ligands (diamondoids) were successfully used as precatalysts for highly selective heterogeneous gold-catalyzed dimethyl allyl(propargyl)malonate cyclization to 5-membered conjugated diene. Such reaction usually suffers from selectivity issues with homogeneous catalysts. This control over selectivity further opened the way to one-pot cascade reaction, as illustrated by the 1,6-enyne cycloisomerization-Diels-Alder reaction of dimethyl allyl propargyl malonate with maleic anhydride. The ability to assemble nanoparticles with controllable sizes and shapes within networks concerns research in sensors, medical diagnostics, information storage, and catalysis applications. Herein, the control of the synthesis of sub-2-nm gold NPs is achieved by the formation of dense networks, which are assembled in a single step reaction by employing ditopic polymantanethiols. By using 1,1'-bisadamantane-3,3'-dithiol (BAd-SH) and diamantane-4,9-dithiol (DAd-SH), serving both as bulky surface stabilizers and short-sized linkers, we provide a simple method to form uniformly small gold NPs (1.3 ± 0.2 nm to 1.6 ± 0.3 nm) embedded in rigid frameworks. These NP arrays are organized alongside short interparticular distances ranging from 1.9 to 2.7 nm. The analysis of gold NP surfaces and their modification were achieved in joint experimental and theoretical studies, using notably XPS, NMR, and DFT modeling. Our experimental studies and DFT analyses highlighted the necessary oxidative surface reorganization of individual nanoparticles for an effective enyne cycloisomerization. The modifications at bulky stabilizing ligands allow surface steric decongestion for the alkyne moiety activation but also result in network alteration by overoxidation of sulfurs. Thus, sub-2-nm nanoparticles originating from networks building create convenient conditions for generating reactive Au(I) surface single-sites-in the absence of silver additives-useful for heterogeneous gold-catalyzed enyne cyclization. These nanocatalysts, which as such ease organic products separation, also provide a convenient access for building further polycyclic complexity, owing to their high reactivity and selectivity.