2,5-Functionalized thieno thiophenes

Ortho-bi functional thiophenes provide the most convenient route to unsubstituted thieno[2,3-6]thiophene (1). 3-Bromothiophene yielded 3-thiophenealdehyde. The corresponding diethyl acetal, using the procedure for thienothiophene 2, led to thieno[2,3-6]thiophene (l) in about 40% yield based on 3-bromothiophene [Eq. (24)]. This method was also used by Gronowitz and Persson for the synthesis of thienothiophene 1. 

It is not necessary for the carbonyl functionality on the thiophene ring to be an ester. The carboxylic acid 399, shown in Scheme 32, is converted by reaction with a variety of acid chlorides (or acid anhydrides) into thieno-oxazinones 400. Reaction of 400 with dry ammonia generates 401 in average yields <2002JCM5>. 

As mentioned earlier (Section 3.18.4) thienopyrroles have been studied much more than other azaheteropentalenes. Thieno[2,3-f ]pyrrole (32) undergoes Vilsmeier formylation to yield a mixture of two products (43) and (44) in 90% and 10% yields, respectively. The major product is the C-5 substituted isomer (43) which is not surprising in view of the higher reactivity of pyrrole compared to thiophene. Upon deactivation of the pyrrole ring by the introduction of an ester function at C-5 as in (45) formylation occurs exclusively at C-2 to give aldehyde (46) in 95% yield (Scheme 8) (78AHC(22)183>. 

The 8//-thieno 2,3-/ indole 181 was prepared from 3-(2-nitrophenyl)thiophene 180 by a palladium-catalyzed regioselective C-H bond functionalization driven by CO as the stochiometric reductant. The compound 181 was obtained as a mixture of regioisomers in the ratio 93 7, determined by H NMR spectroscopic analysis of the crude reaction mixture. After flash chromatography, 181 was isolated as a major isomer in 81% yield [43] (Scheme 32). 

Virtually all of the syntheses of thieno[2,3-synthetic strategy. Thus, thiophenes with a 2-amino- (or (substituted)amino-) function, and either a carboalkoxy, carboxamido, or a cyano group at 03 are the preferred starting points. Typically, when the thiophene has a carbonyl function at 03 (268 R = OEt or NH2) cyclization affords the 4-oxothienopyrimidines (269) (Equation (93)). 

Hetero-/-fused derivatives of indeno[/i]benzopyrans result from the chromenylation of fluoreno[3,4-()][l]benzofuranols and related S andNMe analogues. The furo- 25 and thieno-26 [2,3-7]benzopyrans show absorption bands between 450 - 530 nm and 550 - 640 nm according to the substituents on the gem diaryl function and generally appear as various shades of brown. The half-lives vary between 1 and 6 minutes, with the thiophene analogues exhibiting faster fade rates than the furan derivatives. The closed form of the sole pyrano-... 

Cyclization of isomeric thiophenes 65 and 66 containing the aldehyde function and the SCH2C02Me group in positions 2 and 3 by reaction with l,5-diazabicyclo[4.3.0]-non-5-ene (DBN) affords methyl thieno[2,3- )]- (67) and -[3,2-i ]thiophene-2-carboxylates (68), respectively (91JMC1805). 

Hydroxyfluorenyl)thieno[3,2-Z)]thiophenes 278a-c and their diphenyl analogue 279 were subjected to methanolysis (2001JOC803). Substitution of the hydroxy function with the methoxy group requires specific reactions conditions for each substrate. 

In addition to the various dithienothiophene polymers, the related thieno[3,4-h] thiophene (1) and its derivatives have received considerable attention. The first reported example was the alkyl-functionalized derivative, poly(2-decylthieno[3,4-h]thiophene) (73) reported by Pomerantz and Gu [109]. Polymer 73 was prepared by the FeCls oxidation of 2-decylthieno[3,4-l jthiophene, which was synthesized from 2-thiophenecarboxylic acid as illustrated in Scheme 12.16 [109,110]. Such methods produced a fairly high LLk material LLk = 90,000, M = 52,000, PDI =1.7) that was blue-green in color and soluble in a variety of organic solvents (CHCI3, THF, and chlorobenzene). Solvent cast films exhibited a transition at 925 nm with an onset at 1350 nm (F of 0.92 eV). FeCls-doped films gave a conductivity of 3.1 X 10 . Films doped with either I2 or NOBF4 gave lower conductivities (1.0—4.2 x 10 ). 

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