8-[4-(2-Hydroxypropane-2-yl)phenyl]-1,3,4,4,5,7-hexamethyl-4-boron-3a,4a-diaza-S-indacene

Abstract

During recent years, the BODIPY core became a popular scaffold for designing photoremovable protecting groups (PPG). In this paper, we report the synthesis of a new molecule—8-[4-(2-hydroxypropane-2-yl)phenyl]-1,3,4,4,5,7-hexamethyl-4-boron-3a,4a-diaza-S-indacene—by the treatment of meso-(4-CO₂Me-phenyl)-BODIPY with excess of MeMgI. The product was characterized by ¹H, ¹³C NMR and HRMS. The combination of BODIPY core with tertiary benzilyc alcohol might be promising for utilizing this molecule as visible light removable PPG.

1. Introduction

Boron-dipyrromethene (4,4-difluoro-4-bora-3a,4a-diaza-S-indacene, BODIPY) dyes have been intensively studied in recent years. Their unique photochemical properties, such as narrow absorption bands with tunable wavelength, render these dyes promising in various applications, from optoelectronics to life sciences (see [1] and references therein).

Since seminal work of Winstein’s and Winter’s groups in 2015 [2,3], meso-CH₂OH-BODIPY photoremovable protecting groups (PPGs) gained much popularity for the design of caged compounds, which enable the light-controlled release of small molecules, including those that are biologically relevant [4,5,6]. For biological applications, the light dose should be minimized due to potential phototoxic effects, emphasizing the need for high quantum yield (QY) of photorelease. However, for BODIPY-based PPG, low QYs are commonly observed but could be increased by the introduction of heavy iodine atoms or by changing fluorine atoms with methyl groups (Figure 1A) [7,8]. The mechanism of photorelease is suggested to proceed from the formation of cation 1 [2] by heterolysis of C-X bond in an excited state of BODIPY (Figure 1B). We hypothesized that additional stabilization of the carbocationic center by phenyl conjugation or the addition of methyl groups could be favorable for increasing the effectiveness of the photorelease.

In this paper, we describe the synthesis of 8-[4-(2-hydroxypropane-2-yl)phenyl]-1,3,4,4,5,7-hexamethyl-4-boron-3a,4a-diaza-s-indacene, a BODIPY with fluorine atoms exchanged for methyl groups bearing tertiary benzylic alcohol. Derivatives of this compound potentially may serve as photoremovable protective groups.

2. Results and Discussion

One of the common approaches to the construction of BODIPY core is the one pot procedure starting with the condensation of pyrroles and aldehydes under acidic catalysis, subsequent oxidation with DDQ or chloranil and, finally, treatment with NEt₃ or DIPEA followed by the addition of BF₃∙Et₂O [9]. We started our work with the synthesis of aldehyde 3 by reactions of MeMgI with dimethylacetal 2, which was prepared from 4-formylbenzoic acid and methanol/SOCl₂ (Scheme 1). Nevertheless, the application of the above-mentioned procedure of BODIPY synthesis to aldehyde 3 resulted in a complex mixture of products according to ¹⁹F spectrum. The problems may arise from dehydration of tertiary alcohol with the formation of carbocation, which also might be active in reactions with pyrrole.

At the next step, we synthesized BODIPY 4 according to the literature procedure [10]. The reaction of BODIPY 4 with excess of MeMgI resulted in the exchange of fluorines to methyl groups and the transformation of CO₂Me to 2-hydroxyprop-2-yl fragments (Scheme 2). BODIPY 5 was obtained in good yield.

