Activated Indium

D.J. Milliron, I.G. Hill, C. Shen, A. Kahn, and J. Schwartz, Surface oxidation activates indium tin oxide for hole injection, J. Appl. Phys., 87 572-576, 2000. 

The zinc-mediated Reformatsky reaction is one of the classical methods for carbon-carbon bond formation. To date, various main group metals and transition metals have been used for this reaction. Rieke s activated indium powder mediates readily the coupling of ethyl a-bromoacetate and a variety of carbonyl compounds yielding /3-hydroxy esters in good yields (Scheme 87).3 Later, commercially available indium powder has been found to be equally effective for the indium-based Reformatsky reaction in THF.28 This indium Reformatsky reaction is accelerated by ultrasound irradiation (Scheme 88).322,323 Indium(i) iodide also mediates the Reformatsky reaction of aldehydes and ketones to give /3-hydroxy esters, presumably via organoindium(m) diiodide (Scheme 89).27... 

Chao, L.-C., Rieke, R. D. Activated metals. IX. New reformatsky reagent involving activated indium for the preparation of P-hydroxy esters. J. Org. Chem. 1975, 40, 2253-2255. 

Fig. 3 Activated indium prepared by reduction of ln( l3 with potassium in xylene...

Reaction of the activated indium with alkyl iodides was found to proceed very rapidly producing an almost quantitative yield of a single product, the dialkylindium iodide.6) Furthermore, it was found that the... 

Table 9. Summary of reformatsky reaction of carbonyl compounds with activated indium and ethyl bromoacetate1)...

Activated indium metal reacts with aryl iodides to give a good yield of diphenyl- or ditolyl-indium iodides. ... 

InR3 (R = alkyl or aryl) may be prepared conveniently by the reaction of diorganomercury species with activated indium metal. The latter is formed by reducing indium salts by alkali metals in hydrocarbon solvents. ... 

The activated indium can be used to effect the Reformatsky reaction of carbonyl compounds with ethyl bromoacetate. High yields can be realized when an excess of the carbonyl compound is used. ... 

Further proof of the high reactivity of the indium powder comes from its reaction with iodine. The standard method for the preparation of indium monoiodide involves the heating of stoichiometric amounts of indium metal and iodine at 400°C for 24 h [4]. We find that iodine reacts with the activated indium powder in refluxing xylene quantitatively in 30 min, and the indium monoiodide formed is easily separated and purified. 

The high degree of reactivity can be seen by comparing the literature preparation of these indium compounds. For example, Dennis prepared (CH3)3ln from indium metal and (CH3)2Hg by heating at 100°C for 8 days, and no yield was reported. However, when (CH3)2Hg was allowed to react with the activated indium metal, the reaction reached 100% yields after only 3 h at 100°C. Similarly, the preparation of the triaryl indium compounds is greatly facilitated by using the activated indium metal, and the yields are essentially quantitative. 

Preparation ofOrganoindium Compounds 243 Table 6.1 Reactions of activated indium and diorganomercury. 

The Direct Synthesis of Diphenylindium Iodide and Ditolyindium Iodide from Activated Indium and Aryl Iodides... 

This black indium powder was found to react rapidly with aryl iodides. For example, the activated indium reacted with neat iodobenzenes at 150°C, and in 2 h over 94% yield of diphenylindium iodide was obtained. For comparison, the same reaction was run with commercial indium powder (atomized, 325 mesh) and yielded only 30% diphenylindium iodide after 25 h. It is interesting to note that considerable conglomeration of the indium metal particles occurred with the commercial indium powder. It is felt that this conglomeration is, to a large extent, responsible for the low reactivity of the commercial indium metal. Attempts to prevent the conglomeration by the addition of potassium chloride failed. 

Recently Deacon and Parrott [12] reported the direct reaction of indium metal with iodopentafluorobenzene. The reaction yield was <40%, and the product composition was dependent upon the stoichiometric ratios of reactants trispentafluorophenylindium was prepared with an excess of indium metal but was mixed with bispentafluorophenylindium when the stoichiometric amount was used. We have found that our indium will react with neat iodobenzenes or iodotoluene at 150°C for 2h to give a very high yield, 94% and 93%, respectively, and only a single product. The diphenylindium iodide or ditolylindium iodide is formed regardless of the stoichiometric ratios of reactants. The reactions of aryl halide with activated indium powder at various... 

