Rieke-method

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). 

Alkali metal reductions of metallic salts using an arene as electron carrier, lithium being the most used metallic component (Rieke method) . Although not belonging to this group of metals, magnesium-anthracene has found some apphcations in the activation of other metals . 

Lithium halides (bromide or Iodide) may well modify the Lewis character of the zinc atom, probably via a zincate species [53], and prevent the efficient coordination of the zinc atom to the double bond, coordination which is required for the carbocyclization. Thus, in the Rieke method, it is essential to wash the active zinc thoroughly since the lithium naphthalenide reduction of zinc bromide also generates lithium bromide, which is detrimental to the success of the reaction. Indeed, the insertion of Rieke s zinc in the presence of LiBr leads to the linear organozinc iodide but not to the cyclic product [52]. 

Conceivably, any metal on the periodic table can be activated by the Rieke method. 

An important aspect of the active magnesium by Rieke methods is its convenient preparation. The apparatus required is very inexpensive and simple. The reductions are normally carried out in a two-neck flask equipped with a condenser (if necessary), magnetic stirrer, under an argon atmosphere. In general, it is not possible to prepare anhydrous... 

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. 

Soon after the report of the McCullough method, the Rieke method (Scheme 2.2B) was published (Chen and Rieke, 1992). In this method, the stcuting material is changed to 2,5-dibromo-3-alkylthiophenes 34, which reacts with Rieke zinc at low temperature to yield a mixture of two isomeric organozinc intermediates 35 and 36 in a ratio of 90 10 directly. The intermediates are polymerized in the presence of Ni(dppe)Cl2. The yield of P3ATs 33 is increased to 75%, and the polymer parameters (Mn = 24,000-34,000, PDI = 1.4) are maintained in a comparison to McCullough method. 

Shortly after this initial report, the Rieke method for the synthesis of rr-PAT was reported [31-34], The major innovation in this approach was the generation of an asymmetric organometallic intermediate by treating 2,5-dibromo-3-alkylthiophenes 5 with highly reactive Rieke Zinc (Zn ) [35,36] (Scheme 9.2). The metal reacts quantitatively to yield a mixture of the isomers, 2-bromo-3-alkyl-5-(bromozincio)thiophene 6 and 2-(bromozincio)-3-alkyl-5-bromothiophene 7. The ratio of these... 

Table 9.1 is a quick reference to the large number of side-chain functionalized PTs that have been synthesized to date. All of these have been covered throughout the chapter. The primary methods of synthesis used have been designated as Method A (McCullough), Method B (Rieke), Method C (GRIM), Method D (Stille), Method E (FeCl3), and Method F (derivatization). Please refer to the previous section for the general schemes of these synthesizes. 

Method A (McCullough), Method B (RIeke), Method C (GRIM), Method D (Stille), Method E (FeCIs), Method F (Suzuki) Method C (post-functionallzation) Method H (Ullmann). 

Wurtz-Ullmann processes can be carried out with copper or zinc, prepared by the sonochemical Rieke method. Lindley et al examined the fundamental aspects of the copper-induced reaction of 2-nitro iodobenzene (Eq. 44). 9 Almost quantitative yields are obtained with only a fourfold excess of copper flakes at 60 C in DMF under probe sonication, with a rate multiplied by a factor of ca. 50. 

Substituted allyl Grignard reagents were synthesized in high yields from the corresponding chlorides and Mg-slurry, obtained by evaporation, activated by equilibration with its anthracene adduct in THF [30]. The authors compared this magnesium with Rieke-magnesium and found, on careful experimentation, that also the Rieke-method gave satisfactory results. 

As was the case with magnesium, also highly reactive forms of zinc were made through the Rieke method and through its intercalation compounds with graphite. 

Table 9.5 Polymerization of P3ATs using the Rieke method.

In 1962, Gaudeman used THF as the solvent for the oxidative reactions and found that the reaction could be extended to allylic and benzylic bromides. Also, alkyl iodides were easily reacted. Knochel has since made considerable advances in activating the metal by using 1,2-dibromoethane and chlorotri-methylsilane [12,13]. Recently, Knochel has reported that adding alkali salts such as LiCl can be used to activate zinc metals. The importance of the alkali salts generated in the Rieke method was pointed out in our first reports in the 1970s. 

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). 

When compared with other metals produced by the Rieke method of metal activation, copper suffers from the disadvantage of sintering into larger particles, thereby reducing its reactivity. It was apparent that the main difficulty associated with the copper metal sintering was the exceptionally... 

Considering the high reactivity of the active manganese (Mn ) metal prepared by the Rieke method and the exceptional tolerance to a wide range of functionality in the organic moiety, we explored the possibility of the direct oxidative addition to a variety of carbon-oxygen bonds. Our first approach employed benzyl sulfonates. This was expanded to functionalized and... 

Although iron has been known to man prior to 3000 BC, its use in chemistry had to wait until the twentieth century. Most of its early applications involved catalysis chemistry, primarily in the petrochemical industry. In 1983, we reported the preparation of an extremely reactive iron powder using the Rieke method [1-3]. 

Fig. 17. Rieke method for regiospecific synthesis of poly(3-alkylthio-phene)s with 100% head-to-tail couplings.

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