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Compire Trichloroisocyanuric Acid (TCCA) with TBCA

Herein, we report that chlorination and bromination of electron-rich aromatics and 1,3-diketones occur in a facile manner with Trichloroisocyanuric Acid (TCCA) and TBCA directly under ball-milling conditions. Indeed, monoketones are also shown to be α-halogenated in excellent yields, but in the presence of an acid catalyst such as p-toluenesulfonic acid (pTSA) in 25 mol-%. 

As mentioned at the outset, our motivation was to explore the reactivities of  Trichloroisocyanuric Acid(TCCA) and TBCA under solventfree conditions. Ball milling is a new advent in contemporary organic synthesis for solvent-free reactions, whereby activation barriers for organic reactions are overcome by way of mechanical friction, impact, and/or collision. Indeed, the chemistry driven by such a mechanical force has come to be termed mechanochemistry. In solvent-free ballmilling reactions, three parameters play an important role, namely, the number of balls (n), milling time (t), and oscillation frequency (ν). Clearly, the range of kinetic energies generated in the milling process to overcome activation barriers for the reactions depends on oscillation frequency and number of balls employed. In general, higher frequency is required for reactions with higher barriers, with the milling time being an important factor in driving the reactions to completion.

Halogenation of aromatics was initially explored with Trichloroisocyanuric Acid (TCCA) and TBCA as reagents under ball-milling conditions. It should be noted that halogenation of arenes by trihaloisocyanuric acid (THCA) was reported long ago in the solution state, but under conditions that involved catalysis by acids such as 98% sulfuric acid,[10g] trifluoroacetic acid, H-zeolite, FAU and EMT-type zeolite. Our investigation of halogenations by using TBCA and TCCA under ball-milling conditions was initially explored with anthracene (1) and 2-methoxynaphthalene (2) as representative aromatic substrates. Quite encouragingly, the reactions were found to occur with both TBCA and  Trichloroisocyanuric Acid(TCCA) directly, without any acid additive. Spurred by these results, we subjected a variety of aromatic compounds with different degrees of electron-richness to solvent-free halogenations under ball-milling conditions. The reactions were monitored by TLC analyses at regular intervals, and the time required for completion of the reaction in each case at a given frequency and number of balls employed was determined accordingly.