Molecular Structure of Heterocycles: 6. Solvent Effects on the 17O Nmr Chemical Shifts of 5-Trichloromethylisoxazoles

Abstracts

A multilinear-regression analysis using the Kamlet-Abboud-Taft (KAT) solvatochromic parameters in order to elucidate and quantify the solvent effects on the 17O chemical shifts of three 5-trichloromethylisoxazoles [(1a) non-, (1b) 3-methyl- and (1c) 4-methyl-substituted] is reported. The chemical shifts of ring oxygen atom, O1, of compounds 1a-c show dependencies (in ppm) on the solvent polarity-polarizability of -4.8pi*, -3.2pi*, -8.9pi*, on the solvent hydrogen-bond-donor (HBD) acidities 0.9alpha, -0.2alpha, -2.7alpha and the solvent hydrogen-bond-acceptor (HBA) basicities -0.4beta, 1.9beta, 0.9beta, respectively. The data of net charges of O1 and dipole moment, obtained from MO calculations (AM1), are compared with the solvent effect parameters obtained for compounds 1a-c.

17O NMR; solvent effects; isoxazoles; MO calculations

Com o objetivo de elucidar e quantificar os efeitos do solvente sobre os deslocamentos químicos de 17O de três 5-triclorometilisoxazóis [(1a) não-, (1b) 3-metil- e (1c) 4-metil-substituído] foi realizada uma análise de regressão multilinear, utilizando os parâmetros solvatocrômicos de Kamlet-Abboud-Taft (KAT). Os deslocamentos químicos do átomo de oxigênio do anel, O1, dos compostos 1a-c mostraram dependências (em ppm) em função da polaridade-polarizabilidade do solvente de -4.8pi*, -3.2pi*, -8.9pi*, em função da acidez do solvente (HBD) de 0.9alfa, -0.2alfa, -2.7alfa e em função da basicidade do solvente (HBA) de -0.4beta, 1.9beta, 0.9beta, respectivamente. Os dados de carga líquida de O1 e de momento de dipolo, obtidos por cálculos de orbitais moleculares (AM1), são comparados com os parâmetros de efeitos do solvente determinados para os compostos 1a-c.

a19v12n6

Short Report

Molecular Structure of Heterocycles: 6. Solvent Effects on the ¹⁷O Nmr Chemical Shifts of 5-Trichloromethylisoxazoles

Marcos A. P. Martinsa^*, Rogério A. Freitag^b, Nilo E. K. Zimmermanna, Adilson P. Sinhorin^a, Wilson Cúnico^a, Giovani P. Bastos^a, Nilo Zanatta^a and Helio G. Bonacorso^a

^a Departamento de Química, Universidade Federal de Santa Maria, Santa Maria - RS, Brazil

^b Departamento de Química Orgânica - I.Q., Universidade Federal de Pelotas, Pelotas - RS, Brazil

Com o objetivo de elucidar e quantificar os efeitos do solvente sobre os deslocamentos químicos de ¹⁷O de três 5-triclorometilisoxazóis [(1a) não-, (1b) 3-metil- e (1c) 4-metil-substituído] foi realizada uma análise de regressão multilinear, utilizando os parâmetros solvatocrômicos de Kamlet-Abboud-Taft (KAT). Os deslocamentos químicos do átomo de oxigênio do anel, O1, dos compostos 1a-c mostraram dependências (em ppm) em função da polaridade-polarizabilidade do solvente de -4.8p*, -3.2p*, -8.9p*, em função da acidez do solvente (HBD) de 0.9a, -0.2a, -2.7a e em função da basicidade do solvente (HBA) de -0.4b, 1.9b, 0.9b, respectivamente. Os dados de carga líquida de O1 e de momento de dipolo, obtidos por cálculos de orbitais moleculares (AM1), são comparados com os parâmetros de efeitos do solvente determinados para os compostos 1a-c.

A multilinear-regression analysis using the Kamlet-Abboud-Taft (KAT) solvatochromic parameters in order to elucidate and quantify the solvent effects on the ¹⁷O chemical shifts of three 5-trichloromethylisoxazoles [(1a) non-, (1b) 3-methyl- and (1c) 4-methyl-substituted] is reported. The chemical shifts of ring oxygen atom, O1, of compounds 1a-c show dependencies (in ppm) on the solvent polarity-polarizability of -4.8p*, -3.2p*, -8.9p*, on the solvent hydrogen-bond-donor (HBD) acidities 0.9a, -0.2a, -2.7a and the solvent hydrogen-bond-acceptor (HBA) basicities -0.4b, 1.9b, 0.9b, respectively. The data of net charges of O1 and dipole moment, obtained from MO calculations (AM1), are compared with the solvent effect parameters obtained for compounds 1a-c.

