Effect halide substitution

In contrast to the zirconium compounds the chromium halides are less active than pure Cr(allyl)3 Cr (allyl) Cl2 seems to be completely inactive. The only other element known in which halide substitution of the corresponding allyl compound results in activation is hafnium, suggesting the effect may be confined to group IVa. 

By varying R at C, (which is separated from the reaction site at Q by an oxygen) it is possible to influence the electron density at C in a defined way to change the heterolysis rate constant. Corrected for the incomplete transmission of electronic effects through the C methylene group and assuming that the transmission of the oxygen is 100%, the Taft p value for the effect of substitution on the heterolysis rate constant is equal to — 3.95, comparable to that ip = — 3.29) for the SnI hydrolysis of tertiary alkyl halides [21],... 

Si H M agostic interactions in silylamido complexes have been extensively studied to date. The earlier examples were prepared by halide displacement in the coordination sphere of a metal by a silylated amide, which puts severe limitations on the nature of the substituents at silicon (usually, robust methyl groups are used). More recently, a new route to p-agostic silylamides based on the direct coupling of silanes with imido ligands was discovered that allows one to trace the effect of substitution at silicon on the extent of the Si-H bond complexation (vide infra). 

Fluorination in the electrophilic 2-, 4-, and 6-positions is effected by substitutions of other halides, and this is normally performed by nucleophilic displacement with fluoride ion <1994HC(52)1>. Hydrofluoric acid can also be used, and in the case of 2,4-dichloro-5-trichloromethylpyrimidine 111, replacement of all five chlorine atoms occurred, to give 2,4-difluoro-5-trifluoromethylpyrimidine 112, which was subsequently hydrolyzed to give 5-tri-fluoromethyluracil 113 <1996JFC(77)93>. 

The carbon atom of alkyl halides, R—Hal, is electrophilic, but rarely is it sufficiently so to effect the substitution of aromatic species the presence of a Lewis acid catalyst, e.g. AlHalj is also required. That alkyl halides do react with Lewis acids has been demonstrated by the exchange of radioactive bromine into EtBr from AlBr on mixing and re-isolation also the actual isolation of solid 1 1 complexes, e.g. CHjBr-AlBrj, at low temperatures (-78°). These complexes, though polar, are only faintly conducting. Where R is capable of forming a particularly stable carbocation, e.g. with McsC—Br, it is probable that the attacking electrophile in alkylation is then the actual carbocation, Me3C , as part of an ion pair ... 

The reactivities of aryl halides, such as the halobenzenes, are exceedingly low toward nucleophilic reagents that normally effect displacements with alkyl halides and activated aryl halides. Substitutions do occur under forcing conditions of either high temperatures or very strong bases. For example, chlorobenzene reacts with sodium hydroxide solution at temperatures around 340° and this reaction was once an important commercial process for the production of benzenol (phenol) ... 

Exercise 22-12 Reagents, besides the molecular halogens, that effect halogen substitution include hypochlorous and hypobromous acids. They are most effective when a strong acid is present and care is taken to exclude formation of halide ions. Account for the catalytic effect of acid and the anticatalytic effect of halide ions. 

Because of their increased acidity, it is possible for phenols to effect total substitution of halide ligands in cases where alcohols cause only partial substitution. Hence, although WC16 reacts with ethanol to give the Wv complex [WCl3(OEt)2]2,35 phenol in benzene gives W(OPh)6 in excellent yields.4,36 Similarly, the phenoxides of Ti,37 Nb and Ta38 have been obtained. In cases where only partial replacement by phenol occurs, the use of substituents that increase the phenolic acidity can sometimes aid the reaction, for example as shown in equation (7).39,4(1... 

The effect of substituting fluorine for hydrogen in the methylene halides is to increase the rate of carbanion formation and presumably to stabilize the carbanion and decrease the pK. The effect of fluorine substitution on the acidity of other carbon acids is not so clear. In some cases fluorine substitution leads to a smaller increase in rate than expected from the inductive effect and in other cases results in a decrease in the rate of carbanion formation and an increase in pK values. The results shown in Table 6 were obtained for the rate coefficient (k) for methoxide ion catalysed carbanion formation per hydrogen atom by following the appearance of the MeOH absorbance in the IR spectrum when the esters were allowed to exchange in MeOD [167]. The acidity of the nitro-paraffins in Table 7 is mostly increased by chlorine substitution (p/T... 

Palladium halide complexed to a crown-phosphine effectively promotes substitution of haloarenes with powdered NaCN in refluxing dioxane. ... 

Alkylation of altylic halides and alcohols (8, 334-335). Complete details of the reaction of RCu-BF3 with allylic halides and alcohols are now available. The reagent is probably an ate complex, RBF3 Cu+, at least at low temperatures. In the case of allylic halides, THF is superior to ether for effecting y-substitution. However use of ether is essential for the direct alkylation. Addition of BF3 (even 2 equivalents) has no effect on reactions of C6H5Cu.s... 

---