The syntheses of 4H-indeno[ 1,2-b] thiophene (2) and bH-indeno[ 1,2-c] thiophene (3) are described. Upon metalation with n-butyllithium and treatment with Dry Ice, the former yields exclusively 4H-indeno[ 1,241 thiophene-4-carboxylic acid (13), while the latter gives 8H-indeno[ 1,2-c] thiophene-8-carboxylic acid (14, 38%), 8Hindeno[ 1,2-c] thiophene-1-carboxylic acid (15, 14%), and 8H-indeno[ 1,2-c] thiophene-3-carboxylic acid (16, 48%). These results are discussed in terms of the mode of fusion of the thiophene nuclius and direct bridging between the benzene and thiophene rings.
Reaction of phenyl(3-thienyl)glycolic acid (1) with AICI3 in benzene solution leads to the formation of 4Hindeno [ 1,2-6]thiophene-4-carboxylic acid (2) whereas analogous reaction of phenyl(2-thienyl)glycolic acid (4) produces no indenothiophene but only a mixture of 5 and 6. In the case of di-(2-thienyl)glycolic acid (14b) and di-(3thienybglycolic acid (16b) analogous results are obtained, with the former leading to the formation of 15 and the latter producing 17. In the case of the (benzo[6] thienyl)phenylglycolic analogues of 1 and 4 the acids were unstable to heat so the ethyl esters, ethyl (2-benzo[fc]thienyl)phenylglycolate ( 21) and ethyl (3-benzo[6]thienyl)phenylglycolate ( 20), upon treatment with A1C13 in benzene led to cyclized products only. The former gave 23 which was saponified and decarboxylated to yield 25 and compared with an authentic sample obtained by synthesis. Ester 20 similarly gave 22 which was similarly converted to the known 24. A mechanistic explanation of these findings is proposed.
Borepinodithiophenes 287 ments, that there is an appreciable degree of bonding In a C-C "single" bond which separates two multiple bonds: K. Freí and H. J. Bernstein, J. Chem. Phys., 38, 1216(1963. ( 16) A similar Interpretation may be valid In the case of conjugation between double bonds. Thus, for example, while the C2-C3 distance in 1,3-butadiene Is somewhat less than a typical C-C single bond, 1.48 vs. 1.54 Á, the Ci-C2 and C3-C4 distances are almost exactly the same as the C=C bond length in ethylene, 1.336 vs. 1.337 Á, respectively: L. E.
A solution of 0.067 g (0.22 mmol) of 6 in 10 ml of benzene containing 5 mg of p-toluenesulfonic acid was heated under reflux for 24 hr. Shorter reflux periods resulted in isolation of substantial quantities of unreacted 6. The mixture was washed with a saturated solution of sodium bicarbonate and dried over sodium sulfate. Removal of the solvent in vacuo left a nearly colorless residue which was recrystallized from ethanol to give 0.042 g (65%) of 1,2-dibenzoyl-3,6-dimethylbenzene:
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