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Flexible BODIPY Platform that Offers an Unexpected Regioselective Heterocyclization Reaction Toward Preparation of 2-Pyridone[a]-Fused BODIPYs Natalia O. Didukh, Viktor P. Yakubovskyi, Yuriy V. Zatsikha, Gregory T. Rohde, Victor N. Nemykin, and Yuriy P. Kovtun J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b03119 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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The Journal of Organic Chemistry

Flexible BODIPY Platform that Offers an Unexpected Regioselective Heterocyclization Reaction Toward Preparation of 2-Pyridone[a]-Fused BODIPYs

Natalia O. Didukh,a,b Viktor P. Yakubovskyi,a Yuriy V. Zatsikha,b Gregory T. Rohde,c Victor N. Nemykin,*b Yuriy P. Kovtun*a

a

Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmanska str.,

02660 Kyiv, Ukraine. b

Department of Chemistry, University of Manitoba, Winnipeg, MB, R3T 2N2, Canada.

c

Marshall School, Duluth, MN, 55811 USA

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Abstract We have explored the synthetic routes for regioselective formation of 2-pyridone[a]- and 2pyridone[b]-fused BODIPYs using 1,3,5,7-tetramethyl-2,6-dicarbethoxy-BODIPY as the universal starting platform. While heterocyclization of the 3-(dimethylaminovinyl)-BODIPY and 3,5bis(dimethylaminovinyl)-BODIPY results in the formation of mono-2-pyridone- and bis-2pyridone[b]-fused

BODIPYs,

respectively,

similar

heterocyclization

of

the

1,3-

bis(dimethylaminovinyl)-BODIPY leads to the regioselective formation of the 2-pyridone[a]-fused BODIPY core, which is the first example of heterocycle[a]-fused BODIPYs. The regioselective formation of the 2-pyridone[a]-fused BODIPY was further confirmed by X-ray crystallography and explained on a basis of the DFT and TDDFT calculations that are suggestive of the energy-favorable out-of-plane rotation of the dimethylaminovinyl group located at first position, which accelerates reaction with n-butylamine. Trends in the UV-vis and fluorescence spectra of the BODIPYs 1 – 17 were discussed on the basis of DFT and TDDFT calculations.


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INTRODUCTION Dyes derived from a boron dipyrromethene (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene or BODIPY) have been attracting considerable attention over the past decades because of their excellent thermal, chemical, and photochemical stability, high molar absorption coefficients, high fluorescence quantum yields, tunable optical properties, and a large two-photon cross-section for multiphoton excitation.1-13 Tuning the optical and redox properties of the BODIPYs requires development of a large scope of synthetic procedures for regioselective chemical modification at specific position(s) of the BODIPY core. Not surprisingly, a variety of the chemical transformations of the BODIPY core that include formylation,14,15 Knoevenagel condensation,16-19 halogenation,20-22 cyanination,23-25 nucleophilic substitution,26,27 coupling,28-30 and substitution at the boron hub31-33 have been developed in recent years. One of the most effective synthetic strategies that allows incremental optical tuning of the optical properties of BODIPY system is the annulation of the BODIPY core using - ([b]fused BODIPYs) or ’-carbon atoms ([a]-fused BODIPYs) that can be fused either with aromatic benzene or heterocyclic rings.34,35 Such benzo- or heterocyclic fusion leads to the formation of a rigid, extended -system with different optical and tunable fluorescence properties. For instance, simultaneous benzo-[b]-fusion of the BODIPY core (I, Chart 1) leads to the red shift of the most intense, low-energy BODIPY transition with dramatic reduction of both molar extinction coefficient and fluorescence quantum yield when compared to the parent BODIPY.36,37 On the contrary, prepared by Suzuki and coworkers,38,39 furan-[b]-fused BODIPY II (known as Keio Fluors) exhibits a very strong fluorescence and similar to BODIPY I low-energy optical shift with a very large intensity increase. A large array of such type dyes is obtained up to day.40-53 Additional benzoannulation of II with one- (III) or two (IV) conjugated benzene rings leads to a further red shift of the low-energy BODIPY transition that is accompanied by a significant reduction of the fluorescence quantum yields.54 In our previous work, we have demonstrated that the BODIPY core V (Chart 1) provides a very high degree of flexibility for tuning its redox, optical, and fluorescence properties.23,24,55-63 Indeed, active -methyl groups in V can be easily introduced to Knoevenagel condensation61-63 as ACS Paragon Plus Environment


