Preparation of dipyrrins from F-BODIPYs by treatment with methanesulfonic acids

Abstract

An alternative metal-free soft procedure for the preparation of dipyrrins from F-BODIPYs is reported. The new method makes possible to obtain certain dipyrrin derivatives that were unaccessible from F-BODIPYs to date. To demonstrate the ability of the new procedure, dipyrrins having highly reactive groups, such as chloro, cyano or acetoxyl, have been easily obtained from the corresponding F-BODIPY, which shows the synthetic utility of the reported methodology.

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Introduction

Difluoroboron-dipyrromethene (F-BODIPY) complexes (see 1 in Fig. 1) are widely known as valuable organic dyes for the development of useful photonic tools, such as fluorescent chemical sensors and probes, fluorescent security labels, dye-lasing systems, dyes for bioimaging, light harvesting materials, energy-converting antenna systems, or photodynamic therapy agents, among other interesting photonic applications.¹

Fig. 1 Dipyrrins by BF₂ removal in BODIPYs. Established procedures.

Beyond photonics, F-BODIPYs can be also employed as synthetic precursors in the preparation of dipyrrins (see 2 in Figure 1),² which are important molecules in medicinal chemistry and materials science (e.g., as synthetic intermediates for biologically active porphyrinoids, or for the design of specific cation- or anion-binding supramolecular systems).³ The importance of this synthetic use of F-BODIPYs is due to their higher chemical stability when compared to the corresponding dipyrrins, making possible chemical transformations on the BODIPY core which are impracticable on the dipyrrin.^2a,b In other words, the F-BODIPY difluoroboron (BF₂) group is a very convenient protecting/activating group to take into account when projecting the synthesis of a functionalized dipyrrin. Moreover, the so-obtained dipyrrins can be also used for preparing other BODIPYs (e.g., certain C-BODIPYs which cannot be synthesized directly from the corresponding parent F-BODIPYs by simple fluorine substitution).⁴ However, the known stability of F-BODIPYs, joined to the also known high instability of dipyrrins,^3a makes the decomplexation of F-BODIPYs a non-trivial process, especially when highly reactive dipyrrins are required.

Up to now, two main procedures are available for decomplexing F-BODIPYs (see Fig. 1): The useful “basic” method developed by Thompson and co-workers,^2a,b which involves strong basic conditions (e.g., potassium hydroxide or tert-butoxide in tert-BuOH under microwave, MW, irradiation), and the “Lewis-acid” method, developed first by Ravinkanth and co-workers (e.g., ZrCl₄ or TiCl₄ in refluxing MeCN/MeOH),^2e further improved by the Thompson group under softer conditions (e.g., BF₃ or BCl₃ in CH₂Cl₂ at room temperature, r.t.).^2f

However, the scope of these procedures is not general: the Ravinkanth method seems to be efficient only for 8-aryl-F-BODIPYs;^2e the basic Thompson procedure cannot be applied to F-BODIPYs having functional groups labile under the required strong basic conditions (e.g., halogen at the C3/5 BODIPY positions,^2d or easily enolizable functionalities^2b); the softer (r.t.) Lewis-acid Thompson procedure requires electron-rich enough F-BODIPYs (e.g., polyalkylated F-BODIPYs).^2f Moreover, metal cations are involved in all these procedures, and this can be a limitation when obtaining highly chelating dipyrrins. These drawbacks make necessary the establishment of an alternative wide-scope method for the preparation of dipyrrins from F-BODIPYs, taking place under soft enough reaction conditions and free of metal cations, as well. Herein, we report such a method, in which methanesulfonic acid or trifluoromethanesulfonic acid in methylene dichloride promotes the desired F-BODIPY decomplexation.

Results and discussion

Within the frame of our research work in the chemistry of BODIPYs,⁵ we found that methanesulfonic acid was able to remove the BF₂ group in electron-poor F-BODIPYs under very soft conditions (methylene dichloride at r.t.), despite the known stability of this kind of BODIPYs (electron poor) toward strong acids (e.g., trichloroacetic or trifluoroacetic acid) reported before.⁶ Oppositely to this statement, the electron-rich polyalkylated 1,3,5,7,8-pentamethyl-2,6-diethyl-F-BODIPY has been demonstrated to be unstable in the presence of di- or trichloroacetic acid.⁷ Related to the latter result, Yu et al. have recently reported BF₂ removal in certain 1,3,5,7-tetramethyl-8-aryl-F-BODIPYs by using trifluoroacetic acid or HCl in aqueous organic solvents,⁸ as it was also observed by Liras et al. in F-BODIPYs substituted with amino or amido groups.⁹ However, these procedures are expected to fail for F-BODIPYs with higher electron-poor character (F-BODIPY protonation must be involved in the decomplexation activation, see later), and they have been shown incompatible with certain hydrolyzable functionalities (e.g., acetamide).⁹

