Conformational analysis and stereoelectronic effects in trans-1,2-dihalocyclohexanes: 1H NMR and theoretical investigation

Spectrochimica Acta Part A 61 (2005) 1771–1776

Conformational analysis and stereoelectronic effects in trans-1,2-dihalocyclohexanes: 1 H NMR and theoretical investigation Matheus P. Freitasa , Roberto Rittnera , Cl´audio F. Tormenab∗ , Raymond J. Abrahamc a

b

Physical Organic Chemistry Laboratory, Instituto de Qu´ımica, Universidade Estadual de Campinas, Caixa Postal 6154, 13084-971 Campinas, SP, Brazil Departamento de Qu´ımica, Faculdade de Filosofia Ciˆencias e Letras de Ribeir˜ao Preto, Universidade de S˜ao Paulo, Av. Bandeirantes 3900, 14040-901 Ribeir˜ao Preto, SP, Brazil c Department of Chemistry, The University of Liverpool, PO Box 147, Liverpool L69 3BX, UK Received 11 May 2004; accepted 10 July 2004

Abstract The conformational equilibrium of trans-1,2-difluoro- (1), trans-1,2-dichloro- (2) and trans-1,2-dibromo-cyclohexane (3) was studied through a combined method of NMR, theoretical calculations and solvation theory. The solvent dependence of the 3 JH1 ,H2 NMR coupling constants together with theoretical calculations allow the direct determination of the conformational equilibria without recourse to model compounds. The coupling constants were obtained with the aid of spectrum simulation, since these symmetric molecules present complex coupling systems. The observed couplings, when analysed by solvation theory and utilising DFT geometries (B3LYP/6-311+G**), gave energy values of Eee − Eaa of 0.10, 0.95 and 1.40 kcal mol−1 in the vapour phase for 1, 2 and 3, respectively, decreasing to −0.63, 0.36 and 0.93 kcal mol−1 in CCl4 and to −1.91, −0.80 and −0.05 kcal mol−1 in DMSO solution. The diaxial preference for all compounds is explained by natural bond orbital (NBO) analysis, which shows important hyperconjugative effects in this conformation. The “gauche effect” for compounds with more electronegative substituents, which are in gauche arrangement in the ee conformation, also plays a relevant role in more polar solvents. © 2004 Elsevier B.V. All rights reserved. Keywords: Conformational analysis; NMR; Theoretical calculations; trans-1,2-Dihalocyclohexanes

1. Introduction Conformational analysis of trans-1,2-disubstituted cyclohexanes has been performed by several investigators [1–6] to determine the main factors that lead the equilibrium towards one conformer or the other, but its importance also has a place in biochemistry and in the pharmacological fields [7,8]. The spectroscopic tools which have been utilised for determination of the conformational behaviour of six-membered rings are usually NMR [3,4,9], through the use of tert-butyl derivatives or low-temperature experiments, and infrared ∗

Corresponding author. Tel.: +55 16 602 4382; fax: +55 16 633 8151. E-mail address: [email protected] (C.F. Tormena).

1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.07.007

spectroscopy [9–11]. However, in the case of symmetric molecular systems, such as trans-1,2-difluoro- (1), trans-1,2dichloro- (2) and trans-1,2-dibromo-cyclohexane (3), there are no reports on the conformational analysis through H H coupling constants, since these couplings are known to be complex spin systems, due to the magnetic non-equivalence of the 1- and 2-position hydrogen atoms. These difficulties can be overcome by spectrum simulation, which has been shown to be an important way to obtain coupling constant values for such systems [12]. We report here the conformational analysis of compounds 1–3 (Fig. 1) through the joint methodology of NMR, theoretical calculations and solvation theory, which had been developed in the past by Abraham and Bretschneider [13] and

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The theory has been given in detail previously and was shown to give an accurate account of the solvent dependence for a variety of conformational equilibria [14–18]. The calculations were performed with the MODELS program, using as input the geometries from gaussian98 [19a]. The dipole and quadrupole moments of the molecules are calculated directly from the partial atomic charges in the molecule, obtained from the CHARGE program [20].

Fig. 1. Conformational equilibrium of trans-1,2-dihalocyclohexanes (1, X = F; 2, X = Cl; 3, X = Br).

recently applied, with success, to several systems [14–18]. The NMR method consists in the determination of the coupling constants between H1 and H2 (3 JH1 ,H2 ) in different solvents, without recourse to rigid derivatives, the couplings being obtained with aid of spectrum simulation. The solvation energies are calculated by the MODELS program [13], whose theory follows. The natural bond orbital analysis was applied to get the energies involved in orbital interactions (hyperconjugative interactions) and to evaluate which interaction is more important for the conformational stability. Using these orbital interactions, it is possible to explain the experimental results for the studied compounds.

