3- Hydroxybenzo thiophene, tautomers

Hydroxybenzo[6]thiophenes exist as oxo tautomers and are referred to as thiooxindoles. O- Alkylation of thiooxindole is favoured in polar aprotic solvents thus NaH/HMPT/Me2S04 gives a 90% yield of 2-methoxybenzo[6]thiophene. Treatment of thiooxindole with two equivalents of NaOH and dimethyl sulfate, or... 

IR spectroscopy has been used to show that 2-aminobenzo[6]thio-phenes normally exist solely as the amino tautomer,112-114 and that 2 ii2,ii3 an(j 3-hydroxybenzo[6]thiophenes117,147 exist solely as the keto tautomers, except that compounds with an adjacent carbonyl-containing group exist mainly as the enols.109,116,147> 148 3-Hydroxy-2-nitrobenzo[6]thiophene-l,1-dioxide exists as a mixture of the keto and enol forms.149... 

In the benzo[A] and benzol/] heterocycles 220 and 221, where the loss of ring resonance energy on tautomerism is much less than in the nonannulated heterocycles 217, the oxo tautomers 222 and 223 are energetically preferred. 2-Hydroxyindole 220 (X = NH) exists exclusively as the oxo tautomer 222 (X = NH) (oxindole). The hydroxy form of 2-hydroxybenzo[A]thiophene 220 (X = S) is detectable by NMR spectroscopy when its trimethylsilyl ether precursor is... 

The relative rates of ketonization of hydroxythiophenes and hydroxybenzo[A]thiophenes in acetonitrile-water (9 1) are as follows 2-hydroxybenzo[A]thiophene > 2,5-dihydroxythiophene > 2-hydroxythiophene > 3-hydroxy-benzo[A]thiophene > 3-hydroxythiophene (Table 38). 3-Hydroxythiophene does not ketonize readily in the above solvent system but in 1 1 acetonitrile-water it ketonizes 6.5 times slower than 2-hydroxythiophene <1987PAC1577>. The solvent has a significant effect on the equilibrium constants <1989JA5346>. In general, for the benzo b systems, increasing solvent polarity favors the hydroxy tautomer, which becomes the almost exclusive species in 2-acetyl <1965T3331> and 2-aryl <1976CS(9)216> derivatives, even in nonpolar media. 

In previous reviews, thiooxindole (benzo[b]thiophen-2(3//)-one) and thioindoxyl (benzo[f>]thiophen-3(2/f)-one) have been discussed in the sections dealing with hydroxybenzo[f>]thiophenes.1,2 Because they are now known to exist mainly as the oxo tautomers, tradition has been broken, and they are here discussed as oxo derivatives of 2,3-dihydrobenzo[b]thiophene. To prevent confusion, a few 2-substituted thioindoxyls, which are known to be predominantly enolic, are referred to as derivatives of the oxo... 

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