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Synthesis, conformational preferences in gas and solution, and molecular gear rotation in 1-(dimethylamino)-1-phenyl-1-silacyclohexane by gas phase electron diffraction (GED), LT NMR and theoretical calculations
Tetrahedron 74 (2018) 4299e4307
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Synthesis, conformational preferences in gas and solution, and molecular gear rotation in 1-(dimethylamino)-1-phenyl-1silacyclohexane by gas phase electron diffraction (GED), LT NMR and theoretical calculations Bagrat A. Shainyan a, *, Elena N. Suslova a, Tran Dinh Phien b, c, Sergey A. Shlykov d, Erich Kleinpeter e, * a
A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Division of the Russian Academy of Sciences, 1 Favorsky Street, 664033 Irkutsk, Russian Federation Institute of Research and Development, Duy Tan University, 03 Quang Trung, Da Nang, Viet Nam c Department of Chemistry and Environment, Vietnam-Russian Tropical Centre, 63 Nguyen Van Huyen, Cau Giay District, Ha Noi, Viet Nam d Ivanovo State University of Chemistry and Technology, Research Institute for Thermodynamics and Kinetics of Chemical Processes, 7 Sheremetievskiy Ave, 153000, Ivanovo, Russian Federation e €t Potsdam, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam (Golm), Germany Chemisches Institut der Universita b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 May 2018 Received in revised form 6 June 2018 Accepted 9 June 2018 Available online 19 June 2018
1-(Dimethylamino)-1-phenyl-1-silacyclohexane 1, was synthesized, and its molecular structure and conformational properties studied by gas-phase electron diffraction (GED), low temperature 13C NMR spectroscopy and quantum-chemical calculations. The predominance of the 1-Phax conformer (1-Pheq:1Phax ratio of 20:80%, DG (317 K) ¼ 0.87 kcal/mol) in the gas phase is close to the theoretically estimated conformational equilibrium. In solution, low temperature NMR spectroscopy showed analyzable decoalescence of Cipso and C(1,5) carbon signals in 13C NMR spectra at 103 K. Opposite to the gas state in the freon solution employed (CD2Cl2/CHFCl2/CHFCl2 ¼ 1:1:3), which is still liquid at 100 K, the 1-Pheq conformer was found to be the preferred one [(1-Pheq: 1-Phax ¼ 77%: 23%, K ¼ 77/23 ¼ 2.8; DG ¼ RT ln K (at 103 K) ¼ 0.44 ± 0.1 kcal/mol]. When comparing 1 with 1-phenyl-1-(X)silacylohexanes (X ¼ H, Me, OMe, F, Cl), studied so far, the trend of predominance of the Phax conformer in the gas phase and of the Pheq conformer in solution is confirmed. © 2018 Elsevier Ltd. All rights reserved.
Keywords: 1-(Dimethylamino)-1-phenyl-1silacyclohexane Conformational analysis Gas phase electron diffraction Low-temperature d-NMR DFT MP2 M062X/6-311G** calculations
1. Introduction The conformational analysis of silacyclohexanes and silaheterocyclohexanes revealed principal difference in the conformational preferences, molecular structure, barriers to interconversion between the conformers, and the relative importance of basic factors determining the position of conformational equilibrium, such as steric effects, hyperconjugation, and electrostatic effects [1,2]. Conformational analysis of the Si-monosubstituted sila(hetero)cyclohexanes allows to estimate the conformational energies of
* Corresponding authors. E-mail addresses: [email protected] (B.A. Shainyan), [email protected] (E. Kleinpeter). https://doi.org/10.1016/j.tet.2018.06.023 0040-4020/© 2018 Elsevier Ltd. All rights reserved.
substituents at silicon and to compare them to those at carbon in the corresponding cyclohexanes. The next step is the analysis of sila(hetero)cyclohexanes with two different groups at silicon allowing to determine the relative conformational preferences of the substituents [3]. For the phenyl group at silicon, an additional problem arises due to non-degenerate rotation of an unsymmetrical rotor about the SieCipso bond in both the Phax and Pheq conformers. For the Si-Ph-substituted sila(hetero)cyclo-hexanes with a monoatomic second substituent at silicon, like H [3], or Hlg [4], the phenyl group rotation is affected only by the size of this atom. In contrast to this, for polyatomic substituents, even in the case of symmetric rotors like the methyl group, internal rotation of the two substituents, coupled like cogwheels, is correlated so that rotation about one axis is transferred to another axis. Such a coupling affects the ease of rotation, which, in turn, has an effect on the entropy and,
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hence, the free energy and the ratio of the conformers. So far, we have studied sila(hetero)cyclohexanes with Si(Ph,Me) pair of substituents in gas [5] and in solution [3], and of sila(hetero)cyclohexanes with Si(Ph,OH), and Si(Ph,OMe) pairs of substituents [6]. However, the methyl group is a C3v symmetric rotor and the steric hindrances to the phenyl group rotation should only slightly depend on rotation about the SieMe bond. The OH and OMe groups can adopt the conformation with the OeH or OeMe group turned outward from the Ph group [with the PheSieOeH(Me) torsional angle equal or close to 180 ] thus minimizing the steric hindrances. Therefore, it was of interest to investigate the conformational behavior of a system having two substituents at silicon which would be more “rotationally coupled”. With this in mind, in the present work we performed conformational analysis of the recently synthesized [7] 1-(dimethylamino)-1-phenylsilacyclohexane 1, having an asymmetric rotor NMe2, by gas-phase electron diffraction (GED), low-temperature NMR down to 103 K, and theoretical calculations both in gas and in solution (see Scheme 1).
2. Results and discussion 2.1. Energies The potential energy surface (PES) profiles for phenyl ring and dimethylamino group rotations around the SieCPh and SieN bonds, respectively, are plotted in Fig. 1. Four conformers of this compound were observed and drawn in Fig. 2. From MP2 calculation, conformer IV has CS symmetry. In all forms, due to steric repulsion between methyl hydrogens and the hydrogen atoms in ortho-position of the phenyl ring, the dimethylamino group rotation induces phenyl ring rotation and causes the conformers I and III to transform to II and IV, respectively, and vice versa, with low energy barriers, 0.6 kcal/mol. However, a complicated situation was found for the phenyl ring rotation, which leads to transformations of I to II and of III to IV with high energy barriers, but does not induce an opposite conversion, i.e. II to I and IV to III. For comparison, in our recent work [5], the methyl group rotation in 1-methyl-1-phenyl-silacyclohexane induces phenyl ring rotation in both Phax and Pheq conformers, whereas the phenyl group rotation does not noticeably affect the methyl group orientation. In Fig. 3, the dependence of dihedral angles between the Ph group and С4eSieСipso plane upon the NMe2 group rotation in the two conformers of molecule 1 is depicted. As can be clearly seen, rotation of the two groups is strongly correlated, which is typical for gear motion in molecular motors. Vertical’ lines in Fig. 3 (left) appear because for two scanned angles of the NMe2 group differing by 5 the geometry optimization of the axial Ph group leads the system to two different local minima, causing a ‘jump’ of energy. Relative total energies, Gibbs free energies, and the molar fractions of the conformers are summarized in Table 1. As follows from Table 1, in the gas phase, M062X calculations show that conformer I is energetically most stable; the PheqMe2Nax forms are more preferable than PhaxMe2Neq, the Pheq: Phax ratio being 60e65%: 40e35%. Similar ratio, Pheq: Phax ¼ 70%: 30%, is predicted from
B3LYP-D3/6-311G** calculations. In the case of B3LYP/cc-pVTZ method, the starting geometry of conformer I is optimized to structure II with the ratio Pheq: Phax ¼ 55%: 45%. At the same time, according to MP2 calculations, the Pheq conformers are less stable than their Phax counterparts. Thus, the two theoretical methods give different results. 2.2. GED analysis As follows from optimized geometries of conformers I and II (see Figs. 1 and 2), they differ by 10 and 30 in the angles of the phenyl ring and dimethylamino group rotations, respectively. Moreover, the energy barrier between the I and II is very low, 0.1 kcal/mol, so in the framework of GED method, it is unreal to distinguish them upon structural analysis. Thus, in this section, conformers I and II were combined as the latter (with variable mentioned torsion angles), and the GED analysis was carried out considering three conformers II, III and IV only. The following independent geometric parameters were used to describe the geometry: bond distances SieC1, C1eC2, SieN, CeN, C8eC9 and CeH; bond angles SiC1C2, NSiC1, CNSi, CNC, C8SiC1, C8C9C10, HC1Si, HC1C2, HCN, HCH and, HC9C8; dihedral angles SiC1C2C3, C1C2C3C4, CNSiC1, C9C8SiN for conformer II; dihedral angles CNSiCPh and NSiCPhCortho for conformers III and IV. All other geometrical parameters for the three conformers were described by parameters analogous to those in conformer II and corrected by adding the differences adopted from M062X/6-311G** calculations. For the three conformers, the benzene ring was fixed to be planar since the QC calculations showed very small deviations from planarity. 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. Vibrational amplitudes for the three conformers were refined in eight groups according to the specific regions in the radial distribution: 0e1.21; 1.21e1.68; 1.68e2.06; 2.06e2.60; 2.60e3.8; 3.8e4.5; 4.5e6.1; 6.1e10.0 Å. The differences between the amplitudes within each group were constrained to the calculated values. Vibrational corrections Dr ¼ rh1 e ra and starting root-mean square amplitudes were calculated with the Vibmodule program [10] 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 optimal conformer ratio was found to be II:III:IV ¼ 20(15):25(15):55(15)% with the uncertainty estimated using Hamilton's criterion [11] at 0.05 significance level (Rf, min ¼ 3.39%), see Fig. 5. Thus, from GED analysis, the conformer 1PheqMe2Nax is less stable than 1-Phax Me2Neq with the ratio Pheq: Phax ¼ 20(15): 80(15) %. Relative Gibbs free energies between the axial and equatorial conformers were estimated from the conformer ratio, as DG(317 K) ¼ GaxeGeq ¼ 0.87(48) kcal/mol (or relative DG ¼ 0; 0.14; 0.64 kcal/mol for II:III:IV). The conformer ratio and Gibbs free energies coincide with those predicted by the MP2 calculations. 2.3. Geometry
Scheme 1. Compound 1 studied.
