long-lived 9-cyano-9,10-dimethylphenanthrenyl cation. migration of cyano group in carbocations

7
ISSN 1070-4280, Russian Journal of Organic Chemistry, 2011, Vol. 47, No. 7, pp. 1050–1056. © Pleiades Publishing, Ltd., 2011. Original Russian Text © V.A. Bushmelev, A.M. Genaev, G.E. Sal’nikov, V.G. Shubin, 2011, published in Zhurnal Organicheskoi Khimii, 2011, Vol. 47, No. 7, pp. 1034–1039. 1050 Long-Lived 9-Cyano-9,10-dimethylphenanthrenyl Cation. Migration of Cyano Group in Carbocations V. A. Bushmelev, A. M. Genaev, G. E. Sal’nikov, and V. G. Shubin Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences, pr. Akademika Lavrent’eva 9, Novosibirsk, 630090 Russia e-mail: [email protected] Received November 17, 2010 Abstract—Long-lived 9-cyano-9,10-dimethylphenanthrenyl cation was generated in superacidic medium, and its structure was determined by 1 H and 13 C NMR spectroscopy. The energy barrier to 1,2-shift of the cyano group in 9-cyano-9,10-dimethylphenanthrenyl cation was estimated by NMR and DFT calculations. 1,2-Shifts of atoms and groups in carbocations may be regarded as a specific intramolecular electrophilic substitution reaction [1]. Up to now, data on the rates of rearrangements of long-lived carbocations via 1,2-shift of various migrating atoms or groups were reported [2]. However, as far as we know, there are no published data on rearrangements of carbocations where the migrating group is linked to the cationic center through an sp-hybridized carbon atom, e.g., on 1,2-shift of cyano group (cf. [3–5]). The goal of the present work was to estimate the migrating ability of cyano group in a long-lived carbo- cation. We anticipated that a convenient model may be 9-cyano-9,10-dimethylphenanthrenyl cation (I) gen- erated under conditions ensuring its “long life.” It is known that long-lived 9-R-9,10-dimethylphenan- threnyl cations are convenient models for estimation of migrating ability of various atoms and groups [2]. We planned to generate cation I according to Scheme 1 (cf. [6]). Compound II as precursor was synthesized by reaction of trimethylsilyl cyanide with 10,10-dimethyl- 9,10-dihydrophenanthren-9-one (IV). According to the results of DFT quantum-chemical calculations, cations I and III are almost equally stable, E I E III = 2.2 kJ/mol. However, in keeping with the data of [7], the calculation method used over- estimates the stability of cations like III. Therefore, cation I was presumed to predominate in the equilib- rium mixture. In fact, the 1 H NMR spectrum of a solu- tion of compound II in the acid system HSO 3 F– SO 2 ClF–CD 2 Cl 2 (1 : 3 : 1 by volume), prepared at –95°C, at –105°C contained signals from protons in the 9- and 10-CH 3 groups in cation I (δ 2.18 and 3.66 ppm, respectively) together with signals from aromatic protons (δ 7.6–8.9 ppm). In the 13 C NMR spectrum of the same solution, the cationic carbon atom (C 10 ) resonated at δ C 215.0 ppm, signals from aromatic carbon atoms were located at δ C 120–160 ppm, the CN carbon signal appeared at δ C 113.4 ppm, and signals from C 9 and methyl groups were observed at δ C 52.4, 26.4, and 33.4 ppm, respectively. The carbon signals were assigned by comparing the above spectra with the spectra of cation I in trifluoromethanesulfonic acid, recorded at room temperature (see below). The chem- ical shift of C 10 attracts attention, for it can be used as a measure of positive charge on the carbocationic center. Obviously, the C 10 signal is displaced upfield relative to the corresponding signal of structurally DOI: 10.1134/S107042801107013X Me Me NC Me 3 SiO II H + –Me 3 SiOH Me Me NC III ~Me NC Me Me I 1 2 3 4 4a 4b 5 6 7 8 8a 9 10 10a Scheme 1.

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ISSN 1070-4280, Russian Journal of Organic Chemistry, 2011, Vol. 47, No. 7, pp. 1050–1056. © Pleiades Publishing, Ltd., 2011. Original Russian Text © V.A. Bushmelev, A.M. Genaev, G.E. Sal’nikov, V.G. Shubin, 2011, published in Zhurnal Organicheskoi Khimii, 2011, Vol. 47, No. 7, pp. 1034–1039.

