Halides reaction

B. Reactions.—(/) Halides. Whereas ylides are alkylated in the normal way on treatment with a-bromo- or a-iodo-esters, quite different reactions occur with a-fluoro- and a-chloro-acetates. When salt-free ylides were refluxed in benzene with ethyl fluoroacetate or trifluoroacetate normal Wittig olefin synthesis took place with the carbonyls of the ester groups to give vinyl ethers, e.g. (14). On the other hand, methyl chloroacetate with... 

Babcock, L.M. and Streit, G.E. (1984) third-body effects in termolecular reactions Halide ion addition to boron trifluoride and boron trichloride./. Phys. Chem., 88, 5025. 

In some reactions, halide ions may be retained at E.512 This is potentially useful as the residual halide becomes a functionality that may allow for further reactions. Examples are given in Eqs. (13)53 71 and (14).72... 

Yields are also acceptable for reactions of hydroxide ion with secondary alkyl halides if the compound is especially favorable for SN2 reactions (halides that are allylic, ben-zylic, or adjacent to a carbonyl group), as shown in the following example ... 

One of the main issues with the use of magnesium and zinc as inexpensive co-reductants is their reactivity with alkyl halides. Also, additives such as TMSOTf are relatively expensive. In an attempt to address these limitations, Namy utilised mischmetal (La 33%, Ce 50%, Nd 12%, Pr 4%, Sm and other lanthanides 1%) as the stoichiometric reductant for the regeneration of the Sm(II).31,32 This reagent system provides an important alternative since it does not require the use of additives and mischmetal is relatively inexpensive. This system has been utilised successfully in Barbier and Reformatsky reactions, halide reductions and pinacol couplings (Scheme 7.7).31,32... 

In most Sn2 reactions, halides routinely serve as convenient and readily available leaving groups. They are easily formed from alcohols under a variety of conditions. Because the application of these reactions to sugars is a major component of this chapter, a complete discussion of halogenation reactions will be presented in Section 6.4. 

The formation of halides by the nucleophilic substitution of leaving groups is commonly used in the synthesis of more elaborate structures. The synthesis of inexpensive bulk chemicals like solvents, however, is mostly done by oxidation or addition reactions. Halides are either used as intermediates or are valuable end products. Authors of publications, especially patent literature, often claim one reaction for three or even four halogens. In practice, this rarely holds true and each of the halogens quite often requires different conditions. The usual order of halide nucleophilicity is I" > Br > Cl > F". In dimethyl-formamide the reverse order Cl > Br > I is observed. This order goes along with the order of the basic... 

Organozinc reactions. Halides are converted to organozinc halides in the presence... 

Von Braun reaction Halides from amines via acylamineo... 

Another interesting use of TEMPO has been in free-radical substitution of alkyl halides. In this reaction, halides react with tributyltin hydride and TEMPO to yield A-alkoxyamine substitution products [18. This reaction is especially attractive in cases where anionic nucleophiles are sterically prevented from carrying out substitution reactions. An example of this can be seen in Barrett s synthesis of sucrose [18b], in which a stereoselective iodoetherification reaction was used to produce neopentyl alkyl iodide 13 (Scheme 5). Free radical substitution mediated by tributyltin hydride and TEMPO yielded A-alkoxyamine 14. The mechanism [19] involves TEMPO abstraction of hydrogen from tributyltin hydride [20] the stannyl radical then abstracts iodide from the substrate, and a second equivalent of TEMPO traps the resulting carbon radical. 

Strong reducing agent. Good for halides (Sn2 type reactions), halides and sulfonate esters (C—O cleavage). 

FormylatioH and acylation of pyridine. Direct formylation and acylation at the )S-position of pyridine is possible by reaction with phenyllithium and then with Fe(CO)s. The reactions are formulated in scheme (I). 2-Phenylpyridine is also obtained in these reactions. Halide exchange of 3-bromopyridine with n-butyl-lithium to give 3-pyridinyllithium followed by reaction with Fe(CO)5 gives only a 5% yield of 3-pyridinectu boxaldehyde. ... 

The following factors are variable in HR, and careful qualitative and quantitative optimization of these factors are essential to achieve successful reactions halides alkenes ligands solvents bases additives and temperature, hi order to find optimum conditions of reactions, a careful survey of these factors is crucial. In the literature, numerous recipes of optimum reactions have been reported, in which high turnover numbers (TON) and TOP have been reported. However, in many cases, these results have been obtained imder a narrow range of conditions from specific reactants in small scale experiments, and the results can not always be retraced. At present, it is difficult to find clear-cut recipes for many kinds of reactions from data reported in the literature. In the following sections, some controlling factors are surveyed. 

The pentacarbonylmanganese halides have been known for some time but are still the subject of intense interest Numerous vibrational analyses have been carried out, as well as synthetic and kinetic studies of carbonyl substitution reactions. Halide substitution, which requires elevated temperatures, leads to charged species. Diazocyclopentadienes insert into the manganese-halogen bond. ... 

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