Selected experimental (GED) and calculated (QC) geometric parameters of the three conformers II, III and IV are compiled in Table 2. It is obvious, that the M062X method gives shorter SieN, CeN and CeC bond distances of the phenyl ring and larger pyramidality of the N-vicinity, as compared with MP2 results and experimental data. Comparing the molecular parameters of the three conformers, one can see that the SieCPh bond in conformer IV
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Fig. 1. Lowest energy pathways for axial and equatorial conformers by rotating the dimethylamino group around SieN bond (left) and the phenyl ring around SieCipso bond (right) calculated at M062X/6-311G** level.
Fig. 2. Conformers of compound 1 corresponding to the energy minima (hydrogen atoms omitted).
Fig. 3. Correlation between dihedral angles C1eSieC8eC9 and C4eSieNeMe in the Phax (left) and Pheq conformers (right) of molecule 1.
is longer by 0.007e0.009 Å than that in other conformers due probably to orbital interaction between the electron lone pair on nitrogen atom and the SieCPh bond. At the same time, other bond distances and bond angles in the three conformers are not affected by the position of the substituents. The calculated bond distances from the MP2 method are in good agreement with GED values. Orientations of dimethylamino group and phenyl ring are characterized by LpNSiC8 and NSiC8C9 bond angles, respectively (Lp e lone electron pair on nitrogen atom which was put in the
plane bisecting the CeNeC bond angle of the dimethylamino group). Due to ortho-a-hydrogen interaction, the phenyl ring in conformer Pheq lies in the mirror plane bisecting the silacyclohexane ring, whereas in the Phax conformers, because of 1,3-diaxial and ortho-a-hydrogen interactions, the mirror plane and the plane of the phenyl ring are perpendicular. So, the SieN bond in conformer II lies in the plane of phenyl ring, while in conformers III and IV, the SieN bond lies in the mirror plane and is perpendicular to the plane of the phenyl ring. The LpNSiC8 and NSiC8C9 torsion
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Table 1 Relative energies DE, free Gibbs energies DG(298 K) and molar fractions of all conformers of compound 1 from QC calculations. Method\\conformer
DE, kcal/mol
DG(298 K), kcal/mol
XI:XII:XIII:XIV, %
I
II
III
IV
I
II
III
IV
B3LYP-D3/6-311G** B3LYP-D3/cc-pVTZ
0 0 (I^II)
0.00
0.00 0.19
0.31 0.37
0 0 (I^II)
0.27
0.54 0.39
0.63 0.42
M062X/6-311G** M062X/cc-pVTZ MP2/6-311G** Experiment GED
0 0 0
0.08 0.04 0.03
0.17 0.30 0.83
0.27 0.32 0.65
0 0 0
0.56 0.45 0.65a
0.12 0.08 0.82
0.02 0.11 1.77a
0.14
0.64
a
0
42:27:17:14 55:22:23 (XI≡II:XIII:XIV) 17:44:21:18 21:44:18:17 4:11:14:71 XI≡II:XIII:XIV ¼ 20:25:55(15)
Lowest frequencies of conformers II and IV are 13 and 4 cm1, respectively.
Fig. 4. Molecular scattering intensities sM(s) and radial distribution curves f(r): experimental (dots) and theoretical (black line) for refined mixture of three conformers of II:III:IV ¼ 20(15):25(15):55(15)%; colored lines correspond to refinement of all parameters under assumptions of the individual conformers; the differences “Experim.-Theor.” are given at the bottom.
Fig. 5. Agreement factor Rf as a function of molar fraction of the conformers II and III (a sum of contributions of II, III and IV is equal to 1).
angles, refined from the GED data, have a huge uncertainty, which is caused by a shallow minimum region on the PES profiles (see Fig. 1).
2.4. Low temperature dynamic
13
C NMR study
In Figs. 6 and 7 the 13C NMR spectrum of 1-NMe2-1-Ph-1silacyclohexane 1 is visualized dependent on temperature; at
ambient temperature (Fig. 6), the spectrum in methylene chloride is given, at 103 K, the lowest temperature obtained (Fig. 7) the spectrum is recorded in the freon mixture CD2Cl2/CHFCl2/CHFCl2 in a ratio of 1:1:3 which is employed because the solvent proves to be still liquid down to 100 K and the conformational equilibria of Sisubstituted silacyclohexanes and heterocyclic analogues could be readily studied [1e4,6,12]. As expected, also the conformational equilibrium of 1 can be determined; at 103 K the ring interconversion of the silacyclohexane ring system is slow on the NMR time scale. From the phenyl ring carbon atoms only Cipso decoalesced, CorthoeCpara, obviously, are too far away to be influenced by the different conformation of phenyl (and NMe2) at silicon. The same, and for identical reasons, was obtained for C-3 of the silacyclohexane moiety of 1. The remaining carbon atoms, C-1,5 and C-2,4, and the carbon atoms of the NMe2 substituent at silicon, decoalesced, and, with the exception of C-2,4 and NMe2 carbons (due to impurity signals near to the decoalesced carbon signals of the two participating conformers e cf. Fig. 7) the signals of the C-1,5 and Cipso in the two participating conformers can be evaluated and signal intensities integrated (cf. Fig. 8): C-1,5 (77%: 23%) and Cipso (71%: 29%), with the arithmetic mean of 74%: 26%, resulting in K ¼ 2.8; DG ¼ RT ln K (at 103 K) ¼ 0.44 ± 0.1 kcal/mol (see Fig. 9). Thus, even if the ratio of the two conformers as obtained from the various signals is different (from 77: 23 to 71: 29), the preferred conformer can be clearly assigned. On the other hand, 13C signal intensities were employed to determine the conformation equilibrium of 1, which are still dependent on relaxation times T1 and T2; even if the pulse repetition time used should be long enough for
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Table 2 Selected theoretical (with 6-311G** basis set) and experimental geometrical parametersa of conformers II, III and IV of compound 1. Conf.
