Formation of dications bearing a S(OH)₂⁺ group from long-lived 9,9-dimethyl-10 R-phenanthrenium cations in FSO₃H–SbF₅/SO₂ClF/SO₂: a mechanistic study

Abstract

¹H and ¹³C NMR studies have shown that the long-lived 9,9-dimethyl-10-R-phenanthrenium cations (R = PhC≡C, Me, OH) generated in FSO₃H–SbF₅/SO₂ClF/SO₂/CD₂Cl₂ transform into long-lived 7-dihydroxysulfonio-9,9-dimethyl-10-R-phenanthrenium dications. The effect of the 10-R substituents on the reaction rate suggests that a key step in the reaction mechanism is addition of SO₂ to protonated phenanthrenium cations.

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Introduction

It is known that aromatic compounds in superacids, such as HF–SbF₅ and FSO₃H–SbF₅, containing SO₂, transform into protonated sulfinic acids.^1–3 The mechanism of this reaction may consist of addition of SO₂ to arenium cations formed under protonation of aromatics or electrophilic attack of the cation SO₂H⁺ or complex of SO₂ with SbF₅ on the aromatics involved in equilibrium with arenium ions.^1,4

These mechanisms suggest the opposite effect of the substituent R on the reaction rate. While studying the long-lived 9,9-dimethyl-10-R-phenanthrenium cations^5,6 we have found that these cations are convenient model structures for study of carbocation reactions mechanisms. In the present work we have used these cations to determine the mechanism of sulfination reaction.

Results and discussion

We have found that the long-lived 9,9-dimethyl-10-phenylethynylphenanthrenium cation (1a)⁵ generated from 9-hydroxy-10,10-dimethyl-9-phenylethynyl-9,10-dihydrophenanthrene (2a) (Scheme 1) in FSO₃H–SbF₅/SO₂ClF/SO₂/CD₂Cl₂ at −95 °C, at −71 °C quickly (t_1/2 = 3 min) and totally turns into another long-lived cationic particle. The structure of this particle, as unambiguously determined from 2D COSY, ROESY, HSQC and HMBC correlations (ESI, pp. S2–S6†), corresponds to the structure of the original cation (1a) in which the atom H⁷ is replaced with a substituent containing no carbon atoms. The ¹H NMR spectrum of the new particle (Fig. 1) shows an additional singlet signal at δ 9.43 ppm of 2H intensity area, which correlates via long range spin–spin coupling with the ¹³C NMR signal of C⁷ in the 2D HMBC spectrum. The protons corresponding to this signal are involved in slow exchange with acid protons and the NOE cross-relaxation is observed between these protons and H⁶ and H⁸ signals. By analogy to the literary data^1–3,7 according to which the protons of S(OH)₂⁺ group in cations Ar–S(OH)₂⁺ do not exhibit fast exchange with acidic media and show distinct signals with chemical shifts in the range δ 9.12–9.63 ppm we believe that the new signal in our case belongs to S(OH)₂⁺ group and the new formed particle is 7-dihydroxysulfonio-9,9-dimethyl-10-phenylethynylphenanthrenium dication 3a (Scheme 1). Additional evidences for its dicationic nature are downfield shifts of all signals in the ¹H NMR spectrum (Fig. 1), particularly of the signals of H⁶ and H⁸, relative to that of the original cation (1a), and instability of its solutions containing lock substance CD₂Cl₂ at the temperatures above −60 °C, cf. ref. 8. The remarkable features of the sulfination reaction are its high rate and practically full regioselectivity. The rate constant of dication 3a formation is 4 × 10⁻³ s⁻¹ at −71 °C.

Scheme 1

Fig. 1 ¹H NMR spectra (aromatics area) of cation 1a in FSO₃H–SbF₅/SO₂ClF/CD₂Cl₂ at −91 °C and dication 3a in FSO₃H–SbF₅/SO₂ClF–SO₂/CD₂Cl₂ at −81 °C.