To prove the ability of methanesulfonic acids to activate the BF₂ removal in F-BODIPYs under soft conditions, we came interested in testing a difficult case: the decomplexation of dichlorinated 8-aryl-F-BODIPY 3¹⁰ (Fig. 2). Although the corresponding dichlorinated dipyrrin can be easily obtained by well-established methods from pyrrole derivatives,¹⁰ we chose the study of this case for three main reasons: (1) the high electron-poor character of 3, that should make it highly inert toward standard Lewis acid conditions; (2) the known S_NAr reactivity of 3 toward nucleophiles (chlorine substitution), due to the said electron-poor character; (3) the known instability of the corresponding dichlorinated dipyrrin.

Fig. 2 BF₂ removal in 3 (final water treatment was conducted in all cases).

Indeed, trying to remove the BF₂ group in 3 by the basic Thompson procedure affords decomposition (Fig. 2). This result was expected, due to the presence of chlorine atoms at the C3/5 positions of the starting F-BODIPY. These chlorine atoms must easily undergo S_NAr under the used strong basic reaction conditions.^2d,10 Besides, said chlorines would make the final dipyrrin highly reactive and therefore, unstable under the severe conditions employed for the removal.

On the other hand, 3 was recovered in 60% yield (isolated yield after chromatographic purification) when performing the softer Lewis-acid (BCl₃) Thompson procedure (Fig. 2). It must be noted here that the desired final dipyrrin was not detected by ¹H-NMR analysis of the crude mixture prior purification. The observed result is explained on the basis of the electron-poor character of 3 (note the strong electron-withdrawing inductive effect (−I) exerted by the chlorine atoms), which should diminish the Lewis basicity of its nitrogen centers.^2f

Trying to remove the BF₂ group of 3 by treatment with methanesulfonic acid was also unsuccessful. Thus, 3 does not react with this acid in CH₂Cl₂ solution, even under refluxing conditions (40 °C for 24 h; see Fig. 2). However, using the stronger trifluoromethanesulfonic acid, TfOH, at r.t. gave fairly place to the desired dichlorinated dipyrrin as the corresponding salt 4 in ca. quantitative yield (see Fig. 2).

The latter result clearly shows that the BF₂ removal is activated by the protonation of the F-BODIPY core (1), which requires a strong enough Brönsted acid due to its weak-base character (Fig. 3). After final hydrolysis (nucleophilic water attack to the electrophilic boron in the formed intermediate), the so-obtained free dipyrrin (2) would be protonated by a second mol equiv. of the strong acid present in the reaction medium, to generate the corresponding dipyrrin salt (isolated final product), whereas the formed BF₂OH would react with additional water molecules to generate B(OH)₃.¹¹

Fig. 3 Activation of F-BODIPY decomplexation by protonation.

Encouraged by the result obtained by 3, we decided to carry out a screening of BF₂ removal from a set of differently substituted F-BODIPYs using methanesulfonic and trifluoromethanesulfonic acids (Table 1). We observed that, in agreement with the proposed way of activation (i.e., involving an initial protonation), the use of a softer acid (methanesulfonic) led to the corresponding dipyrrin salt starting from alkyl F-BODIPYs 5a–d (see Table 1), due to the expected higher basic character of the involved F-BODIPY core when compared to 3 (note the expected different effect of the electron-withdrawing chlorines vs. the electron-donor alkyls on the basicity of the F-BODIPY core). The dipyrrins were obtained as the corresponding stable methanesulfonic acid salt.