II
Method
M062X
SieC1 SieN SieC8 C1eC2 C6eN C8eC9 SiC1C2 C1C2C3 NSiC1 C6NSi C8SiC1 C8C9C10 SiC1C2C3 C1C2C3C4 LpNSiC8 NSiC8C9
GEDb MP2
Bond distance, Å 1.878 1.879 1.733 1.739 1.882 1.882 1.541 1.542 1.450 1.454 1.400 1.409 Bond angle 110.2 110.4 113.1 113.0 109.2 109.0 121.1 120.7 112.4 112.0 121.2 121.3 Torsion angle 57.0 56.8 64.4 64.5 176.5 173.7 12.1 13.5
GEDb
III M062X
MP2
1.880(4) 1.739(6) 1.885(4) 1.547(4) 1.449(7) 1.407(9)
1.880 1.735 1.882 1.542 1.453 1.399
1.879 1.741 1.879 1.543 1.457 1.409
110.6(5) 113.5(5) 108.9(9) 118.5(11) 112.5(9) 120.9(15)
109.7 113.1 111.3 121.5 109.2 121.2
59(3) 67(2) 163(55) 46(23)
58.8 67.3 62.7 117.3
GEDb
IV M062X
MP2
1.882(4) 1.741(6) 1.885(4) 1.548(4) 1.452(7) 1.406(9)
1.877 1.739 1.891 1.542 1.454 1.400
1.877 1.745 1.889 1.544 1.458 1.411
1.879(4) 1.744(6) 1.894(4) 1.548(4) 1.452(7) 1.407(9)
109.7 113.1 111.3 120.1 109.1 121.3
110.1(5) 113.5(5) 110.9(9) 118.9(11) 109.2(9) 120.9(15)
109.4 113.4 110.9 119.0 108.5 121.4
109.4 113.3 111.3 117.7 108.6 121.5
109.8(5) 113.7(5) 110.5(9) 116.4(11) 108.5(9) 120.9(15)
59.1 67.0 62.8 115.5
60(3) 69(2) 64(26) 123(34)
56.6 66.5 178.1 102.9
56.7 65.7 180.0 87.4
58(3) 69(2) 179(25) 102(34)
a rh1 values (rh1 ¼ raþDr) are given for GED results. The vibrational corrections Dr were calculated with the Vibmodule program [10] using the so called second approximation, in which harmonic approach with nonlinear relation between Cartesian and internal coordinates were applied on the basis of force field estimated in quantum chemical calculations at the M062X/6-311G** level. b Values in parentheses are full errors estimated as s(rh1) ¼ [s2scaleþ(2.5sLS)2]½, where sscale ¼ 0.002r and sLS is a standard deviation in least-squares refinement for internuclear distances and as 3sLS for vibration amplitudes. The place-value is such that the last digit of the uncertainty lines up with the last digit of the nominal value.
Fig. 6.
13
C NMR spectrum of compound 1 at ambient temperature in CD2Cl2.
CH signals but probably not long enough for complete relaxation of Cipso in between the pulses; the free energy difference between the two participating conformers, 0.44 ± 0.1 kcal/mol, proved to be sufficient to characterize the position of the conformational equilibrium of 1-NMe2-1-Ph-1-silacyclohexane 1. It remains to assign the preferred conformer to the 13C signal sets obtained at 103 K. 1-Phenyl-substituted silacyclohexanes have been already studied by us [3,4,6,12]. In case of 1-phenyl-1-silacyclohexane3 and 1phenyl(1-Me, 1-OMe, 1-F, 1-Cl)silacyclohexanes [4,6,12], conformational equilibria could be frozen and both the equilibrium constant K and the free energy difference DG between the participating conformers could be determined. In all cases studied so far, the conformer with the equatorial phenyl substituent proves to
be the preferred one in solution. The corresponding 13C chemical shifts of C-1,5 and C-2,4 signals, d(13C)/ppm, are collected in Table 3. The b-effect of the 1-Si substituents on C-1,5, which is of electronic influence, was found to be unequivocal initially: in each of the former Si-mono- and di-substituted silacyclohexanes the carbon signal of C-1,5 in the Pheq conformers proves to be at lower field compared with the corresponding d(13C)/ppm value in the Phax conformer. To assign compound 1 accordingly, would mean that the Phax conformer of 1-(dimethylamino)-1-phenylsilacyclohexane 1 is preferred. On the other hand, when employing the g-effect of the Sisubstituents on C-2,4, the result proves to be inconsistent. To begin with the d(13C)/ppm values of 1-phenyl-1-Me-
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Fig. 7.
Fig. 8.
13
13
C NMR spectrum of compound 1 at 103 K in the freon mixture CD2Cl2/CHFCl2/CHFCl2 ¼ 1:1:3.
C NMR spectrum of compound 1 at 103 K in the freon mixture CD2Cl2/CHFCl2/CHFCl2 ¼ 1:1:3; silacyclohexane moiety.
silacyclohexane compared with the one in 1-phenyl-silacyclohexane, C-2,4 is 1.7 ppm high field shifted due to the well-known steric g-effect of methyl; even if due to the longer CeSi bond lengths this steric g-effect is reduced with respect to the same effect in cyclohexanes, it is readily visible in 1-phenyl-1-Me-silacyclohexane [3]. In the remaining compounds in Table 3, including compound 1, the d(13C)/ppm values of C-2,4 only slightly change, Dd(13C)/ ppm ¼ 22.8e23.3 in the Pheq conformer and Dd(13C)/ ppm ¼ 23.8e25.0 in the Phax conformer. The corresponding Dd(13C)/ppm values of 1 are outside the Dd(13C)/ppm ranges (if C1,5 is assigned correctly, vide supra): 24.4 ppm (Pheq) and 23.4 ppm (Phax). Thus, inverted assignment of C-2,4 better fits to the other compounds in Table 3 which means the preference of also the Pheq
conformer of 1. Actually, we trust in the g-effect more than in the beffect and prefer the latter assignment. Thus, if this is correct, the signal position of C-1,5 in 1 is reversed with respect to other compounds in Table 3. As mentioned already, the b-effect which is of electronic origin, and the certain conformation of the dimethylamino substituent in axial position could be the reason for this result. The 13C chemical shifts of the two conformers also have been calculated (Table 4); the following Dd(13C)/ppm values for decoalesced carbon atoms were obtained: In addition to C-2,4, also Cipso and the NMe2 carbon atoms (which also decoalesced, cf. Fig. 8) agree with our assignment employing the g-effect on the 13C chemical shifts of C-2,4.
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Fig. 9.
4305
13
C NMR spectrum of compound 1 at 103 K in the freon mixture CD2Cl2/CHFCl2/CHFCl2 ¼ 1:1:3; phenyl moiety.
Table 3 13 C chemical shifts of C-1,5 and C-2,4 carbon atoms in 1-phenyl-1-X-1-silacyclohexanes. Compound
Signal
1-Phenyl-1-silacyclohexanea 1-Methyl-1-phenyl-silacyclohexane 1-Methoxy-1-phenyl-silacyclohexane 1-Fluoro-1-phenylsilacyclohexane 1-Chloro-1-phenyl-silacyclohexanea 1-(Dimethylamino)-1-phenylsilacyclohexane 1
a
C-1,5 C-2,4 C-1,5 C-2,4 C-1,5 C-2,4 C-1,5 C-2,4 C-2,4 C-1,5 C-2,4 Cipso
Chemical shifts d(13C)/ppm
Ref.
Phax conformer
Pheq conformer (major)
8.4 23.5 10.5 25.0 10.0 25.0 11.3 23.8 24.0 10.2 24.4 137.3
10.4 25.0 11.4 23.3 11.1 23.3 12.1 23.0 22.8 9.3 23.4 138.1
[3] [3] [6] [4] [4] This work
For C-1,5 no decoalescence observed.
To prove the position of the frozen conformational equilibrium of 1 at 103 K, 1D, 2D-NOE, HMQC and HMBC experiments were recorded but failed unfortunately due to the still existing dynamic ring interconversion of 1. Thus, the position of the conformational equilibrium of 1(dimethylamino)-1-phenylsilacyclohexane 1 could not be determined unequivocally. However, we trust more in the steric g-effect of the Si-substituents on C-2,4 more than in the b-effect on C-1,5, and, moreover, there is no intelligible reason why should the conformational equilibrium of compound 1 be inverted with
Table 4 Calculated and experimental differences Dd(13C) in compound 1.
