Application à l’analyse conformationnelle des N-oxy-pyrrolidines

Dans le document Exploration de surfaces d’énergie potentielle par modélisation moléculaire en chimie organique (Page 26-50)

que nous avons modifié afin de traiter :

- la fonction nitroxyle

- la pseudorotation des cycles à cinq chaînons (méthode développée par D. Cremer et J.A. Pople) - la fonction peroxydique

- l’effet anomère

Ces travaux ont fait l’objet de deux publications et trois communications par affiche.

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Large Phosphorus Hyperfine Coupling as a Sensitive Tool for Studying Molecular

Dynamics: ESR and Molecular Mechanics Studies of Ring Interconversion in the

cis-2,5-Diphosphoryl-2,5-dimethyl-pyrrolidinoxyl Radical

Antal Rockenbauer,Anouk Gaudel-Siri,Didier Siri,Yves Berchadsky,§Franc¸ois Le Moigne,§

Gilles Olive,§and Paul Tordo*

Chemical Research Center, Institute of Chemistry H-1075 Budapest, Pusztaszeri 59, Hungary,

Laboratoire de Chimie The´orique et Mode´lisation Mole´culaire, UMR 6517, CNRS-UniVersite´s d’Aix-Marseille I et III, Case D42, 13397 Marseille, Ce´dex 20, France, and Laboratoire Structure et Re´actiVite´ des Espe`ces Paramagne´tiques, UMR 6517, CNRS-UniVersite´s d’Aix-Marseille I et III, Case 521, 13397 Marseille, Ce´dex 20, France

ReceiVed: June 5, 2002; In Final Form: January 20, 2003

ESR spectra of cis- and trans-β-diphosphorylated pyrrolidine-N-oxyl radicals, c-1 and t-1, were studied in liquid and frozen solution. The expected 1:2:1 triplet (aP(2)) of the 1:1:1 triplet (aN) was observed for t-1; however, for c-1, the inner lines of the 1:2:1 triplet showed a dramatic broadening characteristic of chemical exchange between two equivalent conformations. Owing to the large difference in the hyperfine splitting constants (hfsc) of the exchanging phosphorus (∆aP ≈ 21 G), the coalescence temperature was unusually high (193 K, in n-pentane), and the potential barrier for ring interconversion (10.5 kJ mol-1) was easily obtained from the temperature dependence of the exchange rate. This value was in very good agreement with the value obtained for an empirical pseudorotational potential (11 kJ mol-1) that was adjusted to fit the temperature dependence of the phosphorus hfsc. For c-1, molecular mechanics calculations gave similar characteristics for the pseudorotational potential and indicated the existence of two identical minima with distorted geometries lying between3T4and3E or4T3and E3. For t-1, only the3T4conformer was found to be significantly populated. Frozen solution spectra showed that the phosphorus hfsc anisotropy is higher when the C-P bond is pseudoaxial; this result can be explained by a geometry-dependent delocalization of the unpaired electron into the phosphorus 3p orbitals.

Introduction

Because of the biological importance of various five-membered rings such as those of ribose rings in nucleic acids1

and proline residues in peptides and proteins,2 there is still considerable interest in determining the conformational states of five-membered rings. Pyrrolidine-N-oxyl radicals (Scheme 1) form an important class of five-membered ring radicals. If their dismutation is prevented by the absence of hydrogens on the R carbons, then most of these radicals are chemically stable, and different approaches have been developed to investigate their conformational behavior.3,4

The conformers of five-membered rings can be close to either envelope (E) or twist (T) geometries (Figure 1). Conversion among the various conformers is usually thought to occur through pseudorotation. Spectroscopic techniques often fail to give reliable information on the heights and positions of barriers to interconversion, except for few simple molecules when the microwave technique is applicable.5 As compared to six-membered rings containing six σ bonds (sp3carbon or nitrogen atoms) where energetically well-separated (∆G0 is ca. 20 kJ mol-1) and slowly interconverting (the interconversion

fre-quency is less than 5 × 108 s-1 below room temperature) conformers exist,6the analogous five-membered rings are rather flexible, and interconversion is fast. (∆G0does not exceed 10 kJ mol-1, and the interconversion frequency is larger than 7× 109s-1 above 200 K.7,8a) This difference is evident in ESR spectroscopy studies where line-width alternation due to chemi-cal exchange is often observed in the spectra of radichemi-cals centered on a six-membered ring. However, for the five-membered ring analogues, such phenomena were detected only in a few cases using the low-temperature matrix technique.8For five-membered ring radicals, the lack of line-width alternation is caused by the complete averaging of coupling constants due to the fast pseudorotation.