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well as formylation reaction.55 In the latter case, mono- and bis-diacetaldehyde derivatives can be used for cyclocondensation reactions with ester group or boron hub.55-60 In particular, heterocyclization reaction with ester group results in the formation of mono- or bis(2-pyridone)containing BODIPYs (BODIPY VI, Chart 1).64 Unlike benzo-analogue I, BODIPY VI is highly fluorescent, has an intense BODIPY-centered low-energy transition, and compared to I and II undergoes the largest red-shift, which is close to BODIPY III. Moreover, the further significant decrease in the optical gap of BODIPY VI can be achieved by the introduction of the cyano group into its meso-position.23,24

Chart 1.


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Compared to the benzo[b]-fusion, the benzo[a]-fusion leads to the “isoindole” type BODIPY chromophore (for instance BODIPY VII) that has a further red-shifted (compared to the parent BODIPY I) BODIPY absorption of higher intensity and much higher fluorescence quantum yield.34,65-73 The heterocyclic analogous of “isoindole” type BODIPYs VII are unknown, and thus the difference between optical properties of the homologue series of [a]-fused and [b]-fused heterocyclic BODIPYs has not been evaluated. Since phenyl groups in BODIPY platform V preclude any heterocyclization reactions needed for [a]-fusion, we were hoping that its tetramethyl-containing analogue 1 (Scheme 1) can be used for the formation of 2-pyridone[a]-fused and 2-pyridone[b]-fused BODIPYs that would allow us, for the first time, investigate the influence of the fusion position on the optical properties of heterocycle-containing BODIPY core. Although BODIPY core 1 has been known since 1968,74 it was not intensively used as a starting platform for BODIPY core modification perhaps because of its relatively low stability in chemical reactions. To our large surprise discussed below, 2-pyridone[a]-fused BODIPY core results in a larger, not smaller optical gap, which contrasts general trends for benzo[a]-fused and benzo[b]-fused BODIPYs I and VII (Chart 1).

RESULTS AND DISCUSSION

The initial rationale for using of tetramethyl-containing BODIPY 1 was to explore and compare the reactivity of the methyl groups at 1,7- versus 3,5-positions. Although 1,3,5,7tetramethyl-2,6-dicarbethoxy-BODIPY 1 is known from 1968,74 to our surprise, this platform was mentioned only in four publications so far.74-77 Moreover, while the chemical transformations of the diphenyl-analogue V are quite reach and well-explored, only a single paper on BODIPY 1 discussed direct nitromethylation of its meso-position and thus reactivity of four methyl groups in 1 remains completely unexplored.75 This is even more surprising taking into consideration commercial availability and easiness of preparation of the 2,4-dimethyl-3-carbethoxypyrrole, which is a key precursor for synthesis of BODIPY 1. Thus, we carefully explored chemistry of the BODIPY 1. First, ACS Paragon Plus Environment


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we tested a standard Knoevenagel reaction between BODIPY 1 and several aromatic aldehydes since up to four methyl groups can be involved into condensation reaction.78-80 To our surprise, under typical Knoevenagel reaction conditions (piperidine/acetic acid in boiling benzene or toluene) BODIPY 1 undergoes fast degradation with elimination of the chromophoric -system. In a separate control experiment, it was found that the BODIPY 1 even in piperidine/acetic acid/solvent system (no aldehyde added) loses BF2 fragment and undergoes slow degradation, which contrasts a chemistry established for a stable BODIPY V. Further control experiments have shown that the BODIPY 1 is quite sensitive toward organic and inorganic bases which narrows a scope of the chemical reactions accessible for 1 and might explain a lack of the reports that explore chemistry of this BODIPY core. Relatively low stability of the BODIPY 1 cannot be explained by electronic effects as the more electron-donating methyl groups in 1 should stabilize this core toward deborylation reaction compared to V, which is not the case. Although one might speculate that the partial conjugation of the phenyl groups in V can be responsible for its higher stability (the torsion angle between phenyl group and the pyrrolic heterocycle in available X-ray crystal structures of 1,7-diphenyl substituted BODIPYs varies between 39.8 and 56.80) at this time we have no clear proof of this hypothesis.

