Conformational rivalry of geminal substituents in silacyclohexane derivatives: 1-Phenyl vs. 1-OR, R=H or Me

Tetrahedron 75 (2019) 3038e3045

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Conformational rivalry of geminal substituents in silacyclohexane derivatives: 1-Phenyl vs. 1-OR, R¼H or Me Tran Dinh Phien a, Liubov E. Kuzmina b, Elena N. Suslova c, Bagrat A. Shainyan c, Sergey A. Shlykov b, * a b c

Institute of Research and Development, Duy Tan University, 03 Quang Trung, Da Nang, Viet Nam Ivanovo State University of Chemistry and Technology, Sheremetievskiy Ave., 7, Ivanovo, 153000, Russian Federation A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Division of the Russian Academy of Sciences, 1 Favorsky Street, 664033, Irkutsk, Russian Federation

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 March 2019 Received in revised form 4 April 2019 Accepted 17 April 2019 Available online 23 April 2019

Molecular structure and conformational equilibria of 1-methoxy- 1 and 1-hydroxy-1phenylsilacyclohexanes 2 were studied by quantum chemical (QC) calculations and combined gas electron diffraction/mass spectrometry (GED/MS). Both molecules may exist in 5 or 6 forms, differing from each other by the substituents' position: (i) axial or equatorial and (ii) rotational orientation relative to the six-membered ring frame. The contribution of axial forms of both compounds varies from 35 to 60% depending on the theoretical method applied. From the GED data, the summarized molar fractions of the conformers were found to be Phax:Pheq ¼ 70(15):30(20) and 50(20):50(20)% which corresponds to DG ¼ GaxeGeq ¼ 0.55(46) and 0.00(56) kcal/mol, for compound 1 and 2, respectively. The concentration of the Phax forms of 1-phenyl-1-(X)-silacyclohexanes (X ¼ H, HO, Me, MeO and Me2N) increases with the size of the second substituent at the silicon atom: 38(10)<50(20)<58(15)<70(15) <80(15)%. © 2019 Elsevier Ltd. All rights reserved.

Keywords: 1-Methoxy-1-phenyl-1-silacyclohexane 1-Hydroxy-1-phenyl-1-silacyclohexane Conformational analysis Gas phase electron diffraction Quantum chemical calculations

1. Introduction Cyclic compounds, specifically six-membered heterocycles, play an important role in various fields of chemistry and pharmaceutics, so their properties are extensively studied. The physical, chemical and physicochemical properties of compounds are directly related to their molecular structure. The molecular structure and conformational behavior of saturated six-membered heterocycles, containing nitrogen, oxygen, sulfur and/or silica atoms are investigated for a long time and reviewed in, for example, [1]. In our recent studies on the conformational analysis of N-phenylpiperidine as well as N-cyanopiperidine and cyanocyclohexane in gas and liquid phase it was shown that the axial preference of the substituent increases with longer XeR bond distance and conjugation between heteroatom and substituents [2e4]. In going from cyclohexanes to silacyclohexanes, the structural changes are even more pronounced because of much lower electronegativity of silicon, much longer SieX vs. CeX bonds with the substituents, and more flattened ‘silicon part’ of the ring. These structural differences cause different

* Corresponding author. E-mail address: [email protected] (S.A. Shlykov). https://doi.org/10.1016/j.tet.2019.04.044 0040-4020/© 2019 Elsevier Ltd. All rights reserved.

conformational preferences of the substituents, in particular, much lower or even negative conformational energy of the substituents at silicon [5]. The conformational behavior of a large number of 1monosubstituted-1-silacyclohexanes has been investigated by various methods including NMR, gas-phase electron diffraction (GED), microwave, infrared, Raman spectroscopy and quantum chemical (QC) calculations. Also, a considerable number of 1,1disubstitued silacyclohexanes, primarily 1-X-1phenylsilacylohexanes, was studied experimentally and theoretically. In the case of 1-X-1-phenylsilacylohexanes (X ¼ F, Cl, CH3 and MeO) the Pheq conformers predominate in solution because steric effects play a subordinate role in silacyclohexane with respect to electrostatic and hyperconjugation effects [6e8]. The equilibrium between the axial and equatorial structures and the orientation of substituents relative to the cycles are influenced by steric, electrostatic and orbital interactions [8e13]. From the low temperature 13 C NMR spectroscopy, the ratio of the conformers Pheq:Phax is 3:1, 4.5:1, 1.7:1 and 2.2:1 for X ¼ F, Cl, CH3 and MeO [6e8], respectively. However, the opposite situation is observed in the gas phase: from the GED analysis, the Phax conformers are more stable than Pheq with the Pheq:Phax ratio of 40:60 for X ¼ F, 79:21 for X ¼ Cl [6] and 42:58% for X ¼ CH3 [7]. The theoretically calculated conformational preference of the Pheq conformer in the isolated molecule of 1-

