5- Bromo-2,2 -bithiophene

Bromo-2-thienyl-thiophene-2-carbaldehyde (128), obtained from the photochemical coupling of 5-iodothiophene-2-carbaldehyde and 2-bro-mothiophene, can be reduced with NaBH4 129 was treated with propyne, copper iodide, and Pd(PPh3)4 under phase-transfer conditions to give a natural bithiophene 130 isolated from Arctium lappa (87JOC5243). 

The use of methyltriphenylphosphonium bromide and MeSOCH2 on 5-bromo-2-thienyl-thiophene-2-carbaldehyde (128) furnished the corresponding olefin 133. The reaction with propyne in this case furnished a natural bithiophene 134 isolated from Tagetes minuta (86G747). 

The solvent-free, microwave-assisted coupling of thienyl boronic acids and esters with thienyl bromides, using aluminum oxide as the solid support, served to rapidly check the reaction trends on changing times, temperature, catalyst, and base and easily optimize the experimental conditions to obtain the desired product in fair amounts. This procedure offers a novel, general, and very rapid route to the preparation of soluble thiophene oligomers. Quaterthiophene 265 was obtained in 6 min by reaction of 2-bromo-2,2 -bithiophene with bis(pinacolato)diborane(4) in 65% yield, whereas dithiophene 266 was obtained with 70% yield. The synthesis of new chiral 2,2 -bithiophenes also was reported. The detailed... 

Halogen-substituted thiophenium ions have low stability and can be observed by NMR only at temperatures below -30 °C (86MRC699). Disproportionation of the 2,5-dibromo-2H-thiophenium ion with formation of 5-bromo- and 3,5-dibromo-2H-thiophenium ions was observed on increasing the temperature from —50 to —10 °C (86MRC699). For chlorothiophenium ions, the disproportionation could not be observed so distinctly. Note that 2,4-dichloro-2H-thiophenium ion (5) is stable at room temperature and is the main transformation product of the less stable 2,5-dichloro-2H-thiophenium ion (2j). The latter fact was used for the preparation of difficultly available 2,4-dichlorothiophene (6) from the 2,5-isomer 3,5,4 -trichloro-2,2 -bithiophene (7) was isolated as a byproduct resulted, probably, from the reaction of ion 5 with dichloride 6 (Scheme 7) (90G365). 

A variety of 2- or 3-substituted thiophenes, as well as benzothiophenes, have been subjected to the catalytic direct arylation [3, 9, 10]. As expected, 2,2 -bithio-phene can be diarylated at the 5,5 -positions (Equation 10.47) [72], although the use of a bulky phosphine is of key importance for this reaction. 2,2 -Bithiophene protected by benzophenone at the 5-position reacts with aryl bromides, initially with liberation of the ketone (see Scheme 10.6, to give 5-aryl-2,2 -bithiophene, which is then arylated at the 5 -position (Equation 10.48) [72]. 5-Bromo-2,2 -bithio-phenes undergo oxidative homocoupling in the presence of a palladium complex and a silver salt (Equation 10.49) [73]. 

Carboxylic acid additives have been shown to assist in the deprotonation process vide infra). Fagnou and coworkers have used palladium (II) acetate, a phosphine ligand, potassium carbonate and pivalic acid (PivOH) in DM Ac or toluene to prepare several small molecules through the coupling of thiophenes and 2,2 -bithiophenes with arylbromides. An example of a biscoupling of 2,2 -bithiophene with two equivalents of l-bromo-3,5-dimethylbenzene to give 7 is shown in Scheme 19.3. 

The Ni-catalyzed homo-coupling of 2-bromo-3,3 -bithiophene to form the /3,a,l3-linked quaterthiophene isomer 18 also proved to be far more effective (84% yield) than the oxidative coupling of the corresponding lithiated bithiophene 17 with CUCI2 (6% yield) [49]. 

Trihexyl-2,2 5, 2"-terthiophene 196 was similarly prepared, but by Stille-type cross-coupling reactions [177], In this reaction sequence, the less effective step is the Pd(0)-catalyzed coupling of the organotin thiophene and 2-bromo-3-hex-ylthiophene to yield 3,4 -dihexyl-2,2 -bithiophene 163 in only 23%. Bromination of the latter and reaction with the stannylated thiophene under Stille-conditions results in 69% yield of the desired regioregular terthiophene 196 [Eq. (57)]. 

Bromopyrimidine reacts with bithiophene/2-phenylthiophene/[2,2 5, 2 ] terthiophene in the presence of air-stable [PdCl(C3H )dppb] complex as catalyst at 150°C for 16 h to form 5-(het)aryl substituted pyrimidines by C-C coupling and 5-bromo-4-(het)aryl-pyrimidines have been prepared from the same starting materi-... 

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