2-pyridone-2-hydroxypyridine dimer

Figure 29.9 Calculated structures of the equilibrium configuration and the transition state of the 2-pyridone 2-hydroxypyridine dimer in the ground state and the excited state [29], Pertinent bond lengths are given in A the numbers in parentheses refer to the equilibrium configuration of the excited state.

A., and Leutwyler, S. (2002) Hydrogen bonding and tunneling in the 2-pyridone- 2-hydroxypyridine dimer, effect of electronic excitation. Chem. Phys., 112, 3717-3726. 

Interestingly, although 6-diethylamino-2-hydroxypyridine exists primarily in the hydroxy form at ambient temperature, its 111 NMR spectrum in TI IF-<7X is temperature-dependent indicating the presence of two distinct species at —118 °C, the phenomenon being attributed to solvated hydroxypyridine monomer pyridone dimer interconversion (91JA721). Hydroxypyridine dimer and pyridone monomer were not detected. 

The synthesis of cis-1,4 polymers was also tried by e use of monomers with an s-cis conformation. The solid-state photopolymerization of pyridone derivatives, which is a six-membered cyclic diene amide and is a tautomer of 2-hydroxypyridine, was attempted [100]. Pyridones make hydrogen-bonded cocrystals with a carboxylic acid in the crystalline state. Because the cyclic structure fixes its s-cis conformation, if the polymerization proceeds, a cis-2,5 polymer would be obtained. Actually, however, the photopolymerization did not occur, contrary to our expectation, but [4-1-4] photodimerization proceeded when the carbon-to-carbon distance for the dimerization was small (less than 4 A) [101]. A closer stacking distance of the 2-pyridone moieties might be required for the topochemical polymerization of cychc diene monomers. 

Measuring the intensity of the OH and NH stretching vibrations in the IR spectra of 2-hydroxypyridine in the range from 428 to 533 K in the gas phase allowed for determination of AH and AS for the equilibrium. The oxo-hydroxy tautomeric ratio in Ar or N2 matrices has been estimated as 1 2.80 and 1 2.99, respectively. A similar ratio of tautomers has been observed also in the gas phase (92JPC1562). IR has also been used as a quantitative tool to determine the association of pyridone dimer (96MI1). 

The concentration dependence of FTIR and UV spectra of 2-hydroxypyridine and its 6-chloro analog was measured in chloroform and CCI4 in order to elucidate their tautomeric equilibrium and determine the association constants (96MI1). For unsubstituted 2-hydroxypyridine, the hydroxy monomer or dimer was not detected in both solvents, and the equilibrium existed exclusively between oxo monomer and dimer. In contrast, for 6-chloro-2-pyridone both monomeric hydroxy (about 72%) and oxo (about 28%) forms were observed in chloroform. Increase in the substrate concentration led to decrease in the content of the monomeric species in favor of relevant dimers. In CC14, 6-chloro-2-hydroxypyridine was found to exist mainly in hydroxy monomer hydroxy dimer equilibrium. 

Observations analogous to those for carboxylic acid dimers have been reported for 2-pyridone-2-hydroxypyridine (2PY-2HP) [52, 53], illustrated in Fig. 29.9. This dimer, formed from two isomers, is asymmetric, but it has a symmetric double proton transfer potential since the transfer interchanges the isomers. In a collaborative effort, Pratt, Zwier, Leutwyler and their coworkers [52] measured and analyzed the high-resolution fluorescence-excitation spectrum of the origin... 

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