T.D. Phien et al. / Tetrahedron 75 (2019) 3038e3045

methoxy-1-phenyl-1-silacyclohexane 1 varied from 38 to 77%, depending on the method/basis set combination; while for 1hydroxy-1-phenyl-1-silacyclohexane 2, the calculated dispersion was much smaller, from 32 to 38% [8]. The conformational properties of similar compounds, such as 3subsituted-silatetrahydropyrans, were determined by the experimental and theoretical methods [8]. From the QC calculations, the Phax conformers of 3-methoxy-3-phenyl-3-silatetrahydropyran 3 and 3-hydroxy-3-phenyl-3-silatetrahydropyran 4 predominate in gas phase with the ratio Pheq:Phax ~ (15e26):(85e74) and (28e49):(72-51) % at 298 K, respectively [8]. The repulsion of unidirectional dipoles of the endocyclic oxygen lone pair and of the highly polar axial SiO bond leads to destabilization of 3-Pheq conformers, as is the case for other silatetrahydropyrans [13e17]. In solution, as in the case of 2, the conformational ratio of 3 could not be determinate. At the same time, the Pheq conformer of 4 was shown to predominate over Phax conformer with the ratio Pheq:Phax ~82.9:17.1% from 13C NMR spectroscopy at 103 K. In solution, the decoalescence of the 13C signals of the participating conformers of 1 at low temperature could be reached experimentally [8]. In gas phase, the rotation of the phenyl group about the SiePh bond complicates the structural analysis and interpretation of experimental data in the framework of GED method. For compound 1, the NMR experiment gives only axialequatorial ratio in solution, because the rotation about the Si-Ph bond is still fast on the NMR time scale at ca. 100 K; for

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compound 2 e no experimental ratio is given. In addition, the relative orientation of both substituents, phenyl ring and methoxy/ hydroxyl group, can't be defined from NMR experiments. Thus, in this paper, we perform extended QC calculations and GED study covering the entire variety of the conformers of these two compounds. 2. Results and discussion 2.1. Energies The PES profiles of both compounds 1 and 2 were obtained by scanning the CipsoeSieOeH/C (4) and CortheCipsoeSieO (q) dihedral angles describing the rotation of the methoxy/hydroxyl group about the SieO bond and rotation of the phenyl ring about the SieCipso bond are plotted in Fig. 1. All possible conformers (5 and/or 6, neglecting the rotamers) of both compounds are shown in Fig. 3 (see also Supporting Information for more details). In the conformers III and VI, both substituents lie in the plane, that bisects the plane of the silacyclohexane frame, and hence these conformers have Cs symmetry. As can be clearly seen, conformers of both compounds have similar structure. It should be noted, that use of the more sophisticated basis set cc-pVTZ leads to transformation of conformer IV to V in compound 2 under optimization. Thus, both compounds have five analogous forms. As follows from these PES profiles of both compounds, rotation of the two groups is strongly correlated, which is typical for gear

Fig. 1. Lowest energy pathways for axial and equatorial conformers by rotating the methoxy/hydroxyl group around SieO bond (right) and the phenyl ring around SieCipso bond (left) calculated at M062X/6-311G** level.

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motion in molecular motors, and which was demonstrated in our recent work on the related compound, 1-(dimethylamino)-1phenyl-1-silacyclohexane [17]. In order to clarify this influence, we have calculated the energy at the M062X/6-311G** level for the both axial and equatorial forms on a grid of points corresponding to the torsion angles around the SieCPh and SieO bonds scanned with a step of 10 at optimizations of all other geometry parameters, see

Fig. 2. The methoxy/hydroxy group rotation induces the phenyl ring rotation in both Phax and Pheq forms, whereas the phenyl group rotation does not noticeably affect the methoxy/hydroxy group orientation. The phenyl ring rotation only leads to reversible transformation between conformer II and III with low energy barrier of 0.3 kcal/mol at the M062X/6-311G** level. A similar

Fig. 2. Potential energy surface (PES) profiles of the orientation relative to the methoxy/hydroxy group were obtained by synchronous scan of two angles calculated at M062X/6311G** level.

Fig. 3. Conformers of compounds 1 and 2.