Dd(13C)/ppm PheqPhax
Calculated
Experimental
Cipso C-1,5 C-2,4 -NMe2
3.3 ppm 2.4 ppm þ1.2 ppm þ0.3 ppm
0.8 ppm þ0.9 ppm þ1.0 ppm þ0.5 ppm
respect to all other [3,4,6,8] (see Table 3); it was the Pheq conformer which proved to be the preferred one. Theoretical calculations of the conformational equilibrium of 1NMe2-1-Ph-1-silacyclohexane 1 support the latter assignment (cf. Tables 3 and 4); while in the gas phase generally and for the 1·CHCl3 solvate complex (considering methylene chloride as solvent) and in CD2Cl2 at ambient temperature the Phax conformer was found to be the preferred conformer, in solution (considering the latter solvent) the Pheq conformer was found to be the preferred one (cf. Table 5). Without the exception of the d(C-1,5)/ppm value, compared with the relative position in the frozen 13C NMR spectra of 1-Ph and 1-X1-Ph-1-silacyclohexane (X ¼ Me, OMe, F, Cl) [3,4,6,8] all remaining experimental and theoretical evidences argue for 1-PheqNax to be the preferred conformer of 1-(dimethylamino)-1phenylsilacyclohexane 1.
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Table 5 M062X/6-311G** relative energies DE [E(Pheq) e E(Phax)], Gibbs free energies DG [G (Pheq) e G (Phax)] (kcal/mol) and the ratio of conformers of compound 1 and its solvate complexes in gas phase and solution.
3. Experimental section 3.1. General Chemicals and technical grade solvents (hexane, triethylamine) were distilled prior to use over CaH2. Room temperature 1H, 13C, and 29Si NMR spectra were registered on a Bruker DPX 400 spectrometer at working frequencies 400 (1H), 100 (13C) and 79 MHz (29Si) in CDCl3. Low temperature 13C NMR spectra were recorded on a Bruker AV-600 instrument (at 150.95 MHz) and low temperature 19 F NMR spectra on a Bruker AV-500 instrument (at 564.2 MHz). Chemical shifts were determined relative to residual CHCl3 (1H, d 7.27 ppm), internal CDCl3 (13C, d 77.0 ppm), and internal CD2Cl2 (13C, d 53.73 ppm) downfield from TMS (for 1H, 13C). Analysis and assignment of the 1H NMR data were supported by homonuclear (COSY) and heteronuclear (HSQC and HMBC) correlation experiments. A solvent mixture of CD2Cl2, CHFCl2, and CHF2Cl in a ratio of 1:1:3 was employed for the low temperature measurements because of being still liquid at around 100 K. The probe temperature was calibrated by means of a thermocouple PT 100 inserted into a dummy tube. The low temperature measurements were estimated to be accurate to ±1. The HRMS ESI spectrum of 1 was recorded using a QTOFmicro mass spectrometer in positive electrospray mode. The capillary voltage was 3.2 kV, with a cone voltage between 20 and 25 V. Elementary composition was determined by accurate mass measurement with standard deviation <5 ppm H3PO4 used as reference compound. 1-(Dimethylamino)-1-phenyl-1-silacyclohexane 1 was prepared in 56% yield by the reaction of 1-chloro-1-phenyl-1silacyclohexane with excess of dimethylamine in hexane solution and purified by vacuum distillation (140e142 С/5 mm Hg). The details of the experiment, physico-chemical characteristics and spectral data were described recently.7 Compound 1 is highly sensitive to moisture: after one month, the content of the corresponding siloxane reached 35%, and after two months, the compound is fully hydrolyzed. As a result, even when the freshly prepared sample was used, the formation of impurities (silanol and/or siloxane) could not be fully avoided. 3.2. 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 [8,9]. The sample of 1 was evaporated from stainless steel cell with inner sizes diameter length of 7 18 mm2 (cell) and of 0.6 1.2 mm2 (cylindrical effusion nozzle) which was kept at 317(3) K in the course of the experiments. The conditions of the GED/MS experiment and data refinement details are given in Supporting Information.
Mass spectra (EI, 50 eV) of the effusing molecular beam were recorded simultaneously with the collection of the diffracted electrons (see Fig. S1 in the Supporting Information). The major þ þ peaks corresponds to ions SiC6Hþ 5 , H2Si(C6H5)N , H2Si(C6H5)NMe2 , þ þ þ MeSi(C6H5)NMe2 , C5H10Si(C6H5) and molecular ion M with relative intensities 52, 32, 40, 43, 32 and 100%, respectively. The intensities of other peaks are less than 20%. The relative intensities of the ions in the mass spectra showed no change during the course of the experiment. No ions with m/z higher than 219 Da (Mþ) were detected. In addition, we recorded mass spectra at different ionizing electron energies. The contribution of the fragment ions decreases with lower electron energy. At the energy as low as ca. Ei ¼ 10 eV, the electron impact produces the molecular ion Mþ, m/ z ¼ 219 Da, exclusively (see Fig. S2 in the Supplementary Data) which is the evidence that the only species present in the gas is 1.
3.3. Computational details All QC calculations were performed with Gaussian 09 program package [13]. Geometry and vibrational calculations were performed with 6-311G** and cc-pVTZ basic sets by using DFT method with B3LYP-D3 and M062X functionals. Geometry of 1 also was optimized by MP2/6-311G** level. The potential energy surface (PES) profiles were obtained at M062X/6-311G** level by varying the NeSieCPheCorth angle for the phenyl ring rotation and Ce NeSieCPh angle for the dimethylamino group rotation with a step of 5 and optimization of all other geometrical parameters calculated.
4. Conclusions In summary, 1-(dimethylamino)-1-phenylsilacyclohexane 1 was synthesized and its conformational analysis was performed by GED, low-temperature 13C NMR and quantum-chemical calculations. In gas phase, the GED determined geometry is well reproduced at both DFT and MP2 levels, whereas the experimentally determined predominance of the 1-PhaxNeq conformer conformers is reproduced by MP2 and not by B3LYP or M062X calculations. Inversely, in solution LT 13C NMR spectra at 103 K showed the predominance of the 1-Pheq,Nax conformer; the latter assignment from low temperature NMR spectra and theoretical calculations is not completely unequivocal. With the exception of d(C-1,5)/ppm value, compared with the relative position in the frozen 13C NMR spectra of 1-Ph and 1-Ph,1-X-silacyclohexane (X ¼ Me, OMe, F, Cl) [3,4,6,8] all remaining experimental and theoretical evidences argue for 1PheqNax to be the preferred conformer of 1-(dimethylamino)-1phenylsilacyclohexane 1.
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Acknowledgement 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); the low temperature 13C NMR measurements of 1-(dimethylamino)-1-phenylsilacyclohexane 1 by Dr. Matthias Heydenreich and Angela Krtitschka from the University of Potsdam is gratefully acknowledged.