Scheme 1. Preparation of 2-pyridone[b]-fused BODIPY 5 from 1. Regents and conditions: (i) DMA DMF, C2H4Cl2, 3h reflux; (ii) 40% aq. AcOH, 2 h reflux; (iii) CHCl3, AcOH, n-BuNH2, 20 min; (iv) i-PrOH, AcONa, reflux overnight; (v) MeCN, n-BuNH2, 40 min, r.t. ACS Paragon Plus Environment

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Despite the lower stability and sensitivity of BODIPY 1 toward bases, it still can be involved into a variety of chemical transformations that results in preparation of 2-pyridone[b]-fused and more importantly 2-pyridone[a]-fused BODIPYs outlined in Schemes 1 – 3. Indeed, the reaction between BODIPY 1 and DMF dimethyl acetal conducted under mild conditions results in the formation of enamine-containing BODIPY 2 (Scheme 1). Hydrolysis of the BODIPY 2 in acidic media leads to the formation of aldehyde 3, which is unlike its 1,7-diphenyl analogue V55 exists only as aldehyde (no enol tautomeric form was observed in the 1H NMR spectrum of 3). Reaction between aldehyde 3 and n-butylamine in chloroform at room temperature leads to the formation of enamine 4, which undergoes heterocyclization reaction and forms the target BODIPY 5. BODIPY 5 can also be prepared by the direct reaction between enamine 2 and n-butylamine in acetonitrile but with significantly lower yield (Scheme 1). Next, we were expected that the monopyridone-containing BODIPY 5 can be further transformed into dipyridone BODIPY 11 (Scheme 2) by a second aminoformylation reaction with the remaining 5-methyl group in 5 that would follow chemistry similar to that outlined in Scheme 1 and proven in formation of dipyridone derivative of BODIPY V.58 However, the reaction between BODIPY 5 and DMF dimethyl acetal was found to produce several unstable dyes that cannot be separated by conventional methods in reasonable yields. Thus, the target BODIPY 11 was prepared according to a general synthetic approach56 outlined in Scheme 2. All synthetic steps shown in Scheme 2 proceed smoothly and allow preparation of the target BODIPY 11 in a reasonable yield. Again, aldehydes 8 and 10, that were prepared by stepwise hydrolysis of BODIPYs 7 and 9, respectively, exist entirely in aldehyde tautomeric form without any detectable by the NMR spectroscopy enol tautomeric form. We also tried to form bis-2-pyridone[b]-fused BODIPY 11 directly from 9 (that can be obtained in a single step from 7) without much success.



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Scheme 2. Synthetic route for preparation of bis-2-pyridone[b]-fused BODIPY 11. Reagents and conditions: (i) Ac2O, DMF, 110 ℃, 2h; (ii) DMA DMF, Ac2O, toluene, 3 min reflux; (iii) TFA, H2O, reflux, 20 min; (iv) 40% aq. AcOH, reflux 20 min; (v) n-BuNH2, 3 h, r.t.; (vi) MeCN, (CH3)2NH, reflux 3 min.

During preparation of the enamine 2 from BODIPY 1 we always observed the formation of unknown by-product. In order to minimize this impurity, the reaction should be conducted under mild temperature (boiling DCE, dichloroethane) and impurity can be removed from the reaction product by a simple recrystallization. However, when starting BODIPY 1 was reacted with DMF dimethyl acetal at higher temperature (boiling toluene), we have obtained an unexpected bis-enamine 12 as a single pure product (Scheme 3). No formation of tris- or tetrakis-enamine containing BODIPYs was observed in this reaction. Initial analysis of the NMR and UV-Vis spectra of 12 was inconclusive. Indeed, two different enamine fragments can be clearly seen in the 1H NMR spectrum of 12. However, the NMR spectrum of 12 does not differentiate between 1,3- and 1,5-regioisomers. Further hydrolysis of bis-enamine 12 in acidic conditions results in dialdehyde 13, which also has two different aldehyde signals in the 1H NMR spectrum, which again does not differentiate between 1,3- and 1,5regioisomers. Room-temperature reaction of bis-enamine 12 with an excess of n-butylamine in acetonitrile results in formation of BODIPY 14, which upon prolonged reaction time (3 days) ACS Paragon Plus Environment

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transforms into BODIPY 15 (Scheme 3). In order to clarify structures of BODIPYs 12 – 15, we have tried to grow corresponding single crystals suitable for X-ray analysis. Although in majority of cases, single crystals turned out to be too small for a single-crystal data collection, we were able to obtain a crystal structure of BODIPY 14 that clarified an unusual reactivity of the BODIPY 1.