T.D. Phien et al. / Tetrahedron 75 (2019) 3038e3045

situation was found for 1-phenyl-1-R-silacyclohexanes (R ¼ CH3 or Me2N) [7,17]. From the M062X/6-311G** results, the energy barriers for the RO group rotation between conformers I / II and V / VI in compound 1 are ca. 2.3 and 1.6 kcal/mol, respectively. These values are higher than those in compound 2, 1.5 and 0.3 kcal/mol. Thus, the substitution of the hydrogen atom in the hydroxyl group by a methyl group leads to increase of repulsion between two substituents and, hence, increases energy barrier and could be one of reasons for the absence of decoalescence of the signals in the 13C NMR spectra of compound 2 [8]. The relative total electron and Gibbs free energies (kcal/mol) of all conformers of both compounds are given in Table 1. According to the QC calculations, conformers I and V of compound 1 have very similar DE values and are more stable than others. Since the phenyl group is a bulkier substituent than the methoxy group, the Pheq conformers are entropically more preferable. Of the QC computation levels applied, the B3LYP-D3 calculations predict the conformer VI as one of the most stable ones with DG value of 0.05 to 0.05 kcal/mol relative to I. However, it is to be mentioned that because of low vibrational frequency for the methoxy group rotation around the SieO bond in this conformer, 10e12 cm1, the DG values predictions performed ‘by default’ in harmonic approximation may appear to be incorrect. From the M062X calculations, the values DE ¼ 1.14e1.40 and DG ¼ 0.87e1.20 kcal/mol for the conformers II, III and VI are higher than those of the conformer I. According to the MP2 results, the most stable conformers are I, II and V. Total contribution of the Phax conformers summed over all conformers varies from 40 to 60%. It should be noted that calculations with more sophisticated the basis set leads to higher contribution for not only Pheq conformers V and VI, but also for Phax conformers II and III. The conformer V is predicted to be of the same or even higher preference than I. In the case of compound 2, the axial conformer I is most energetically stable. However, more easy rotation of the phenyl ring in Pheq conformers makes them entropically more favorable and hence decreases the DG values (sometimes up to inversion of the sign), so that the contribution of Pheq conformers may reach 60e66%. 2.2. GED analysis Resuming the discussion in the previous section, the QC predictions of the conformational preference of compound 1 are rather

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contradictive, Table 1. Based on the DG values of QC results, conformer III is unfavorable as compared with conformers I, II and V, because of strong repulsion between the phenyl and silacyclohexane rings (i.e. 1,3-diaxial repulsion). The results of theoretical methods M062X and MP2 show that conformer VI is less stable than the others. Indeed, as it follows from Fig. 3, the structures of the conformers V and VI differ by orientation of the methoxy group. For this reason, we assume the investigated vapor over liquid 1 at 325(3) K to consist of three conformers, namely I, II and IV. In the case of compound 2, the optimized geometries of the pairs of conformers I/II and V/VI differ in orientation of the hydroxyl group. In the framework of GED method, it is unreal to distinguish them by means of structural analysis. Sophistication of the basis set leads to transformation of conformer IV to V. Thus, in this section, conformers I/II and V/VI are considered as single conformers denoted as I and V, respectively, and the GED analysis was carried out considering three conformers I, III and V. The following independent geometric parameters were used to describe the geometry: e for compound 1: bond distances SieC2, C2eC3, C8eC9, SieO, CeO and CeH; bond angles SiC2C3, C7SiC2, C8C7Si, OSiC2, HC2Si and HC2C3; dihedral angles SiC2C3C4, C2C3C4C5, C8C7SiC2 and COSiC7 for conformer I; dihedral C8C7SiC2 and COSiC7 for conformers II and V; e for compound 2: bond distances SieC2, C2eC3, C8eC9, SieO, CeO, HeO and CeH; bond angles SiC2C3, C7SiC2, C8C7Si, OSiC2, HC2Si, HC2C3, HC8C7 and HOSi; dihedral angles SiC2C3C4, C2C3C4C5, C8C7SiC2 and HOSiC for conformer I; dihedral C8C7SiC2 and HOSiC for conformer III, C8C7SiC2 and HOSiC for conformer V. Following parameters are fixed: HeO, HOSi, HOSiC, HOSiC, HOSiC for conformers I, III, V, respectively. All other geometrical parameters of both compounds were described by parameters analogous to those in conformer I and corrected by adding the differences adopted from M062X/6-311G** calculations. For three conformers, the benzene ring was fixed to be planar since the QC calculations showed very small deviations from planarity. Vibrational amplitudes for all three conformers were refined in eight groups according to the specific regions in the radial distribution: 0e1.21; 1.21e1.74; 1.74e2.05; 2.05e2.60; 2.60e3.83; 3.83e4.5; 4.5e6.1; 6.1e10.0 Å for compound 1 and 0e1.21; 1.21e1.73; 1.73e2.07; 2.07e2.61; 2.61e3.7; 3.7e4.44; 4.44e4.89;

