Iodine-copper exchange

A number of pyridazines have been prepared by standard condensations of enediones with hydrazine but a general synthesis of the intermediate enediones is notable. This involved iodine-copper exchange of an iodoenone 3, followed by reaction of the resulting cuprate with acid chlorides. However, only a few of these enediones were actually converted into pyridazines <06OL1941>. 

Another method for the regioselective functionalization of indoles is the halogen-metal reaction. The regioselective iodine-copper exchange reaction of 2,3-diiodoindole 203 with dineophylcuprate (neophil = nphyl) provided cuprate 204 which underwent reactions with electrophiles to produce 2-substituted indoles 205 <04OL1665>. A second iodine-copper exchange then afforded a synthesis of 2,3-disubstituted indoles. 

Preparation of dioctylzinc by iodine-zinc exchange and its copper-mediated coupling with an electrophile preparation of ethyl 2-nonylacrylate... 

The extension of this method to heterocyclic products took advantage of nickel catalysis for the iodine-zinc exchange, for example, of iodoacetals 22, and the organozinc intermediates 23 formed by radical cyclization were again transmetallated to the corresponding copper species which could be reacted with the usual Michael acceptors to afford the products 24 with high diastereoselectivity (Scheme 6).33>33a... 

The iodine-zinc exchange of an alkyl iodide with EtjZn is proi oted by the addition of small amounts of copper(I) salts such as CuCN or Cul. Although the exact reason for this copper catalysis is now known, it has been speculated that the presence of copper(I) salts promotes a radical chain-reaction resulting in the formation of a dialkylzinc species (Scheme 9-32) [22b]. Similarly, the addition of other transition metals such as nickel and palladium salts promotes radical reactions. 

A few modifications of this reaction are the reaction between alkyl iodide and copper-zinc alloy, the reaction between zinc and a mixture of alkyl iodide and alkyl bromide, the transmetalation between zinc and lithium, magnesium, or tin, and the iodine-zinc exchange. ... 

Some results are already described in the literature with nickel- or copper catalysts (refs. 3,71-78). It was us possible to develop this exchange and to show that it is under thermodynamic control the equilibrium lies about 60 to 40 % for bromine and iodine, and is much more shifted to the left (95 to 5 %) for chlorine and iodine. 

In the method for [17] inorganic arsenic the sample is treated with sodium borohydride added at a controlled rate (Fig. 10.1). The arsine evolved is absorbed in a solution of iodine and the resultant arsenate ion is determined photometrically by a molybdenum blue method. For seawater the range, standard deviation, and detection limit are 1—4 xg/l, 1.4%, and 0.14 pg/1, respectively for potable waters they are 0-800 pg/1, about 1% (at 2 pg/1 level), and 0.5 pg/1, respectively. Silver and copper cause serious interference at concentrations of a few tens of mg/1 however, these elements can be removed either by preliminary extraction with a solution of dithizone in chloroform or by ion exchange. 

In a similar reaction, Knochel and co-workers synthesized allylpyrimidine 63 through a magnesium-iodine exchange with iodopyrimidine 62, followed by copper (I) promoted coupling with allylbromide . 

Unusual exchange reactions are observed with or on the metal surfaces of a source itself. Sherwood and Turner (118) in their studies of the spectra of iodine pentoxide observed peaks due to copper oxygen, copper iodine, and copper oxygen iodine species (Cu2 , Cu2l, Cu3l2", ... 

In the copper catalyzed aromatic nucleophilic substitution of aryl halides bromoindole derivatives were converted to the appropriate cyanoindoles. Both 5-bromoindole and its 7V-tosyl derivative gave excellent yields, when a substoichiometric amount potassium iodide was added to the reaction mixture (6.80.), Pyrazole and benzothiophene showed a similar reactivity. The role of the added iodide is to activate the aromatic system through a bromine-iodine exchange.111... 

---