[5]
[6]
[7]
Appendix A. Supplementary data [8]
Supplementary data related to this article can be found at https://doi.org/10.1016/j.tet.2018.06.023. References [1] B.A. Shainyan, E. Kleinpeter, Silacyclohexanes and silaheterocyclohexanes e why are they so different from other heterocyclohexanes? Tetrahedron 69 (2013) 5927e5936. [2] B.A. Shainyan, Structure and conformational analysis of silacyclohexanes and 1,3- silaheterocyclohexanes, Tetrahedron 72 (2016) 5027e5035. [3] B.A. Shainyan, E. Kleinpeter, Conformational preferences of SiePh,H and SiePh,Me silacyclohexanes and 1,3-thiasilacyclohexanes. Additivity of conformational energies in 1,1-disubstituted heterocyclohexanes, Tetrahedron 68 (2012) 114e125. [4] B.A. Shainyan, A.V. Belyakov, Yu F. Sigolaev, A.N. Khramov, E. Kleinpeter, Molecular structure and conformational analysis of 1-phenyl-1-X-1-
[9]
[10]
[11] [12]
[13]
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silacyclohexanes (X ¼ F, Cl) by electron diffraction, low-temperature NMR, and quantum chemical calculations, J. Org. Chem. 82 (2017) 461e470. 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 (2017) 1127e1134. B.A. Shainyan, S.V. Kirpichenko, E. Kleinpeter, Conformational preferences of the phenyl group in 1-Phenyl-1-X-1-silacyclohexanes (X ¼ MeO, HO) and 3phenyl-3-X-3- silatetrahydropyrans (X ¼ HO, H) by low temperature 13CNMR spectroscopy and theoretical calculations, J. Org. Chem. 82 (2017) 13414e13422. E.N. Suslova, B.A. Shainyan, 1-Phenyl-1-X-1-silacyclohexanes (X ¼ MeO, OH, Me2N), Russ. J. Gen. Chem. 87 (2017) 1645e1648. G.V. Girichev, A.N. Utkin, Y.F. Revichev, Instruments and experimental techniques 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. 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. W.C. Hamilton, Significance tests on the crystallographic R factor, Acta Crstallogr. 18 (1965) 502e510. 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 (2015) 3810e3818. M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford, CT, 2009.
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Synthesis, conformational preferences in gas and solution, and molecular gear rotation in 1-(dimethylamino)-1-phenyl-1silacyclohexane by gas phase electron diffraction (GED), LT NMR and theoretical calculations Bagrat A. Shainyan a, *, Elena N. Suslova a, Tran Dinh Phien b, c, Sergey A. Shlykov d, Erich Kleinpeter e, * a
A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Division of the Russian Academy of Sciences, 1 Favorsky Street, 664033 Irkutsk, Russian Federation Institute of Research and Development, Duy Tan University, 03 Quang Trung, Da Nang, Viet Nam c Department of Chemistry and Environment, Vietnam-Russian Tropical Centre, 63 Nguyen Van Huyen, Cau Giay District, Ha Noi, Viet Nam d Ivanovo State University of Chemistry and Technology, Research Institute for Thermodynamics and Kinetics of Chemical Processes, 7 Sheremetievskiy Ave, 153000, Ivanovo, Russian Federation e €t Potsdam, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam (Golm), Germany Chemisches Institut der Universita b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 May 2018 Received in revised form 6 June 2018 Accepted 9 June 2018 Available online 19 June 2018
1-(Dimethylamino)-1-phenyl-1-silacyclohexane 1, was synthesized, and its molecular structure and conformational properties studied by gas-phase electron diffraction (GED), low temperature 13C NMR spectroscopy and quantum-chemical calculations. The predominance of the 1-Phax conformer (1-Pheq:1Phax ratio of 20:80%, DG (317 K) ¼ 0.87 kcal/mol) in the gas phase is close to the theoretically estimated conformational equilibrium. In solution, low temperature NMR spectroscopy showed analyzable decoalescence of Cipso and C(1,5) carbon signals in 13C NMR spectra at 103 K. Opposite to the gas state in the freon solution employed (CD2Cl2/CHFCl2/CHFCl2 ¼ 1:1:3), which is still liquid at 100 K, the 1-Pheq conformer was found to be the preferred one [(1-Pheq: 1-Phax ¼ 77%: 23%, K ¼ 77/23 ¼ 2.8; DG ¼ RT ln K (at 103 K) ¼ 0.44 ± 0.1 kcal/mol]. When comparing 1 with 1-phenyl-1-(X)silacylohexanes (X ¼ H, Me, OMe, F, Cl), studied so far, the trend of predominance of the Phax conformer in the gas phase and of the Pheq conformer in solution is confirmed. © 2018 Elsevier Ltd. All rights reserved.
Keywords: 1-(Dimethylamino)-1-phenyl-1silacyclohexane Conformational analysis Gas phase electron diffraction Low-temperature d-NMR DFT MP2 M062X/6-311G** calculations
1. Introduction The conformational analysis of silacyclohexanes and silaheterocyclohexanes revealed principal difference in the conformational preferences, molecular structure, barriers to interconversion between the conformers, and the relative importance of basic factors determining the position of conformational equilibrium, such as steric effects, hyperconjugation, and electrostatic effects [1,2]. Conformational analysis of the Si-monosubstituted sila(hetero)cyclohexanes allows to estimate the conformational energies of
* Corresponding authors. E-mail addresses: [email protected] (B.A. Shainyan), [email protected] (E. Kleinpeter). https://doi.org/10.1016/j.tet.2018.06.023 0040-4020/© 2018 Elsevier Ltd. All rights reserved.
substituents at silicon and to compare them to those at carbon in the corresponding cyclohexanes. The next step is the analysis of sila(hetero)cyclohexanes with two different groups at silicon allowing to determine the relative conformational preferences of the substituents [3]. For the phenyl group at silicon, an additional problem arises due to non-degenerate rotation of an unsymmetrical rotor about the SieCipso bond in both the Phax and Pheq conformers. For the Si-Ph-substituted sila(hetero)cyclo-hexanes with a monoatomic second substituent at silicon, like H [3], or Hlg [4], the phenyl group rotation is affected only by the size of this atom. In contrast to this, for polyatomic substituents, even in the case of symmetric rotors like the methyl group, internal rotation of the two substituents, coupled like cogwheels, is correlated so that rotation about one axis is transferred to another axis. Such a coupling affects the ease of rotation, which, in turn, has an effect on the entropy and,
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hence, the free energy and the ratio of the conformers. So far, we have studied sila(hetero)cyclohexanes with Si(Ph,Me) pair of substituents in gas [5] and in solution [3], and of sila(hetero)cyclohexanes with Si(Ph,OH), and Si(Ph,OMe) pairs of substituents [6]. However, the methyl group is a C3v symmetric rotor and the steric hindrances to the phenyl group rotation should only slightly depend on rotation about the SieMe bond. The OH and OMe groups can adopt the conformation with the OeH or OeMe group turned outward from the Ph group [with the PheSieOeH(Me) torsional angle equal or close to 180 ] thus minimizing the steric hindrances. Therefore, it was of interest to investigate the conformational behavior of a system having two substituents at silicon which would be more “rotationally coupled”. With this in mind, in the present work we performed conformational analysis of the recently synthesized [7] 1-(dimethylamino)-1-phenylsilacyclohexane 1, having an asymmetric rotor NMe2, by gas-phase electron diffraction (GED), low-temperature NMR down to 103 K, and theoretical calculations both in gas and in solution (see Scheme 1).
