Synthesis of Anthryl-Substituted Bithienylpyridines and their N-Methylpyridinium Salts

Abstract

The synthesis of bithiophenes with anthracene and pyridine as terminal groups is described. 3- and 4-thienylpyridines 3, derived from thiopheneboronic acid 1 and the respective bromopyridines 2a, b, were coupled with 2-(9-anthryl)-5-iodothiophene 4 to give the isomeric (9-anthryl)bithienylpyridines 5a, b. In order to introduce a methylene spacer between the donor anthracene and bithiophene, 5-(2,2′-bithienyl)magnesium bromide 7 was reacted with 2 to yield the bithienylpyridines 9a, b. The reaction of 9 with 9-hydroxymethylanthracene 10 gave 3- and 4-(9-anthrylmethyl)bithienylpyridines 11a, b. The linkage of pyridine to bithiophene via a methylene spacer was reached by reacting lithiobithiophene with the isomeric pyridinecarbaldehydes 14a, b. Dehydroxylation of the resulting carbinols 15a, b with NaBH3CN/ZnI2, however, afforded the N-cyanoborane adducts 16a, b, which were transformed to the desired 3- and 4-(bithienylmethyl)pyridines 18a, b by heating either in EtOH or in 6 N HCl. Pyridines 5, 9, 11, and 18 were methylated with CF3SO3Me in CH2Cl2 or Et2O as solvent to yield the corresponding pyridinium triflates 6, 12, 13, and 19 in high purity.