Introduction:

Benzofuran is basically a heterocyclic compound comprising of fused benzene and furan ring. This colourless solid is a component of the coal tar. Benzofuran is the 'parent' of many correlated compounds with more complex structures. For illustration, psoralen is a benzofuran derivative which takes place in various plants. Benzothiophene A bicyclic aromatic heterocycle in which a benzene ring is fused to that of a thiophene molecule; having a molecular formula C₈H₆S and an odor identical to naphthalene (mothballs). It takes place naturally as a constituent of petroleum-related deposits like lignite tar. Benzothiophene has no household utilization. It is primarily used in industry and research.

The aromaticity of benzothiophene makes it comparatively stable, however as a heterocycle, it consists of reactive sites that allow for functionalization.

General Physical and Chemical Properties:

The aromaticity of benzofuran is weaker as compare to in indole and this ring is simply cleaved via reduction and oxidation. Electrophilic reagents tend to react by benzofuran at C-2 in preference to C-3 (figure shown below), reflecting the reduced capability of the heteroatom to stabilize the intermediate for 3-substitution. Attack in the heterocycle is often accompanied via substitution in the benzenoid ring. Nitration by nitric acid in acetic acid provides mostly 2-nitrobenzofuran, plus the 4-,6-,7-isomers. Whenever the reagent is N₂O₄ in benzene maintained at 10°C, both 3- and 2- nitrofurans are made in the ratio of 4:1. Under Vilsmeier reaction conditions, benzofuran provides 2-formylbenzofuran in 40% yield. Chlorine or bromine add across the C=C bond of the furan ring providing 2,3-dihalo-2,3-dihydrobenzo-furans. Base-promoted ehydrohalogenation of the dihalides affords mixture of the corresponding 2-and 3-halobenzofurans.

Fig: Reactions of Benzofuran

For benzothiophene, the heterocycle is instead more resistant to ring opening and oxidation by hydrogen peroxide in acetic acid at around 95°C, for illustration, provides the 1,1-dioxide. Reduction with either sodium and ethanol or triethylsilane in trifluoroacetic acid affords 2,3-dihydrobenzothiophene. Electrophiles provide mostly 3-substituted benzothiophenes, however such products are often accompanied through smaller amounts of the 2-isomers (figure shown below).

Fig: Reactions of Benzothiophene

Synthesis of Benzofuran and Benzothiophenes:  

A general route to both benzofurans and benzothiophenes based on the cyclodehydration of either 2-hydroxyl or 2-sulfanylbenzyl ketones or aldehydes (figure shown). 2-Acetylbenzofuran can be acquired by reacting sodium salt of 2-formylphenolwith chloroacetone (chloropropanone) (figure shown). A similar reaction by using sodium 2-formylbenzenethiolate results 2-acetylbenzothiophene. 

3-Methylbenzothiophene is available via the interaction of sodium benzenethiolate and chloracetone, followed via the cyclisation of the initial product via the action of a Lewis acid, zinc chloride (figure shown).

Fig: Synthesis of Benzofuran and Benzothiophene

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