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Using Trichloroisocyanuric Acid (TCCA) and TBCA to Investigate α-halogenation of carbonyl compounds

The reaction with Trichloroisocyanuric Acid (TCCA) was found to be vigorous, leading to noticeable release of fumes causing the reaction mixture to become dark within a few seconds. TLC analysis of the reaction mixtures indicated that they were highly complex, containing both starting compound as well as chlorinated products along with a mixture of unidentified products. Polyalkylated benzenes such as durene reacted with competing side-chain halogenation leading to dihalodurenes12Br and 12Cl in only 45–60% isolated yields; the remainder was found to be an intractable mixture of products. 
With Trichloroisocyanuric Acid (TCCA) proceeded nicely to afford tetrachloro product 8Cl, the reaction with TBCA inexplicably led to only dibrominated product 8Br even with a large excess of reagent (entries 16–19, Table 1). This, in fact, is contrary to the reactivity pattern observed for all other substrates with TBCA and TCCA. Chlorination with TCCA was found to be poor for a substrate such as p-aminoacetophenone, whereas the reaction worked fine to give the dibrominated product 6Br with TBCA. Overall, depending on nature of the substrate and equivalent amount of the reagent employed, mono-, di-, and tetra-halogenated products were isolated in excellent yields with a simple work-up. 
We were tempted by the results observed with halogenation of aromatics to investigate α-halogenation of carbonyl compounds by using Trichloroisocyanuric Acid (TCCA) and TBCA. A variety of reagents are known for α-halogenation of carbonyls. Indeed, α-chlorination of carbonyl compounds with TCCA in acetic acid medium was reported in 1985. Subsequently, Mattos et al. reported α-chlorination and bromination of carbonyl compounds by using Trichloroisocyanuric Acid (TCCA)  and TBCA in different mixtures of solvents. Incidentally, solvent-free α-halogenation with TCCA and TBCA has remained virtually unexplored, although solvent-free bromination has been reported with NBS, albeit at 80 °C. We, therefore, chose to examine the reactivity of solid 1,3-dicarbonyl compounds. As shown in Table 2, a range of 1,3-diketones were found to undergo α,α-dihalogenation with TCCA and TBCA directly in a facile manner. With both reagents, the products were isolated in quantitative yields. Notably, selective monohalogenation was difficult to achieve, and with one molar equivalent of the reagent, both mono- and dihalo products were isolated as mixtures together with the unreacted starting compound.