## 2 Direct \(\alpha\)-Halogenation of Aryl Ketones
### Metal Halides (MX)  
Similarly to their NaNO\({}_{2}\)-I\({}_{2}\) system (Scheme 25), [PERSON] and co-workers designed a NaNO\({}_{2}\)-KI-based procedure for the \(\alpha\)-iodination of aromatic ketones, which yielded nine \(\alpha\)-iodoketones in moderate to excellent yields (Scheme 26) [83]. KI exhibited similar iodination efficiency to molecular iodine when compared to the NaNO\({}_{2}\)-I\({}_{2}\)-mediated reaction [55]. Nonetheless, potassium iodide remains a much better choice than I\({}_{2}\) as an iodination agent, as it is cheaper and safer to handle. It should be noted that a similar effect of the bromination solvent on the regioselectivity was observed, as described for the I\({}_{2}\) reaction. When carried out in aqueous ethanol, selective \(\alpha\)-bromination occurred. When the iodination was performed in dry acetonitrile, the aromatic ring was exclusively halogenated. Another MX-based procedure was developed by [PERSON] and co-workers using NaCl and K\({}_{2}\)SO\({}_{8}\) to perform the chlorination of arenes in acetonitrile [84]. The selectivity of the method was altered to exclusively lead to the \(\alpha\)-chlorination w used. Since the scope only consisted of one aryl ketone, broader screening is required for a better assessment of this route's efficacy. A similar sodium chloride-based halogenation was reported in 2004 (Scheme 27) [85], with a broad range of alkanes, arenes and ketones being converted into their chlorinated derivatives. However, the method only showed moderate chlorination efficiency with aryl ketones. In 2005, [PERSON] and co-workers reported the synthesis of \(\alpha\)-bromoketones mediated by the combination of UHP and NaBr with acetic acid-functionalised silica. After microwave irradiation at 300 W, 12 \(\alpha\)-bromoaryl ketones could be isolated in good yields (Scheme 28) [86; 87]. The presence of electron-withdrawing, -donating and acid-sensitive groups on the phenyl ring was well tolerated. This route's main features are its solvent-free conditions and the extremely short reaction times. Quaternary Ammonium Halides (NH4X)  
Over the past two decades, a significant amount of work has been dedicated to the development of NH4X- and Oxone(r)-mediated halogenation reactions. The high stability, water solubility and non-toxic nature make Oxone(r) an attractive oxidation agent. In 2012, [PERSON] and [PERSON] independently described the chlorination of aromatic ketones in MeOH using a NH4 Cl and Oxone(r) system (Scheme 29) [88]. The mechanism follows a similar path to MX-based oxidation approaches, generating in situ the X2 active reagents. Both protocols established a broad substrate scope although [PERSON]'s method showed the best chlorination efficiency. The two procedures reported a similar trend, i.e., electron-poor aryl ketones were less efficiently chlorinated. No polychlorination or ring chlorination was observed for either route, stressing their excellent selectivity. Using the same system, [PERSON] and co-workers reported the \(\alpha\)-bromination of 21 aromatic ketones in low to excellent yields (24-97%) (Scheme 30) [89]. Electron-deficient substrates (i.e., bearing CN and NO2 groups) appeared to be deactivated under these conditions, as they were isolated in only 21 and 35% yield. Furthermore, this route showed a rather moderate selectivity for these substrates, since \(\alpha\)-bromo-dimethylketals were obtained as the major products. Additionally, some substrates showed signs of ring halogenation, adding to the system's selectivity issues. **Scheme 30. ** The Oxone-based bromination of aromatic ketones by [PERSON] and co-workers. [PERSON] and co-workers went on to develop another procedure for the \(\alpha\)-iodination of ketones, generating a wide variety of \(\alpha\)-iodokettones (Scheme 31) [90; 91]. Again, a similar trend was observed; aryl ketones bearing electron-withdrawing groups were less efficiently iodinated.