If the same superacid system does not contain SO₂, cation 1a remains stable up to −40 °C, at higher temperatures giving cyclization product dication 4 (Scheme 2).^5b

Scheme 2

An analogous sulfination reaction occurs with the long-lived 9,9,10-trimethylphenanthrenium cation 1b⁹ (Scheme 1). At −71 °C a mixture of this cation and 7-dihydroxysulfonio-9,9,10-trimethylphenanthrenium dication 3b is formed (ESI, pp. S10–S15†), the content of the latter being 34%, and maintaining practically not increased upon standing or by adding an additional amount of SO₂. As in the case of 3a, all ¹H NMR signals of 3b are downfielded relative to that of the precursor monocation 1b. The downfield displacement of NMR signals of the phenanthrene fragment of 3b is more pronounced than that of 3a (ESI, p. S23†), which is apparently due to lower opportunity for positive charge delocalization. Sulfination of 1b proceeds faster than that of 1a. The rate constant of dication 3b formation measured at −100 °C is 6 × 10⁻⁴ s⁻¹ (0.2 s⁻¹ for −71 °C as recalculated using the Eyring equation).

9,9-Dimethyl-10-hydroxyphenanthrenium cation 1c¹⁰ (Scheme 3) also undergoes sulfination reaction (ESI, pp. S16–S17†). This cation exists as a mixture of exchanging E- and Z-isomers (equilibrium constant Z/E is 0.3, exchange rate constant Z → E is 30 s⁻¹ at −81 °C). ROESY spectrum shows cross peaks due to NOE cross relaxation of hydroxyl proton with protons of methyl groups for the E-isomer, and with the H¹ proton for the Z-isomer. Hydroxyl proton of the Z-isomer is strongly downfielded relative to that of the E-isomer (δ 11.92 ppm vs. 11.31 ppm) due to deshielding anisotropy of the aromatic ring. Significant chemical shifts difference for H¹ proton of E- and Z-isomers is observed (δ 8.73 ppm and 8.28 ppm, respectively). The probable reason of the downfield shift of H¹ of the E-isomer is its spatial proximity to the lone pair of oxygen atom, cf. ref. 11.

Scheme 3

Sulfination of cation 1c results in dication 3c formation as the main product (ESI, pp. S18–S19†). Dication 3c also exists as an equilibrium mixture of slowly exchanging E- and Z-isomers (equilibrium constant Z/E is 0.4, exchange rate constant Z → E is 30 s⁻¹ at −71 °C). All the ¹H NMR signals of dications 3c are downfielded relative to the corresponding signals of cations 1c, the differences between ¹H NMR spectra of E- and Z-isomers being analogous to those observed for E- and Z-isomers of cation 1c. In this case the sulfination reaction is slower than that of 1a and 1b. The rate constant of dication 3c formation is 7 × 10⁻⁵ s⁻¹ at −71 °C.

Both cations 1d and 1e (Scheme 1) do not yield observable amounts of sulfination products. Cation 1d^5a,6 at −71 °C transforms into a complex mixture of unidentifiable products (ESI, p. S21†). On the contrary, cation 1e^5a,6 is quite stable up to −39 °C, content of the sulfination products being less than 2%. At higher temperatures this cation is decomposed (ESI, p. S22†).

Possible mechanisms of the sulfinated dications formation are presented in Schemes 4–6, cf. ref. 1,4. The mechanism, in which the protonated SO₂ is an attacking electrophile (Scheme 4), seems to be unlikely, as this mechanism assumes an approach of two positively charged particles to the distance necessary for the formation of a chemical bond. In Scheme 5 complex SO₂·SbF₅ (ref. 12) serves as electrophile. Its presence in the reaction mixture has been established by ¹⁹F NMR spectroscopy (ESI, p. S8†), cf. ref. 13. Both Schemes 4 and 5 assume that cation 1 is a nucleophilic particle. The nucleophilicity of the aromatic fragment should be in line with the electron donating properties of 10-R substituent. Apparently the nucleophilicity should decrease in the order R = OH > CCPh > Me. This assumption is confirmed by chemical shifts of aromatic fragments of cations 1a,b,c in ¹H and ¹³C NMR spectra (ESI, p. S23†). The rate of dications 3 formation according to Scheme 4 or 5, as determined by the nucleophilicity of cations 1, would decrease in the order 3c > 3a > 3b. However, the rate decreases just in the opposite order: 3b > 3a > 3c (the rate constants ratio is approx. 50 : 1 : 0.02 at −71 °C). Moreover, neither of Schemes 4 and 5 can explain the observed regioselectivity of the reaction, as according to these mechanisms an electrophile has to attack the C⁷ atom bearing a partial positive charge.

Scheme 4

Scheme 5

Scheme 6

Scheme 6 assumes that the superelectrophilic intermediates – dications 6 and/or 7 play a key role in the reaction. The inactivity of cation 1e in the sulfination reaction may result from a low concentration of the respective dication intermediates owing to low basicity of C⁸ and C⁶ atoms of cation 1e.