Table 1 Relative total electron energy and Gibbs free energy of conformers of 1 and 2. Conformer

DEb, kcal/mol

DG (298 K), kcal/mol Pheq

Phax I Method/basis set B3LYP-D3/6-311G** B3LYP-D3/cc-pVTZ M062X/6-311G** M062X/cc-pVTZ MP2(FC)/6-311G** CCSD(T)/CBS B3LYP-D3/6-311G** B3LYP-D3/cc-pVTZ M062X/6-311G** M062X/cc-pVTZ MP2(FC)/6-311G** a b

II

PhMeOSiC, 1 0 1.07 0 1.03 0 1.25 0 1.15 0 1.18 0 PhHOSiC, 2 0 1.10 0 0.86 0 1.17 0 0.92 0 0.99

Phax

Pheq

III

IV

V

VI

I

II

III

IV

V

VI

0.89 0.83 1.40 1.39 1.83

a

0.06 0.13 0.04 0.15 0.50 0.05

0.91 0.77 1.25 1.14 2.02

0 0 0 0 0

0.88 0.60 1.08 0.87 0.60

1.03 0.79 1.13 1.10 0.97

a

0.06 0.21 0.06 0.33 0.04

0.05 0.05 1.20 1.05 1.68

0.59 0.13 0.68 0.17 1.08

0.90 0.42 0.97 0.44 1.53

0.29 0.05 0.02 0.20 0.62

0.14 0.26 0.18 0.23 0.44

a a a a a

1.18 0.77 1.48 1.19 1.82

0.51 a

0.52 a

0.77

a a a a a

0 0 0 0 0

1.01 0.65 1.03 0.78 0.73

1.17 0.78 1.05 0.79 0.81

0.22 a

0.05 a

0.41

The conformer IV converts into conformer V. The energies are given for the minima; those but with the ZPE correction do not differ noticeably. The latter are represented in Table S2 in Supporting Information.

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4.89e6.14; 6.14e10.0 Å for compound 2. The differences between the amplitudes within each group were constrained to the calculated values. Vibrational corrections Dr ¼ rh1 e ra and starting rootmean square amplitudes were calculated with the Vibmodule program [18] using the so-called second approximation, in which a harmonic approach with nonlinear relation between Cartesian and internal coordinates was applied on the basis of the force field estimated in the QC calculations at the M062X/6-311G** level. The experimental and theoretical molecular scattering intensities sM(s) and radial distribution curves f(r) with the corresponding differences “Experim.eTheor.” are plotted in Fig. 4. The optimal conformer ratio of compound 1 and 2 were found to be I:II:V ¼ 60(15):10(10):30(15)% and I:III:V ¼ 20(20):30(20): 50(15)%, respectively, with the uncertainty estimated using Hamilton's criterion at 0.05 significance level (Rf,min ¼ 4.17 and 4.79%), see Fig. 5. Thus, from GED analysis, the conformer Pheq is less stable than Phax with the ratio Pheq:Phax ¼ 30(15):70(15)% for 1 and 50(20):50(15)% for 2. Relative Gibbs free energies between axial and equatorial conformers were estimated from the conformer ratio, as DG(325 K) ¼ GaxeGeq ¼ 0.55(46) and 0.0(20) kcal/mol (the relative free energies DG of the conformers I:II:V of compound 1 are 0; 1.15; 0.45 kcal/mol; of the conformers I:III:V of compound 2 0.54; 0.30; 0 kcal/mol). For comparison, the ratios Pheq:Phax of 1phenyl-1-(X)-silacyclohexanes are 62(10):38(10), 42(15):58(15) and 20(15):80(15)% for X of H, Me and Me2N group, respectively. In addition, a goodness of fit was checked by running the KCED

program with no refinement of geometry and vibrational parameters of all six conformers of the compounds for the theoretical models built on the M062X/6-311G** calculations; the tests resulted in the following R-factors: 12.7, 13.4, 15.1, 13.3, 15.0 and 11.6, 12.0, 14.4, 17.4, 13.8, 14.0 in the series of I to VI conformers, respectively.

