Rieke procedure

With special techniques for the activation of the metal—e.g. for removal of the oxide layer, and the preparation of finely dispersed metal—the scope of the Refor-matsky reaction has been broadened, and yields have been markedly improved." The attempted activation of zinc by treatment with iodine or dibromomethane, or washing with dilute hydrochloric acid prior to use, often is only moderately successful. Much more effective is the use of special alloys—e.g. zinc-copper couple, or the reduction of zinc halides using potassium (the so-called Rieke procedure ) or potassium graphite. The application of ultrasound has also been reported. ... 

This activated zinc would be an aggregation of very fine zinc particles dispersed in the DMF solution. The size of these particles is smaller than that obtained in the previous process, which was performed in the absence of naphthalene12. This electrochemical method is comparable to the chemical Rieke procedure in which the activated zinc is prepared by reduction of zinc halide with alkali metal naphthalenide in THF13. 

A very reactive form of a finely divided metal is a so-called Rieke powder [79]. These materials are produced as fine powders by chemical precipitation during the reduction of various metal halides ivith potassium metal in refluxing tetrahydrofuran. Obviously this is a potentially hazardous laboratory procedure and ultrasound has provided an alternative method of preparation of these extremely valuable reagents [80]. The sonochemical technique involves the reduction of metal halides with lithium in TH F at room temperature in a cleaning bath and gives rise to metal powders that have reactivities comparable to those of Rieke powders. Thus powders of Zn, Mg, Cr, Cu, Ni, Pd, Co and Pb were obtained in less than 40 min by this ultrasonic method compared with reaction times of 8 h using the experimentally more difScult Rieke method (Tab. 3.1). 

Nevertheless, this electrochemical zinc activation procedure, which looks like the chemical activation developed by Rieke, does not appear to be very convenient for the electrochemical synthesis of organozinc compounds in one step (equation 24) without preparation of the active zinc in a preliminary step4,13. 

General procedure for the eyelization involving Rieke s activated zinc (Scheme 7-39)... 

Another interesting variation of the potassium-magnesium chloride reduction is to carry out the reduction in a mixed solvent system of THF and triethylamine (ratio 1 1). This method also yields a form of magnesium that is nearly as reactive as the lithium reduction procedure. Table 3 shows reactions of p-chlorotoluene with Rieke magnesium prepared in THF-Et N. Few reports have appeared using this form of highly reactive magnesium. 

A primary halide, 1-bromooctane, reacted as expected to give 100% yield in 10 min and was converted to pivalic acid in 52% yield after 1 hr of reaction. l-Chlorobicyclo[2.2.1]heptane reacted slowly at room temperature, so this was rerun in refluxing THF. Again, the Grignard preparation was slow, giving 74% yield after 6 hr of reflux. The Grignard was then quenched with CO2 to give l-bicyclo[2.2.1]heptane-carboxylic acid in 63% yield. Bixler and Niemann [64] prepared l-bicyclo[2.2.1]-heptanecarboxylic acid from l-chlorobicyclo[2.2.l]heptane by conversion of the chloride to the lithium salt, followed by CO2 quench. The Rieke method appears to be superior, since it obviates the preparation of the lithium salt used in the procedure of Bixler and Niemann. 

Although 1,2-dimethylenecyclopentane and 1,2-dimethylenecycloheptane dienes have been used in combination with Rieke magnesium, most work has concentrated on the 1,2-dimethylenecyclohexane-magnesium complex. All three of these cyclic dienes have been synthesized by established procedures [14]. 

Rieke, R. D., Uhm, S. J. Activated metais. Xi. improved procedure forthe preparation of 3-hydroxy esters using activated zinc. Synthesis 1975,452453. 

Rieke powders are normally prepared by reducing the corresponding metal halides with potassium metal in refluxing tetrahydrofuran, which is clearly a hazardous procedure. Much of the hazard can be avoided if lithium metal is used in place of potassium and the reaction is carried out at room temperature... 

Metal carbonyl complexes are accessible by this procedure from transition metal halides, sodium, and carbon monoxide (Fig. 7). This reaction requires temperatures of 100-300°C under high pressure, even when using freshly prepared Rieke s metals. The use of a horn emitter facilitates the reduction step. 

Reaction Procedure (Scheme 4.4) Aryl bromide (1.0 mmol) in THF (2 mL) was added to Rieke zine (4 mL, 5 g/100 mL suspension in THF). After the addition, the reaetion mixture was heated at reflux and then concentrated in vacuo. The aryl zine was dissolved in DMA (4 mL) and transferred via cannula onto a solid copper(i) bromide-dimethyl sulfide complex (20 mg, 0.1 mmol). Oxidant (147 mg, 0.5 mmol) in DMA (2 mL) was then added and the solution was kept stirring for 1 h at room temperature. The reaction mixture was filtered through a plug of silica eluting with hexane and EtOAc. The filtrate was concentrated in vacuo and the residue purified by flash eolumn chromatography on silica gel. 

A representative procedure to a slurry of Rieke zinc (7.98 mmol) in THF (25 ml) under argon was added 2-bromobutane (7.95 mmol), and the mbcture was refluxed for 2.5 h. The resulting light brown solution was cooled to rt and was transferred via cannula to a THF (10 ml) solution of CuCN (1.5 mmol) and LiBr (1.5 mmol) at -45°C. Benzoyl chloride (7.95 mmol) was added neat, and the mixture was warmed slowly to rt over 4h. The reaction mbcture vras quenched with 3M HCl (20 ml) and extracted with ether (3 x 20 ml), and the combined organic layers were washed with water (20 ml), dried over MgS04, and concentrated. 2-Methyl-l-phenylbutanone (7.52 mmol, 95%) was isolated from the crude reaction mixture by flash chromatography (hexanes/ethyl acetate). 

In general, the preparation of 2-pyridyl organometallics is mostly performed by lithiation of 2-halopyridine at cryogenic conditions followed by transmetal-lation with an appropriate metal halide. As mentioned previously, this procedure causes some limitations on the use of the 2-pyridyl organometallics. In our study, readily available 2-bromopyridine was treated at it with active zinc prepared by the Rieke method [138]. The oxidative addition of the active zinc to carbon-bromine bond was completed in an hour at refluxing temperature to give rise to the corresponding 2-pyridylzinc bromide (PI). 

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