In order to verify the possibility of intermediates 6 and 7 formation, we carried out the sulfination reaction of 1a in FSO₃D–SbF₅/SO₂ClF/SO₂ (ESI, p. S9†). The methyl groups of both the precursor 1a and the product 3a do not undergo deuteration, as unambiguously follows from the absence of the corresponding signals in the ²H NMR spectrum. The ²H NMR signals in the aromatics region are too broad and indistinguishable from each others because of severe overlaps. Integrals of ¹H NMR signals of phenanthrene rings taken relative to the integrals of methyl group signals clearly indicate that protons belonging to the more positively charged ring (protons H¹⁻⁴ of both 1a and 3a) do not take part in deuteration to any visible degree. On the contrary, integrals of proton signals of the other phenanthrene ring both in the precursor monocation 1a and in the sulfinated product 3a (protons H⁵⁻⁸ of 1a and protons H^5,6,8 of 3a) are significantly reduced due to their partial deuteration. Deuteration of 1a into the positions H⁵⁻⁸ proceeds practically immediately even at −100 °C, i.e. much faster than sulfination. Therefore dications 6 and 7 in superacid FSO₃H–SbF₅ can be intermediates in the sulfination reaction. The regioselectivity of sulfination can be explained by the higher stability of the dications sulfinated at C⁷ in comparison with other sulfination products. In this case the second formal positive charge (at C⁷–S(OH)₂⁺) is located most distantly along the delocalized π bond system from the first one (at C¹⁰).

It is important to note that keeping the monocation precursors in FSO₃D in the absence of SbF₅ even up to −15 °C did not lead to any deuteration, and the reaction with SO₂ under these conditions also does not proceed. Therefore we believe that the role of SbF₅ in the sulfination reaction consists solely in ensuring high acidity of the medium necessary for formation of the dication intermediates 6 and/or 7.

In ¹H DNMR and ROESY spectra (ESI, pp. S14–S15†) of dication 3b chemical exchange between 9- and 10-methyl groups is observed (k = 6.4 s⁻¹ at −61 °C, DNMR), the signals of aromatic ring protons being intact. Probable mechanisms of this exchange are shown in Schemes 7 and 8. As the exchange rate between protons of S(OH)₂⁺ group of dication 3b and protons of superacid medium (k = 0.2 s⁻¹ at −61 °C, ROESY, pseudomonomolecular approximation) is significantly less than the rate of methyl groups exchange, Scheme 8 cannot be the preferred one. The chemical exchange between 9- and 10-methyl groups of dication 3b is slower than that of monocation 1b (k = 12.3 s⁻¹ at −61 °C, DNMR) probably due to lower stability of the dication intermediate 8 (Scheme 7) compared to dication 3b. In contrast to the latter monocation 1b undergoes degenerate rearrangement, so that there is no such a retarding effect.

Scheme 7

Scheme 8

Conclusions

Long-lived carbocations of the phenanthrene series are fruitful models for solving mechanistic problems of the organic reactions proceeding with formation of carbocation intermediates. The advantage of these models in study of the sufination reaction is the opportunity to explore kinetics and mechanism of the reaction, based on dependence of the rate on the substituents in aromatic ring.

Experimental

General methods and materials

NMR spectra were obtained at the Chemical Service Centre of Siberian Branch of the Russian Academy of Sciences on Bruker AV-600, using the residual proton and carbon signals of deuterated methylene chloride as internal references (CHDCl₂, δ_H 5.33 ppm; CD₂Cl₂, δ_C 53.6 ppm) and CFCl₃ (δ_F 0 ppm) as external standard. For structure elucidation and NMR signal assignment 2D correlation spectra ¹H–¹H (COSY, ROESY) and ¹H–¹³C (HSQC, HMBC) were used.

Analysis of SO₂ClF was performed on Hewlett-Packard G1800A device, equipped with gas chromatograph HP 5890 of II series having capillary column HP-5MS and mass selective detector HP 5971. The column temperature was 30 °C.

