Add time:08/21/2019         Source:sciencedirect.com

13C- and 1H-NMR spectroscopy was used to obtain njHH, nJCH and δC-13 for a series of substituted 2-hydroxypyridines (3-hydroxy; 4-hydroxy; 5-chloro; 3-methyl-5-chloro; 6-chloro; 6-amino; 6-hydroxy). It is shown that the additivity of δC-13 and 1JCH values provides a valuable criterion for differentiating between quinoidal and aromatic structures and in defining conformations. The first conclusion is based on the fact that for 3-substituted compounds there is accordance between experimental values and the additivity 1JCH and δC-13 values calculated for 2-pyridone. In the 6-substituted compounds there is marked deviation from additivity in 1JCH. On the other hand, it seems that the substituent effect on δC-13 in the hypothetical 2-hydroxypyridine and the real 2-pyridone are markedly different for C6, since there is no accordance between additivity and experimental values for the chemical shifts of this carbon. These conclusions are in agreement with results obtained from IR and pK studies and quantum-chemical calculations which indicate that a substituent in position 3 favours a pyridone structure, while a pyridinol structure is favoured when the substituent is at ring-position 6.

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