2. Results and discussion 2.1. Energies The potential energy surface (PES) profiles for phenyl ring and dimethylamino group rotations around the SieCPh and SieN bonds, respectively, are plotted in Fig. 1. Four conformers of this compound were observed and drawn in Fig. 2. From MP2 calculation, conformer IV has CS symmetry. In all forms, due to steric repulsion between methyl hydrogens and the hydrogen atoms in ortho-position of the phenyl ring, the dimethylamino group rotation induces phenyl ring rotation and causes the conformers I and III to transform to II and IV, respectively, and vice versa, with low energy barriers, 0.6 kcal/mol. However, a complicated situation was found for the phenyl ring rotation, which leads to transformations of I to II and of III to IV with high energy barriers, but does not induce an opposite conversion, i.e. II to I and IV to III. For comparison, in our recent work [5], the methyl group rotation in 1-methyl-1-phenyl-silacyclohexane induces phenyl ring rotation in both Phax and Pheq conformers, whereas the phenyl group rotation does not noticeably affect the methyl group orientation. In Fig. 3, the dependence of dihedral angles between the Ph group and С4eSieСipso plane upon the NMe2 group rotation in the two conformers of molecule 1 is depicted. As can be clearly seen, rotation of the two groups is strongly correlated, which is typical for gear motion in molecular motors. Vertical’ lines in Fig. 3 (left) appear because for two scanned angles of the NMe2 group differing by 5 the geometry optimization of the axial Ph group leads the system to two different local minima, causing a ‘jump’ of energy. Relative total energies, Gibbs free energies, and the molar fractions of the conformers are summarized in Table 1. As follows from Table 1, in the gas phase, M062X calculations show that conformer I is energetically most stable; the PheqMe2Nax forms are more preferable than PhaxMe2Neq, the Pheq: Phax ratio being 60e65%: 40e35%. Similar ratio, Pheq: Phax ¼ 70%: 30%, is predicted from
B3LYP-D3/6-311G** calculations. In the case of B3LYP/cc-pVTZ method, the starting geometry of conformer I is optimized to structure II with the ratio Pheq: Phax ¼ 55%: 45%. At the same time, according to MP2 calculations, the Pheq conformers are less stable than their Phax counterparts. Thus, the two theoretical methods give different results. 2.2. GED analysis As follows from optimized geometries of conformers I and II (see Figs. 1 and 2), they differ by 10 and 30 in the angles of the phenyl ring and dimethylamino group rotations, respectively. Moreover, the energy barrier between the I and II is very low, 0.1 kcal/mol, so in the framework of GED method, it is unreal to distinguish them upon structural analysis. Thus, in this section, conformers I and II were combined as the latter (with variable mentioned torsion angles), and the GED analysis was carried out considering three conformers II, III and IV only. The following independent geometric parameters were used to describe the geometry: bond distances SieC1, C1eC2, SieN, CeN, C8eC9 and CeH; bond angles SiC1C2, NSiC1, CNSi, CNC, C8SiC1, C8C9C10, HC1Si, HC1C2, HCN, HCH and, HC9C8; dihedral angles SiC1C2C3, C1C2C3C4, CNSiC1, C9C8SiN for conformer II; dihedral angles CNSiCPh and NSiCPhCortho for conformers III and IV. All other geometrical parameters for the three conformers were described by parameters analogous to those in conformer II and corrected by adding the differences adopted from M062X/6-311G** calculations. For the three conformers, the benzene ring was fixed to be planar since the QC calculations showed very small deviations from planarity. 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. Vibrational amplitudes for the three conformers were refined in eight groups according to the specific regions in the radial distribution: 0e1.21; 1.21e1.68; 1.68e2.06; 2.06e2.60; 2.60e3.8; 3.8e4.5; 4.5e6.1; 6.1e10.0 Å. The differences between the amplitudes within each group were constrained to the calculated values. Vibrational corrections Dr ¼ rh1 e ra and starting root-mean square amplitudes were calculated with the Vibmodule program [10] 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 optimal conformer ratio was found to be II:III:IV ¼ 20(15):25(15):55(15)% with the uncertainty estimated using Hamilton's criterion [11] at 0.05 significance level (Rf, min ¼ 3.39%), see Fig. 5. Thus, from GED analysis, the conformer 1PheqMe2Nax is less stable than 1-Phax Me2Neq with the ratio Pheq: Phax ¼ 20(15): 80(15) %. Relative Gibbs free energies between the axial and equatorial conformers were estimated from the conformer ratio, as DG(317 K) ¼ GaxeGeq ¼ 0.87(48) kcal/mol (or relative DG ¼ 0; 0.14; 0.64 kcal/mol for II:III:IV). The conformer ratio and Gibbs free energies coincide with those predicted by the MP2 calculations. 2.3. Geometry
Scheme 1. Compound 1 studied.
Selected experimental (GED) and calculated (QC) geometric parameters of the three conformers II, III and IV are compiled in Table 2. It is obvious, that the M062X method gives shorter SieN, CeN and CeC bond distances of the phenyl ring and larger pyramidality of the N-vicinity, as compared with MP2 results and experimental data. Comparing the molecular parameters of the three conformers, one can see that the SieCPh bond in conformer IV
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Fig. 1. Lowest energy pathways for axial and equatorial conformers by rotating the dimethylamino group around SieN bond (left) and the phenyl ring around SieCipso bond (right) calculated at M062X/6-311G** level.
Fig. 2. Conformers of compound 1 corresponding to the energy minima (hydrogen atoms omitted).
Fig. 3. Correlation between dihedral angles C1eSieC8eC9 and C4eSieNeMe in the Phax (left) and Pheq conformers (right) of molecule 1.
is longer by 0.007e0.009 Å than that in other conformers due probably to orbital interaction between the electron lone pair on nitrogen atom and the SieCPh bond. At the same time, other bond distances and bond angles in the three conformers are not affected by the position of the substituents. The calculated bond distances from the MP2 method are in good agreement with GED values. Orientations of dimethylamino group and phenyl ring are characterized by LpNSiC8 and NSiC8C9 bond angles, respectively (Lp e lone electron pair on nitrogen atom which was put in the
plane bisecting the CeNeC bond angle of the dimethylamino group). Due to ortho-a-hydrogen interaction, the phenyl ring in conformer Pheq lies in the mirror plane bisecting the silacyclohexane ring, whereas in the Phax conformers, because of 1,3-diaxial and ortho-a-hydrogen interactions, the mirror plane and the plane of the phenyl ring are perpendicular. So, the SieN bond in conformer II lies in the plane of phenyl ring, while in conformers III and IV, the SieN bond lies in the mirror plane and is perpendicular to the plane of the phenyl ring. The LpNSiC8 and NSiC8C9 torsion
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Table 1 Relative energies DE, free Gibbs energies DG(298 K) and molar fractions of all conformers of compound 1 from QC calculations. Method\\conformer
DE, kcal/mol
DG(298 K), kcal/mol
XI:XII:XIII:XIV, %
I
II
III
IV
I
II
III
IV
B3LYP-D3/6-311G** B3LYP-D3/cc-pVTZ
0 0 (I^II)
0.00
0.00 0.19
0.31 0.37
0 0 (I^II)
0.27
0.54 0.39
0.63 0.42
M062X/6-311G** M062X/cc-pVTZ MP2/6-311G** Experiment GED
0 0 0
0.08 0.04 0.03
0.17 0.30 0.83
0.27 0.32 0.65
0 0 0
0.56 0.45 0.65a
0.12 0.08 0.82
0.02 0.11 1.77a
0.14
0.64
a
0
42:27:17:14 55:22:23 (XI≡II:XIII:XIV) 17:44:21:18 21:44:18:17 4:11:14:71 XI≡II:XIII:XIV ¼ 20:25:55(15)
Lowest frequencies of conformers II and IV are 13 and 4 cm1, respectively.
Fig. 4. Molecular scattering intensities sM(s) and radial distribution curves f(r): experimental (dots) and theoretical (black line) for refined mixture of three conformers of II:III:IV ¼ 20(15):25(15):55(15)%; colored lines correspond to refinement of all parameters under assumptions of the individual conformers; the differences “Experim.-Theor.” are given at the bottom.
Fig. 5. Agreement factor Rf as a function of molar fraction of the conformers II and III (a sum of contributions of II, III and IV is equal to 1).
angles, refined from the GED data, have a huge uncertainty, which is caused by a shallow minimum region on the PES profiles (see Fig. 1).
2.4. Low temperature dynamic
13
C NMR study
In Figs. 6 and 7 the 13C NMR spectrum of 1-NMe2-1-Ph-1silacyclohexane 1 is visualized dependent on temperature; at
ambient temperature (Fig. 6), the spectrum in methylene chloride is given, at 103 K, the lowest temperature obtained (Fig. 7) the spectrum is recorded in the freon mixture CD2Cl2/CHFCl2/CHFCl2 in a ratio of 1:1:3 which is employed because the solvent proves to be still liquid down to 100 K and the conformational equilibria of Sisubstituted silacyclohexanes and heterocyclic analogues could be readily studied [1e4,6,12]. As expected, also the conformational equilibrium of 1 can be determined; at 103 K the ring interconversion of the silacyclohexane ring system is slow on the NMR time scale. From the phenyl ring carbon atoms only Cipso decoalesced, CorthoeCpara, obviously, are too far away to be influenced by the different conformation of phenyl (and NMe2) at silicon. The same, and for identical reasons, was obtained for C-3 of the silacyclohexane moiety of 1. The remaining carbon atoms, C-1,5 and C-2,4, and the carbon atoms of the NMe2 substituent at silicon, decoalesced, and, with the exception of C-2,4 and NMe2 carbons (due to impurity signals near to the decoalesced carbon signals of the two participating conformers e cf. Fig. 7) the signals of the C-1,5 and Cipso in the two participating conformers can be evaluated and signal intensities integrated (cf. Fig. 8): C-1,5 (77%: 23%) and Cipso (71%: 29%), with the arithmetic mean of 74%: 26%, resulting in K ¼ 2.8; DG ¼ RT ln K (at 103 K) ¼ 0.44 ± 0.1 kcal/mol (see Fig. 9). Thus, even if the ratio of the two conformers as obtained from the various signals is different (from 77: 23 to 71: 29), the preferred conformer can be clearly assigned. On the other hand, 13C signal intensities were employed to determine the conformation equilibrium of 1, which are still dependent on relaxation times T1 and T2; even if the pulse repetition time used should be long enough for
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Table 2 Selected theoretical (with 6-311G** basis set) and experimental geometrical parametersa of conformers II, III and IV of compound 1. Conf.