Table 1 BF₂ removal in selected F-BODIPYs promoted by methanesulfonic acids

BODIPY	R^1	R^2	R^3	R	Met.^a	Dipyrrin salt (yield)
a Method A: methanesulfonic acid in refluxing CH2Cl2 overnight; Method B: TfOH in CH2Cl2 at r.t. overnight; Method C: TfOH in aqueous CH2Cl2 at r.t. overnight; Method D: TfOH in water at r.t. overnight.b 7 is the corresponding hydroxylated dipyrrin salt coming from the ester hydrolysis (6e/7 = 5/6 determined by ^1H NMR).
5a	Me	Me	H	Me	A	6a (92%)
5b	Me	Me	Et	Me	A	6b (99%)
				CF3	B	6b′ (98%)
				CF3	C	6b′ (98%)
				CF3	D	6b′ (trace)
5c	Nonyl	Me	tBu	Me	A	6c (97%)
5d	Me	Me	Bu	Me	A	6d (100%)
5e	CH2OAc	Me	Et	Me	A	No reaction
				CF3	B	6e/7^b (99%)
				CF3	C	Complex mixt.
5f	CN	Me	Et	Me	A	No reaction
				CF3	B	6f (98%)
5g	Mesityl	H	H	CF3	B	6g (98%)

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To check the expected presence of a single unit of organic acid in the structure of the obtained dipyrrin salts, the amount of fluorine in salt 6b (involving strong-acid and -base partners) was determined by ¹⁹F-NMR (p-trifluoromethylacetophenone was used as internal standard; see ESI†), showing a single unit of TfOH in its structure.

Also in agreement with the mechanism shown in Fig. 3, it was possible to remove the BF₂ group in the presence of water. Thus, 5b was converted to the corresponding dipyrrin salt by treatment with TfOH in aqueous CH₂Cl₂ (Method C) instead of dry CH₂Cl₂ (Method B). However, using water as the only solvent (Method D) noticeably reduces the effectiveness of the process, probably due to the insolubility of both the starting BODIPY and the final dipyrrin salt in water (see Table 1).

Noticeably, the essayed decomplexations of meso-alkyl-F-BODIPYs took place without detecting dypyrrin tautomerization to the corresponding 1,1-di-1H-pyrrol-2-ylethene derivative (Fig. 4), despite the known tautomerizable character of the involved meso alkyl groups.¹² In regard to this, it is known that the decomplexation of 5b under basic conditions affords the corresponding 1,1-dipyrrolylethene instead of the dipyrrin.^2b

Fig. 4 Dipyrrin – 1,1-di-1H-pyrrol-2-ylethene tautomerization in meso-methyldipyrrins.

Interestingly, the sulfonic acid-promoted decomplexation took place without tautomerization even for acetoxylated 5e (see Table 1), despite the higher reactivity towards tautomerization expected for its meso methylene group. However, TfOH instead of methanesulfonic acid was required to remove the BF₂ group in this case, probably due to the electron-withdrawing effect exerted by the acetoxyl group, which lowers the key BODIPY basicity. For the same reasons, the decomplexation of electronically deactivated (electron-poor) 5f required the use of TfOH, as well (see Table 1).

It is worth to mention that, despite the highly hydrolyzable character of the acetoxyl group of 5e, ca. 45% mol equiv. of this functional group remained unaltered (final product 6e) when using Method B for the removal process (see Table 1). However, when the aqueous-solvent procedure (Method C) is applied instead, a complex mixture of reaction products is obtained, being 6e and 7 minor products (see Table 1) as shown by the ^IH NMR analysis of the said mixture (see ESI†). This result clearly shows the interest of Methods A or B for straightforwardly decomplexing F-BODIPYs having easily hydrolyzable functions.

Significantly, it was also possible to remove the BF₂ group in electronically deactivated 5g (meso-aryl group as the only F-BODIPY substituent), in the absence of metals and under soft reaction conditions (Method B; see Table 1). It must be noted that, up to date, this kind of F-BODIPYs could only be decomplexed by using the Ravinkanth method,^2e but not under soft (r.t.) Lewis-acid conditions.^2f

Conclusions

In summary, methanesulfonic acids are able to remove BF₂ in F-BODIPYs under soft reaction conditions (r.t. or refluxing CH₂Cl₂), which constitutes a valuable alternative to obtain dipyrrins from F-BODIPYs.

The new procedure is efficient for both meso-alkyl and meso-aryl F-BODIPYs, and its main goals are: (1) possibility of undergoing BF₂ removal in electron-poor F-BODIPYs under metal-free soft acid conditions, (2) compatibility with labile functional groups (e.g., easily hydrolyzable functionalities), and (3) possibility of synthesizing highly reactive dipyrrins. We are convinced that the herein reported new procedure to obtain dipyrrins from F-BODIPYs has a great potential for the future development of new porphyrinoids and ion binding systems through dipyrrins having reactive groups which can only be straightforwardly introduced by using a F-BODIPY intermediate (e.g., by regiocontrolled S_NAr reactions in halo-F-BODIPYs).

