Thiophene synthesis cross-coupling reactions

The 2-bromo-4,6-dihydrothieno[3,4-, ]thiophene-5,5-dioxide 211 was used in the synthesis of 5-(5,5-dioxo-5,6-dihydro-477-5A -thieno[3,4-, ]thiophen-2-yl)-3,3"-dipentyl[2,2 5, 2"]terthiophene 213 by Suzuki cross-coupling reaction with 5-bromo-3,3"-dipentyl-[2,2 5, 2"]terthiophene 212 (Scheme 20) <1998S1372>. 

Much work has been directed towards the synthesis of thiophene oligomers and polymers. This is due to the current interest in research on conducting polymers and molecular electronics (92CRV711). Two main approaches have been used for making such polymers (i) chemical (e.g. FeCl3) or electrochemical oxidation of monomeric thiophenes and (ii) transition metal-catalyzed cross-coupling reactions. 

Pyridine ring construction according to strategy P is based on the one-step formation of the C(4)-N(5) and C(7)-C(7a) bonds. For example, this approach provides the basis for the synthesis of the simplest isomeric dithienopyridines (1988CS281). The Stille cross-coupling reaction of 2-tributylstannyl-3-thiophene-carbaldehyde (256) with thienylcarbamates 115, 257, and 258 affords the target compounds 259-261, respectively, in different yields. 

In the decade since then, the major focus has been twofold improving the scope of such reactions and application of the cross-coupling reactions for the synthesis of thiophene dendrimers, thiophene polymers, and thiophenes as optoelectronic materials. 

The cross-coupling reactions easily lend themselves to use in controlled synthesis of polymers containing thiophene nuclei <2002JOM(653)195, 1994MM6620, 1997T10357>. Equations (35)-(37) provide representative examples. 

Cross-coupling reactions of 1,2,4-triazines are less common due to rather poor availability of suitable substituted 1,2,4-triazine derivatives. Nevertheless, they attract attention from researchers resulting in development of new synthetic methods for 1,2,4-triazine functionalization. Thus, the synthesis of novel substituted 1,2,4-triazines 33 and 34 containing thiophene and furan rings using palladium-catalyzed cross-coupling reactions of 3-alkylsulfanyl- or 5-cyano-l,2,4-triazine 35, 36 was described (14T4697). 

Extensive use has been made of palladium(0)-catalysed cross-coupling reactions in thiophene chemistiy, for the synthesis of both small molecules and polymers (cross-coupUng reactions are discussed in detail in Schatz and Hoffmann [11] and polymers in Livi et al. [12]). Schemes 7 and 8 give two examples in the first, regioselective metallation at the thiophene 5-position (an a-position) proceeds nickel(0)-catalysed cross-coupling [13] the second (Scheme 8) illustrates a highly efficient StiUe cross-coupling [14]. 

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