3. Experimental 3.1. Synthesis Compound 1 was obtained through the procedure described in the literature [21], while compound 3 was obtained through the classical reaction between cyclohexene and bromine. Compound 2 was commercially available (Acros). 3.2. NMR experiments The 1 H NMR spectra were obtained on a Varian Gemini 300 spectrometer operating at 300.07 MHz. Spectra were of ca. 20 mg cm−3 solutions with a probe temperature of 296 K. Spectra were all referenced to Me4 Si and the typical conditions were: spectral width 2000 Hz with 32 K data points and zero-filled to 128 K to give a digital resolution of 0.03 Hz, and Gauss window of 0.450 s centred at 0.450 s. For the gCOSY experiment, a Varian standard pulse sequence was used. Typical conditions were 16 transients, accumulated into 2 K data points with 128 experiments, with a pulse width of 12.9 ␮s, sweep width of ca. 5800 Hz and AT of 0.17 s. The FID was zero-filled to 2 K data points (F2) and 2 K data points (F1). Spectrum simulations were performed using the Varian VNMR program [22] and considering an ABCDEF spin system, corresponding to the hydrogens H1 , H2 , H3 , H3  , H6 and H6  , respectively. For the difluoro compound, a ABCDEFXY system was considered. The coupling constants for compounds 1–3 are listed in Table 1. trans-1,2-Difluorocyclohexane. 1 H NMR (CCl4 , 300.07 MHz; δ in ppm): δ 1.32 (2H, m, H4 and H5 ), 1.63 (2H, m, H3 and H6 ), 1.95 (2H, m, H4  and H5  ), 2.77 (2H, m, H3  and H6  ), 4.51 (2H, m, H1 and H2 ). 13 C NMR (CCl4 ,

2. Theory The DFT calculations were performed using the gaussian98 program [19a] and the solvation calculations using the MODELS program [13]. The solvation theory has been fully described elsewhere [13–18], thus only a brief description is given here. The solvation energy of any molecule in state A (a given conformation) is the difference between the V ) and in any solvent (ES ) of relenergy in the vapour (EA A ative permittivity . This is given in terms of the dipolar (kA ) and quadrupolar (hA ) reaction-field terms plus a direct dipole–dipole term to take account of the breakdown of the Onsager reaction-field theory in very polar media [13]. The input for the program is simply the dipole and quadrupole moments plus the solute radius and refractive index, both of which are calculated in the program. In state B (the other conformation) a similar equation is obtained, differing only in the values of the dipole and quadrupole terms. Subtraction of the S − ES ), the energy difference two equations gives ES (EA B between the two conformers, in any solvent of known relative permittivity, in terms of EV and calculable parameters. Table 1 Experimental coupling constants (Hz) for 1–3, in selected solvents Solvent

1 3J

CCl4 CDCl3 Pure liquid Pyridine-d5 Acetone-d6 CD3 CN DMSO-d6

2 H1 ,H2

7.87 8.14 8.28 8.29 8.4 8.64 8.79

3J

H1 ,H6

9.6 10.48 10.62 10.85 10.95 11.11 11.18

3J

H1 ,F

17.88 18.49 18.67 18.83 19.07 19.31 19.55

2J

H1 ,F

49.48 49.53 49.48 49.44 48.82 49.53 49.54

3J

3 H1 ,H2

5.27 6.91 7.14 7.83 8.15 8.36 8.42

3J

H1 ,H6

7.11 8.23 8.42 9.14 9.36 9.67 9.7

3J

H1 ,H2

4.55 5.47 5.22 6.28 6.62 7.37 7.53

3J

H1 ,H6

6.47 7.43 7.39 8.5 8.98 9.54 9.77

M.P. Freitas et al. / Spectrochimica Acta Part A 61 (2005) 1771–1776

75.45 MHz; δ in ppm): δ 21.0 (C4 and C5 ), 28.4 (C3 and C6 ), 89.6 (C1 and C2 ). trans-1,2-Dichlorocyclohexane. 1 H NMR (CCl4 , 300.07 MHz; δ in ppm): δ 1.38 (2H, m, H4 and H5 ), 1.70 (2H, m, H3 and H6 ), 2.25 (4H, m, H3  , H4  , H5  and H6  ), 3.99 (2H, m, H1 and H2 ). 13 C NMR (CCl4 , 75.45 MHz; δ in ppm): δ 21.7 (C4 and C5 ), 31.3 (C3 and C6 ), 61.2 (C1 and C2 ). trans-1,2-Dibromocyclohexane. 1 H NMR (CCl4 , 300.07 MHz; δ in ppm): δ 1.49 (2H, m, H4 and H5 ), 1.80 (2H, m, H3 and H6 ), 1.86 (2H, m, H4  and H5  ), 2.40 (2H, m, H3  and H6  ), 4.43 (2H, m, H1 and H2 ). 13 C NMR (CCl4 , 75.45 MHz; δ in ppm): δ 21.2 (C4 and C5 ), 30.2 (C3 and C6 ), 53.5 (C1 and C2 ).

