AROMATIC-SUBSTITUTION .59. C-NITRATION OF 2,6-DI-TERT-BUTYLPYRIDINE AND 2,4,6-TRI-TERT-BUTYLPYRIDINE WITH NITRONIUM TETRAFLUOROBORATE

Electrophilic substitution of pyridine occurs preferentially on the heteroatom (N-substitution). Nitration of pyridine with mixed nitric-sulfuric acid gives a low yield of the 3-nitro product, indicative that the nitration is that of the pyridinium ion formed by protonation in the acid medium.2 The most convenient method of preparing 4-nitropyridine is that through nitration of pyridine N-oxide, followed by deoxygenation (for example with phosphorous trichloride).3 Nitronium salts when reacted with pyridine gave N-nitropyridinium salts (N-nitration).4 Nitration of 2,6-dimethylpyridine and 2,4,6-trimethyl-pyridine also occurred on nitrogen. The resulting pyridinium salts are effective transfer nitration reagents.5,6Whereas 2,6-di-tert-butylpyridine7has found extensive use as proton trap in synthetic chemistry,8 electrophilic chemistry of hindered pyridines has not been explored. © 1990, American Chemical Society. All rights reserved.