Preparation of Organoindium Compounds 245 Table 6.2 Reactions of aryl halides with activated indium. 

The activated indium not only reacts with iodobenzenes and iodotoluene but also reacts with bromobenzene however, with bromobenzene the yield is low... 

One note of caution is necessary regarding the preparation of the activated indium. Excess potassium or indium trichloride should be avoided as they can catalyze side reactions. Thus the ratio of potassium to indium trichloride should be 3 1 under these conditions GC analysis indicated little or no byproducts. Thus the use of activated indium allows the ready preparation of diaryl indium iodides directly from the aryl iodide. 

The activated indium powder was prepared by reducing anhydrous InCls (Alfe ultrapure) with freshly cut potassium metal in dry, freshly distilled xylene. The mixture is heated to reflux and stirred with a magnetic stirrer under argon for 4-6 h, yielding a light black powder ... 

Reaction of this activated indium metal with a-haloesters is rapid. For example, reaction of a mixture of ethyl-a-bromoacetate and cyclohexanone in xylene at 55°C for 2 h gives an almost quantitative yield of the corresponding P-hydroxyester. The activated indium will also react with ethyl-a-chloroace-tate. In this case reaction for 7 h at 55°C in xylene gave a 42% yield of the p-hydroxy ester. In contrast, commercial indium metal (325 mesh) was... 

In order to obtain good yields of p-hydroxy esters, several reaction conditions must be carefully controlled. In the preparation of the activated indium, it is very important that the molar ratio of InCls to K be exactly 1 3. If there is an excess of either InCls or K, side reaction products become considerable. The ratio of the carbonyl compound to the a-bromoacetate is also very critical. In order to obtain high yields, a onefold excess of the carbonyl compound is... 

Table 6.3 Solvent effect on the yield of the reactions with activated indium. ...

Table 6.4 Effect of stoichiometric ratio of ethyl bromoacetate to carbonyl compounds on the yield of the reaction with activated Indium." ...

Finally, the results of reactions of a variety of carbonyl compounds with the activated indium and ethyl-a-bromoacetate are summarized in Table 6.5. In general, the yields of p-hydroxy esters are good with ketones and also with benzaldehyde in xylene and diethyl ether. However, alkyl aldehydes give relatively low yields. In conclusion, highly activated indium has been prepared and some interesting new results have been reported. However, it is clear that much work remains to be done to fully explore the utility of this new material. 

A Experimental Details for Preparation and Reactions of Activated Indium... 

Preparation of Active Indium and Reaction with Alkyl Iodides... 

Sublimed iodine (1.27 g, 5 mmol) was added to activated indium (prepared from 2.21 g of InCls and 1.17 g of potassium) in xylene (25 ml). After refluxing for 30 min, the reaction was complete. The red indium monoiodide was collected by filtration, washed with water to remove the alkali salts, and dried under reduced pressure. The product analyzed correctly for Ini (Calcd. In, 47.50 Found In 46.80, 47.91). The indium was determined by EDTA titration using 4-(2-pyridylazo) resorcinol as indicator or atomic absorption spectroscopy. 

In a typical experiment 20 mmol of activated indium metal in 25 ml xylene, which was prepared by following the above procedure, was allowed to react with 7.10g of diphenylmercury under an argon atmosphere. The mixture was heated to reflux for 2.5 h, and then the solution was allowed to cool to room temperature, and 0.5 ml of solution was withdrawn and hydrolyzed by concentrated HNO3. This sample was tested for the presence of R2Hg and was found to contain none. The solution was then warmed up again and filtered in the dry box when it was hot. The filtrate was allowed to cool, and the mass of needles which separated was collected and washed with dry petroleum ether and dried under vacuum, mp 206-208°C (lit. [6] 208°C). Anal. Calcd. for (QHsisIn CeHs 66.83, In 33.17 found for C5H5 67.75, In 33.98 [14]. 

The following was found to be the best procedure. The xylene is stripped off after the activated indium was generated, and then the light black powder was allowed to react with (CH3)2Hg at 100°C for 3 h, followed by sublimation of the product under vacuum. The product is conveniently trapped in a U-tube equipped with a ground glass stopcock and cooled in liquid nitrogen (mp 88 C) (lit. [5] 89-89.5 C). 

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