II
Method
M062X
SieC1 SieN SieC8 C1eC2 C6eN C8eC9 SiC1C2 C1C2C3 NSiC1 C6NSi C8SiC1 C8C9C10 SiC1C2C3 C1C2C3C4 LpNSiC8 NSiC8C9
GEDb MP2
Bond distance, Å 1.878 1.879 1.733 1.739 1.882 1.882 1.541 1.542 1.450 1.454 1.400 1.409 Bond angle 110.2 110.4 113.1 113.0 109.2 109.0 121.1 120.7 112.4 112.0 121.2 121.3 Torsion angle 57.0 56.8 64.4 64.5 176.5 173.7 12.1 13.5
GEDb
III M062X
MP2
1.880(4) 1.739(6) 1.885(4) 1.547(4) 1.449(7) 1.407(9)
1.880 1.735 1.882 1.542 1.453 1.399
1.879 1.741 1.879 1.543 1.457 1.409
110.6(5) 113.5(5) 108.9(9) 118.5(11) 112.5(9) 120.9(15)
109.7 113.1 111.3 121.5 109.2 121.2
59(3) 67(2) 163(55) 46(23)
58.8 67.3 62.7 117.3
GEDb
IV M062X
MP2
1.882(4) 1.741(6) 1.885(4) 1.548(4) 1.452(7) 1.406(9)
1.877 1.739 1.891 1.542 1.454 1.400
1.877 1.745 1.889 1.544 1.458 1.411
1.879(4) 1.744(6) 1.894(4) 1.548(4) 1.452(7) 1.407(9)
109.7 113.1 111.3 120.1 109.1 121.3
110.1(5) 113.5(5) 110.9(9) 118.9(11) 109.2(9) 120.9(15)
109.4 113.4 110.9 119.0 108.5 121.4
109.4 113.3 111.3 117.7 108.6 121.5
109.8(5) 113.7(5) 110.5(9) 116.4(11) 108.5(9) 120.9(15)
59.1 67.0 62.8 115.5
60(3) 69(2) 64(26) 123(34)
56.6 66.5 178.1 102.9
56.7 65.7 180.0 87.4
58(3) 69(2) 179(25) 102(34)
a rh1 values (rh1 ¼ raþDr) are given for GED results. The vibrational corrections Dr were calculated with the Vibmodule program [10] using the so called second approximation, in which harmonic approach with nonlinear relation between Cartesian and internal coordinates were applied on the basis of force field estimated in quantum chemical calculations at the M062X/6-311G** level. b Values in parentheses are full errors estimated as s(rh1) ¼ [s2scaleþ(2.5sLS)2]½, where sscale ¼ 0.002r and sLS is a standard deviation in least-squares refinement for internuclear distances and as 3sLS for vibration amplitudes. The place-value is such that the last digit of the uncertainty lines up with the last digit of the nominal value.
Fig. 6.
13
C NMR spectrum of compound 1 at ambient temperature in CD2Cl2.
CH signals but probably not long enough for complete relaxation of Cipso in between the pulses; the free energy difference between the two participating conformers, 0.44 ± 0.1 kcal/mol, proved to be sufficient to characterize the position of the conformational equilibrium of 1-NMe2-1-Ph-1-silacyclohexane 1. It remains to assign the preferred conformer to the 13C signal sets obtained at 103 K. 1-Phenyl-substituted silacyclohexanes have been already studied by us [3,4,6,12]. In case of 1-phenyl-1-silacyclohexane3 and 1phenyl(1-Me, 1-OMe, 1-F, 1-Cl)silacyclohexanes [4,6,12], conformational equilibria could be frozen and both the equilibrium constant K and the free energy difference DG between the participating conformers could be determined. In all cases studied so far, the conformer with the equatorial phenyl substituent proves to
be the preferred one in solution. The corresponding 13C chemical shifts of C-1,5 and C-2,4 signals, d(13C)/ppm, are collected in Table 3. The b-effect of the 1-Si substituents on C-1,5, which is of electronic influence, was found to be unequivocal initially: in each of the former Si-mono- and di-substituted silacyclohexanes the carbon signal of C-1,5 in the Pheq conformers proves to be at lower field compared with the corresponding d(13C)/ppm value in the Phax conformer. To assign compound 1 accordingly, would mean that the Phax conformer of 1-(dimethylamino)-1-phenylsilacyclohexane 1 is preferred. On the other hand, when employing the g-effect of the Sisubstituents on C-2,4, the result proves to be inconsistent. To begin with the d(13C)/ppm values of 1-phenyl-1-Me-
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Fig. 7.
Fig. 8.
13
13
C NMR spectrum of compound 1 at 103 K in the freon mixture CD2Cl2/CHFCl2/CHFCl2 ¼ 1:1:3.
C NMR spectrum of compound 1 at 103 K in the freon mixture CD2Cl2/CHFCl2/CHFCl2 ¼ 1:1:3; silacyclohexane moiety.
silacyclohexane compared with the one in 1-phenyl-silacyclohexane, C-2,4 is 1.7 ppm high field shifted due to the well-known steric g-effect of methyl; even if due to the longer CeSi bond lengths this steric g-effect is reduced with respect to the same effect in cyclohexanes, it is readily visible in 1-phenyl-1-Me-silacyclohexane [3]. In the remaining compounds in Table 3, including compound 1, the d(13C)/ppm values of C-2,4 only slightly change, Dd(13C)/ ppm ¼ 22.8e23.3 in the Pheq conformer and Dd(13C)/ ppm ¼ 23.8e25.0 in the Phax conformer. The corresponding Dd(13C)/ppm values of 1 are outside the Dd(13C)/ppm ranges (if C1,5 is assigned correctly, vide supra): 24.4 ppm (Pheq) and 23.4 ppm (Phax). Thus, inverted assignment of C-2,4 better fits to the other compounds in Table 3 which means the preference of also the Pheq
conformer of 1. Actually, we trust in the g-effect more than in the beffect and prefer the latter assignment. Thus, if this is correct, the signal position of C-1,5 in 1 is reversed with respect to other compounds in Table 3. As mentioned already, the b-effect which is of electronic origin, and the certain conformation of the dimethylamino substituent in axial position could be the reason for this result. The 13C chemical shifts of the two conformers also have been calculated (Table 4); the following Dd(13C)/ppm values for decoalesced carbon atoms were obtained: In addition to C-2,4, also Cipso and the NMe2 carbon atoms (which also decoalesced, cf. Fig. 8) agree with our assignment employing the g-effect on the 13C chemical shifts of C-2,4.
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Fig. 9.
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13
C NMR spectrum of compound 1 at 103 K in the freon mixture CD2Cl2/CHFCl2/CHFCl2 ¼ 1:1:3; phenyl moiety.
Table 3 13 C chemical shifts of C-1,5 and C-2,4 carbon atoms in 1-phenyl-1-X-1-silacyclohexanes. Compound
Signal
1-Phenyl-1-silacyclohexanea 1-Methyl-1-phenyl-silacyclohexane 1-Methoxy-1-phenyl-silacyclohexane 1-Fluoro-1-phenylsilacyclohexane 1-Chloro-1-phenyl-silacyclohexanea 1-(Dimethylamino)-1-phenylsilacyclohexane 1
a
C-1,5 C-2,4 C-1,5 C-2,4 C-1,5 C-2,4 C-1,5 C-2,4 C-2,4 C-1,5 C-2,4 Cipso
Chemical shifts d(13C)/ppm
Ref.