1050

Long-Lived 9-Cyano-9,10-dimethylphenanthrenyl Cation. Migration of Cyano Group in Carbocations

V. A. Bushmelev, A. M. Genaev, G. E. Sal’nikov, and V. G. Shubin

Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences, pr. Akademika Lavrent’eva 9, Novosibirsk, 630090 Russia

e-mail: [email protected]

Received November 17, 2010

Abstract—Long-lived 9-cyano-9,10-dimethylphenanthrenyl cation was generated in superacidic medium, and its structure was determined by 1H and 13C NMR spectroscopy. The energy barrier to 1,2-shift of the cyano group in 9-cyano-9,10-dimethylphenanthrenyl cation was estimated by NMR and DFT calculations.

1,2-Shifts of atoms and groups in carbocations may be regarded as a specific intramolecular electrophilic substitution reaction [1]. Up to now, data on the rates of rearrangements of long-lived carbocations via 1,2-shift of various migrating atoms or groups were reported [2]. However, as far as we know, there are no published data on rearrangements of carbocations where the migrating group is linked to the cationic center through an sp-hybridized carbon atom, e.g., on 1,2-shift of cyano group (cf. [3–5]).

The goal of the present work was to estimate the migrating ability of cyano group in a long-lived carbo-cation. We anticipated that a convenient model may be 9-cyano-9,10-dimethylphenanthrenyl cation (I) gen-erated under conditions ensuring its “long life.” It is known that long-lived 9-R-9,10-dimethylphenan-threnyl cations are convenient models for estimation of migrating ability of various atoms and groups [2]. We planned to generate cation I according to Scheme 1 (cf. [6]). Compound II as precursor was synthesized by reaction of trimethylsilyl cyanide with 10,10-dimethyl-9,10-dihydrophenanthren-9-one (IV).

According to the results of DFT quantum-chemical calculations, cations I and III are almost equally

stable, EI – EIII = 2.2 kJ/mol. However, in keeping with the data of [7], the calculation method used over-estimates the stability of cations like III. Therefore, cation I was presumed to predominate in the equilib-rium mixture. In fact, the 1H NMR spectrum of a solu-tion of compound II in the acid system HSO3F–SO2ClF–CD2Cl2 (1 : 3 : 1 by volume), prepared at –95°C, at –105°C contained signals from protons in the 9- and 10-CH3 groups in cation I (δ 2.18 and 3.66 ppm, respectively) together with signals from aromatic protons (δ 7.6–8.9 ppm). In the 13C NMR spectrum of the same solution, the cationic carbon atom (C10) resonated at δC 215.0 ppm, signals from aromatic carbon atoms were located at δC 120–160 ppm, the CN carbon signal appeared at δC 113.4 ppm, and signals from C9 and methyl groups were observed at δC 52.4, 26.4, and 33.4 ppm, respectively. The carbon signals were assigned by comparing the above spectra with the spectra of cation I in trifluoromethanesulfonic acid, recorded at room temperature (see below). The chem-ical shift of C10 attracts attention, for it can be used as a measure of positive charge on the carbocationic center. Obviously, the C10 signal is displaced upfield relative to the corresponding signal of structurally

DOI: 10.1134/S107042801107013X

MeMe

NCMe3SiO

II

H+

–Me3SiOH

MeMe

NC

III

~Me

NCMe

Me

I

1

2

34

4a4b

56

7

8 8a

9 1010a

Scheme 1.

LONG-LIVED 9-CYANO-9,10-DIMETHYLPHENANTHRENYL CATION.

RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 7 2011

1051

(a)

(b)

160 155 150 145 140 135 130 δC, ppm 9.0 8.8 8.6 8.4 8.2 8.0 δ, ppm

1-H 5-H

4-H 3-H

8-H 7-H 6-H

2-H C3

C3a C8a

C1 C7

C10a

C2

C6 C5 C8 C4

C4b

Fig. 1. Schematic representation of signals from aromatic rings in the 13C and 1H NMR spectra of cations (a) V [9] and (b) I.

Fig. 2. Calculated steric structure of cation I.

shift of the CN group was observed at 48°C (ΔG# > 76 kJ/mol).

