Tautomerism in some aromatic Schiff bases and related azo compounds: an LSER study

An LSER study on the tautomerism of some aromatic Schiff bases and related azo compounds has vindicated the picture presented by previous ab initio studies and added further detail of its own. As predicted, the aminoenone tautomer is always the more polar (positive π* term) and, in addition, is specifically favoured by proton donor solvents (α term positive) through binding to the second lone pair on carbonyl. As expected for chelates, the Schiff bases are unaffected by proton acceptors (β term zero) but owing to the poor proton acceptor properties of the azo linkage, those of the azo compounds are partially broken. Tautomer preference as earlier calculated by ab initio methods is linearly related to log KT in the gas phase as estimated by LSER. Comparison with other compounds clearly shows the effect of aromatization in reducing KT; for 1-[(E)- methyliminomethyl]naphthalen-2-ol this is quantitatively similar to that for 2-pyridone, of which it is a kind of vinylogue. However, the effect of benzo fusion is similar only in part, and a further apparent anomaly lies in the near-identical KT values, for water, of Schiff bases and azo compounds. This is discussed in terms of their differing ‘intrinsic’ tautomeric bias, leading to a fortuitous cancellation of effects.