Halides, alkyl from halide exchange

The difficulties associated with the presence of an alkyl halide coproduct from an exchange reaction may be avoided altogether by the use of t-BuLi rather than n-BuLi. To date, however, there are no reports in the open literature of the use of t-BuLi on large scale for the production of aryllithiums. 

Halide exchange, sometimes call the Finkelstein reaction, is an equilibrium process, but it is often possible to shift the equilibrium." The reaction is most often applied to the preparation of iodides and fluorides. Iodides can be prepared from chlorides or bromides by taking advantage of the fact that sodium iodide, but not the bromide or chloride, is soluble in acetone. When an alkyl chloride or bromide is treated with a solution of sodium iodide in acetone, the equilibrium is shifted by the precipitation of sodium chloride or bromide. Since the mechanism is Sn2, the reaction is much more successful for primary halides than for secondary or tertiary halides sodium iodide in acetone can be used as a test for primary bromides or chlorides. Tertiary chlorides can be converted to iodides by treatment with excess Nal in CS2, with ZnCl2 as catalyst. " Vinylic bromides give vinylic iodides with retention of configuration when treated with KI and a nickel bromide-zinc catalyst," or with KI and Cul in hot HMPA." ... 

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]+. 

Although no explanation was given for the spontaneous, unexpected oxidation steps 79- 78a and 78a- 34, it is reasonable to assume that the initially present tri-valent phosphorus compounds (phosphorus trichloride and 5-chloro-5H-dibenzo-phosphole 85, respectively), and/or the alkyl halide stemming from the halogen metal exchange reaction used for preparing 2,2 -dilithiobiphenyl, may play an active role here. 

Primary alkyl bromides react well in this sequence except for particularly reactive compounds (e.g., methyl bromide, allyl bromide) which give the vinyl halide by metal-halogen exchange. Secondary halides, as expected, suffer from elimination as a side reaction. Other electrophiles have been used successfully including D20, aldehydes and ketones, di-methylformamide,47 chlorotrimethylsilane,4 8 1,2-dibromoethane,4 and... 

The reactivity of alkenes toward dichlorocarbene also decreases if there is a halogen in an allylic position. In this case side reactions, such as alkylation of trichloromethyl anion, halide exchange in the case of allylic bromides etc., take place. Although the yields of the products from these side reactions are low, they impede the isolation of the desired product, e.g. formation of 3, 4, and... 

Ion exchangers have been produced from crosslinked xanthates, applied as either their sodium or magnesium salts.2176,2186-2193 Crosslinked xanthates are also used to make molded articles and films.2155 Reported crosslinkers include aldehydes, urea, isocyanantes, acids, acyl halides, alkyl and aryl halides, organosilyl halides,2194... 

Several routes are currently applied to synthesize cationic organolanthanide species, including the halide abstraction from heteroleptic Ln(III) compounds [Eq. (25)] [152], the oxidation of divalent metallocenes [Eqs. (26) and (27)] [153], the protolysis of lanthanide alkyl and amide moieties [Eqs. (28) and (29)] [154,155], and anion exchange [Eqs. (30) and (31)] [84,156]. In the absence of a coordinating solvent such extremely electrophilic species attain stabilization via arene interactions with the BPh4 anion (Sect.5.1) [153b]. Cationic rare earth species have been considered as promising candidates for Lewis acid catalysis [157]. 

Metal Halides and Other Salts. The more reactive organometallic types will exchange their alkyl groups for the halogen or acid anion of the salt of some less reactive metal or metalloid. Thus the reactive lithium and magnesium (Grignard) alkyls can form metal alkyls from many other metal salts. For example,... 

This reaction was initially reported by Finkelstein in 1910. It is a preparation of alkyl iodide from alkyl bromide or chloride with potassium or sodium iodide in acetone. Therefore, this reaction is generally known as the Finkelstein reaction. Occasionally, it is also referred to as the Finkelstein halide exchange, Finkelstein displacement, or Conant-Finkelstein reaction. Mechanistically, this reaction is a simple nucleophilic substitution (often via Sn2), as iodide is a stronger nucleophile than bromide or chloride. The yield of this reaction is very high and can be quantitative if occurs in DMF. It was found that the trifluoromethyl group retards the displacement of bromide when it presents as an a- or /3-substituent but accelerates the reaction as a substituent in an allylic chloride. Under normal conditions, this type of halide displacement does not occur for aryl halides. For dihalides, unsaturated or cyclic compounds may form via carbocation intermediates, which form transient covalent iodides or are reduced directly by iodide to free radicals. However, the aromatic halide exchange reacts smoothly when 10% Cul is present in the reaction... 

Shilov reported some of the earliest evidence that transition metal complexes could selectively cleave the C-H bonds of alkanes in a catalytic fashion. Shilov showed that H/D exchange would occur between alkanes and deuterated acid in the presence of platinum complexes (Equation 18.5 and Table 18.1). In addition, Shilov showed that the oxidation of alkanes occurred in the presence of a platinum(II) catalyst, although a platinum(IV) complex was needed as the oxidant. These reactions led to a mixture of alkyl halides formed from the halide of the Pt(IV) oxidant (Equation 18.6) and trifluoroacetate from the trifluoroacetic acid solvent. The cost of platinum(IV) as an oxidant makes this reaction impractical. However, these results provided hope that selective alkane functionalization could be developed because H/D exchange occurred faster at primary C-H bonds than at secondary C-H bonds (Table 18.1), and some selectivity for oxidations of primary C-H bonds over secondary C-H bonds was observed. As noted in Chapter 6, these results motivated a large number of groups to seek transition metal complexes that would insert into, or by other means selectively cleave, the C-H bond of alkanes and create products from this bond cleavage that could be observed directly. 

---