DFT quantum-chemical calculations showed that the energy of the transition state for 1,2-shift of the CN group in cation I is higher by 87 kJ/mol than the energy of the initial cation. Assuming that this energy is equal to ΔG#, the rate constant calculated by the Eyring equation should be equal to 4 × 10–3 s–1 at 25°C. The Gibbs energy of activation for 1,2-shift of the CN group in cation I was estimated at ΔG≠ = 89.5 kJ/mol by the equation proposed in [12]. In this case, measur-able signal broadening in dynamic NMR experiments (by several Hertzes, which corresponds to slow ex-change) would be attained at 100°C. Unfortunately, cation I turned out to be thermally unstable: its half-conversion period in CF3SO3H at room temperature amounts to several hours, and at 48°C, to ~1 h; after 30 min at 60°C, even traces of I were not detected.

related 9,9,10-trimethylphenanthrenyl cation (V, δC 232.6 ppm [8, 9]; cf. [10]) due to electron-with-drawing effect of the cyano group. This means that the positive charge on C10 in cation I is smaller than in cation V. As might be expected, delocalization of posi-tive charge leads to downfield shift of signals from aromatic carbon atoms and protons relative to the cor-responding signals of cation V (Fig. 1). An exception is the signal of C8a which, as well as C10, resides in the β-position with respect to the cyano group.

On the other hand, the signal of the 10-CH3 group (δ 3.66 ppm) directly attached to the cationic center is displaced downfield relative to the corresponding signal of cation V (δ 3.47 ppm [8]). Presumably, this is the result of field effect of the cyano group in the α-position. It is known [11] that such effect depends on mutual orientation of the interacting fragments, the maximal effect (~0.5 ppm downfield) being observed when proton appears in the vicinity of the plane ortho-gonal to the C≡N bond. Quantum-chemical calcula-tions showed that the structure of cation I conforms to the above conditions: the 9-CN group occupies pseudoequatorial position, approximately opposite to the 10-CH3 group (Fig. 2).

Raising the temperature from –105 to –15°C through a step of 10 deg does not change the 1H NMR spectrum of cation I, indicating a fairly high energy barrier to 1,2-shift of the cyano group. In order to estimate the possibility for observing 1,2-shift by dynamic NMR, cation I was generated by dissolution of trimethylsilyl ether II in CF3SO3H–CD2Cl2 (4 : 1 by volume) and in pure CF3SO3H, which allowed the NMR spectra to be recorded at higher temperature. However, no signal broadening which could indicate

RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 7 2011

BUSHMELEV et al. 1052

* This term was proposed in [15].

Fig. 3. Calculated transition state for 1,2-shift of the CN group in cation I; some bond lengths are given in Å.

1.49

1.67

1.16

Using the saturation labeling* [13] and NOESY techniques [14] we succeeded in detecting chemical exchange between methyl groups in cation I (ΔG≠ = 79 kJ/mol). However, this process does not involve 1,2-shift of the CN group but successive 1,2-shifts of methyl groups (Scheme 2), for it is not accompanied by exchange of protons in the aromatic rings (accord-ing to the NOESY data). The absence in the NOESY spectrum of cross peaks between protons in the aro-matic rings in cation I, corresponding to chemical ex-change, led us to conclude that the barrier to CN group migration (ΔG≠) at 48°C exceeds 92 kJ/mol. Marx et al. [3] also failed to observe 1,2-shift of the cyano group in long-lived 1-cyano-4-hydroxy-1-methylben-zenonium ion. According to the authors, the high energy barrier to this reaction is determined by sp hybridization of atoms in the migrating group, while the transition state is structurally similar to vinyl cation. The calculated (DFT) structure of the transition state for CN group migration in cation I is shown in Fig. 3.

Scheme 2.

~Me

NCMe

Me

I

MeMe

NC

III

~Me

NCMe

Me

I'

The high barrier to 1,2-migration of the cyano group in cation I may be determined in part by its

unfavorable orientation with respect to the p orbital of the cationic center (Fig. 2; cf. [16]) and formation of hydrogen bond with acid medium (cf. [17–19]). However, the assumption that the rearrangement is hindered as a result of protonation of cyano group (cf. [19, 20]) is untenable. This is supported by the follow-ing. As shown in [20], the degree of protonation of the methoxy group in structurally related 9-methoxy-9,10-dimethylphenanthrenylium in HSO3F–SO2ClF is rela-tively small (20–30%). Acetonitrile is a weak base; it undergoes protonation by half in 99.6% H2SO4, i.e., the H0 value for the conjugate acid is –10.12 [21]. Dimethyl ether is a much stronger base (H0 = –3.85 for the conjugate acid [22]). Therefore, we can presume that the degree of protonation of the cyano group in cation I in CF3SO3H is considerably lower than the degree of protonation of the methoxy group in 9-me-thoxy-9,10-dimethylphenanthrenylium in a medium with analogous acidity (HSO3F–SO2ClF).