2.3. Geometry Selected experimental (GED) along with the calculated (QC) geometric parameters of the three most stable conformers I, II/III and V of compounds 1 and 2 are compiled in Table 2. As follows from QC results of both compounds, due to orbital interaction between the electron lone pairs of oxygen atom and SieC bonds, the SieC bond distances of conformer II (in which the 4 angle is ~180 ) are longer and shorter by 0.010 and 0.005e0.010 Å, respectively, than those in conformers I and V. This interaction also leads to narrower C7SiO bond angle by 2e3 in 1 and 5e7 in 2. As a rule, the MP2 method predicts longer CeC bond distances of phenyl ring and CeO bond length by ~0.009e0.012 and 0.005e0.008 Å, respectively, than DFT calculations. In both compounds, the SieO bond in Pheq conformers is slightly longer than that in the Phax conformers. In conformer II, the steric repulsion between the methyl group and two methylene groups in the a-positions of the silacyclohexane ring increases the SiOC angle by ca. 1. The substitution of the hydrogen atom in the hydroxy group by a methyl

Fig. 4. Molecular scattering intensities sM(s) (left) and radial distribution curves f(r) (right): experimental (dots) and theoretical (black line) for refined mixture of three conformers of I:II:V ¼ 60(15):10(10):30(15)% for 1 and I:III:V ¼ 20(20):30(20):50(15)% for 2; colored lines correspond to refinement of all parameters under assumptions of the individual conformers; the differences “Experim.-Theor.” are given at the bottom.

T.D. Phien et al. / Tetrahedron 75 (2019) 3038e3045

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Fig. 5. Agreement factor Rf as a function of molar fraction of the most stable conformers of compounds 1 (left) and 2 (right).

Table 2 Theoretical (with 6-311G** basis set) and experimental parametersa of the conformers of 1 and 2. Conf.

I

Method

M062X

MP2

GED

M062X

MP2

GED

M062X

MP2

GED

1.865 1.882 1.662 1.399 1.414 angles, 108.8 122.3 53.6 62.4

1.863 1.875 1.660 1.403 1.419

1.866(4) 1.882(4) 1.657(4) 1.403(3) 1.407(7)

1.877 1.871 1.662 1.399 1.411

1.878 1.870 1.668 1.410 1.419

1.878(4) 1.871(4) 1.657(4) 1.403(3) 1.404(7)

1.866 1.875 1.667 1.401 1.413

1.868 1.875 1.672 1.410 1.420

1.866(4) 1.875(4) 1.663(4) 1.405(3) 1.406(7)

109.3 121.0 53.0 65.8

110.5(16) 123.3(13) 65(19) 67(10)

106.5 123.3 173.0 65.4

106.6 122.3 176.9 76.9

108.1(16) 124.3(13) 162(168) 78(100)

107.9 121.8 69.5 22.6

108.6 120.7 64.6 33.2

108.1(16) 122.8(13) 82(18) 32(15)

1.867 1.880 1.673 1.410 0.959

1.866(4) 1.881(4) 1.659(4) 1.404(3) 0.958(2)

1.878 1.872 1.667 1.401 0.957

1.879 1.872 1.674 1.410 0.959

1.878(4) 1.872(4) 1.660(4) 1.405(3) 0.957(2)

1.866 1.875 1.669 1.400 0.958

1.868 1.875 1.675 1.410 0.959

1.866(4) 1.875(4) 1.662(4) 1.405(3) 0.958(2)

110.5 114.8 35.1 73.6

112.0(14) 117.6(14) 37(36) 73(36)

103.2 118.3 180.0 0.0

103.7 115.2 171.8 8.8

105.3(14) 118.3(14) 180(111) 4(111)

108.7 117.9 63.8 19.1

109.3 115.2 60.3 20.4

111.0(14) 118.0(14) 64(24) 23(24)

PhMeOSiC, 1 Bond distance, Å SieC2 SieC7 SieO C7eC8 CeO Bond and torsion C7SiO SiOC 4

q

PhHOSiC, 2 Bond distance, Å SieC2 SieC7 SieO C7eC8 HeO Bond and torsion C7SiO SiOH 4

q

a

1.866 1.881 1.666 1.400 0.958 angles, 109.7 117.7 37.1 66.7

II/III

V





See Fig. 3 for atom numbering and text above for angles 4 and q definition.

group slightly increases the SieO bond by ~0.004 Å. At the same time, the SieCipso bond does not change. The q angles of both Phax conformers I and II of both compounds 1 and 2 differ by 3e11, i.e. the rotation of the hydroxy or methoxy group does not strongly influence the orientation of the phenyl ring. In going from the SieOMe compound 1 to the SieOH compound 1 the 4, q angles increase by 16e18 , 4e8 , respectively, in conformer I and by 4e6 , 3e13 in conformer V. In conformer II, the methoxy group lies in a plane, that bisects the plane of the silacyclohexane ring. At the same time, in conformers III and VI, both substituents lie in the bisecting plane. The calculated bond distances and bond angles are in good agreement with the GED values. The SieO bond distances derived from the QC calculations are longer than the experimental value.

