2-Pyridone is the chemical compound with the formula C5H4NH(O). This colourless crystalline solid is used in peptide synthesis. It is well known to form hydrogen bonded structures somewhat related to the base-pairing mechanism found in RNA and DNA. It is also a classic case of a molecule that exists as tautomers.
This pattern is responsible for some remarkable physical and chemical properties.
The hydrogen bond to the nitrogen is also suitable to move to the oxygen. Through hydrogen and electron shift the second tautomer form of the substance is formed. 2-hydroxypyridine is the name for this tautomer. This lactam lactim tautomerism can also be found in other molecules with a similar structure.
Tautomerism in Solid State
The predominant form in solid state is the 2-pyridone. This fact has been clarified by X-ray crystallography which shows that the hydrogen in solid state is closer to the nitrogen than to the oxygen (because of the low electron density at the hydrogen the exact positioning is difficult) and IR-spectroscopy in which the C=O longitudinal frequency is present and the O-H frequencies are absent.
Tautomerism in Solution
To determine which of the two tautomeric forms is present in solution has been the subject of many publications. The energy difference seems to be very small and depending on the polarity of the solvent. Nonpolar solvents favour the 2-hydroxypyridine whereas in polar solvents like alcohols and water the 2-pyridone is favoured.
The energy difference for the two tautomers in the gas phase was measured by IR-spectroscopy to be 2.43 to 3.3 kJ/mol for the solid state and 8.95 kJ/mol and 8.83 kJ/mol for the liquid state.
Tautomerisation Mechanism A
The single molecular tautomerisation has a forbidden 1-3 suprafacial transition state and therefore has a high energy barrier for this tautomerisation, which was calculated with theoretical methods to be 125 or 210 kJ/mol. The direct tautomerisation is energetically not favoured. There are other possible mechanisms for this tautomerisation.
The 2-pyridone and the 2-hydroxypyridine can form dimers with two hydrogen bonds.
Aggregation in Solid-state
In solid state the dimeric form is not present; the 2-pyridones form a helical structure over hydrogen bonds. Some substituted 2-pyridones form the dimer in solid state, for example the 5-methyl-3-carbonitrile-2-pyridone. The determination of all these structures was done by X-ray crystallography. In solid state the hydrogen is located closer to the oxygen so it could be considered to be right to call the colourless crystals in the flask 2-pyridone.[1-5]
Aggregation in Solution
In solution the dimeric form is present; the ratio of dimerisation is strongly dependent on the polarity of the solvent. Polar and protic solvents interact with the hydrogen bonds and more monomer is formed. hydrophobic effects in unpolar solvents lead to a predominance of the dimere. Also the ratio of the tautomeric forms is dependent on the solvent. All possible tautomers and dimmers can be present and form an equilibrium. The exact measurement of all the equilibrium constants in the system are extremely difficult.[17-27]
Some publications only focus one of the two possible patterns, and neglect the influence of the other. For example to calculate the energy difference of the two tautomeres in nonpolar solution, leads too a wrong results if a large quantity of the substance is on the side of the dimer in an equilibrium.
Tautomerisation Mechanism B
The direct tautomerisation is energetically not favoured, but a dimerisation followed by a double proton transfer and a dissociation of the dimer is a self catalytic way from one tautomer to the other. In solution, the tautomerisation can be done over the dimer. Protic solvents also mediate the proton transfer during the tautomerisation. Like the deprotonation and reprotonation during autoprotolyse can leads to both tautomers.
Main Research Interests
After the discovering that 2-Pyridone catalyses the mutarotation of sugars and that 2-pyridone has a large effect on the reaction from activated esters with amines in nonpolar solvent. Acid or base catalysed reactions should depend in first order on pKa value, but as relative weak acid or base enhances the reaction far more than expected. 4-Pyridone shows no such effect, this leads to the conclusion that the special structure and tautomerisation is the cause for this catalysis.
Neither sugar mutarotation nor ester aminolysis in nonpolar solvent have big impact in synthesis. Because sugar mutarotation takes place even without catalysis and ester aminolysis as source for peptide bonds was seldom used and with activated esters the reaction itself is fast. Polar solvents enhance the reaction more than the use of 2-pyridone. The normal synthesis for peptides with DCCI or DMAP give good yields and the previous synthesis of phenyl or nitro-phenyl esters can be avoided. Because of this a direct use of 2-pyridone in ester aminolysis was never the goal of the research. But understanding the simple proton transfer catalysis would be a big step in understanding the principle which is also present in enzyme catalysed reaction. Most of the research was done to understand the activation of the transition state by the 2-pyridone. Isotope labelling, kinetics and quantum chemical methods were used on the mechanism to determine the rate determining step in the reaction.
The cyclisation of a macrocycle was catalysed with 2-pyridone. A synthesis trick for unwilling substrates is to use molten 2-pyridone as solvent.
Relation to Base Pairs
These structures are closely related to the base pairs present in the DNA or RNA. These dimers are sometimes used as simple models for base pairs (in experimental and theoretical studies). The strength of the hydrogen bonds is important for the two strands in DNA and RNA sticking to each other. For the 2-pyridone dimer there are direct measurements of the dimerisation constant and the dimerisation energy which are compared to the calculated ones. Because of the multiple possible base pair combinations, measurements with the natural base pairs are difficult. If the results of the simple 2-pyridone model give good agreement, these theoretical methods are also suitable for base pairs.
2-Pyridone and some derivatives where used as ligands in coordination chemistry. The main point of this chemistry was that 2-pyridone functions as a 1,3-bridged ligand like carboxylate. There is a large number of dimeric complexes. A review with a literature overview can be found at Rawson and Winpenny.
1H-NMR (400 MHz, CD3OD): /ρ = 8.07 (dd,3J = 2.5 Hz,4J = 1.1 Hz, 1H, C-6), 7.98 (dd,3J = 4.0 Hz,3J = 2.0 Hz, 1H, C-3), 7.23 (dd,3J = 2.5 Hz,3J = 2.0 Hz, 1H, C-5), 7.21 (dd,3J = 4.0 Hz,4J = 1.0 Hz, 1H, C-4).
(100.57 MHz, CD3OD): ρ = 155.9 (C-2), 140.8 (C-4), 138.3 (C-6), 125.8 (C-3), 124.4 (C-5)
(MeOH):νmax (lg ε) = 226.2 (0.44), 297.6 (0.30).
(KBr): ν = 3440 cm-1–1 (br, m), 3119 (m), 3072 (m), 2986 (m), 1682 (s), 1649 (vs), 1609 (vs), 1578 (vs), 1540 (s), 1456 (m), 1433 (m), 1364 (w), 1243 (m), 1156 (m), 1098 (m), 983 (m), 926 (w), 781 (s), 730 (w), 612 (w), 560 (w), 554 (w), 526 (m), 476 (m), 451 (w).
EI-MS (70 eV): m/z (%) = 95 (100) [M+], 67 (35) [M+ - CO], 51 (4)[C4H3+].
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