Exchange reactions alkyl-halide

Substitution by the Sn2 mechanism and jS-elimination by the E2 and Elcb mechanisms are not the only reactions that can occur at C(sp )— X. Substitution can also occur at C(sp )—X by the SrnI mechanism, the elimination-addition mechanism, and metal insertion and halogen-metal exchange reactions. Alkyl halides can also undergo a-elimination reactions to give carbenes. 

Catalysis of the interlayer anions in the halide exchange between alkyl halides (eg. 1) was examined. A reaction between benzyl chloride (33 mmol) and butyl bromide (33 mmol) in toluene at 373 K was carried out using a 1.0 g portion of H.T.-C1 . The reaction proceeded and, as listed in Table 1, the yield of benzyl bromide was 31% at 4 h, increasing to 37% after 20 h. The reaction did not proceed in the absence of the H.T.-C1. ... 

Many ionic liquids, including the still widely used tetrafluoroborate and hexaflu-orophosphate systems, are synthesized in a two-step synthesis. In the first step a nucleophile, such as a tertiary amine or phosphine, is alkylated to form the cation. For this reaction alkyl halides are frequently used as alkylating agents, forming the halide salts of the desired cation. To obtain a non-halide ionic liquid, the halide anion is exchanged in a second reaction step. This can either be realized by addition of the alkali salt of the desired anion (with precipitation of the alkali halide salt) or by reaction with a strong acid (with removal of the corresponding hydrohalic... 

Aikyl fluorides by exchange from alkyl halides or methanesulfonates. The resin used for the reaction is the F form of Amberlyst-A26 (Rohm and Haas), a macroreticular anion-exchange resin containing ammonium groups. When this material and primary alkyl halides or sulfonates are refluxed in a solvent (pentane, hexane, ether), alkyl fluorides are formed, usually in satisfactory yields. Alkenes accompany fluorides in the reaction of secondary substrates. This reaction has been conducted previously under phase-transfer catalysis (5, 322). ... 

Alkoxyl exchange in redistribution reactions of alkyl alkoxyl silanes is catalyzed by several interhalogen compounds, notably iodine monobromide, and by iodine. Halogen exchange between alkyl halides and trimethylsilicon iodide is also catalyzed in some cases by iodine. ... 

Substitution at the Carbon—Chlorine Bond. Vinyl chloride is generally considered inert to nucleophilic replacement compared to other alkyl halides. However, the chlorine atom can be exchanged under nucleophilic conditions in the presence of palladium [7440-05-3] Pd, and certain other metal chlorides and salts. Vinyl alcoholates, esters, and ethers can be readily produced from these reactions. 

The formation of bicyclic imines (263,264) from piperidine enamines and y-bromopropyl amines may appear at first sight to be a simple extension of the reactions of enamines with alkyl halides. However, evidence has been found that the products are formed by an initial enamine exchange, followed by an intramolecular enamine alkylation. Thus y-bromodiethylamino-propane does not react with piperidinocyclohexene under conditions suitable for the corresponding primary amine. Furthermore, the enamine of cyclopentanone, but not that of cyclohexanone, requires a secondary rather than primary y-bromopropylamine, presumably because of the less favorable imine to enamine conversion in this instance. 

Secondary or tertiary alkyl halides are much less reactive. For example an alkyl dichloride with a primary and a secondary chloride substituent reacts selectively by exchange of the primary chloride. The reactivity with respect to the Finkelstein reaction is thus opposite to the reactivity for the hydrolysis of alkyl chlorides. For the Finkelstein reaction on secondary and tertiary substrates Lewis acids may be used," e.g. ZnCla, FeCls or MesAl. 

The SH2y mechanism is most clearly shown by the exchange of RaSn-(155,156), and by the reaction with alkyl halides (157, 158). 

It seems likely that the mechanism of the Wurtz reaction consists of two basic steps. The first is halogen-metal exchange to give an organometallic compound (RX -(- M —+ RM), which in many cases can be isolated (12-36). Following this, the organometallic compound reacts with a second molecule of alkyl halide (RX + RM —> RR). This reaction and its mechanism are considered in the next section (10-94). 

Coupling of certain lithiated reagents with aryl and vinyl halides is also possible.82 These reactions probably proceeds by a fast halogen-lithium exchange, generating the alkyl halide, which then undergoes substitution. This reaction has been applied to P-lithiobenzamides.83... 

Quantitative assessment of the electrophilic character of various types of phosphenium ions has been attempted using computational studies on hydride and halide exchange reactions, and the results attribute to 1,3,2-diazaphospholenium ions a lower electrophilicity (and thus higher stability) than other types of phosphenium ions [20, 66], The gain in stability due to aromatic -delocalization is predicted to be somewhat larger than inductive stabilization resulting from exhaustive A-alkylation of the parent diaminophosphenium ion, [P(NH2)2]+. 

The nitrogen atom in quinolizidine derivatives behaves as a tertiary amine and hence it can undergo quaternization by reaction with alkyl halides. For instance, berberine derivative 101 was transformed into 102 by treatment with 3-iodopropanol followed by anion exchange. Compound 102 was then transformed into intermediate 103, which was employed as a precursor for the the preparation of bis-ammonium salt 104 (Scheme 10). This compound showed ultrashort curare-like activity in rhesus monkeys <2001JOC3495>. 

---