1.3.4.6- Tetraphenylthieno thiophene

Continuing their search for ways to synthesize this system. Cava and Husbands " allowed tetrabenzoylethane (146) to react with phosphorus pentasulfide in refluxing xylene to obtain 46% of 1,3,4,6-tetraphenyl-li/,3if-thieno[3,4-c]thiophene (147) oxidized with periodate the latter yielded the sulfoxide (148), which was dehydrated with acetic anhydride to produce the stable l,3,4,6-tetraphenylthieno[3,4-c]thiophene (149) (87%). The latter is obtained in one step, in about 3% yield, by the reaction of tetrabenzoylethane (146) with phosphorus pentasulfide in refluxing xylene, along with the formation of 147 (30%). A mixture of two ad-... 

In 1973 Cava et al. reported the synthesis of 4,6-dimethyl-l/f,3if-thieno[3,4-c]thiophene (142) and l,3,4,6-tetraphenylthieno[3,4-cl-thiophene (149) ° as well as data on some chemicaJ conversions of the latter and the dehydration of 4,6-dimethoxycarbonyl-l/f,3H-thieno-[3,4-c]thiophene sulfoxide. Thienothiophene (149) was also obtained (42%) by Potts and McKeough by condensation of anhydro-4-hydroxy-2,3,5-triphenylthiazolium hydroxide with dibenzoylacetylene followed by reaction of the product with P S,. 

Pig. 3. Bond lengths (A) and angles of the thienothiophene fragment of the 1,3,4,6-tetraphenylthieno[3,4-c]thiophene molecule. 

Mass spectrometry was also used to analyze mixtures of deuterated and nondeuterated thiophenes and thienothiophenes 1 and 2, as well as to confirm the structure of l,3,5,6-tetraphenylthieno[3,4-c]thiophene (149) (see also Bugge ). 

The ability to form adducts was also found in the so-called non-classical thienothiophene (4). As described above (see Section II,B), the sulfoxide of 4,6-dimethyl-l//,3i/-thieno[3,4-c]thiophene (143) and l,3,4,6-tetraphenylthieno[3,4-c]thiophene 049) react with A -phenyl-maleimide to form endolexo pairs of adducts 144, 145 and 150,... 

The nonclassical thiophenes (6), (12) and (13) show intense molecular ions (over 70%) as well as pronounced doubly charged M2+ ions. Other characteristic fragment ions result from the loss of phenyl or methyl substituents as the case may be. The thiobenzoyl fragment at m/e 121 (10-20%) is another common fragment in these molecules. The mass spectral fragmentation of tetraphenylthieno[3,4-c]thiophene (6) is presented in Scheme 6. 

Tetraphenylthieno[3,4-c]thiophene (6) and the thieno[3,4-c]pyrrole (12) were reduced catalytically to the corresponding cis dihydro derivatives (24 X = S, NMe). The addition of hydrogen occurred in cis fashion across the 1,3-positions in the thiophene system (Scheme 46). Both thienopyrroles (12) and (13) were resistant to chemical reduction (75ACR139). 

Phosphorus pentasulfide (P4S10) has also been used in construction of a new thiophene ring by the introduction of sulfur. Dipolar addition of dibenzoylacetylene to the mesoionic anhydro-4-hydroxy-2,3,5-triphenylthiazolium hydroxide (260) gave 3,4-dibenzoyl-2,5-diphenylthiophene (151), which upon reaction with P4Si0 in dry pyridine gave tetraphenylthieno[3,4-c]thiophene (6 Scheme 89) (77HC(30)317). 

Vigorous oxidation (Cr03/H0Ac) of l,3,4,6-tetraphenylthieno[3,4-c]thiophene (149) opens a ring, to yield 3,4-dibenzoyl-2,5-diphenyl-thiophene (60%).103... 

Cava and Pollack s elegant synthesis of benzo[c]thiophene52 has been extended to the synthesis of, for example, naphtho[l,2-c]thio-phene (47),52 methyl benzo[c]thiophene-5-carboxylate,54 and 1,3-dimethyl- (48)55 and l,3,4,6-tetraphenylthieno[3,4-c]thiophene (49).56 The last compound is of particular interest because it is a remarkably stable (cf. compound 48 which only exists transiently) nonclassical thiophene containing ten n electrons for which the only uncharged resonance structure (49) contains a tetravalent sulfur atom. When the sulfoxide (50) is pyrolyzed over aluminum oxide, it gives the parent cyclic sulfide and 51 by disproportionation.57 However, when 50 is heated in acetic anhydride in the presence of iV-phenylmaleimide,... 

The PE spectra of tetraphenylthieno[3,4-c]thiophene (26) and triphenylthieno[3,4-c]isothiazole (29) have been recorded (76JA7187, 78JOC3893, 79CB260) and the low first ionization energies (26 6.2 eV) and (29 6.9 eV) are consistent with the high HOMO energy expected from theory (see Section 4.37.2.1). Aza substitution (29) lowers the HOMO energy (ca. 0.7 eV). 

The crystal structure of tetraphenylthieno[3,4-c]thiophene (26) has been determined (see Chapter 3.18). 

Photodimerization of 2,3-diphenylcyclopropenethione (90) in benzene (71TL4993) occurs regioselectively to give the expected 2,3,5,6-tetraphenylthieno[3,2- >]thiophene... 

The structures of l,3,4,6-tetraphenylthieno[3,4-Z)]thiophene (272) (83JA1705), 1,4-bis(rerr-butylthio)-3,6-diphenylthieno[3,4-c]thiophene, 4-(terr-butylthio)-3,6-diphe-nylthieno[3,4-c]thiophene-l(3//)-thione, 4-( rerr-butylthio)-3,6-diphenylthieno[3,4-... 

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