NH-Tautomerism of Isocorrole — Solvent Mediated Proton Transfer

Dominik Bremm and Georg Hohlneicher

Isocorrole is a contracted porphyrin, which has recently been synthesized in the group of E. Vogel (University of Cologne). Contary to the parent compound porphyrin, this molecule posesses three iminoprotons in it's central N4 cavity. These protons can be arranged in two different ways and consequently two NH-tautomers are possible (A and B, see scheme below).

According to the NMR spectra of the compound, tautomer A is the predominant form in solution at room temperature and below. Presumably because of their low intensity, no signals of the tautomer B could be detected. Electronic spectra however show clearly, that a small amount of this second tautomer is indeed present. Apart from intense bands attributalble to tautomer A, bathochromically shifted shoulders of low intensity are observed in the low-energy and the high-energy part of the spectrum (Q band region and Soret band region). These bands are seen most clearly in the spectrum of a matrix isolated sample of isocorrole (see Fig. 1). Using fluorescence spectroscopy and temperature dependent measurements (see below), an unambiguous assignment of these signals to tautomer B was possible.

The temperature dependent measurements of the UV/Vis spectrum in solution reveal remarkable differences with respect to the solvent used (Fig. 2). In ethanol, a reduction of the intensities of bands assigned to tautmer B in relation to the bands of A is observed on cooling of the sample, which can be attributed to a shift of the tautomeric equilibrium towards tautomer A. In aprotic solvents like 2-methyl-tetrahydrofurane and dichloromethane however there is almost no change in the relative intensity of the bands of B. Obviously, in the second case the tautomeric equlibrium is not attained.

These spectroscopic results can be interpreted in terms of a solvent-mediated proton transfer process. The energy barrier of the “direct ” proton transfer leading to tautomerization is obviously too high to permit the reaction to proceed at measurable speed at room temperature and below. A protic solvent like ethanol can form a bridge between the two pyrrole rings participating in the proton transfer and may thus act as a catalyst for the reaction (see Scheme below). A similar catalysis of a proton transfer step by a protic solvent has been demonstrated in a number of other cases [2].

In order to corroborate the experimental results, the energetics of the proton transfer reaction with and without catalysis were calculated at DFT level using the B3LYP exchange correlation functional and the 6-31G(d,p) basis set [3]. The energy barrier of the direct transfer was determined from geometry optimizations of the ground states and the transition state in vacuum. In the calculations on the catalysed reaction, one alcohol molecule was included in the DFT calculation, whereas the remainder of the surrounding solvent was included by a SCRF calculation. The results of these computations are displayed in Fig. 3. The calculated energy barrier of the uncatalysed reaction is 84 kJ/mol (including ZPE), which is a rather large value compared to similar compounds like porphyrin. This energy barrier is sharply reduced to 33 kJ/mol, when the solvent is taken into account. Thus, the catalytic influence of a protic solvent can be modelled very well by quantum chemical calculations. The calculated energy difference between tautomer B and tautomer A of 6.6 kJ/mol (vacuum) agrees reasonably well with the experimental reaction enthalpy of 5.1 kJ/mol which was determined from the temperature dependent measurements using the van't Hoff equation.

[1] Y. Hellwig, Dissertation , Universität zu Köln 1993.
[2] for example: O. Bensaude, M. Chevrier, J.-E. Dubois, J. Am. Chem. Soc. 101, 2423 (1979).
[3] B3LYP: A. D. Becke, J. Chem. Phys. 96, 2155 (1992); A. D. Becke, J. Chem. Phys. 98, 5648 (1993); C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37, 785 (1988); 6-31G(d,p) Basis set: P. C. Hariharan, J. A. Pople, Theoret. Chim. Acta 28, 213 (1973).