Some structural specificities of compound II (pre-cursor of I) deserve attention. Signals of the methyl groups in the 1H and 13C NMR spectra of II at room temperature are broadened as a result of conforma-tional exchange (Scheme 3) which averages signals from pseudoaxial and pseudoequatorial groups in con-formers IIa and IIb (cf. [23]). The conformational exchange becomes restricted at –13°C, and the NMR spectra contain two sets of signals belonging to struc-tures IIa and IIb at a ratio of 4 : 1 (Fig. 4).

Scheme 3. MeMe

NCMe3SiO

IIa

MeMeNC

Me3SiO

IIb

The exchange pattern was determined on the basis of the NOESY spectrum (Fig. 5). The kinetic param-eters of the conformational exchange shown in Scheme 3 were estimated by dynamic NMR by com-paring the experimental 1H NMR spectra with those calculated using MEX program [24] (Fig. 6): Ea = 45.7 kJ/mol, log A = 10.4, ΔH≠ = 43.3 kJ/mol, ΔS≠ = –53 J mol–1 K–1.

To conclude, in the present work we calculated at the DFT level the energy barrier to 1,2-shift of the cyano group in 9-cyano-9,10-dimethylphenanthren-ylium (87 kJ/mol) and determined by NMR spectros-

LONG-LIVED 9-CYANO-9,10-DIMETHYLPHENANTHRENYL CATION.

RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 7 2011

1053

Fig. 4. Calculated (DFT) structures of conformers (a) IIa and (b) IIb and chemical shifts (δ, ppm) of methyl protons.

(a) (b) 1.86

1.11

–0.19

0.54

1.70

0.87

Fig. 5. NOESY 1H NMR spectrum of compound II at –13°C (600 MHz, methyl proton region). Signal of unidentified im-purity is marked with an asterisk.

1.5 1.0 0.5 0.0 δ, ppm

1.5

1.0

0.5

0.0

δ, ppm

*

*

(dried over 4 Å molecular sieves), CF3SO3H (97%, from Aldrich), and AlCl3 (from Fluka).

The structures of cation I and ether II were op-timized by the DFT method with PBE functional [28] using PRIRODA program [29] (L1 basis set, Λ01 [30], an analog of cc-pVDZ). Visualizations and Cartesian coordinates are available at http://limor1.nioch.nsc.ru/quant/phen-CN/. The chemical shifts were calculated at the GIAO/DFT/PBE level of theory with L22 basis set (Λ22 [30], an analog of cc-pCVTZ) using PRIRODA program. The exchange NMR spectra were

copy the lower limit of that barrier (ΔG≠ > 92 kJ/mol). It should also be noted that the reaction of trimethyl-silyl ether II with acids, apart from long-lived cation I, gives rise to other products whose structure and mech-anism of formation were considered in [25].

EXPERIMENTAL

The 1H and 13C NMR spectra were recorded on Bruker AV-300 (300.13 and 75.47 MHz), AV-400 (400.13 and 100.61 MHz), and AV-600 spectrometers (600.30 and 150.95 MHz, respectively). The chemical shifts were determined relative to CD2Cl2 (CHDCl2, δ 5.33 ppm; CD2Cl2, δC 53.3 ppm; carbocations) or CDCl3 (CHCl3, δ 7.24 ppm; CDCl3, δC 76.9 ppm; neutral compounds). The probes of NMR spectrom-eters were calibrated against pentane (mp –130°C) and methanol standard sample (above –90°C); linear inter-polation was used for intermediate temperatures. Signals in the NMR spectra were assigned using two-dimensional 1H–1H correlations with double quantum filtering (COSYDQF) and 13C–1H correlations through direct (HSQC) and long-range couplings (HMBC). The molecular weight of ether II was determined from the high-resolution mass spectrum recorded on a DFS Thermo Scientific instrument.

