5- Methylthiophenium salts

Diels-Alder reactivity of thiophene and benzothiophene remains poorly understood. AMI semiempirical studies examining the activation of thiophene for this thermally allowed [4+2] cycloaddition process have shown that the usual synthesis approaches (use of highly reactive dienophiles, substitution on thiophene, increased reaction pressures) have only small effects on rate enhancement. However, use of the corresponding S-methylthiophenium salts, which have little aromaticity, should provide excellent activation for Diels-Alder reactions of thiophenes even with poor dienophiles such as ethylene <95JHC483>. This AMI approach has been applied to examine Diels-Alder reactions of benzo[6] and benzo[c]thiophenes the theoretical data agree with experimental results <95JCS(P1)1217>. 

H-NMR spectra of S-alkylthiophenium salts have been examined in detail (Table III). Generally, small differences in the chemical shift of the ring protons are observed (relative to the parent thiophene derivatives), but since solubilities usually preclude direct comparison of the spectra, any significance may be lost. The methyl group in S-methylthiophenium salts usually resonates in the range <5 3.0-3.4 ppm. 

Ultraviolet Spectra of Simple Thiophenes and Their 1-Methylthiophenium Hexafluorophosphate Salts" ... 

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