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Table 3 Parametersa for reaction-field calculations for 1–3 Compound

µ (D)

k (kcal mol−1 )

h (kcal mol−1 )

l

VM (cm3 mol−1 )

1aa 1ee 2aa 2ee 3aa 3ee

0.859 3.392 1.11 3.267 1.06 3.001

0.2318 3.6104 0.3378 2.9277 0.2885 2.3147

2.7593 1.2834 2.2464 1.4231 1.8256 1.277

0.4713 0.4713 0.5659 0.5659 0.612 0.612

115.852 115.852 132.532 132.532 141.492 141.492

µ is the dipole moment, k and h are the dipolar and quadrupolar terms, respectively, l is the term related to the solute refractive index and VM is the molar volume. a

4. Results and discussion 4.1. Calculations and NMR experiments The geometries of the conformers were optimised at the B3LYP/6-311+G(d,p) level, available in the gaussian98 program [19a]. These calculations gave the axial–axial conformer for 2 and 3 as the most stable one in the vapour phase, while for 1 both conformers present very similar stabilities, as shown in Table 2. Natural bond orbital [19b] (NBO) calculations were carried out at the same level. The CHARGE routine [20], using the optimised geometries, gave dipole moments, which are in good agreement with the theoretical (Gaussian) calculations. Hence the CHARGE partial atomic charges may be used with confidence in the MODELS solvation calculations [13]. The parameters required to calculate the solvation energy are given in Table 3. The coupling constants were obtained with the aid of spectrum simulation, since the molecules studied here present complex spin systems in the 1 H NMR spectra, due to symmetry. Comparison between experimental and calculated spectra, as illustrated in Fig. 2, allowed the measurement of the coupling constants, which are presented in Table 1. The observed trend, i.e. the couplings increase with increasing solvent polarity, denotes a high sensitivity of the conformer population with changes in the medium, where the population of the most polar conformer (ee) increases with increasing solvent polarity. This was expected, since the conformers present reasonably different dipole moments and the intrinsic cou-

Fig. 2. 1 H NMR signal for H1 (and H2 ) of 3 in DMSO-d6 : (a) calculated and (b) experimental ABCDEF coupling system.

pling 3 JH1a ,H2a (ee conformer) is usually much larger than 3J H1e ,H2e (aa conformer). The NMR data in Table 1 may be combined with the solvation calculations to provide a detailed account of the conformational equilibrium via Eq. (1): Jobs = naa Jaa + nee Jee , naa + nee = 1,
 nee −E = exp , E = Eee − Eaa naa RT

(1)

With these considerations, the solvent data may be used with the solvation theory to search for the best solution for both the conformer energy difference and the values of Jee and Jaa . Using the parameters from MODELS, the obtained EV values were 0.10, 0.95 and 1.40 kcal mol−1 for 1, 2 and

Table 2 Calculated parameters for 1–3 at the B3LYP/6-311+G(d,p) level Parameters

µ (D) Erel (kcal mol−1 )a ˚ rC X (A) ∠X C1 C2 (◦ ) θ X C C X (◦ ) θH2 C C H2 (◦ ) a

1

2

3

aa

ee

aa

ee

aa

ee

1.02 0.02 1.415 106.26 168.83 60.62

3.88 0 1.406 109.20 64.60 166.94

1.35 0 1.846 107.22 161.89 62.30

3.93 0.45 1.827 111.24 62.19 168.72

1.44 0 2.024 107.01 160.33 60.11

3.87 1.16 1.995 111.88 62.61 167.14

The relative energies were calculated taking into account the zero-point energy (ZPE) correction.

