Copper bromide methoxide

As discussed in the last sections, addition of sodium methoxide-alkenylborane complexes to copper bromide-dimethyl sulfide at 0 °C immediately gives conjugated dienes. However, if the temperature is lowered to —15 °C, the dark blue-black complex formed is stable and can be trapped by allylic halides to afford a stereochemically defined synthesis of (4 )-1,4-dienes (Eq. 93) 148). 

One aspect of the copper catalytic system that has received attention is the identity of the active catalytic species. In the case of displacement of aryl bromides by methoxide ion in the presence of CuBr, it has been suggested that the active species is Cu(I)(OCH3)2, an anionic cuprate.153... 

A more milder procedure is, where sodium methoxyalkenyldialkylborates obtained by the simple treatment of alkenyldialkylboranes with sodium methoxide, react readily with cuprous bromide-methylsulfide at 0 °C to afford symmetrical conjugated dienes (Eq. 87) 144). Although the intermediacy of a copper(I) borate complex formed... 

As in benzenoid chemistry, some nucleophilic displacement reactions can be copper catalyzed. Illustrative of these reactions is the displacement of bromide from 3-bromothiophene-2-carboxylic acid and 3-bromothiophene-4-carboxylic acid by active methylene compounds (e.g., AcCH2C02Et) in the presence of copper and sodium ethoxide (Scheme 136). Analogously, 2-methoxythiophene can be prepared in 83% yield by refluxing 2-bromothiophene in methanol containing excess sodium methoxide, along with copper(I) bromide as catalyst. For the analogous preparation of 3-methoxythiophene, addition of a polar cosolvent (e.g., l-methyl-2-pyrrolidone) is beneficial. In the case of halothiophenes, an SrnI mechanism is involved. 

Discrete copper compounds can also be used as catalysts for the synthesis of arylamines (Scheme 3.52) [58]. Venkataraman used a neocuproine-ligated copper(I) species to promote the coupling of aryl halides with secondary amines. A practical advantage to this chemistry was that only air-stable materials were needed to construct the catalyst needed for the cross-coupling. A base was needed to promote the reaction, and potassium tert-butoxide was found to be more effective in the cross-coupling than other common bases including potassium phosphate, sodium methoxide, or cesium carbonate. Curiously, cesium carbonate was not as active in this chemistry, but it was quite effective in the preparation of diaryl ethers. Several aryl halides were screened for activity, and aryl bromides and iodides afforded moderate to good yields of the arylamines. It should be noted that an electron-neutral aryl chloride was converted into the triarylamine, albeit in lower yield (49%). 

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