Mechanochemical halogenation of unsymmetrically substituted azobenzenes

Abstract

The direct and selective mechanochemical halogenation of C–H bonds in unsymmetrically substituted azobenzenes using N-halosuccinimides as the halogen source under neat grinding or liquid-assisted grinding conditions in a ball mill has been described. Depending on the azobenzene substrate used, halogenation of the C–H bonds occurs in the absence or only in the presence of PdII catalysts. Insight into the reaction dynamics and characterization of the products was achieved by in situ Raman and ex situ NMR spectroscopy and PXRD analysis. A strong influence of the different 4,4’-substituents of azobenzene on the halogenation time and mechanism was found.

Disentangling the Effect of Pressure and Mixing on a Mechanochemical Bromination Reaction by Solid‐State NMR Spectroscopy**

“…In light of the observed clean and fast bromination of the sulfoximine in the 4-position of the S-aryl group, a double bromination of 1 was attempted in analogy to previously reported mechanochemical halogenations with NBS 2. 76,80 Thus, milling 2 equiv. of NBS with respect to the sulfoximine 1 (molar ratios) was tested.…”

“…This result motivated us to investigate this halogenation in more detail. We herein describe a bromination of 2-methyl-3H-2λ 4 benzo[c]isothiazole 2-oxide with N-bromosuccinimide (NBS) performed in a mixer mill and its detailed solid-state NMR spectroscopic investigation (for previous halogenation reactions of arenes, see [75][76][77][78][79][80] ). 13 C-detected solid-state MAS NMR has been used to study the reaction products ex situ.…”

Dissecting Mechanochemistry III

“…Ćurić and co-workers [4]. They demonstrated how, depending on the azobenzene structure, the halogenation of the C-H bonds with NBX occurred in the presence of Pd(II) catalysts or under metal-free conditions (Scheme 1b).…”

“…N -Halosuccinimides were also key reagents to develop the mechanochemical halogenation of azobenzenes as studied by Ćurić and co-workers [ 4 ]. They demonstrated how, depending on the azobenzene structure, the halogenation of the C–H bonds with NBX occurred in the presence of Pd(II) catalysts or under metal-free conditions ( Scheme 1b ).…”

The Role of Crystalline Intermediates in Mechanochemical Cyclorhodation Reactions Elucidated by in‐Situ X‐ray Powder Diffraction and Computation

“…Since its introduction, [27] this approach has been used to follow in real-time mechanochemical milling reactions permitting the observation of multistep transformations, reaction kinetics, and the discovery of new materials. [25][26][27][28][29][30] However, in-situ monitoring of mechanochemical reactions such as metal-mediated CÀ H activations has only been developed by Raman spectroscopy, [31][32][33][34] while real-time monitoring by synchrotron powder X-ray diffraction remains unexplored. Thus, to deepen our understanding in this area we began this study.…”