Thiophenes protodesilylation

Until a few years ago, no quantitative comparison of overall reactivities in electrophilic substitution of the fundamental five-membered rings was available. Only the reactivity of thiophene relative to benzene had been measured quantitatively in several electrophilic substitutions bromination,72 chlorination,72 nitration,121 hydrogen exchange,57 protodesilylation,174 and mercura-tion.121... 

Electrophilic Substitution Reactions of Monocyclic Thiophens. - A careful kinetic investigation of the protodesilylation of substituted trimethylsilyl-thiophens, either with aqueous HCIO4 in methanol or aqueous H2SO4 in acetic acid, gave values of log k which, except for the nitro-compounds, showed excellent linear correlations with the corresponding ortho-, meta-, or para-substituted trimethylsilylbenzenes. The effect of substituents was larger in the thiophen than in the benzene series. Mixtures of 2- and 3-... 

Lithiation of commercially available bromoarenes (281) followed by treatment with zirconocene (methyl) chloride affords zirconocene complexes of substituted benzynes (283), which react with symmetric alkynes and l-(trimethylsilyl)propyne to give single regioisomeric zirconacycles, (284) and (285), respectively (Scheme 66). Both (284) and (285) react with disulfur dichloride to produce benzothiophenes (286) and (287), respectively, in 60-80% isolated yields in a one-pot procedure <89JOC2793>. Protodesilylation of (287) can be accomplished in >90% yield to give the corresponding 2-unsubstituted benzothiophenes by treatment with tetrabutylammonium fluoride in tetrahydrofuran. Application of the procedure to the preparation of 2,3-dihydrobenzo[6]thiophenes was also reported <9iOM537>. 

It is worth noting that the j -eflfect of silicon can also enhance a side-reaction such as protodesilylation of silylated thiophene. As noted in references [10] and [13], electropolymerization of silyl monomers must be performed in an anhydrous medium. Moreover, the... 

Two SiMea substituents in the 2,5 positions of the thiophene ring are necessary. Polymerization of mono-silylated-3-alkylthiophene undergoes coupling either with loss of the silyl group or loss of the proton. The increased acidity of the reaction medium resulted in competing protodesilylation of the monomer. Thus, the polymerization of mono-silylated-3-alkylthiophene is complex and gives a broad distribution of molecular weight. 

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