Carbon-halogen bond forming reactions

Chlorination of alcohols with trimethylsilyl chloride in the presence of a catalytic amount of bismuth chloride typical procedure 

To a stirred mixture of cinnamyl alcohol (6.71 g, 0.05 mol) and bismuth chloride (0.79 g, 25 mmol), trimethylsilyl chloride (5.43 g, 0.05 mol) was added in small portions. After 5 min, the liquid phase was decanted and the resulting cinnamyl chloride was separated by distillation (7.25 g, 95%) [94SL723]. 

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Scheme 5.43. Bismuth(III) salt-based carbon-halogen bond forming reactions.

Organocatalytic asymmetric carbon-halogen bond-forming reactions have been combined with other catalytic asymmetric reactions in a single flask to effect cascade reactions. Quintard and Alexakis [78] developed a cascade reaction in which a Cu-catalyzed conjugate addition to an a,p-unsaturated aldehyde was followed by an enamine-catalyzed a-fluorination or chlorination. 

As can be seen from the diversity of transformations presented in this chapter, there has been tremendous progress in the development of organocatalytic asymmetric carbon-halogen bond-forming reactions since the turn of the twenty-first century. This area is still in its relative infancy, however, and many more breakthroughs are surely still to come. 

Carbon-Halogen Bond-Forming Reactions in Oxidative Pd Catalysis. 71... 

Carbon - Halogen Bond Forming Reactions at Room Temperature... 

Benzimidazoles bearing a halogen in the activated 2-position show a remarkable reactivity in palladium catalyzed carbon-nitrogen bond forming reactions. TV-protected 2-chlorobenzimidazoles reacted smoothly with a series of amines (6.81.). The activity of the aryl halide, besides the ready coupling of the chloro derivative, is also emphasized by the low catalyst loading used.112... 

Significantly, the pre-exponential factors decrease with increasing reactivity, and this suggests that the Wheland intermediate is more nearly formed in the transition state, the more reactive the compound. Or, considered another way, the position along the reaction co-ordinate at which a given amount of carbon-halogen bond formation occurs is nearer to the ground state the more reactive the compound. 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

Similar reactions occur with all aliphatic halides and the rates of substitution are related to the degree of ionic character of the carbon-halogen bond. For preparation purposes, trityl bromide or propargyl bromide are more convenient than allyl bromide. The compounds obtained are listed in Table XI. They were obtained pure and characterized fully. Zr (allyl) 3Br and Zr(allyl)2Br2 are sufficiently soluble in toluene for polymerizations to be initially homogeneous. Their relative reactivities are listed in Table XI. In all cases hydrogen was used to reduce the molecular weight of the polymer formed. In this respect the polymer derived from Zr (allyl )3Br was more readily modified than that from Zr (allyl) 4, but in order to avoid... 

The iron slurries show exceptional reactivity toward oxidative addition reactions with carbon halogen bonds. In fact, the reaction with C.FcI is so exothermic that the slurry has to be cooled to 0 °C before the addition of C F L The reaction of iron with C F Br is also quite exothermic, hence, even for this addition, the iron slurry is cooled to about 0 ° C. The organoiron compound formed in the above reactions, solvated Fe(C.F )2, reacts with CO at room temperature and ambient pressure to yiela Fe(C,F3)2(CO)2(DMEL. 

The polarity of carbon-halogen bond of alkyl halides is responsible for their nucleophilic substitution, elimination and their reaction with metal atoms to form organometallic compounds. Nucleophilic substitution reactions are categorised into and on the basis of their kinetic properties. Chirality has a profound role in understanding the reaction mechanisms of Sj l and Sj 2 reactions. Sj 2 reactions of chiral all l halides are characterised by the inversion of configuration while Sj l reactions are characterised by racemisation. 

You have read (Unit 10, Class Xll) that the carbon - halogen bond In alkyl or benzyl haUdes can be easily cleaved by a nucleophile. Hence, an allqrl or ben l haUde on reaction with an ethanollc solution of ammonia undergoes nucleophilic substitution reaction m which the halogen atom Is replaced by an amino (-NHJ group. This process of cleavage of the C-X bond by ammonia molecule Is known as ammonolysis. The reaction Is carried out In a sealed tube at 373 K. The primary amine thus obtained behaves as a nucleophile and can further react with allqrl halide to form secondary and tertiary amines, and finally quaternary ammonium salt. 

Electrochemical oxidation of alkyl bromides and iodides leads to loss of a nonbonding electron from the halogen substituent, followed by cleavage of the carbon-halogen bond to form a carbonium ion and a halogen atom. The products isolated are formed by further reactions of the carbonium ion while two of the halogen at-... 

In these reactions (Scheme 3.1), the first electron addition is to the alkene giving a radical-anion. This interacts with the alkyl halide to transfer an electron, in a process driven by simultaneous cleavage of the carbon-halogen bond. The alkyl radical formed in this manner adds an alkene radical-anion [25]. Aluminium ions generated at the anode are essential to the overall process. They coordinate with the intermediate carbanion, which then interacts with the second halogen substituent in an Sn2 process to form the carbocycle. 

In the absence of a proton donor, the alkoxide ion generated by carbanion addition to the carbonyl function can interact with a carbon-halogen bond in the 8 2 displacement reaction. Reactions of this type have led to some novel carbon chain forming processes. Ketones are converted to homologated enones in good yield by... 

Reduction of the carbon-halogen bond is an important synthetic reaction in organic chemistry. The importance of the reduction has been enhanced in recent years by the application of a wide range of newly available reductants. However, in the case of fluorine, the reduction is of less importance than for the other halogens. Reviews on the reduction of fluorine bonds have appeared mainly in the more specialized literature,1 4 though some aspects of the subject have formed parts of reviews of the broader topic of hydrogenation, reduction, or substitution.5,6 This section does not include patent literature and all literature data. It is thus selective, not exhaustive, and includes representative examples of the most commonly used procedures for the reduction of C-F and element-fluorine bonds. 

In the second step of the chain reaction (Equation 14-2) the propenyl radical can form a carbon-halogen bond at either end by abstracting a halogen atom from the halogenating agent ... 

As to the next step, namely, the reaction of aryl radicals with nucleophiles, we should take into account the fact that air molecular orbital, which initially accommodates the incoming electron, is available in the aryl halide. The electron is subsequently transferred in-tramolecularly from the it to the o molecular orbital of the carbon-halogen bond. Aryl radicals effectively scavenge H atoms. Therefore, an abstraction of a hydrogen atom from the solvent may occur. However, in the case of nucleophiles that can act as effective traps of aryl radicals, the addition of a nucleophile to the phenyl radical takes place. At this point, let us focus on the step of addition of the nucleophile (Y ) to the intermediate radical (Ph). When a new a bond begins to form between the sp3 carbon-centered radical (H5C6) and... 

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