CBr has been employed as a halogen bond donor catalyst for the selective activation of aldehyde, to achieve an efficient solvent- and metal-free C═C bond forming reaction in the presence of strong acid sensitive groups such as methoxy, cyanide, ester, and ketal for the synthesis of α,β-unsaturated ketones. This unique capability of CBr to act as a halogen bond donor has been explored and established using UV-vis as well as IR spectroscopy. Moreover, this unprecedented methodology enables the synthesis of the pharmaceutically important molecule licochalcone A.
An
efficient protocol for the synthesis of thioether directly from
heteroarenes has been developed in the presence of visible light in
a one-pot manner at room temperature. This method involves two sequential
reactions in a single pot where the formation of the iodinated heteroarene
is followed by a transition-metal-free C–S coupling reaction.
A wide range of heteroarene and thiol partners (including aliphatic
thiols) have been used for the synthesis of thioethers. NMR studies
and DFT calculations revealed the presence of a halogen bond between
the thiolate anion (halogen bond acceptor) and iodoheteroarene (halogen
bond donor). This halogen bonded complex on photoexcitation facilitates
the electron transfer from the thiolate anion to the iodoheteroarene
at room temperature.
The widespread use of halogen‐based reagents in organic synthesis is well known. Hypervalent halogens (oxidation states III and V) have been explored to a significant extent. Among them, hypervalent iodine reagents have found major use and applications. In contrast, understanding of the reactivity of halogen(I) species in the presence of Lewis bases, and their reaction mechanisms, are very limited. This microreview sheds light on the importance of halogen(I) species and their applications in organic synthesis. In commercially available halogen(I) precursors such as N‐halosuccinimides, the halogen atom is attached to the nitrogen atom through a covalent bond with low electrophilicity and thus low reactivity. Interestingly, the reactivity increases if the electrophilic halogen(I) atom is attached to a Lewis base through a polarized covalent bond. The halogen‐bonding interaction between the Lewis base and the halonium ion (or halonium ion equivalent) results in highly reactive and thermodynamically stable halogen(I) intermediates that act as versatile reagents. This microreview unravels the evolution of the unstable halogen(I) intermediates to become versatile reagents.
A CBr4 catalyzed simple, mild, and efficient protocol has been developed for the synthesis of 2-substituted benzothiazole from 2-aminothiophenols and N-methylthioamides under solvent free conditions.
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