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Table 4 Conformer energy differences (kcal mol−1 ), experimental and calculateda coupling constants (Hz) for 1–3, in selected solvents, and mole fraction of axial–axial conformer Solvent

ε

S − ES Eee aa

Vapor CCl4

1 2.2

0.1 −0.63

0.95 0.36

1.4 0.93

CDCl3

4.8

−1.07

−0.05

0.58

Pure liquid

b

−1.35

−0.14

0.59

Pyridine-d5

12.4

−1.45

−0.41

0.27

Acetone-d6

20.7

−1.63

−0.56

0.15

CD3 CN

37.5

−1.83

−0.73

0.01

DMSO-d6

46.7

−1.91

−0.8

1

a b

3J

2

3

−0.05

naa

H1 ,H2

1

2

3

1

2

3

– 7.87 7.66 8.14 8.18 8.28 8.38 8.29 8.44 8.4 8.53 8.64 8.6 8.79 8.62

– 5.27 5.47 6.91 6.69 7.14 6.96 7.83 7.73 8.15 8.10 8.36 8.49 8.42 8.64

– 4.55 4.49 5.47 5.32 5.22 5.36 6.28 6.33 6.62 6.77 7.37 7.28 7.53 7.49

0.54 0.25

0.83 0.65

0.91 0.83

0.14

0.48

0.73

0.09

0.44

0.73

0.08

0.33

0.61

0.06

0.28

0.56

0.04

0.22

0.5

0.04

0.2

0.48

Second entries. Interpolated ε value for 1 is 12.1, for 2 is 5.9 and for 3 is 4.7.

3, respectively. The 3 JH1e ,H2e intrinsic couplings were 4.38, 2.99 and 3.02 Hz, while the corresponding 3 JH1a ,H2a values were 8.78, 10.07 and 11.61 Hz, for 1, 2 and 3, respectively, with an r.m.s. error of 0.14, 0.18 and 0.10 Hz. These individual couplings are in qualitative agreement with the ones calculated through the molecular mechanics PCMODEL program [23], and also with the well-established relationship between coupling values and substituent electronegativities [24], i.e., the large couplings increase with increasing halogen volume (or with decreasing halogen electronegativity). The PCMODEL values were 3 JH1e ,H2e = 3.97, 2.87 and 2.59 Hz, and 3 JH1a ,H2a = 7.36, 9.38 and 9.98 Hz, for 1, 2 and 3, respectively. The energy differences in the different solvents and in the vapour phase are also included in Table 4 . The conformer population results agree to a great extent with previous literature data [1,2,5,25–30]. For compound 1, Zefirov et al. [5] observed the 19 F {1 H} spectrum at vapour low temperatures to obtain Gee–aa = −0.44 kcal mol−1 . In CS2 and acetone-d6 the respective values were −1.03 and −1.62 kcal mol−1 . Two years later, Zefirov et al. [28] applying a different technique, obtained a value of Gvapour = −0.59 kcal mol−1 . For dichloro and dibromo compounds, the literature data are more numerous than for the difluoro derivative. One of the conformational studies for these compounds was performed by Klaeboe et al. [10], through infrared spectroscopy, who found 34% of ee in cyclohexane and 71% of ee in CH3 NO2 for 2 and 19% of ee in cyclohexane and 63% of ee vapour in CH3 NO2 for 3. In general, a good agreement for Gee–aa has been found in the literature for compound 2, with values ranging from 0.8 to 1.0 kcal mol−1 [1,2,28], which are also in accordance with our results. The same good parallelism between literature results and our data for compound vapour 3 is observed, by a comparison of Gee–aa values. Thus, the literature vapor energies range from 1.5 to 1.6 kcal mol−1

[1,2,28], while our result is 1.4 kcal mol−1 . The values in solution also agree with ours [1,25,26,30]. These results allow us to consider the methodology here applied as a reliable tool to describe the conformational equilibrium of trans-1,2dihalocyclohexanes and as an improved method since it does not utilise rigid derivatives. In addition, spectrum simulations have been demonstrated to predict with confidence the coupling constant values of these second-order spin systems. 4.2. Conformational preferences The conformational behaviour of trans-1,2-dihalocyclohexanes (1, F; 2, Cl; 3, Br), excluding the diiodo derivative not reported here due to its fast decomposition [31], can be interpreted as follows. The preference for the aa conformation in the ascending order 1 < 2 < 3 suggests a high steric interaction between the substituents in the ee conformation, which also increases in that order. MODELS [13] gave values for the difsteric − Esteric ) of 0.11, ference of steric energy (Esteric = Eee aa −1 0.47 and 0.62 kcal mol for 1, 2 and 3, respectively. Dipolar repulsion between the substituents in the ee conformation is also an important factor governing the conformational equilibrium. The differences in the electrostatic energies (Eelect elect − Eelect ) given by MODELS [13] are 0.94, 0.52 = Eee aa and 0.27 kcal mol−1 for 1, 2 and 3, respectively, indicating stronger dipolar repulsion in the order 1 > 2 > 3. The sum of dipolar and steric energy differences, Esteric + Eelect = 1.1, 1.0 and 0.9 kcal mol−1 for 1, 2 and 3, respectively, indicates that the aa conformation should be more stable in the order 1 > 2 > 3, which is in disagreement with the experimental results, since the ee conformation is more stable for 1 than for 2 and 3. The extra stability of 1ee in comparison to 2ee and 3ee can be attributed to the “gauche effect”, which is stronger for more electronegative substituents. In moderate or polar