Doubly distilled FSO₃H (b.p. 158–161 °C), freshly distilled SbF₅, SO₂ (from Na₂S₂O₅ and H₂SO₄) drained by a transmission through conc. H₂SO₄, and CD₂Cl₂ drained by 4 Å molecular sieves were used. SO₂ClF was prepared by the method of Woyski.¹⁴ According to GC-MS data it contained SO₂ (8%). Pure SO₂ClF was obtained by distillation from SbF₅.¹⁵

7-Dihydroxysulfonio-9,9-dimethyl-10-phenylethynylphenanthrenium dication (3a)

Solution of carbinol 2a^5a (20 mg, 0.062 mmol.) in 0.12 mL of CD₂Cl₂ was added dropwise to solution of FSO₃H (130 mg, 1.3 mmol) and SbF₅ (250 mg, 1.2 mmol) in 0.3 mL of SO₂ClF–SO₂ (8% SO₂) placed into NMR tube under stirring at −95 °C. Formation of cation 1a⁵ was observed. The tube was kept for 10 min at −71 °C and then cooled down to −81 °C. ¹H NMR spectrum (δ, ppm): 2.13 s (6H, 9-Me), 7.82 t (2H, H_m, J 7.6 Hz), 8.04 t (1H, H_p, J 7.5 Hz), 8.11 t (1H, H², J 7.6 Hz), 8.21–8.26 m (3H, H_o, H⁶), 8.45 s (1H, H⁸), 8.61 t (1H, H³, J 7.6 Hz), 8.72 d (1H, H⁴, J 8.7 Hz), 8.88 d (1H, H⁵, J 9.2 Hz), 8.94 d (1H, H¹, J 8.2 Hz), 9.43 s (2H, OH). ¹³C NMR spectrum: (δ, ppm): 31.7 q (9-Me), 52.2 s (C⁹), 105.5 s (C_a), 119.8 s (C_i), 126.2 d (C⁶), 126.3 d (C⁸), 126.9 d (C⁴), 128.7 d (C⁵), 130.7 d (C_m), 133.5 d (C²), 134.0 s (C^4b), 134.7 s (C⁷), 135.5 s (C^10a), 138.1 d (C_o), 140.1 d (C_p), 140.1 d (C¹), 141.4 s (C^4a), 147.0 s (C^8a), 150.5 d (C³), 161.2 s (C_b), 199.2 s (C¹⁰).

Analogous NMR spectra were obtained for solution in SO₂ at −61 °C (ESI, p. S7†). The signal of S(OH)₂⁺ group is observed at δ 9.86 ppm. ¹⁹F NMR spectrum (δ, ppm): −101.8 d (4F, J 97 Hz), −134.9 quintet (1F, J 97 Hz) (SO₂·SbF₅). The ¹⁹F NMR assignments were made using COSY ¹⁹F–¹⁹F and the data of the paper.¹³

7-Dihydroxysulfonio-9,9,10-trimethylphenanthrenium dication (3b)

Solution of carbinol 2b^9b (20 mg, 0.084 mmol.) in 0.12 mL of CD₂Cl₂ was added to solution of FSO₃H (115 mg, 1.15 mmol) and SbF₅ (235 mg, 1.22 mmol) in 0.25 mL of SO₂ClF–SO₂ (1 : 17 by volume) placed into NMR tube at −95 °C, the mixture was stirred and then warmed to −71 °C. Formation of the mixture of cation 1b⁹ and dication 3b was observed, the content of the latter being 34%. Addition of SO₂ (0.1 mL) to the solution did not result in increase of the content. ¹H NMR spectrum (δ, ppm): 2.01 s (6H, 9-Me), 3.65 s (3H, 10-Me), 8.16 t (1H, H², J 7.7 Hz), 8.28 dd (1H, H⁶, J 8.8, 1.8 Hz), 8.58 d (1H, H⁸, J 1.8 Hz), 8.82 t (1H, H³, J 7.6 Hz), 8.89 d (1H, H⁴, J 8.1 Hz), 8.98 d (1H, H⁵, J 9.0 Hz), 9.03 d (1H, H¹, J 8.3 Hz), 9.86 s (2H, OH). ¹³C NMR spectrum: (δ, ppm): 27.8 q (9-Me), 26.7 q (10-Me), 54.6 s (C⁹), 126.3 d (C⁶), 127.0 d (C⁸), 127.7 d (C⁴), 129.4 d (C⁵), 133.8 d (C²), 133.5 s (C^4b), 135.7 s (C⁷), 133.8 s (C^10a), 140.2 d (C¹), 145.1 s (C^4a), 145.2 s (C^8a), 156.0 d (C³), 239.9 s (C¹⁰).