The structure of BODIPY 5 was confirmed by NMR studies (Supplementary Materials). In ¹H NMR, spectrum signals of 1,4-disubstituted benzene were observed at 7.25 and 7.57 ppm with coupling constant J = 8.3 Hz. Pyrrole fragments produce singlet signals at 5.96 ppm for C-H and at 1.32 and 1.62 ppm for CH₃-groups. Moreover, the signals of CH₃ groups of C(CH₃)₂OH fragment are observed at 2.47 ppm, and B-CH₃ groups have up-field shifted signals at 0.27 ppm. The OH-group produced a broad singlet signal at 1.27 ppm. In the ¹³C NMR spectrum, all signals of aromatic and heterocyclic fragments could be observed in range of 121–153 ppm, and alkyl fragments produce signals at 14–73 ppm region. However, the signals of B(CH₃)₂ fragment are not visible in the ¹³C spectrum. The same peculiarity was previously described for other BODIPY containing B-C bond [11].

To characterize optical properties of 5, we first measured the absorption spectrum of its 1 μM solution in DMSO. Figure 2A shows the obtained result. The maximum absorption wavelength in DMSO is λ_max = 503 nm, with the absorbance 0.093, producing the molar extinction coefficient ε(λ_max) = 93,000 M⁻¹cm⁻¹. In water with 1% DMSO, the absorption peak is blue-shifted and wider, probably indicating the formation of aggregates [12,13].

Figure 2B shows the emission spectra of the same solutions excited at 500 nm. For comparison, we also show a fluorescence spectrum of Rhodamine 6G solution in DMSO, with the absorption at 500 nm matched to the sample of 5 in DMSO. The known fluorescence QY of Rhodamine 6G (0.56; [14]) allows us to estimate the fluorescence QY of 5 in DMSO to be 0.33.

3. Materials and Methods

All chemicals were purchased from commercial sources and used without additional purification unless otherwise noted. The progress of reactions was monitored by thin-layer chromatography (TLC) on Sorbfil Silica 60 F254 on aluminum sheets with UV visualization. Column chromatography was performed by using 100–200 mesh silica gel. NMR spectra were recorded on Bruker Avance-300 (300.13 MHz for ¹H) and Avance-400 (400.13 MHz for ¹H, 100.62 MHz for ¹³C) spectrometers; chemical shifts of ¹H and ¹³C{¹H} are provided in ppm, with solvent signals serving as the internal standard (¹H = 7.24 ppm and ¹³C = 77.16 ppm for CDCl₃). The masses of molecular ions were determined by high-resolution mass spectrometry (HRMS) by means of a DFS Thermo Scientific instrument (EI, 70 eV). The UV–Vis spectrum was registered with a Shimadzu UV-1900 spectrophotometer, and fluorescence spectra were obtained on a Shimadzu RF-6000 fluorometer for 10⁻⁵–10⁻⁶ M solutions in DMSO.

• Methyl 4-(dimethoxymethyl)benzoate (2).

To a solution of 7.50 g (0.05 mol) 4-formylbenzoic acid in 150 mL of dry methanol cooled to 0 °C, 30 mL (0.41 mol) of thionyl chloride was added dropwise. The resultant solution was kept at 0 °C for 3 days and then evaporated in vacuo to dryness. Colorless crystals were obtained at 10.50 g (~100%).

¹H NMR (δ, 400 MHz, CDCl₃): 3.29 (s, 6H), 3.88 (s, 3H), 5.41 (s, 1H), 7.50 (d, J = 7.8 MHz, 2H) and 8.01 (d, J = 7.8 MHz, 2H). The spectrum is in agreement with literature data [15].

• 4-(2-hydroxypropan-2-yl)benzaldehyde (3).

To an ice-cooled solution of CH₃MgI prepared from 3.70 g (0.16 mol) of Mg and 21.15 g (0.15 mol) of CH₃I in 100 mL of dry diethyl ether, a solution of 10.50 g (0.05 mol) ester 1 in 20 mL of dry Et₂O was added. After the addition, the ice bath was removed, and the mixture was refluxed for 1h. The resulting slurry was quenched with 5% HCl in water and kept with intensive stirring for 12 h. Then, the ether layer was separated, and the aqueous layer was extracted with ether (2 × 50 mL). Combined ether fractions were dried over Na₂SO₄, filtered and evaporated under reduced pressure. The residue was dissolved in CHCl₃ and passed through a silica pad. After removal of CHCl₃, 6.40 g (78%) of yellowish oil was obtained.

