## 2 Direct \(\alpha\)-Halogenation of Aryl Ketones
### Metal Halides (MX)
This route yielded a significant amount of either the ring iodinated or the \(\alpha\),\(\alpha\)-diiodinated product for substrates carrying OH- and OMe-substituents. The iodination thus appeared to be less selective than the chlorination, which ultimately hinders its applicability. One of the most interesting approaches employing ammonium halide salts is its combination with electrochemical oxidation conditions [92]. In an electrochemical cell equipped with a Pt/Pt electrodes, aryl ketones were efficiently brominated (Scheme 32). Under these conditions, Br- was oxidised to Br2, which then reacted with water to generate the HOBr active species. The broad substrate scope highlighted the versatile nature of this protocol, and despite the acidic conditions, acid-sensitive groups (i.e., OH) were well tolerated. [PERSON] and co-workers developed a solvent-free NH4 Br-mediated \(\alpha\)-bromination of alkanones [93]. After grinding a mixture of the starting material, NH4 Br (2 equiv.) and (NH4)2S2O8 (2.5 equiv.) for 15 min at room temperature, 10 (hetero)aromatic ketones were isolated in good to excellent yields (83-92%). The procedure tolerated the presence of electron-withdrawing (i.e., Cl, NO2), -donating (i.e., OMe, Me) and acid-sensitive (i.e., OH) substituents on the aryl ketones. It should be noted that trace amounts of water were required to ensure complete homogenisation of the solid reaction mixture. Hydrogen Halides (HX)  
At first sight, HX might seem a valuable halogenation alternative to bromine, as it will ensure a high halide atom economy. However, the highly corrosive and toxic nature of this reagent still limits its applicability [94]. Nonetheless, a significant body of work has been reported on this topic and several noteworthy advances have been made. [PERSON] and co-workers developed an aqueous HBr-based halogenation system for aromatic ketones [95]. In this system, the combination of NaNO\({}_{2}\) and KI acted as the bromination catalyst (Scheme 33). The versatility of this route was demonstrated through the bromination of a broad range of aromatic ketones, bearing electron-withdrawing or -donating substituents. [PERSON] and co-workers demonstrated the versatility of an NH\({}_{4}\)NO\({}_{3}\)- and I\({}_{2}\)-based system in air [96]. The procedure yielded seven \(\alpha\)-chloroketones in moderate to good yields (Scheme 34). This protocol stands out due to its excellent selectivity, as no ring chlorination or dichloromethane was observed. [PERSON] and co-workers developed an HBr, NaNO\({}_{2}\) and O\({}_{2}\)-mediated ring bromination, yielding a diverse set of halogenated arenes (Scheme 35) [97]. When using aryl ketones, \(\alpha\)-bromination occurred instead, generating five \(\alpha\)-bromoketones in moderate to excellent yields (68-92%). The NO\({}_{2}\)-substituted aryl ketones underwent a less efficient bromination, which aligns well with previous observations on the deactivation of electron-poor substrates. The use of molecular oxygen as co-oxidant should also be highlighted. The use of a more environmentally friendly oxidant system, such as H\({}_{2}\)O\({}_{2}\), has also been reported in combination with HBr, by [PERSON] and co-workers (Scheme 36) [98]. Interestingly, the bromination was carried out in water, further highlighting the sustainabilityof this system. 1,3-Diketones, \(\beta\)-ketoesters, and cyclic ketones were brominated in good to excellent yields. However, the scope only consisted of one aromatic ketone, again not making it possible to perform an evaluation of the strength of the protocol. 2-Bromo-1-phenylethanone was isolated in 78% yield. This method exhibited moderate selectivity, since for some of the substrates, unwanted dibromination was a serious issue. [PERSON] and co-workers performed a comparative study between a H\({}_{2}\)O\({}_{2}\) and a TBHP system with HBr as the bromine source [99]. A set of substrates, bearing electron-withdrawing (i.e., NO\({}_{2}\), Cl) and -donating (i.e., OH) groups, were brominated in good to excellent yields (75-95%).