Note that the M062X method underestimated the CeC bond distances of phenyl ring. The dihedral angles 4 and q were refined for the conformers I, II or III and V with big uncertainty, in agreement with the theoretical predictions. The q angles in conformers I and V can be compared to the experimental values of q in phenylcyclohexane, 1-X-1-phenylsilacyclohexanes (X ¼ H, Me, Me2N), which are equal to 70, 74, 81, 87 for the axial, and 0, 0, 0, 14 for the equatorial conformers, respectively. Clearly, the methoxy and hydroxyl groups slightly affect the orientation of the phenyl ring. The SieO bond is not perpendicular to the phenyl ring plane in the axial form, neither it lies in the phenyl ring plane in the equatorial form. Experimental and theoretical geometric and vibrational parameters of all conformers are summarized in Supporting Information, Tables S4 e S9.

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3. Conclusion In summary, conformational analysis of 1-methoxy-1-phenylsilacyclohexane 1 and 1-hydroxy-1-phenyl-silacyclohexane 2 was performed by QC calculations and GED experiment. According to the QC calculations, compounds 1 and 2 may exist as five or six conformers, three axial forms and three or two equatorial, with the fraction of the axial forms of 39e58% and 35e54%, respectively, depending on the methods of calculations and basis sets used. From the GED data, the Phax conformers predominate in gas phase. The substitution of the hydrogen atom in the OH group by a methyl group slightly increases the q angles and decreases the SieO bond, but does not affect the SieCipso bond length. For the theoretical prediction of the Phax or Pheq conformer preference in compounds 1 and 2 one must take into account at least two oppositely acting effects e the preferable axial location of more electronegative group, and steric effects. To estimate the former effect, let's consider the relative polarities of the CePh and CeO bonds in the structures in question. The polarization of the SiePh bond calculated as Dq ¼ qSi  qCipso is nearly the same in the axial and equatorial conformers of both compounds, 1 and 2, with DDq ¼ Dq(Phax) e Dq(Pheq) only slightly varying within 0.002e0.007e in all used methods (B3LYP-D3, M062X or MP2 with 6-311G** basis set). In contrast, the difference in polarization of the SieO bonds DDq in the conformers of 1 and 2, calculated as Dq ¼ qSi  qO [or q(OR), where R ¼ H or Me] is more sensitive to the axial or equatorial position of the OH or OMe group. The value of DDq, depending on the method, varies from 0.022 to 0.043 for compound 1 and from 0.055 to 0.071 for compound 2. In total, in 1Phax, q(Ci) > q(OMe) in DFT methods and q(Ci) ~ q(OMe) in MP2 method. In 1-Pheq, q(Ci) < q(OMe) in all methods. In 2-Phax, q(Ci) > q(OH) in all methods, whereas q(Ci) > q(OH) in M062X, q(Ci) ~ q(OH) in B3LYP-D3, and q(Ci) < q(OH) in MP2. The general trend is that in both compounds the SieO bond is relatively more polarized in the Pheq conformer with respect to Phax conformer. Also, in the conformers of compound 1 the SieO bonds are relatively more polarized than in the corresponding conformers of compound 2. Hence, the first effect must favor the Pheq conformer predominance in 2, which contradicts the experiment. To resolve this contradiction, one can assume that steric factors outweigh small differences in electronic effects in the conformers of molecules 1 and 2. Sterically, the OMe group creates more hindrances in the conformers of 1, which is independently proved by the aforementioned larger barriers to rotation in molecule 1 (2.3 and 1.6 kcal/mol) than in molecule 2 (1.5 and 0.3 kcal/mol). This may explain the observed Phax predominance in molecule 1 (70:30) as compared to molecule 2 (1:1). 4. Experimental section The synthesis of 1 and 2 was described in Ref. [19]. 4.1. GED-MS experiment The diffraction patterns were recorded during a combined gasphase electron diffraction and mass-spectrometric experiment carried out using the EMR-100/APDM-1 unit at ISUCT [20,21]. The samples were loaded into molybdenum effusion cell filled with crushed pieces of Schott filter (the details of the experiment see in the Supporting Information) and kept at 325(5) K and 335(3) K in the course of the experiments for compounds 1 and 2, respectively. The conditions of the GED/MS experiment and data refinement details are given in Supporting Information, Table S1 and related chapters. Mass spectra (EI, 50 eV) of the effusing molecular beam were