Experimental

General methods

Common solvents were dried and distilled by standard procedures. All starting materials and reagents were obtained commercially and used without further purifications. NMR spectra were recorded at 20 °C and the residual solvent peaks were used as internal standards. High resolution mass spectrometry (HRMS) was performed using electrospray ionization. Fourier transform infrared (FTIR) spectra were recorded without solving the sample, by using the attenuated total reflection (ATR) technique.

General synthetic procedures

Method A. Methanesulfonic acid (480 mg, 1.00 mmol) was added to the corresponding F-BODIPY (0.05 mmol) in CH₂Cl₂ (2 mL). The resulting mixture was stirred at 40 °C for 24 h. Then, the reaction mixture was allowed to cool to room temperature and them H₂O was added (5 mL). The layers were separated and the aqueous layer was extracted with CH₂Cl₂ (2 × 5 mL). The combined organic layers were dried over anhydrous Na₂SO₄ and the solvent was removed under reduced pressure to obtain the corresponding pure dipyrrin, as pyrrolium methanesulfonate salt (the known chemical instability of the free dipyrrins usually makes necessary their isolation as acid salts^2a,b). If further purification is necessary, it can be achieved by washing the salt with a proper solvent (e.g., cold pentane).

Method B. Analogous to Method A, but using TfOH (150 mg, 1.00 mmol) instead of methanesulfonic acid, and conducting the reaction at room temperature.

Method C. Analogous to Method B, but using aqueous CH₂Cl₂/water (40 : 1 in volume) instead of CH₂Cl₂.

Method D. Analogous to Method B, but using water instead of CH₂Cl₂.

(2Z)-5-Chloro-2-[(5-chloro-1H-pyrrol-2-yl)(4-methylphen-yl)methylene]-2H-pyrrolium trifluoromethanesulfonate (4). Prepared according to Method B. Bright orange solid. 20.8 mg (92%). Mp: 194–199 °C. ¹H NMR (CDCl_3, 300 MHz) δ 7.33 (d, J = 7.9 Hz, 2H), 7.25 (d, J = 7.9 Hz, 2H), 6.57 (d, J = 4.3 Hz, 2H), 6.26 (d, J = 4.3 Hz, 2H), 2.44 (s, 3H, CH₃), ppm. ¹³C NMR (CDCl_3, 75 MHz) δ 141.5, 139.6, 138.3, 132.5, 130.9, 130.2, 128.6, 116.8, 21.4 (Me) ppm. ¹⁹F NMR (CDCl_3, 282 MHz) δ −78.53 (s, 3F) ppm. ATR-FTIR ν 3233, 2924, 1578, 1443, 1424, 1366, 1343, 1316, 1252, 1038 cm⁻¹. HRMS (m/z): [M + H]⁺ calcd for C₁₆H₁₃Cl₂N₂ 330.0452 found 330.0444.

(2Z)-2-[1-(3,5-Dimethyl-1H-pyrrol-2-yl)ethylidene]-3,5-di-methyl-2H-pyrrolium methanesulfonate (6a). Prepared according to Method A. Bright orange solid. 14.3 mg (92%). Mp: 160–163 °C. ¹H NMR (CDCl_3, 300 MHz) δ 11.62 (br s, 2H), 6.18 (s, 2H), 2.77 (s, 3H), 2.76 (s, 3H), 2.47 (s, 6H), 2.13 (s, 6H) ppm. ¹³C NMR (CDCl_3, 75 MHz) δ 151.2, 149.3, 141.3, 132.3, 119.3, 39.5, 22.6, 14.4, 13.9 ppm. ATR-FTIR ν 3186, 2925, 1546, 1449, 1294, 1179, 1040, 973 cm⁻¹. HRMS (m/z): [M + H]⁺ calcd for C₁₄H₁₉N₂ 215.1541 found 215.1533.

