Amine-imine tautomerism in adenines

Molecular Energies in Adenine Tautomers (CNDO/2 Calculation) with Respect to the Most Stable Tautomer 

112 See, e.g., B. Pullman and A. Pullman, Les Theories Electroniques de la Chimie Organique. Masson, Paris, 1952. 

it is particularly remarkable that this delicate theoretical result is confirmed by experiment. Thus, chemical evidence indicates 

Examples of a tautomeric shift toward the imine form are relatively scarce among conjugated heterocycles. In our laboratory such calculations have been carried out, always by the CNDO/2 method, for two other cases, which seem worthwhile mentioning. 

Singer and H. Fraenkel-Conrad, Progr. Nucl. Acid Res. Mol. Biol. 9, 1 (1969). 

A number of physicochemical properties which have been investigated for a series of simple derivatives of the different tautomeric forms of adenine, prominent among which are dipole moments and ultraviolet absorption spectra, throw additional light on the problem. Table VIII indicates the results of a theoretical evaluation of the dipole moments for the different tautomeric forms (12-19) of adenine as well as the experimental values of the moments in methyl and benzyl derivatives of these forms with the exception of the derivatives of the N(1)H form for which, unfortunately, no experimental data could be obtained because of lack of sufficiently soluble 1-substituted derivatives. 

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The amine-imine tautomerism, liable to occur in aminopurines which may be illustrated in the particularly important case of adenine by the structures 6 and 7. 

There are various forms of tautomerism which operate in the different purine species. (1) Prototropy which involves attachment of the proton to any one of the four ring nitrogen atoms (Scheme 5). Corresponding CH tautomers, for example (52), seem to be of little significance. (2) Amine-imine tautomerism which operates in the aminopurines such as adenine (Scheme 6). (3) Lactam-lactim tautomerism as in the hydroxypurines such as hypoxanthine (Scheme 7) and the related thioxo-thiol tautomerism (53) and (54) in the biologically imporfant mercaptopurines (Scheme 8). The subject has recently been discussed in some detail <76AHC(Si)502>. 

Conformational equilibria of amines and aminoacids have been studied in low-temperature matrix under UV or IR irradiation." Amine-imine tautomerism and formation of a radical was observed from adenine. New studies on the aniline excited states has evidenced the role of Rydberg states. A re-examination of the Oilman reaction has demonstrated that this is an electron transfer process. The photo-cyclization of halogenated (iodine is by far to be preferred) N-ben lani-lines to phenanthridine has been studied with particular attention at the preparative value and the tolerance of alko)q, dio)q methylene and halogen substituents, to which it was found to be quite tolerant. ... 

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