7-Dihydroxysulfonio-9,9-dimethyl-10-hydroxyphenanthrenium dication (3c)

Solution of ketone 5¹⁶ (20 mg, 0.090 mmol.) in 0.12 mL of CD₂Cl₂ was added to solution of FSO₃H (105 mg, 1.07 mmol) and SbF₅ (230 mg, 1.06 mmol) in 0.25 mL of SO₂ClF placed into NMR tube at −95 °C and the mixture was stirred. Formation of cation 1c¹⁰ (as an equilibrium mixture of E- and Z-isomers) was observed by NMR. Liquid SO₂ (0.05 mL) was added into the NMR tube, the resulting solution was mixed, kept at −70 °C for 5 h and then at −61 °C for 2 h. The reaction results in formation of a multicomponent mixture. The content of (E)-3c is approx. 35% and that of (Z)-3c is approx. 15%.

(E)-1c ¹H NMR spectrum (δ, ppm, −91 °C): 1.92 s (6H, 9-Me), 7.72 t (1H, H⁶, J 7.3 Hz), 7.82 t (1H, H⁷, J 8 Hz), 7.85 d (1H, H⁸, J 8 Hz), 7.85 t (1H, H², J 8 Hz), 8.44 t (1H, H³, J 7.7 Hz), 8.49 d (1H, H⁵, J 8.1 Hz), 8.61 d (1H, H⁴, J 8.5 Hz), 8.73 d (1H, H¹, J 8.1 Hz), 11.31 s (OH). ¹³C NMR spectrum: (δ, ppm, −91 °C): 27.6 q (9-Me), 46.8 s (C⁹), 121.4 s (C^10a), 125.5 d (C⁴), 125.9 d (C⁵), 126.0 s (C^4b), 127.3 d (C⁸), 129.5 d (C⁶), 130.7 d (C²), 133.2 d (C¹), 133.3 d (C⁷), 141.7 s (C^8a), 148.2 s (C^4a), 148.3 d (C³), 215.5 s (C¹⁰).

(Z)-1c ¹H NMR spectrum (δ, ppm, −91 °C): 1.96 s (6H, 9-Me), 7.88 d (1H, H⁸, J 7.9 Hz), 7.92 t (1H, H², J 7.5 Hz), 8.28 d (1H, H¹, J 8.1 Hz), 8.67 d (1H, H⁴, J 8.3 Hz), 11.92 s (OH); signals of H^3,5,6,7 are hidden by multiplets of the respective signals of E-isomer. ¹³C NMR spectrum: (δ, ppm, −91 °C): 28.9 q (9-Me), 47.8 s (C⁹), 120.2 s (C^10a), 126.4 d (C⁴), 127.1 d (C⁸), 129.3 d (C¹), 129.3 d (C⁶), 130.8 d (C²), 133.4 d (C⁷), 142.4 s (C^8a), 147.6 d (C³), 147.9 s (C^4a), 217.0 s (C¹⁰); signals of C^4b,5 are hidden by the respective signals of E-isomer.

(E)-3c ¹H NMR spectrum (δ, ppm, −91 °C): 2.00 s (6H, 9-Me), 8.04 t (1H, H², J 8 Hz), 8.24 d (1H, H⁶, J 9 Hz), 8.34 s (H⁸), 8.57 t (1H, H³, J 8 Hz), 8.71 d (1H, H⁴, J 8 Hz), 8.83 d (1H, H⁵, J 9 Hz), 8.86 d (1H, H¹, J 8 Hz), 9.72 s (2H, S(OH)₂), 12.22 s (OH). ¹³C NMR spectrum: (δ, ppm, −91 °C): 27.1 q (9-Me), 47.4 s (C⁹), 122.4 s (C^10a), 126.3 d (C⁸), 126.8 d (C⁴), 127.0 d (C⁶), 128.0 d (C⁵), 132.9 d (C²), 133.3 s (C^4b), 134.3 d (C¹), 134.9 s (C⁷), 142.8 s (C^8a), 143.5 s (C^4a), 148.6 d (C³), 216.1 s (C¹⁰).

(Z)-3c ¹H NMR spectrum (δ, ppm, −91 °C): 2.00 (9-Me), 8.10 (H²), 8.24 (H⁶), 8.36 (H⁸), 8.57 (H³), 8.75 (H⁴), 8.80 (H⁵), 8.47 (H¹), 9.71 (S(OH)₂), 12.82 (OH). The ¹³C NMR assignments for (Z)-3c are unavailable because of its low concentration.

Data on generation and attempts to sulfinate cations 1d,e are given in ESI, p. S20.†

Acknowledgements

We thank the Russian Academy of Sciences (Project 5.1.4) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra11673a

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