Keywords:¹⁷O NMR, solvent effects, isoxazoles, MO calculations

Introduction

Several papers have been devoted to the empirical and theoretical studies of solvent effect on the ¹⁷O chemical shifts in different organic compounds^1,2. Special attention has been devoted to the study of solvent effects in amides, where the ¹⁵N and ¹⁷O nuclei are observed². Recently we applied a multilinear-regression analysis using the Kamlet-Abboud-Taft (KAT)³ solvatochromic parameters in order to elucidate and quantify the solvent effects on the ¹⁷O chemical shifts of 1,1,1-trichloro-4-methoxy-3-alken-2-ones⁴ and 5-hydroxy-4,5-dihydroisoxazoles^5a. According to the KAT formalism, the observed chemical shift of compound X at infinite dilution in solvent Y, d^X_Y, would be given by the relationship³ shown in Equation 1.

The solvent effects are described by the solvent parameters d^X_CH, p*_Y, d_Y, a_Y and b_Y. The p*_Y scale is an index of solvent dipolarity/polarizability, which measures the ability of the solvent to stabilize a charge or a dipole due to its dielectric effect. The a_Y scale of solvent hydrogen-bond-donor (HBD) acidities describes the ability of the solvent to donate a proton in a solvent-to-solute hydrogen bond. The b_Y scale of hydrogen-bond-acceptor (HBA) basicities measures the ability of the solvent to accept a proton (i.e., to donate an electron pair) in a solute-to-solvent hydrogen bond. The d_Y parameter is a polarizability correction term for polychlorinated (d_Y = 0.5) and aromatic (d_Y = 1.0) solvents. The coefficients s^X, a^X and b^X in Equation 1 define the sensitivity of d^X_Y to solvent dipolarity/polarizability, acidity and basicity, respectively. The product of coefficients s^Xd^X defines the sensitivity of d^X_Y for the polarizability correction term. The term d^X_CH is the chemical shift of substrate X measured in cyclohexane since this reference solvent does not form hydrogen bond (a_CH = b_CH = 0) and was selected to define the origin of p*_Y scale (p*_CH = 0). The term s^X (p*_Y + d^Xd_Y) accounts for the difference between the contribution to d^X_Y in solvent Y and in cyclohexane from the solute-solvent interactions other than hydrogen bonding. The terms a^Xa_Y and b^Xb_Y represent the contributions from hydrogen bonds of substrate X with solvents HBD and HBA, respectively.

As a part of our research program we have studied the synthesis^6-9, structure⁵ and the multi-nuclear NMR chemical shifts¹⁰ of 5-, 6- and 7-membered heterocycles. The aim of this work is to elucidate and quantify the solvent effects on the ¹⁷O chemical shifts of 5-trichloromethyl-isoxazoles 1a-c using the Kamlet-Abboud-Taft (KAT) solvatochromic parameters³ (Scheme).

Experimental

Compounds

The synthesis of compounds 1a-c was developed in our laboratories^6a.

NMR spectroscopy

The ¹⁷O NMR spectra were recorded on a Bruker DPX 400 at 54.25 MHz. The sample temperature was set at 323 ± 1 K. The instrumental settings were as follows: spectral widths 38 KHz (705 ppm), 8K data points, pulse width 12 ms (90^o), acquisition time 54 ms, preacquisitions delay 10 ms, 16000-90000 scans, LB of 100 Hz, sample spinning 20 Hz. The spectra were recorded with a RIDE (RIng Down Eliminate) sequence¹³ for suppression of acoustic ringing. The general reproducibility of chemical shift data is estimated to be better than ± 1.0 ppm (± 0.2 within the same series). The half-height widths were in the range 150-800 Hz.

All spectra were acquired in a 10mm tube, at natural abundance, in acetone, methanol, acetonitrile, dimethyl-sulfoxide, toluene, chloroform and dichloromethane as solvents. The concentration of the samples used in these experiments was 0.5, 1.0, 2.0, 3.0, 4.0 and 6.0 mol L^-1, and the signals were referenced to external H₂O (in a capillary coaxial tube).

Semiempirical MO calculations

The MO calculations were carried out by the Austin Model 1 (AM1) semiempirical method¹¹, implemented in the HyperChem 6.03 package (1999)¹². Geometries were completely optimized without fixing any parameter, thus bringing all geometric variables to their equilibrium values. The energy minimization protocol employs the Polak-Ribiere algorithm, a conjugated gradient method^11,12. Convergence to a local minimum is achieved when the energy gradient is < 0.01 kcal mol^-1. The calculations were performed on a PC Pentium IV 1.4 GHz computer equipped with a printer.