(2Z)-4-Ethyl-2-[1-(4-ethyl-3,5-dimethyl-1H-pyrrol-2-yl)eth-ylidene]-3,5-dimethyl-2H-pyrrolium methanesulfonate (6b). Prepared according to Method A. Bright red solid. 18.1 mg (99%). Mp: 154–157 °C. ¹H NMR (CDCl_3, 300 MHz) δ 11.87 (br s, 2H), 2.82 (s, 3H), 2.79 (s, 3H), 2.46 (s, 6H), 2.39 (q, J = 7.5 Hz, 4H), 1.99 (s, 6H), 1.04 (t, J = 7.5 Hz, 6H) ppm. ¹³C NMR (CDCl_3, 75 MHz) δ 148.8, 146.9, 137.0, 132.0, 131.6, 39.7, 23.2, 17.5, 14.4, 12.2, 11.7 ppm. ATR-FTIR ν 3169, 2927, 1570, 1514, 1304, 1226, 1178, 1062, 1041, 958 cm⁻¹. HRMS (m/z): [M + H]⁺ calcd for C₁₈H₂₇N₂ 271.2169 found 271.2163.

(2Z)-4-Ethyl-2-[1-(4-ethyl-3,5-dimethyl-1H-pyrrol-2-yl)eth-ylidene]-3,5-dimethyl-2H-pyrrolium trifluoromethanenesulfo-nate (6b′). Prepared according to Method B. Bright red solid. 20.6 mg (98%). Mp: 139–143 °C. ¹H NMR (CDCl_3, 300 MHz) δ 10.97 (br s, 2H), 2.75 (s, 3H), 2.48–2.37 (m, 10H), 2.03 (s, 6H), 1.07 (m, 6H) ppm. ¹³C NMR (CDCl_3, 75 MHz) δ 149.1, 146.2, 137.6, 132.1, 131.8, 22.5, 17.5, 14.4, 12.1, 11.8 ppm. ¹⁹F NMR (CDCl_3, 282 MHz) δ −78.73 (s, 3F) ppm. ATR-FTIR ν 3229, 2966, 2928, 1569, 1514, 1290, 1224, 1161, 1031, 958 cm⁻¹. HRMS (m/z): [M + H]⁺ calcd for C₁₈H₂₇N₂ 271.2169 found 271.2158.

(2Z)-4-tert-Butyl-2-[1-(4-tert-butyl-3,5-dimethyl-1H-pyrr-ol-2-yl)decylidene]-3,5-dimethyl-2H-pyrrolium methanesulfo-nate (6c). Prepared according to Method A. Bright red solid. 25.9 mg (97%). Mp: 161–165 °C. ¹H NMR (CDCl_3, 300 MHz) δ 11.91 (br s, 2H), 3.28 (t. J = 7.1 Hz, 2H), 2.77 (s, 3H), 2.68 (s, 6H), 2.09 (s, 6H), 1.40 (s, 18H), 1.45–1.10 (m, 14H), 0.85 (t, J = 7.2 Hz, 3H) ppm. ¹³C NMR (CDCl₃, 75 MHz) δ 152.1, 148.7, 136.9, 136.8, 132.2, 36.3, 33.3, 31.9, 31.6, 31.4, 29.7, 29.4, 29.3, 29.2, 22.6, 17.0, 14.9, 14.1 ppm. ATR-FTIR ν 3181, 2925, 1557, 1464, 1291, 1172, 1042, 970 cm⁻¹. HRMS-ESI (m/z): [M + H]⁺ calcd for C₃₀H₅₁N₂ 439.4049 found 439.4037.

(2Z)-4-Butyl-2-[1-(4-butyl-3,5-dimethyl-1H-pyrrol-2-yl)eth-ylidene]-3,5-dimethyl-2H-pyrrolium methanesulfonate (6d). Prepared according to Method A. Bright orange solid. 21.1 mg (ca. quantitative). Mp: 159–162 °C. ¹H NMR (CDCl_3, 300 MHz) δ 12.00 (br s, 2H), 2.84 (s, 3H), 2.80 (s, 3H), 2.47 (s, 6H), 2.36 (t, J = 7.3 Hz, 4H), 1.98 (s, 6H), 1.45–1.22 (m, 8H), 0.92 (t, J = 7.1 Hz, 6H) ppm. ¹³C NMR (CDCl_3, 75 MHz) δ 149.0, 146.9, 137.2, 132.0, 130.2, 39.8, 32.1, 24.0, 23.5, 22.5, 13.9, 12.3, 11.9 ppm. ATR-FTIR ν 3163, 2956, 2929, 1570, 1546, 1513, 1442, 1419, 1307, 1177, 1041, 924 cm⁻¹. HRMS (m/z): [M + H]⁺ calcd for C₂₂H₃₅N₂ 327.2795 found 327.2787.