recorded simultaneously with the collection of the diffracted electrons (see Fig. S1 and Table S2 in the Supporting Information). For compound 1, the major peaks correspond to ions C5H10SiOCHþ 2 and SiOCHþ 3 with intensities 55 and 100%, respectively. The intensities of other peaks, such as C6H5Siþ, C6H5SiHOCHþ 3, þ C6H5SiCH2OCHþ 3 and C6H5Si(CH2)2OCH , do not exceed 25%. For compound 2, the major peaks correspond to ions C5H10SiCCHþ, C5H10SiOþ and SiOHþ with intensities 29, 90 and 100%, respectively. þ The intensities of other peaks, such as C6Hþ 6 , C2H4SiCH2O , SiC5H4, þ C6H5SiCH2OHþ, C5H10SiC4Hþ and C H SiOH do not exceed 25%. 3 11 15 The relative intensities of the ions in the mass spectra showed no change during the course of the experiment. 4.2. Computational details All calculations of molecules 1 and 2 were performed with Gaussian 09 program suite [22]. The geometry and vibrational calculations were performed using DFT (with B3LYP-D3 and M062X functionals) and MP2 methods with the 6-311G** and cc-pVTZ basic sets. High-level coupled cluster calculations were carried out using the ORCA package, version 3.0.3 [23] on MP2/def2-TZVP [24] optimized geometries in order to get accurate potential energy differences between the conformers of 1 (for detail see the Supporting Information). The potential energy surface (PES) profiles were obtained by scanning CortheCipsoeSieO (q) and CipsoeSieOeС/H (4) dihedral angles with a step of 5 and optimization of all other geometrical parameters at M062X/6-311G** level of theory. Also, the energies at the M062X/6-311G** level for the both conformers have been calculated on a grid of points corresponding to the SieCipso and SieO torsion angles varied with a step of 10 with optimizations of all other geometric parameters. Acknowledgments S.A.S. is thankful to the Ministry of Education and Science of the Russian Federation through Project Supporting Program (Project No. 4.3232.2017/4.6). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.tet.2019.04.044. References [1] E. Kleinpeter, Conformational analysis of saturated heterocyclic sixmembered rings, Adv. Heterocycl. Chem. 86 (2004) 41e127. [2] S.A. Shlykov, T.D. Phien, Y. Gao, P.M. Weber, Structure and conformational behavior of N-phenylpiperidine studied by gas-phase electron diffraction and quantum chemical calculations, J. Mol. Struct. 1132 (2017) 3e10. [3] S.A. Shlykov, T.D. Phien, P.M. Weber, Intramolecular inversions, structure and conformational behavior of gaseous and liquid N-cyanopiperidine. Comparison with other 1-cyanoheterocyclohexanes, J. Mol. Struct. 1138 (2017) 41e49. [4] T.D. Phien, L.E. Kuzmina, A. Kvaran, S. Jonsdottir, I. Arnason, S.A. Shlykov, Cyanocyclohexane: axial-to-equatorial “seesaw” parity in gas and condensed phases, J. Mol. Struct. 1168 (2018) 127e134. [5] B.A. Shainyan, Structure and Conformational Analysis of Silacyclohexanes and 1,3-Silaheterocyclohexane, Tetrahedron 72 (2016) 5027e5035. [6] B.A. Shainyan, A.V. Belyakov, Y.F. Sigolaev, A.N. Khramov, E. Kleinpeter, Molecular structure and conformational analysis of 1-phenyl-1-X-1silacyclohexanes (X ¼ F, Cl) by electron diffraction, low-temperature NMR, and quantum chemical calculations, J. Org. Chem. 82 (1) (2016) 461e470. [7] T.D. Phien, S.A. Shlykov, B.A. Shainyan, Molecular structure and conformational behavior of 1-methyl-1-phenylsilacyclohexane studied by gas electron diffraction, IR spectroscopy and quantum chemical calculations, Tetrahedron 73 (8) (2017) 1127e1134. [8] B.A. Shainyan, S.V. Kirpichenko, E. Kleinpeter, Conformational preferences of the phenyl group in 1-Phenyl-1-X-1-silacyclo-hexanes (X ¼ MeO, HO) and 3Phenyl-3-X-3-silatetrahydropyrans (X ¼ HO, H) by low temperature 13C NMR

