Mechanistic Study of the Mechanochemical PdII-Catalyzed Bromination of Aromatic C–H Bonds by Experimental and Computational Methods

Controlling the regioselectivity of the bromolactonization reaction in HFIP

The halolactonization reaction provides rapid access to densely functionalized lactones from unsaturated carboxylic acids. The endo/exo regioselectivity of this cyclization reaction is primarily determined by the electronic stabilization of alkene substituents, thus making it inherently dependent on substrate structures. Therefore this method often affords one type of halolactone regioisomer only. Herein, we introduce a simple and efficient method for regioselectivity-switchable bromolactonization reactions mediated by HFIP solvent. Two sets of reaction conditions were developed, each forming endo-products or exo-products in excellent regioselectivity. A combination of computational and experimental mechanistic studies not only confirmed the crucial role of HFIP, but also revealed the formation of endo-products under kinetic control and exo-products under thermodynamic control. This study paves the way for future work on the use of perfluorinated solvents to dictate reaction outcomes in organic synthesis.

Elemental Sulfur-Mediated Aromatic Halogenation

A method for aromatic halogenation using a combination of elemental sulfur (S8) and N-halosuccinimide has been developed. A catalytic quantity of elemental sulfur (S8) with N-bromosuccinimide (NBS) and N-chlorosuccinimide (NCS) effectively halogenated less-reactive aromatic compounds, such as ester-, cyano-, and nitro-substituted anisole derivatives. No reaction occurred in the absence of S8, underscoring its crucial role in the catalytic activity. This catalytic system was also applicable to aromatic iodination with 1,3-diiodo-5,5-dimethylhydantoin.

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

The occurrence of crystalline intermediates in mechanochemical reactions might be more widespread than previously assumed. For example, a recent study involving the acetate-assisted C-H activation of N-Heterocycles with [Cp*RhCl2 ]2 by ball milling revealed the formation of transient cocrystals between the reagents prior to the C-H activation step. However, such crystalline intermediates were only observed through stepwise intervallic ex-situ analysis, and their exact role in the C-H activation process remained unclear. In this study, we monitored the formation of discrete, stoichiometric cocrystals between benzo[h]quinoline and [Cp*RhCl2 ]2 by ball milling using in-situ synchrotron X-ray powder diffraction. This continuous analysis revealed an initial cocrystal that transformed into a second crystalline form. Computational studies showed that differences in noncovalent interactions made the [Cp*RhCl2 ]2 unit in the later-appearing cocrystal more reactive towards NaOAc. This demonstrated the advantage of cocrystal formation before the acetate-assisted metalation-deprotonation step, and how the net cooperative action of weak interactions between the reagents in mechanochemical experiments can lead to stable supramolecular assemblies, which can enhance substrate activation under ball-milling conditions. This could explain the superiority of some mechanochemical reactions, such as acetate-assisted C-H activation, compared to their solution-based counterparts.

Iron complexes of bridging azo ligands in aqueous solution: changes in the thermal switching mechanism on coordination and oxidation state of metal centres

Three azobenzenes CN(C₆H₄)-NN-(C₅H₄N) (py-iso), CN(C₆H₄)-NN-(C₆H₄)CN (cyano-iso) and CN(C₆H₄)-NN-(C₆H₄)NC (iso-iso) with good coordinating groups (pyridine, phenylcyano or phenylisocyano) at the ends of the diazenyl unit have been synthesized and fully characterised. These compounds have been used as ligands in the synthesis of water-soluble metallic species by coordination to {Fe^II(CN)₅^3-} units, either in one or two of the anchoring groups of the derivatives. Both the azo derivatives and their complexes are photochemically active with respect to their trans-to-cis isomerisation process. Their cis-to-trans reverse thermal reaction has been thoroughly studied as a function of the donor groups, solvent, temperature and pressure, in order to gain insight into the rotation or inversion mechanisms involved in the process. A comparison of the isomerisation mechanism between the iron complexes and the corresponding free ligands revealed an interesting fine tuning of the process on coordination of the {Fe^II(CN)₅^3-} moieties, which may even produce, in some cases, non-photoswitchable species containing typically photoactive units.

Mechanochemical halogenation of unsymmetrically substituted azobenzenes

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 Pd^II 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.