Syntheses and chemical and physical properties of thiophenetriptycenes

Abstract

Synthesis of 8-hydroxy-4-methylthiophenetriptycene 1 was performed by treatment of the trilithium salt, prepared from 1,1,1-tris(2-bromo-4-methyl-3-thienyl)ethane 13, with diethyl carbonate. In a similar manner, the 8-hydroxy-4-ethylthiophenetriptycene 27 was prepared. The isomeric 4-hydroxy-8-methyl derivative 11 was also obtained by reaction of the trilithium salt, derived from 1,1,1-tris(3-bromo-5-methyl-2-thienyl)ethane 40, with dimethyl carbonate. Attempts to prepare 8-hydroxy-4-tert-butyl- 31 and 8-hydroxy-4-unsubstituted thiophenetriptycenes resulted in the formation of ketone 29 and hydroperoxide 32, respectively. The 8-hydroxy-4-methylthiophenetriptycene 1 decomposed to the corresponding ketone 26 on heating. Attempts to generate the carbocation at the bridgehead of compound 1 by dissolution in conc. H₂SO₄ or by acetolysis of methanesulfonate 44 were unsuccessful. 8-Acetoxy (45) and 8-methoxy (46) derivatives of compound 1 were prepared by treatment of compound 1 with acetic anhydride in triethylamine in the presence of DMAP and by methylation of the lithium salt of compound 1 with trimethyloxonium tetrafluoroborate, respectively. Comparison of IR spectra of regioisomers 1 and 11 indicated that hydrogen bonding of the bridgehead hydroxy group in compound 1 is somewhat hampered by the steric hindrance of the sulfur atoms of the three thiophene rings.