Results and Discussion

The ¹⁷O chemical shifts of 5-trichloromethylisoxazoles 1a-c in various solvents are listed in Table 1. These values were determined by extrapolation to infinite dilution from spectral data obtained in several concentrations (0.5 to 6 mol L^-1) relative to external water, at 323 K (see experimental). The Kamlet-Abboud-Taft (KAT) solvatochromic parameters (p*_Y, a_Y, b_Y and d_Y) used in the present work are also given in Table 1. Considering the ¹⁷O NMR chemical shifts of the oxygen atom of the hetero-cyclic ring (O1) of compounds 1a-c and according to the KAT formalism, we can re-write Equation 1 as Equation 2 (where X = O1).

Table 2 presents the least-squares-fitted solute (1a-c) estimates using Equation 2. The chemical shifts of ring oxygen atom, O1, of compounds 1a-1c show dependencies (in ppm) on the solvent polarity-polarizability of -4.8p*, -3.2p*, -8.9p*, on the solvent hydrogen-bond-donor (HBD) acidities 0.9a, -0.2a, -2.7a and on the solvent hydrogen-bond-acceptor (HBA) basicities -0.4b, 1.9b, 0.9b, respectively.

Preliminary comparison shows that the response values of the oxygen chemical shifts to the solvent-solute dipolarity-polarizability (s^O1) are of shielding effect for O1. The response to the solvent HBD acidities (a^O1) is of deshielding for O1 of 1a and shielding for 1b and 1c.

The influence of the solvent hydrogen-bond-acceptor (HBA) basicities (b^O1) is of shielding effect for 1a and deshielding for 1b and 1c.

The contributions (in ppm) to the ¹⁷O chemical shifts of O1 for compounds 1a-c from the terms of Equation 2 are listed in Table 3. The contribution of solvent-solute dipolarity-polarizability (s^O1p*) shows a shielding effect for chemical shift of O1 groups (1a-c) in all solvents, in the following order: dimethylsulfoxide > dichloromethane > chloroform > acetonitrile > acetone > methanol > toluene. The contribution of solvent HBD acidities (a^O1a) shows a negligible (< 1.0 ppm) deshielding (1a) or deshielding (1b and 1c) effect on the chemical shift of O1, except for methanol and chloroform in compound 1c. The contribution response to the solvent HBA basicities (b^O1b) show a negligible (< 1.0 ppm) shielding (1a) or deshielding (1b and 1c) effect for chemical shift of O1 oxygen atom, except for methanol and dimthyl-sulfoxide in compound 1b.

Considering that the terms s^O1, a^O1 and b^O1 are a measurement of the sensitivity of the studied compound to the solvent dipolarity/polarizability (p*_Y), the solvent hydrogen-bond-donor acidities (a_Y) and the solvent hydrogen-bond-acceptor basicities (b_Y), respectively, the differences between these parameters for compounds 1a-c must reflect the differences in some intramolecular properties of these molecules. In order to better understand the differences of the sensitivity of each compound to the solvent effects the MO calculations were performed. Selected data of the most stable molecular structure of compounds 1a-c were determined by energy minimization calculations using the AM1 semiempirical method^11,12 are listed in Table 4.

Although the solvent effect was obtained only for three compounds, and therefore we can not make a statistical treatment, when these data were compared with molecular data obtained by MO calculations, it was possible to observe some reasonable trends.

The solvent-solute dipolarity-polarizability (s^O1) does not show any trend when compared with the dipole moments of compounds 1a-c (Table 4). The other parameters show a tendency to decrease (a^O1) and increase (b^O1) with dipole moments of compounds 1a-c. All absolute values of solvent parameters (s^O1, a^O1, b^O1) for O1 decrease with the decrease of the net charge in this atom (Table 4).

Conclusion

This work show the validaty to use the Kamlet-Abboud-Taft (KAT) model for complete evaluation of the solvent effects on the ¹⁷O chemical shifts of compounds 1a-c. From a multi-linear-regression analysis using the Kamlet-Abboud-Taft (KAT) solvatochromic parameters (p*_Y, a_Y, b_Y and d_Y) and the observed ¹⁷O chemical shifts of compounds 1a-c, at infinite dilution, it was possible to determine the terms s^O1, a^O1 and b^O1. These terms are a measurement of the sensitivity of the studied compounds to the solvent dipolarity/polarizability, the solvent hydrogen-bond-donor acidities and the solvent hydrogen-bond-acceptor basicities, respectively.

Acknowledgments

The authors thank the Conselho Nacional de Desen-volvimento Científico e Tecnológico (CNPq / PADCT) and Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS) for financial support. The fellowships from CNPq, CAPES and FAPERGS are also acknowledged.

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Received: June 11, 2001

Published on the web: August 10, 2001

Publication Dates

• Publication in this collection
  15 Apr 2002
• Date of issue
  Dec 2001

History

• Received
  11 June 2001
• Accepted
  10 Aug 2001