Ketone IV was synthesized according to the proce-dure described in [26]. Commercial trimethylsilyl cya-nide (97%, from Fluka, or 98%, from Acros Organics) was used without additional purification. Cation I was generated using doubly distilled fluorosulfonic acid (bp 158–161°C), SO2ClF [27] (dried by passing its vapor through concentrated sulfuric acid), CD2Cl2

RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 7 2011

BUSHMELEV et al. 1054

Fig. 6. (a) Experimental (600 MHz, methyl proton region) and (b) calculated (MEX [24]) 1H NMR spectra of compound II. Signal of unidentified impurity is marked with an asterisk.

calculated using MEX program [24] with xsim inter-face (ftp://nmr.nioch.nsc.ru/pub/nmr/).

10,10-Dimethyl-9-trimethylsiloxy-9,10-dihydro-phenanthrene-9-carbonitrile (II). a. Ketone IV, 0.5 g (2.25 mmol), was dissolved in 3 ml of benzene, 0.35 ml (2.4 mmol) of trimethylsilyl cyanide and 0.040 g (0.3 mmol) of powdered aluminum chloride were added (cf. [31]), and the resulting yellow mixture was stirred for 3 h at 60°C on a magnetic stirrer and left overnight at room temperature. The mixture was then treated with 5 ml of 3 N hydrochloric acid and extracted with diethyl ether, and the extracts were washed with a saturated aqueous solution of sodium chloride, dried over magnesium sulfate, and evaporat-ed on a rotary evaporator to isolate 0.69 g of com-pound II as an oily substance containing ~20% of the initial ketone (1H NMR data); the product solidified with time. Crude compound II was purified by freez-ing out from a solution in hexane at ~3°C.

b. A mixture of 0.222 g (1 mmol) of ketone IV, 0.023 g of tetraethylammonium 2-carbamoybenzoate [32], and 0.146 g (1.43 mmol) of trimethylsilyl cya-nide was stirred for 4 h at room temperature and left overnight. The resulting dark mixture was treated with 3–4 ml of water and 2 ml of ethyl acetate under stirring on a magnetic stirrer. The organic layer was separated,

the aqueous layer was extracted with ethyl acetate, and the extracts were combined with the organic phase, washed with a saturated aqueous solution of sodium chloride, dried over magnesium sulfate, and evaporat-ed to isolate 0.32 g of a light oily substance which gradually solidified. The product was dissolved in 15 ml of hexane, the solution was filtered, and the sol-vent was removed to obtain 0.30 g (93%) of compound II which was almost free from initial ketone IV. IR spectrum (CHCl3), ν, cm–1: 2200 w (C≡N), 1100 s (C–O). 1H NMR spectrum (600 MHz, acetone-d6, δ 2.06 ppm, –13°C), δ, ppm: conformer IIa: –0.30 (ax′-OSiMe3), 1.05 (10′ax-CH3), 1.75 (10′eq-CH3), 7.34 (2-H, 3-H), 7.46 (1-H), 7.47 (7-H), 7.58 (6-H), 7.84 (8-H), 7.85 (4-H), 7.91 (5-H); conformer IIb: 0.43 (eq′-OSiMe3), 0.84 (10′ax-CH3), 1.67 (10′eq-CH3), 7.20–8.00 m (8H). 13C NMR spectrum (151 MHz, acetone-d6, δC 28.2 ppm, –13°C), δC, ppm: conformer IIa : –1.3 (ax ′-OSiMe3), 20.2 (10 ′eq-CH3), 24.2 (10′ax-CH3), 42.5 (C10), 79.1 (C9), 118.5 (CN), 122.7 (C4), 123.5 (C5) 124.5 (C1), 126.0 (C2 or C3), 126.7 (C7), 126.8 (C8), 127.8 (C3 or C2), 129.7 (C4a or C8a), 129.7 (C6), 132.0 (C4b), 132.0 (C8a or C4a), 139.4 (C10a); conformer IIb: –0.3 (eq′-OSiMe3), 20.1 (10′ax-CH3), 24.0 (10′eq-CH3), 118.7 (CN), 123.2 d, 123.5 d, 123.5 d, 124.2 d, 126.8 d, 127.1 d, 127.9 d, 128.3 d, 130.5 s,

(a) (b)

1.5 1.0 0.5 0.0 δ, ppm 1.5 1.0 0.5 0.0 δ, ppm

–13°C

10°C

29°C

61°C 2000 s–1

350 s–1

100 s–1

18 s–1 *

*

*

*

LONG-LIVED 9-CYANO-9,10-DIMETHYLPHENANTHRENYL CATION.

RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 7 2011

1055

131.6 s, 133.7 s, 139.8 s. Found: m/z 321.1545 [M]+. C20H23NOSi. Calclated: M 321.1543.

Generation of carbocation I. Compound II, 30– 40 mg, in 0.1 ml of CD2Cl2 or without a solvent was added to the corresponding acid system or pure acid cooled to a required temperature and placed into an NMR ampule or reaction flask. The mixture was stirred with a glass rod cooled with liquid nitrogen (in an NMR ampule at low temperature) or on a magnetic stirrer (in a flask at 0°C) until it became homogeneous. Solutions prepared in a flask were transferred into an NMR ampule at room temperature.

The NMR spectra (AV-400) of a solution of com-pound II in HSO3F–SO2ClF–CD2Cl2, recorded at low temperatures, were discussed above. 1H NMR spec-trum (600 MHz, CF3SO3H, 26°C), δ, ppm: 2.30 (9-CH3), 3.74 (10-CH3), 7.98 (6-H), 8.03 (2-H), 8.08 (7-H), 8.20 (8-H), 8.72 (5-H), 8.81 (3-H, 4-H), 8.85 (1-H). 13C NMR spectrum (151 MHz), δC, ppm: 26.0 (10-CH3), 34.3 (9-CH3), 54.3 (C9), 116.2 (CN), 127.7 (C4b), 127.8 (C4), 128.3 (C8), 129.8 (C5), 132.4 (C6), 133.7 (C2), 134.9 (C10a), 137.0 (C7), 140.3 (C8a), 140.6 (C1), 152.2 (C4a), 160.1 (C3), 215.5 (C10).

Kinetic measurement for chemical exchange of methyl groups in cation I were performed by the saturation labeling technique [13] (drop in the signal intensity of one methyl group was determined upon saturation of signal of the other methyl group) and NOESY [14] (the intensities of the corresponding diagonal and cross peaks were compared). The formulas for the calculation of the rate constants k are available at http://nmr.nioch.nsc.ru/noekin/. The required spin–lattice relaxation times for protons in the 9- and 10-CH3 groups in the examined temperature range were 0.4–0.6 and 0.9–1.0 s, respectively. The ΔG≠ values were calculated by the Eyring equation.

This study was performed under financial support by the Russian Foundation for Basic Research (project no. 09-03-00 116-a) and by the Chemistry and Mate-rials Science Department of the Russian Academy of Sciences (project no. 5.1.4).

REFERENCES

1. Brouwer, D.M. and Hogeveen, H., Prog. Phys. Org. Chem., 1972, vol. 9, p. 179. 2. Shubin, V.G. and Borodkin, G.I., Carbocation Chemistry, Olah, G.A. and Prakash, G.K.S., Hoboken: Wiley, 2004, p. 125; Bushmelev, V.A., Genaev, A.M., and Shu- bin, V.G., Russ. J. Org. Chem., 2004, vol. 40, p. 966;

Bushmelev, V.A., Genaev, A.M., and Shubin, V.G., Russ. J. Org. Chem., 2006, vol. 42, p. 100; Artamoshkin, V.G., Bushmelev, V.A., Genaev, A.M., and Shubin, V.G., Russ. J. Org. Chem., 2006, vol. 42, p. 1257; Genaev, A.M., Sal’nikov, G.E., and Shubin, V.G., Russ. J. Org. Chem., 2010, vol. 46, p. 311.

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14. Ernst, R.R., Bodenhausen, G., and Wokaun, A., Principles of Nuclear Magnetic Resonance in One and Two Dimensions, Oxford: Clarendon, 1987.

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18. Laurence, C., Brameld, K.A., Graton, J., Le Ques- tel, J.-Y., and Renault, E., J. Med. Chem., 2009, vol. 52, p. 4073.

19. Chikinev, V.A., Bushmelev, V.A., and Shubin, V.G., Izv. Ross. Akad. Nauk, Ser. Khim., 1992, p. 1315.

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21. Deno, N.C., Gaugler, R.W., and Wisotsky, M.J., J. Org. Chem., 1966, vol. 31, p. 1967.

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