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Table 5 Significant NBO interactions (in kcal mol−1 ) for diaxial and diequatorial conformations of 1, 2 and 3 Diaxial Interaction ␴C1 ␴C6 ␴C1 ␴C1 ␴C1 ␴C5 nX1 nX1 nX1 nX1

∗ X1 → ␴C2 X2 ∗ → ␴ H6ax C1 X1 ∗ X1 → ␴C6 H6ax ∗ H1 → ␴C5 C6 ∗ H2 → ␴C1 C6 ∗ → ␴ C6 C1 H 1 → σC∗ 1 C6 → σC∗ 1 H1 → σC∗ 1 C2 → σC∗ 2 X2

Diequatorial 1 3.5 11 2.1 6 5.1 3.2 9.8 6 9.1 2.9

2 7.8 13.3 3.9 5.9 6.4 3.6 5.3 5.9 6 3.7

3 10.7 14.9 4.7 5.9 6.7 3.8 3.8 4 4.2 3.6

solvents, the ee conformation becomes the more stable one, since the dipole–dipole repulsion is minimised by the solvent dipole. Recent interpretations for the gauche effect have been properly discussed as an hyperconjugative interaction [32], as an “attractive interaction” between lone pairs of electronegative atoms [33] or as an anti destabilisation due to poorer overlap between the C C ␴-bond-forming orbital, caused by bond bending at the carbon nuclei [34,35]. Natural bond orbital (NBO) analyses [19b] were carried out at the B3LYP/6-311+G(d,p) level in order to evaluate the hyperconjugation effect, previously supposed to explain the origin of the gauche effect [32]. The significant NBO interactions (>0.5 kcal mol−1 ) of 1, 2 and 3 for both diaxial and diequatorial conformations are shown in Table 5, indicating that hyperconjugative interactions can be used to explain the great stability of the diaxial conformation. It can be observed (Table 5) that these hyperconjugative interactions are larger for 2 and 3 than for 1. In such compounds, the main interaction usually invoked to justify the diequatorial preference, and consequently the gauche effect, is between the antiperiplanar C5 C6 (or C2 C3 ) bonding orbital and the C1 X1 antibonding orbital (␴C5 C6 → ␴∗ or ␴C2 C3 → ␴∗ ) [32], but this interaction corresponds just to 5.7 kcal mol−1 for 1, while a corresponding ␴C6 H6ax → ␴∗ and ␴C1 X1 → ␴∗ interaction is 11.0 and 3.5 kcal mol−1 , respectively, in the diaxial conformation for the same compound. In addition, the energies of Table 5 show that all the vicinal hyperconjugative interactions involving the C X bond increase according to 1 < 2 < 3. This trend had been observed for the haloethanes [36] and it is due to the lower energy of the antibonding C X orbital in C Br and C Cl than in C F. Thus, C Br and C Cl antibonding orbitals are more suitable for an interaction with a vicinal electron donor than C F, suggesting that hyperconjugation is not responsible for the gauche effect. Wiberg et al. [34,35] have proposed a model to explain the origin of the gauche effect, based on bond bents, and it is the only one still not contested in literature, and also seems quite reasonable to us.

Interaction ␴ C5 ␴ C1 ␴ C1 ␴ C2 ␴ C1 ␴ C6 nX1 nX1

∗ C6 → ␴C1 X1 ∗ → ␴ C6 C2 X2 ∗ X1 → ␴C5 C6 ∗ X2 → ␴C1 C6 ∗ H1 → ␴C2 H2 ∗ H6eq → ␴C1 C2 → σC∗ 1 H1 → σC∗ 1 C2

1

2

3

5.7 5.6 2.3 2 4.7 5.1 10.7 11

7 7.5 4.6 4.6 5.4 6.4 7.9 6.8

7.8 8.5 5.5 5.6 5.5 6.8 5.8 4.7

Acknowledgements We acknowledge FAPESP for financial support of this research and for a scholarship (to M.P.F.) and a fellowship (to C.F.T.), CNPq for a fellowship (to R.R.) and CENAPAD-SP for computer facilities (gaussian98). Professor C.H. Collins’ assistance in revising this manuscript is also gratefully acknowledged.

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