Line broadening resulting from chemical exchange depends on the square of the differences in the coupling constants for * Corresponding author. E-mail: tordo@srepir1.univ-mrs.fr. Tel: 33 4

91 632 851.

†Institute of Chemistry.

‡Laboratoire de Chimie The´orique et Mode´lisation Mole´culaire, CNRS-Universite´s d’Aix-Marseille I et III.

§Laboratoire Structure et Re´activite´ des Espe`ces Paramagne´tiques, CNRS-Universite´s d’Aix-Marseille I et III.

SCHEME 1

3851

J. Phys. Chem. A 2003, 107, 3851-3857

10.1021/jp0213351 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/11/2003

interconverting sites, and since β-phosphorus couplings are typically two or three times larger than the respective β-hydro-gen couplings, the presence of a β-31P atom in a free radical should enhance the chance to observe chemical exchange.

In our earlier studies, dramatic line-broadening effects were indeed observed on the ESR spectra of peroxyl radical adducts of 2-diethoxyphosphoryl-2-methyl-3,4-dihydro-2H-pyrrole-1-oxide (DEPMPO)9a,b as well as on those of geminal diphos-phoryl-substituted pyrrolidinoxyl radicals.9c Interconversion between conformers with different ring geometries resulting from hindered rotation around the O-OR bond in the first case or the C-P bond in the second case9ccan explain the observed line broadening. However, in both cases, the barrier for ring inversion does not represent the major contribution to the total barrier for interconversion, and thus the rate of ring inversion is not available from ESR experiments.

Because of the preference of the pyrrolidinoxyl radicals for

3T4 and 4T3 twist conformations3 and the preference of the diethoxyphosphoryl group for axial geometry,3cno ring inter-conversion was observed for trans-2,5-diethoxy-phosphoryl-2,5-dimethyl-pyrrolidinoxyl nitroxide t-1 (Scheme 1, Figure 2). However, for cis-2,5-diethoxyphosphoryl-2,5-dimethyl-pyrro-lidinoxyl nitroxide c-1 (Scheme 1), one phosphoryl substituent occupies the preferred axial geometry, and the second is forced to the less stable equatorial orientation. As a consequence, a double minimum is expected in the pseudorotational itinerary of the pyrrolidinoxyl ring.

In the present paper, we give a complete analysis of the ESR spectra of c-1 recorded over a large temperature range. To obtain reliable calculated data, liquid and frozen solution spectra were simulated by an automatic parameter-fitting procedure.10 The study of the influence of temperature on the interconversion rate yielded the thermodynamic parameters of the process. We also considered the influence of solvent molecules on the chemical exchange. Moreover, in frozen solution, where dif-ferent ring geometries can be fixed by solvation, we examined the possibility of phase transition due to molecular rearrange-ments in the coordination sphere.

Finally, to have a complete description of the pseudorotational itinerary, conformational analysis of t-1 and c-1 was performed

using molecular mechanics calculations. In this approach, the influence of rotation around the C-P bonds was considered.

Results and Discussion

ESR Spectra. The ESR spectrum of t-1 consisted of a large

1:2:1 triplet due to coupling with two equivalent phosphorus nuclei, each line being further split into the 1:1:1 triplet of14N. As shown in Figure 2, no chemical exchange was observed in the temperature range between 193 and 313 K.9c

At elevated temperature, the ESR spectra of t-1 and c-1 are similar. However, for the cis isomer, the inner lines of the 1:2:1 triplet showed a dramatic broadening that is characteristic of a chemical exchange (Figure 3). A decrease in the temperature increased the broadening of the inner lines, whereas lines in the wings remained sharp. Inner and outer lines had nearly identical amplitudes at 253 K, and coalescence was achieved at 193 K. Below this temperature, new lines evolved in the center of the spectrum, indicating the presence of frozen conformers.

Figure 1. Usual representation of the pseudorotation of five-membered

rings. Figure 2. Experimental and calculated ESR spectra for t-1: 193 K

(top) and 313 K (bottom).

Figure 3. Experimental and calculated ESR spectra for c-1 at different

temperatures.

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