Phax conformer
Pheq conformer (major)
8.4 23.5 10.5 25.0 10.0 25.0 11.3 23.8 24.0 10.2 24.4 137.3
10.4 25.0 11.4 23.3 11.1 23.3 12.1 23.0 22.8 9.3 23.4 138.1
[3] [3] [6] [4] [4] This work
For C-1,5 no decoalescence observed.
To prove the position of the frozen conformational equilibrium of 1 at 103 K, 1D, 2D-NOE, HMQC and HMBC experiments were recorded but failed unfortunately due to the still existing dynamic ring interconversion of 1. Thus, the position of the conformational equilibrium of 1(dimethylamino)-1-phenylsilacyclohexane 1 could not be determined unequivocally. However, we trust more in the steric g-effect of the Si-substituents on C-2,4 more than in the b-effect on C-1,5, and, moreover, there is no intelligible reason why should the conformational equilibrium of compound 1 be inverted with
Table 4 Calculated and experimental differences Dd(13C) in compound 1.
Dd(13C)/ppm PheqPhax
Calculated
Experimental
Cipso C-1,5 C-2,4 -NMe2
3.3 ppm 2.4 ppm þ1.2 ppm þ0.3 ppm
0.8 ppm þ0.9 ppm þ1.0 ppm þ0.5 ppm
respect to all other [3,4,6,8] (see Table 3); it was the Pheq conformer which proved to be the preferred one. Theoretical calculations of the conformational equilibrium of 1NMe2-1-Ph-1-silacyclohexane 1 support the latter assignment (cf. Tables 3 and 4); while in the gas phase generally and for the 1·CHCl3 solvate complex (considering methylene chloride as solvent) and in CD2Cl2 at ambient temperature the Phax conformer was found to be the preferred conformer, in solution (considering the latter solvent) the Pheq conformer was found to be the preferred one (cf. Table 5). Without the exception of the d(C-1,5)/ppm value, compared with the relative position in the frozen 13C NMR spectra of 1-Ph and 1-X1-Ph-1-silacyclohexane (X ¼ Me, OMe, F, Cl) [3,4,6,8] all remaining experimental and theoretical evidences argue for 1-PheqNax to be the preferred conformer of 1-(dimethylamino)-1phenylsilacyclohexane 1.
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Table 5 M062X/6-311G** relative energies DE [E(Pheq) e E(Phax)], Gibbs free energies DG [G (Pheq) e G (Phax)] (kcal/mol) and the ratio of conformers of compound 1 and its solvate complexes in gas phase and solution.
3. Experimental section 3.1. General Chemicals and technical grade solvents (hexane, triethylamine) were distilled prior to use over CaH2. Room temperature 1H, 13C, and 29Si NMR spectra were registered on a Bruker DPX 400 spectrometer at working frequencies 400 (1H), 100 (13C) and 79 MHz (29Si) in CDCl3. Low temperature 13C NMR spectra were recorded on a Bruker AV-600 instrument (at 150.95 MHz) and low temperature 19 F NMR spectra on a Bruker AV-500 instrument (at 564.2 MHz). Chemical shifts were determined relative to residual CHCl3 (1H, d 7.27 ppm), internal CDCl3 (13C, d 77.0 ppm), and internal CD2Cl2 (13C, d 53.73 ppm) downfield from TMS (for 1H, 13C). Analysis and assignment of the 1H NMR data were supported by homonuclear (COSY) and heteronuclear (HSQC and HMBC) correlation experiments. A solvent mixture of CD2Cl2, CHFCl2, and CHF2Cl in a ratio of 1:1:3 was employed for the low temperature measurements because of being still liquid at around 100 K. The probe temperature was calibrated by means of a thermocouple PT 100 inserted into a dummy tube. The low temperature measurements were estimated to be accurate to ±1. The HRMS ESI spectrum of 1 was recorded using a QTOFmicro mass spectrometer in positive electrospray mode. The capillary voltage was 3.2 kV, with a cone voltage between 20 and 25 V. Elementary composition was determined by accurate mass measurement with standard deviation <5 ppm H3PO4 used as reference compound. 1-(Dimethylamino)-1-phenyl-1-silacyclohexane 1 was prepared in 56% yield by the reaction of 1-chloro-1-phenyl-1silacyclohexane with excess of dimethylamine in hexane solution and purified by vacuum distillation (140e142 С/5 mm Hg). The details of the experiment, physico-chemical characteristics and spectral data were described recently.7 Compound 1 is highly sensitive to moisture: after one month, the content of the corresponding siloxane reached 35%, and after two months, the compound is fully hydrolyzed. As a result, even when the freshly prepared sample was used, the formation of impurities (silanol and/or siloxane) could not be fully avoided. 3.2. 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 [8,9]. The sample of 1 was evaporated from stainless steel cell with inner sizes diameter length of 7 18 mm2 (cell) and of 0.6 1.2 mm2 (cylindrical effusion nozzle) which was kept at 317(3) K in the course of the experiments. The conditions of the GED/MS experiment and data refinement details are given in Supporting Information.
Mass spectra (EI, 50 eV) of the effusing molecular beam were recorded simultaneously with the collection of the diffracted electrons (see Fig. S1 in the Supporting Information). The major þ þ peaks corresponds to ions SiC6Hþ 5 , H2Si(C6H5)N , H2Si(C6H5)NMe2 , þ þ þ MeSi(C6H5)NMe2 , C5H10Si(C6H5) and molecular ion M with relative intensities 52, 32, 40, 43, 32 and 100%, respectively. The intensities of other peaks are less than 20%. The relative intensities of the ions in the mass spectra showed no change during the course of the experiment. No ions with m/z higher than 219 Da (Mþ) were detected. In addition, we recorded mass spectra at different ionizing electron energies. The contribution of the fragment ions decreases with lower electron energy. At the energy as low as ca. Ei ¼ 10 eV, the electron impact produces the molecular ion Mþ, m/ z ¼ 219 Da, exclusively (see Fig. S2 in the Supplementary Data) which is the evidence that the only species present in the gas is 1.
3.3. Computational details All QC calculations were performed with Gaussian 09 program package [13]. Geometry and vibrational calculations were performed with 6-311G** and cc-pVTZ basic sets by using DFT method with B3LYP-D3 and M062X functionals. Geometry of 1 also was optimized by MP2/6-311G** level. The potential energy surface (PES) profiles were obtained at M062X/6-311G** level by varying the NeSieCPheCorth angle for the phenyl ring rotation and Ce NeSieCPh angle for the dimethylamino group rotation with a step of 5 and optimization of all other geometrical parameters calculated.
4. Conclusions In summary, 1-(dimethylamino)-1-phenylsilacyclohexane 1 was synthesized and its conformational analysis was performed by GED, low-temperature 13C NMR and quantum-chemical calculations. In gas phase, the GED determined geometry is well reproduced at both DFT and MP2 levels, whereas the experimentally determined predominance of the 1-PhaxNeq conformer conformers is reproduced by MP2 and not by B3LYP or M062X calculations. Inversely, in solution LT 13C NMR spectra at 103 K showed the predominance of the 1-Pheq,Nax conformer; the latter assignment from low temperature NMR spectra and theoretical calculations is not completely unequivocal. With the exception of d(C-1,5)/ppm value, compared with the relative position in the frozen 13C NMR spectra of 1-Ph and 1-Ph,1-X-silacyclohexane (X ¼ Me, OMe, F, Cl) [3,4,6,8] all remaining experimental and theoretical evidences argue for 1PheqNax to be the preferred conformer of 1-(dimethylamino)-1phenylsilacyclohexane 1.
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Acknowledgement 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); the low temperature 13C NMR measurements of 1-(dimethylamino)-1-phenylsilacyclohexane 1 by Dr. Matthias Heydenreich and Angela Krtitschka from the University of Potsdam is gratefully acknowledged.
[5]
[6]
[7]
Appendix A. Supplementary data [8]
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