T.D. Phien et al. / Tetrahedron 75 (2019) 3038e3045

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

spectroscopy and theoretical calculations, J. Org. Chem. 82 (24) (2017) 13414e13422. D.S. Ribeiro, R. Rittner, The role of hyperconjugation in the conformational analysis of methylcyclohexane and methylhetero-cyclohexanes, J. Am. Chem. Soc. 68 (17) (2003) 6780e6787. s-Guzm ndez-Trujillo, G. Cuevas, The nonexistence of F. Corte an, J. Herna repulsive 1, 3-diaxial interactions in monosubstituted cyclohexanes, J. Phys. Chem. A 107 (44) (2003) 9253e9256. F. Taddei, E. Kleinpeter, The anomeric effect in substituted cyclohexanes. I. The role of hyperconjugative interactions and steric effect in monosubstituted cyclohexanes, J. Mol. Struct. (Theochem.) 683 (1e3) (2004) 29e41. F. Taddei, E. Kleinpeter, The anomeric effect in substituted cyclohexanes. II. The role of hyperconjugative interactions and steric effect in 1,4-disubstituted cyclohexanes, J. Mol. Struct. (Theochem.) 718 (1e3) (2005) 141e151. B.A. Shainyan, S.V. Kirpichenko, E. Kleinpeter, S.A. Shlykov, D. Yu Osadchiy, Molecular structure and conformational analysis of 3-methyl-3-phenyl-3silatetrahydropyran. Gas-phase electron diffraction, low temperature NMR and quantum chemical calculations, Tetrahedron 71 (23) (2015) 3810e3818. B.A. Shainyan, S.V. Kirpichenko, E. Kleinpeter, Stereochemistry of 3isopropoxy-3-methyl-1,3-oxasilinane e the first 3-silatetrahydropyran with an exocyclic ROeSi bond, Tetrahedron 71 (38) (2015) 6720e6726. B.A. Shainyan, S.V. Kirpichenko, N.N. Chipanina, L.P. Oznobikhina, E. Kleinpeter, S.A. Shlykov, D. Yu Osadchiy, Synthesis and conformational analysis of 3-Methyl-3-silatetrahydropyran by GED, FTIR, NMR, and theoretical calculations: comparative analysis of 1-Hetero-3-methyl-3silacyclohexanes, J. Org. Chem. 80 (24) (2015) 12492e12500. S.V. Kirpichenko, B.A. Shainyan, E. Kleinpeter, S.A. Shlykov, T.D. Phien, A.I. Albanov, Synthesis of 3-fluoro-3-methyl-3-silatetrahydropyran and its conformational preferences in gas and solution by GED, NMR and theoretical calculations, Tetrahedron 74 (15) (2018) 1859e1867. B.A. Shainyan, E.N. Suslova, T.D. Phien, S.A. Shlykov, E. Kleinpeter, Conformational preferences in gas and solution, and molecular gear rotation in 1-

[18]

[19] [20] [21]

[22]

[23] [24]

3045

(dimethylamino)-1-phenyl-1-silacyclohexane by gas phase electron diffraction (GED), LT NMR and theoretical calculations, Tetrahedron 74 (32) (2018) 4299e4307. Y.V. Vishnevskiy, Y.A. Zhabanov, New implementation of the first-order perturbation theory for calculation of interatomic vibrational amplitudes and corrections in gas electron diffraction, J. Phys. Conf. Ser. A 633 (2015), 012076. E.N. Suslova, B.A. Shainyan, Russ. J. Gen. Chem. 87 (2017) 1645e1648. G.V. Girichev, A.N. Utkin, Y.F. Revichev, Instrum. Exp. Tech. 27 (1984) 457e461. N.I. Giricheva, G.V. Girichev, S.A. Shlykov, V.A. Titov, T.P. Chusova, The joint gas electron diffraction and mass spectrometric study of GeI4(g)þGe(s) system; molecular structure of germanium diiodide, J. Mol. Struct. 344 (1995) 127e134. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford, CT, 2009. K.A. Peterson, T.B. Adler, H.J. Werner, J. Chem. Phys. 128 (2008), 084102. (a) F. Weigend, R. Ahlrichs, Phys. Chem. Chem. Phys. 7 (2005) 3297e3305; (b) K.E. Yousaf, K.A. Peterson, J. Chem. Phys. 129 (2008) 184108.