Halides source

A variety of halide sources have been shown to be capable of displacing the nitro group of the 1,2,4-triazolo[5,l-c][l, 2,4] triazines (180). Unexpectedly, chlorine and bromine in acetic acid gave the same products, presumably via electrophilic pathways (82CHE992). 

Whereas the Prins-type cyclizations reported in this and the preceeding chapter were performed using stoichiometric amounts of Fe salts as Lewis acids, a breakthrough in the field of catalysis was reported in 2009 when the first iron-catalyzed Prins- and aza-Prins cyclization was reported. The catalytic system, which is obtained by combining catalytic amounts of an iron salt with trimethylsilyl halides as a halide source, is widely applicable and promotes the construction of substituted six-membered oxa- and aza-cycles (Scheme 33) [44]. 

The reaction can be used for making either chlorides or bromides by using the appropriate tetraalkylammonium salt as a halide source. 

This reaction is similar to the Sandmeyer reaction, but the halide source is potassium iodide (Kl). Figure 13-28 illustrates this reaction. 

The nature of the chlorinated reagent is crucial for promoting the Kharasch addition reaction (Equation 8.11). The results showed that carbon tetrachloride could be added to various olefins in a regioselective way. Under these reaction conditions, no polymerization products were detected. In contrast, when chloroform was used as the halide source the methyl methacrylate and styrene conversions reached only 33% and 40% with the best performing system (VIII), and a significant fraction of polymers was observed [61]. 

The synthesis of halo-substituted cyclopropanes using zinc carbenoids can be accomplished using three different approaches by the cyclopropanation of a halo-substituted alkene, by the cyclopropanation using a halo-substituted zinc carbenoid, or by the cyclopropanation using, gem-dizinc carbenoids followed by trapping the cyclopropylzinc with an electrophilic halide source. 

The characteristics of the addition of HBr to double bonds are similar to those of the addition of HC1. However, in acetic acid 1,2-dimethylcyclohexene gives more anti addition if HBr is the addend.25 Also, as Figure 7.1 shows, when HX is added to a double bond in acetic acid, the ratio of alkyl halide to alkyl acetate increases sharply as the concentration of HBr is increased but is almost independent of the concentration of HC1. Fahey suggests that the much larger acid dissociation constant of HBr (AXd = 103-104) is responsible for both of these facts. Hydrobromic acid acts as a better halide source, and Ad3 addition is favored.26... 

E. N. Jacobsen, J. Am. Chem. Soc. 1998, 329, 10780-10781 c) Apparently, ARO with other halide sources does not proceed with high enantioselectivity neither (ref 24a). 

Quite evidently, for binary catalyst systems of the type NdCl3 D/A1R3 an additional catalyst component which acts as a halide source is not required. NdCb constitutes an appropriate halide source which provides halogen at a fixed molar ratio of nx/ Nd = 3. 

For ternary catalyst systems a vast number of halide donors was investigated which renders a complete quotation impossible. It is important to note, however, that for a given halide, the actual halide source neither has a strong influence on catalyst activity nor on cis- 1,4-contents. As halogen sources which are found in the literature cover the whole range from ionic halides to covalently bound halogen atoms the strength by which the halide is bound to the donor is not a critical factor. 

Additives are also used to improve the solubility of halide donors [382, 383]. Metal(II) halides such as magnesium chloride, calcium chloride, barium chloride, manganese chloride, zinc chloride and copper chloride etc. are used as halide sources. In order to increase the solubility of the halides they are reacted with electron donors which have been previously described for the increase of solubility of Nd-components [338,339]. The number of catalyst components is further increased if two Al-compounds (alumoxane + aluminum (hydrido) alkyl) are used. In addition, a small amount of diene can also be present during the preformation of the different catalyst components as described by JSR. In some catalyst systems the total number of components reaches up to eight [338,339]. Such complex catalyst systems are also referred to in other JSR patents [384,385] (Sect. 2.2.6). 

Some reactions that appear otherwise may proceed via redox processes. For example, the reaction of [E Fe(CO)4 4]3 with EX3 leads to the formation of [XE Fe(CO)4 3]2 [Eq. (257)].70 At face value this would appear to be a simple substitution of [Fe(CO)4]2 by X, but the reaction does not proceed if the halide source cannot undergo redox chemistry. Conventional [NR4]X and [PPN]X do not promote the exchange reaction while T1C13 will. 

Jones and co-workers investigated reactions of the stable carbene 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene with a series of halide sources... 

Thus, the reaction of both aryl (Table 9, entries 1-3) and alkyl diazo ketones (entries 5-7, 9-11) with (HF) /pyridine and a halide source gives a-fluoro-a-halo ketones. The best yields are generally obtained in the case of the fluoroiodo derivatives (entries 3, 7 and 11). Diazo... 

Alkalimetal halides or the more highly soluble tetraalkylammonium salts are frequently used as the halide sources. The weakly nucleophilic character of the fluoride ion strongly affects its displacement efficiency. In some cases, a cryptand is used to increase its effective nucle-ophilicity (O Scheme 23) [36]. 

The Stille coupling involves heating a halide, a stannane and a catalyst in a suitable solvent, such as toluene or DMF. No base is required. The conditions are relatively straightforward, with little overall variation, apart from the catalyst. However, if a triflate is used instead of a halide, the reaction may not succeed, as transmetallation of aryltin compounds with arylpalladium triflates is often difficult. Addition of a halide source, such as lithium chloride, usually solves this problem as it allows the formation of the arylpalladium chloride, which can undergo transmetallation. 

One disadvantage of arc lamps is the high heat output, as seen by the continued output above 500 nm however, this can be dissipated by the using a heat filter, which usually contains water. Because photochemical reactions are generally initiated by UV radiation, adjustment of the intensity above 500 nm is most unlikely to lead to erroneous photostability data. On the other hand, overheating of the sample by the lamp may lead to thermal decomposition processes that will complicate the issue. The other principal disadvantage of xenon and metal-halide sources is their... 

Many other types of light sources can also be used for photopolymerization reactions, for example, low-pressure mercury arcs, flash lamps, fluorescent lamps, tungsten halide sources, and even lasers. A complete review of light sources used in photopolymerization reactions can be found in Reference 18,... 

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