¹H NMR (δ, 400 MHz, CDCl₃): 1.60 (s, 6H), 3.58 (br s, 1H), 7.54 (d, 2H), 7.68 (d, 2H) and 9.80 (s, 1H). The spectrum is in agreement with literature data [16].

• Interaction of aldehyde 2 with 2,4-dimethylpyrrole.

A solution of 1.20 g (12.6 mmol) of pyrrole and 1.00 g (6.1 mmol) of aldehyde 2 in 200 mL of dry CH₂Cl₂ was degassed with the “freeze-pump-thaw” technique, and then 2 drops of TFA was added. The resultant solution was stirred ~2 h until full consumption of aldehyde according TLC was achieved. Then, 1.67 g (7.3 mmol) of DDQ was added in small portions, and the mixture was additionally stirred for 30 min. After that, the mixture was cooled to 0 °C, and 20 mL of NEt3 was added followed with 20 mL BF₃∙Et₂O. The resultant solution was kept at RT overnight with stirring. Then, it was quenched with 100 mL of H₂O, and the organic phase was separated, washed with 100 mL H₂O, 100 mL 5% HCl in H₂O and 100 mL of saturated Na₂CO₃ and dried over Na₂SO₄. After the removal of CH₂Cl₂, the residue was analyzed by ¹⁹F NMR. The complex mixture of BODIPY derivatives was observed. The analysis of isolated fractions has not revealed any useful products.

• 4,4-difluoro-8-[4-(methoxycarbonyl)phenyl]-1,3,5,7-tetramethyl-4-boron-3a,4a-diaza-s-indacene (4).

BODIPY 3 was prepared according to published procedure [10].

¹H NMR (δ, 300 MHz, CDCl₃): 1.33 (s, 6H), 2.53 (s, 6H), 3.94 (s, 3H), 5.96 (s, 2H), 7.37 (d, J = 8.1 Hz, 2H) and 8.15 (d, J = 8.1 Hz, 2H). The spectrum is in agreement with literature data: https://doi.org/10.1021/acs.inorgchem.1c01279 (Accessed on 15 September 2021).

¹⁹F NMR (δ, 282 MHz, CDCl₃): −149.4 (q, J = 32.8 Hz, 2F).

• 8-[4-(2-hydroxypropane-2-yl)phenyl]-1,3,4,4,5,7-hexamethyl-4-boron-3a,4a-diaza-s-indacene (5).

To a solution of 0.0320 g (0.084 mmol) of BODIPY 4 in 20 mL of dry diethyl ether, 0.28 mL (0.84 mmol) of 3M solution of MeMgI in diethyl ether was added under argon atmosphere. The mixture was stirred for 4 h and then quenched with a saturated solution of NH₄Cl in water. The ether was separated and dried over Na₂SO₄ and removed. The resultant residue was dissolved in CH₂Cl₂ and passed through a pad of Al₂O₃. The evaporation of solvent produced dark red crystals, 0.0210 g (67%). Decomposition was >143 °C.

¹H NMR (δ, 300 MHz, CDCl₃): 0.25 (s, 6H), 1.27 (br s, 1H), 1.32 (s, 6H), 1.62 (s, 6H), 2.47 (s, 6H), 5.96 (s, 2H), 7.25 (d, J = 8.3 Hz, 2H) and 7.57 (d, J = 8.3 Hz, 2H).

¹³C NMR (δ, 101 MHz, CDCl₃): 14.7, 16.6, 32.2, 72.7, 121.7, 124.9, 128.3, 130.0, 134.8, 139.0, 142.2, 149.8 and 152.2 (B-CH₃ groups are not visible in the spectrum).

HRMS (EI, m/z): calculated for C₂₄H₃₁O₁N₂¹¹B—374.2529; found—374.2533.