Iodide as nucleophilic catalyst

Additionally, acetylene itself is a useful two-carbon building block but is not very convenient to handle as it is an explosive gas. Trimethylsilyl acetylene is a distillable liquid that is a convenient substitute for acetylene in reactions involving the lithium derivative as it has only one acidic proton. The synthesis of this alkynyl ketone is an example. Deprotonation with butyl lithium provides the alkynyl lithium that reacted with the alkyl chloride in the presence of iodide as nucleophilic catalyst (see Chapter 17). Removal of the trimethylsilyl group with potassium carbonate in methanol allowed further reaction on the other end of the alkyne. 

With an acceptor-substituted alkene moiety tethered to the molecule, the intermediate silyl enol ether may undergo an intramolecular [2-I-2] cycloaddition.The silyl-assisted addition of hydrogen halides to cyclopropanes is not restricted to ketones with carbonyl groups as activating function or iodide as nucleophile. Esters and other acid derivatives underwent similar reactions when treated with iodotrimethylsilane alone or in the presence of an additional catalyst such as mercury(II) or zinc(II) chloride.Subsequent treatment of the y-iodo ester with potassium carbonate in tetrahydrofuran gave the respective y-butyrolactones in good yield. 

Although the reaction of p-lactones with various carbon nucleophiles has been well studied, the corresponding reaction with diketene is less known, yet is potentially useful. The use of cobalt(ii) iodide as a catalyst for the reaction of Grignards to effect P-carbon-oxygen bond fission is therefore timely (Scheme 34). The 3-methylenealkanoic acid products are readily converted into various terpenoid derivatives. ... 

In Section 8.2.3.2, we discussed arylation of enolates and enolate equivalents using palladium catalysts. Related palladium-phosphine combinations are very effective catalysts for aromatic nucleophilic substitution reactions. For example, conversion of aryl iodides to nitriles can be done under mild conditions with Pd(PPh3)4 as a catalyst. 

The rate-determining step in this process is the oxidative addition of methyl iodide to 1. Within the operating window of the process the reaction rate is independent of the carbon monoxide pressure and independent of the concentration of methanol. The methyl species 2 formed in reaction (2) cannot be observed under the reaction conditions. The methyl iodide intermediate enables the formation of a methyl rhodium complex methanol is not sufficiently electrophilic to carry out this reaction. As for other nucleophiles, the reaction is much slower with methyl bromide or methyl chloride as the catalyst component. 

Although the initial report included amine nucleophiles, the scope was limited to activated amines such as indole (which actually undergoes C-alkylation at the 3-position), phthalimide, and 7/-methylaniline. Furthermore, enantioselectivities were inferior to those observed with alcohols as nucleophiles. Lautens and Fagnou subsequently discovered a profound halide effect in these reactions. The exchange of the chloride for an iodide on the rhodium catalyst resulted in an increased enantioselectivity that is now comparable to levels achieved with alcoholic nucleophiles ... 

As is true for other classes of aromatic nucleophilic substitution, the halogen displacement can frequently be catalyzed by copper or copper(I) salts. Using sodium hydride as the base and copper(I) iodide as catalyst, a series of o-bromophenylethylamine derivatives, including both amides and carbamates, have been cyclized. Oxidation to the indole can be effected with manganese dioxide (81JCS(P1)290). 

The reaction has some similarity to the hydroformylation reaction described in Section 31-4B. The hydrogen iodide is required to transform methanol to methyl iodide. The rhodium catalyst then reacts with the methyl iodide as a nucleophilic reagent ... 

They suggested that the terra in [I-] in equation (61) corresponded to a mechanism in which the iodide ion acted as a nucleophilic catalyst. These results of Gielen and Nasielski have been disputed by Pilloni and Tagliavini51 who declare that in the iodinolysis of tetramethyllead in solvent acetonitrile. . the product /t°2bs-[r] is fairly constant within the limit of experimental error . Their results are given in Table 28. It may be seen that the values of A 5bs [I ] are not quite constant, and can be fitted to an expression of the same type as equation (61), viz. ... 

Carbonylation of alkyl halides is rare. As an exception, AcOH is produced commercially by the Monsanto process from MeOH and CO using Rh as a catalyst in the presence of HI. In this process (Scheme 3.10), Mel is generated in situ from MeOH and HI and undergoes oxidative addition. Insertion of CO generates an acetylrhodium intermediate, and nucleophilic attack of water produces AcOH, regenerating the Rh catalyst and HI (or reductive elimination to give acetyl iodide and hydrolysis). 

The iodide reacts as a better nucleophile than Ph3P and then as a better leaving group than Br. Each iodide ion goes round and round many times as a nucleophilic catalyst. 

Ley and Barton s observation that di-4-methoxyphenyltelluride could be used catalytically was the first entry into the use of in situ generated selenoxides or telluroxides as catalysts. As shown in Fig. 8, a variety of different nucleophiles can be introduced via the selenoxide or telluroxide followed by reductive elimination to generate oxidized product and reduced selenide or telluride. If the nucleophile is relatively inert to oxidation by hydrogen peroxide, then the reduced selenide or telluride can be reoxidized by hydrogen peroxide and the overall oxidation of the nucleophile becomes catalytic in the selenide or telluride. In the case of thiols, disulfides are the final product and the selenides or tellurides exhibit thiolperox-idase-like activity 60-62 64 82 83 If halide salts (chloride, bromide, iodide) are the nucleophiles, then positive halogen sources are the oxidized products and the selenides and tellurides exhibit haloperoxidase-like activity.84-88 The phenoxypro-pyltelluride 59 has been used as a catalyst for the iodination and bromination of a variety of organic substrates as shown in Fig. 24.87... 

Iodide ion may be used as a nucleophilic catalyst, because it is both a good nucleophile and a good leaving group. 

The reaction is impressively sped up by adding a small amount of KI. Potassium iodide is not an acid or a base, and KCl does not speed the reaction at all. What is happening here It must be due to the iodide anion, acting as a nucleophilic catalyst. 

A key to the success of the living cationic polymerization of vinyl ethers is the stabilization of the unstable carbocations via suitable nucleophilic counterion. There are two ways to stabilize the carbocations (1) generation of suitable nucleophilic counterion resulted from the initiator and the catalyst, and (2) addition of nucleophilic agents to the polymerization media. In the first way, Bronsted acids such as hydrogen iodide are employed as the initiators, while Lewis acids such as zinc iodide are employed as the catalysts (Scheme 11.41) [140-143],... 

In the following reaction, iodide ion increases the rate of conversion of ethyl chloride into ethyl alcohol by acting as a nucleophilic catalyst ... 

Iodide ion is a nucleophilic catalyst because it reacts as a nucleophile, forming a covalent bond with the reactant. The iodide ion that is consumed in the first reaction is regenerated in the second, so it comes out of the reaction unchanged. 

Compounds structurally similar to thiamine, e.g. 5-(2-hydroxyethyl)-3,4-dimethylthiazolium iodide 13, in the presence of triethylamine, catalyse the condensation of aldehydes to acyloins [100]. By analogy to a cyanide ion, the ylide acts as a nucleophilic catalyst. 

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