3- Bromothiophene reaction with Grignards

Bromothiophenes give cross-coupled products by reaction with Grignard reagents in the presence of a nickel catalyst (Scheme 71) (80TL4017). 

Recently, nitration of organolithiums and Grignards with N204 has been developed for the preparation of certain kinds of nitro compounds (Eqs. 2.14 and 2.15).31 The success of this process depends on the reaction conditions (low temperature) and the structure of substrates. For example, 3-nitrothiophene can be obtained in 70% overall yield from 3-bromothiophene this is far superior to the older method. 3-Nitroveratrole cannot be prepared usefully by classical electrophilic nitration of veratrole, but it can now be prepared by direct o>7/ o-lithiation followed by low-temperature N204 nitration. The mechanism is believed to proceed by dinitrogen tetroxide oxidation of the anion to a radical, followed by the radical s combination. 

The allylic germanes and stannanes obtained are useful synthons. The hydrogermylation of acetylenes can also be performed with dimethyl(2-thiophenyl)germane58. This compound can be prepared quantitatively by the Grignard reaction of Cl2GeMe2 with 2-bromothiophene (Scheme 8). 

As already mentioned, unsubstituted polythiophene is an insoluble and infusible material. Once the polymer is prepared, it is difficult (if not impossible) to process it further as a material for electronic applications. The solubility can be greatly enhanced by the introduction of side-chains at position 3 (or at both positions 3 and 4). The most widely studied side-chains are n-alkyl substituents that can be easily introduced into the thiophene core by reaction of 3-bromothiophene with alkyl-Grignard reagents from alkyl halides [21]. 

In general, the Pd-complexes are less reactive, but more selective. Rossi et al. discovered that depending on the ratio of the reacting compounds, the Pd(dppf)Cl2-promoted reaction of the Grignard reagents of 2- or 3-bromothiophene with dibromothiophenes results in either mono-coupling to synthetically very valuable bromobithiophenes or in a twofold reaction to terthiophene isomers [76]. 

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