Metal catalyzed halogenation

It is the metal catalyzed halogen exchange for haloarenes, that is to say the arylation of halides anions (chloride, bromide, iodide), acting as nucleophiles (Fig. 21). 

Supported platinum metal-catalyzed halogenation of methane ... 

Transition-Metal Catalyzed Cyclizations. o-Halogenated anilines and anilides can serve as indole precursors in a group of reactions which are typically cataly2ed by transition metals. Several catalysts have been developed which convert o-haloanilines or anilides to indoles by reaction with acetylenes. An early procedure involved coupling to a copper acetyUde with o-iodoaniline. A more versatile procedure involves palladium catalysis of the reaction of an o-bromo- or o-trifluoromethylsulfonyloxyanihde with a triaLkylstaimylalkyne. The reaction is conducted in two stages, first with a Pd(0) and then a Pd(II) catalyst (29). 

Halogen-substituted 2-pyridones are key intermediates for further metal-catalyzed coupling reactions and the halogenation of these scaffolds has already been described in previous sections. In the following section, a variety of C - C and C - N cross-coupling reactions under microwave-assisted conditions are described with some illustrative examples. 

In order to get reliable information about the yield of organoaluminum compounds formed in a transition metal-catalyzed hydroalumination reaction it is essential to derivatize the organometallic products by quenching the reaction mixture with electrophiles like D2O, O2 or halogens. It is often observed that hydrogenation... 

The presence of a remote C=C double bond in the alkyl halide was critical saturated analogs underwent nickel-catalyzed halogen-zinc exchange.248 It has been suggested that the double bond coordinates to the nickel atom. As a 7r-acceptor, the C=C bond removes some electron density from the metal, thus facilitating the reductive coupling (Scheme 154).246... 

No examples of such reactions have been disclosed. Displacement of halogens on the parent heterocycle through metal-catalyzed processes have surprisingly not been reported to our knowledge on the neutral heterocycle. Recently, Suzuki-Miyaura cross-coupling reactions of imidazolium bromide 113 with various boronic acids or esters were reported <2005T6207> to proceed in good yield, without deprotonation at the C-3 position (Scheme 35). 

Traditional methods for the halogenation of Ge-H bonds are still widely used.1 116 A versatile palladium-metal-catalyzed method has been reported (Equation (93), Table 14).125... 

The section Replacement of halogen by metal or by coupling in CHEC-II(1996) <1996CHEC-II(6)1>, mentioning only a few examples of metal-catalyzed reactions, is divided in this edition in Sections 8.01.7.15.1 and 8.01.7.15.2. 

One of the most frequently studied transition metal catalyzed transformations of azoles and indole is their participation in cross-coupling reactions. Due to the abundance of examples in this field we only present some representative examples of the different reaction classes. In this chapter reactions where a halogenated azole is used to introduce the five membered ring onto the palladium in the oxidative addition and processes,... 

This chapter discusses the transition metal catalyzed functionalization of such systems that fall outside the topic of Chapters 6 and 7, as well as certain other compound classes (e.g. purines, pyrones). In contrast to the abundant literature of the chemistry of five and six membered systems, the transition metal catalyzed transformations of other heterocycles have not been studied so far in the same depth, probably due to the limited availability of their halogen derivatives compared to haloazines and haloazoles. Purine compounds and their structural analogues constitute an exception, since their biological importance proved to be a strong drive for synthetic chemist worldwide.1... 

In a series of reactions similar to those described for amino acid complexes, pyruvic acid undergoes metal-catalyzed condensation with aldehydes and also halogenation (Scheme 14). Again, the metal ion presumably functions by accelerating the formation of the methylene carbanion. 

For electrophilic and more bulky radicals, for example CnHal2n+i, selectivity increases dramatically. Whereas the selectivity for the radical chlorination (Table 1, entry 7) and bromination (Table 1, entry 8) of adamantane with Hal2 in CC14 is only approximately 5, for chlorinations with CC14 (Table 1, entries 9 and 10) and brominations with BrCCl3 (Table 1, entry 11) the selectivity increases to 21.5-27. Halogenations under metal-catalyzed GIF conditions (Table 1, entry 12) or in the presence of dioxirane (Table 1, entry 13) are less selective, because of the participation of oxygen-centered (GIF) or methyl (DMD) radicals. 

The increased development of transition metal-catalyzed cross-coupling methods to form C-C bonds has served as an impetus to find methods to synthesize 3-halochromones and 3-haloflavones. The synthesis of 3-halochromones and flavones can be achieved with the addition of halogens across the double bond of the pyrone ring by reaction with a halogenating reagent (e.g., Br2) followed by spontaneous, or base-induced, elimination (Scheme 48). Synthesis of these important compounds has been recently reviewed <2003RCR489>. 

Susuki and Tsuji reported the first Kharasch addition/carbonylation sequences to synthesize halogenated acid chlorides from olefins, carbon tetrachloride, and carbon monoxide catalyzed by [CpFe(CO)2]2 [101]. Its activity is comparable to or better than that of the corresponding molybdenum complex (see Part 1, Sect. 7). Davis and coworkers determined later that the reaction does not involve homolysis of the dimer to a metal-centered radical, which reduces the organic halide, but that radical generation occurs from the dimeric catalyst after initial dissociation of a CO ligand and subsequent SET [102]. The reaction proceeds otherwise as a typical metal-catalyzed atom transfer process (cf. Part 1, Fig. 37, Part 2, Fig. 7). 

Just as in aryl halides, the halogen can be replaced by hydrogen and by a metal, or be involved in transition metal-catalyzed processes (covered in Section 3.2.3.11.2). Three of the mechanisms of such nucleophilic substitutions are familiar from benzene chemistry via arynes, SrnI processes, and Pd(0)-catalyzed sequences. However, of the two further mechanisms of nucleophilic replacement, the ANRORC (Addition of Micleophile, Ring Opening, Ring Closure) is unique to heterocycles, and Sae reactions occur only with strongly activated benzenoid systems. 

---