(2E)-2-[2-(Acetyloxy)-1-(4-ethyl-3,5-dimethyl-1H-pyrrol-2-yl)ethylidene]-4-ethyl-3,5-dimethyl-2H-pyrrolium trifluoro-methanesulfonate (6e) and (2E)-4-ethyl-2-(1-(4-ethyl-3,5-dimethyl-1H-pyrrol-2-yl)-2-hydroxyethylidene)-3,5-dimethyl-2H-pyrrolium trifluoromethanesulfonate (7). Prepared according to Method B. 22.5 mg, mixture of 6e (44%) and 7 (55%). ¹H NMR (mixture, CDCl_3, 300 MHz) δ 11.03 (br s, 2H), 10.97 (br s, 2.4), 5.38 (s, 2H), 4.98 (s, 2.4H), 2.47 (s, 6H), 2.49–2.39 (m, 8.8H), 2.37 (s, 7.2H), 2.07 (s, 3H), 2.04 (s, 6H), 2.02 (s, 7.2H), 1.10 (t, J = 7.5 Hz, 6H), 1.08 (t, J = 7.5 Hz, 7.2H) ppm. ¹³C NMR (mixture, CDCl_3, 75 MHz) δ 170.0, 151.6, 150.0, 145.8, 139.1, 138.8, 133.8, 132.9, 130.5, 130.1, 120.2 (q, J = 319.0 Hz), 62.5, 61.6, 20.7, 17.6, 17.5, 14.3, 14.2, 12.6, 12.2, 11.7, 11.6 ppm. ¹⁹F NMR (CDCl_3, 282 MHz) δ −78.69 (s, 3F) ppm.

(2E)-2-[Cyano(3,4,5-trimethyl-1H-pyrrol-2-yl)methylene]-3,4,5-trimethyl-2H-pyrrolium trifluoromethanesulfonate (6f). Prepared according to Method B. Bright purple solid. 19.8 mg (98%). Mp: 177–179 °C. ¹H NMR (CDCl_3, 300 MHz) δ 11.10 (br s, 2H), 2.59 (s, 6H), 2.49 (s, 6H), 2.04 (s, 6H) ppm. ¹³C NMR (CDCl_3, 75 MHz) δ 158.0, 142.1, 129.2, 129.0, 120.1 (q, J = 319.4 Hz), 114.5, 105.2, 13.4, 12.8, 9.2 ppm. ¹⁹F NMR (CDCl_3, 282 MHz) δ −78.67 (s, 3F) ppm. ATR-FTIR ν 3210, 2925, 2197, 1613, 1565, 1287, 1242, 1157, 1029, 940 cm⁻¹. HRMS (m/z) [M + H]⁺ calcd for C₁₆H₂₈N₃ 254.1650 found 254.1642.

(2Z)-2-[(Mesityl)(1H-pyrrol-2-yl)methylene]-2H-pyrrolium trifluoromethanesulfonate (6g). The title compound was prepared according to Method B. Bright green solid. 20.2 mg (98%). Mp: 174–176 °C. ¹H NMR (CDCl_3, 300 MHz) δ 12.22 (br s, 2H), 8.08 (s, 2H), 6.98 (s, 2H), 6.77 (m, 2H), 6.61 (m, 2H), 2.38 (s, 3H), 2.03 (s, 6H) ppm. ¹³C NMR (CDCl_3, 75 MHz) δ 149.9, 144.7, 139.9, 136.7, 135.5, 132.4, 131.6, 128.4, 118.1, 21.1, 19.7 ppm. ¹⁹F NMR (CDCl_3, 282 MHz) δ −78.47 (s, 3F) ppm. ATR-FTIR ν 3194, 2927, 1565, 1449, 1331, 1284, 1250, 1038 cm⁻¹. HRMS (m/z) [M + H]⁺ calcd for C₁₈H₁₉N₂ 263.1543 found 263.1533.

Acknowledgements

Financial support from the MINECO (grant MAT2014-51937-C3-2-P) and UCM (grants GR3/14-910107 and-910150) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H, ¹³C and ¹⁹F NMR spectra. See DOI: 10.1039/c5ra14914b

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