Scheme 3. Preparation of the 2-pyridone[a]-fused BODIPY 17. Reagents and conditions: (i) DMA DMF, toluene, 100 ℃, 24 h; (ii) MeCN, n-BuNH2, 48 h, r.t.; (iii) AcOH, H2O, MeOH, 3 h, r.t.; (iv) MeCN, n-BuNH2, 72 h, r.t.; (v) HOCH2CH2OH, reflux, 1 min; (vi) 25 % aq. AcOH, 55 ℃, 1 h.



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C16 C15 C11 C21 C14 C20 C5 C3 C19 O3 C18 C4 C6 C7 C17 N3 C8 C13 N1 C12 C2 B1 O1 C9 N2 C1 O2 C23 C10 F1 F2 C22 C24 N4 C25 Figure 1. X-ray structure of BODIPY 14. Thermal ellipsoids are at 50% probability level. Solvent toluene molecule and hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (0): F1-B1 1.415(8), F2-B1 1.378(9), N1-B1 1.567(9), N2-B1 1.514(9), O1-C17 1.230(7), C1-C22 1.392(9), C22-C23 1.380(8), N4-C23 1.364(7), O1-H23 2.006(12); F1-B1-F2 108.4(6), N1-B1-N2 106.7(6), N1-C1-C22 119.9(7), C1-C22-C23 126.4(7), N4-C23-C22 124.5(7), N1-C1-C22-C23 176.70(14), C1-C22-C23-N4 178.32(12).

The X-ray analysis of BODIPY 14 (Figure 1) is clearly indicative of the regioselective formation of 1,3-bis-enamine 12, which upon heterocyclization forms 2-pyridone[a]-fused BODIPY 14. X-ray structure of 14 is indicative of the planar BODIPY core and essentially co-planar with the BODIPY enamine fragment. C=O bond in pyridone fragment of 14 has clear double bond character and forms a short hydrogen bond with the hydrogen atom of enamine group. The B-F and B-N bond distances were found in the typical for BODIPYs range. The –C(pyrr)-CH=, –CH=CH-, and =CH-Nbond distances in 14 are clearly indicative of the enamine fragment -delocalization rather then classic alternation of single and double bonds. Thus, a second aminoformylation reaction that transforms BODIPY 1 via 2 into bis-enamine 12 is regioselective and dictates a formation of the regioselective products 13 – 17 (Scheme 3). The regioselective formation of 1,3-bis-enamine product 12 is very unusual and contradicts current ACS Paragon Plus Environment

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understanding of BODIPY’s chemistry. Indeed, it is generally accepted that the methyl groups at positions (3,5-positions) of BODIPY core have higher reactivity of the C-H bonds compared to the methyl groups located at -positions (1,7-positions) of the BODIPY core.78-80 At this moment, we do not have a clear explanation on the regioselective formation of the BODIPY 12. Indeed, DFT calculations on BODIPY 2 (mono-enamine compound) are indicative of very similar Mulliken charges on carbon as well as hydrogen atoms in all three remaining methyl groups although one might speculate that the Mulliken charge on the most acidic proton of the methyl group at position 1 (+0.245) is slightly higher that that at position 5 (+0.241) and position 7 (+0.237). Similarly, the DFTpredicted Mulliken charge at the methyl group carbon atom located at position 1 of the BODIPY core in BODIPY 2 (-0.662) is slightly lower than that at positions 5 (-0.705) and 7 (-0.669).

Figure 2. 1H NMR spectrum of bis-enamine 12 in CDCl3 (bottom) and fragment of spectrum in DMSO-d6 (top). Unusual regioselective formation of the BODIPY 14 from bis-enamine 12 can be explained more easily. Indeed, the 1H NMR spectrum of BODIPY 12 is indicative of two quite different enamine groups (Figure 2). One set of the signals (in particular doublets at 8.3 and 6.2 ppm) is almost identical ACS Paragon Plus Environment


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to that observed in the mono-enamine 2 (Supporting Information) and was assigned to the enamine fragment located at position 3 (-position) of the BODIPY core. Two doublets at 5.9 and 7.0 ppm belong to the –CH=CH- protons of the second enamine fragment located at the position 1 (-position) of the BODIPY core. Similarly, two different enamine fragments were observed in DMSO-d6 but in this solvent the observed differences in chemical shifts are smaller (Figure 2). Conformational search on the potential energy surface of BODIPY 12 reveals a presence of four conformers with ~65:25:9:1% contributions at room temperature (Figure 3). The NMR DFT calculations on the most stable conformation of 12 suggest that the =C(H)-N proton at position 3 (predicted at 9.29 ppm) should be quite different from that located at position 1 (predicted at 6.98 ppm).

Figure 3. DFT-predicted conformers of the BODIPY 12. Room-temperature Boltzmann distribution is shown below of each structure. In two major conformers that account for ~90% contribution, the enamine fragment at position 1 (-position) is rotated away from the BODIPY plane by ~30o (Figure 3), while the second enamine fragment remains almost co-planar with the BODIPY core. In this case, one might expect a considerably higher reactivity of the non-conjugated enamine toward nucleophilic attack by an excess of n-butylamine, which explains fast and regioselective formation of BODIPY 14. Further reaction of the BODIPY 14 with an excess of n-butylamine results in the formation of BODIPY 15. Lower reactivity of the better conjugated enamine fragment in BODIPY 12 at position 3 (-position) correlate well with that observed in BODIPY 2 (Scheme 1) as in both cases, cross-enamination reaction with n-butylamine requires harsher reaction conditions. ACS Paragon Plus Environment

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A higher reactivity of the enamine fragment at position 1 (-position) in BODIPY 12 allowed us to prepare first heterocycle[a]-fused (“isoindole” type) BODIPYs 14 – 17, which can be directly compared with more common heterocycle[b]-fused (“indole” type) BODIPYs. Indeed, remaining enamine fragment in BODIPY 14 can be transformed into the methyl group of BODIPY 17 by stepwise hydrolysis followed deformylation reaction in ethylene glycol (Scheme 3). Although target BODIPY 17 can be prepared in this synthetic pathway, the yield of the final product is rather low and we were not able to find reaction conditions that improve its preparation. Moreover, our attempts to prepare bis-2-pyridone[a]-fused BODIPY from 17 have also failed. Optical properties of the new BODIPYs are summarized in Table 1 with typical examples shown in Figures 4 - 6. For the majority of BODIPYs modified at positions 3 and 5 (-positions), energy of the characteristic and the most intense BODIPY band in the visible region correlates well with the properties of 1,7-diphenyl-containing analogues based on BODIPY platform V (Chart 1) with the exception of aldehydes 3, 8, 10, and 13 which have indistinguishable from methyl analogues UV-vis spectra. The observed difference for the 1,7-dimethyl BODIPY aldehydes 3, 8, 10, and 13 compared to the 1,7-diphenyl analogues can be attributed to the pure aldehyde tautomeric form observed in the former cases and predominantly enol tautomeric form observed in the later compounds. In the case of di-substituted BODIPYs the low-energy shift of the most intense BODIPY band has an additive nature as shown in Figure 4 for BODIPYs 2 and 9.

1.0 1 2 9 12

0.8

e / 10-5 M-1cm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.4

0.2

0.0 400

500

600

700

Wavelength, nm ACS Paragon Plus Environment

800


Figure 4. UV-vis spectra of BODIPYs 2, 9 and 12 in comparison with the parent BODIPY 1 in DCM.

It is interesting to note that unlike all other BODIPYs mentioned in this work, the 1,3bisenamine 12 is fairy solvatochromic (Figure 5). Moreover, a relative intensity of the higher-energy band observed at ~540 nm increases with an increase of solvent polarity, which is explained below on a basis of TDDFT calculations.

0.8 DCM DMF MeOH Toluene

0.7

e / 10-5 M-1cm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.5 0.4 0.3 0.2 0.1 0.0 400

450

500

550

600

Wavelength, nm

Figure 5. UV-vis spectra of BODIPY 12 in different solvents.


650

700

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Table 1. Photophysical properties of BODIPYs 1 - 17 in DCM. No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

λabs, nm

ε, M-1.cm-1

Δ λabs, nma

504 92000 603 67000 99 502 89000 -2 592 70000 88 561 84000 57 534 76000 30 629 45000 125 535 72000 31 705 76000 201 502 68000 -2 623 103000 119 587 59000 183 504 74000 0 583 54000 81 577 60000 73 540 70000 36 535 78000 31 a spectral shift in comparison with BODIPY 1

λem, nm

Φf, %

514 632 514 628 582 556 680 549 750 515 652 515 610 602 554 548

93

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