Organotrifluoroborate Salts as Complexation Reagents for

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Organotrifluoroborate Salts as Complexation Reagents for Synthesizing BODIPY Dyes Containing Both Fluoride and an Organo Substituent at the Boron Center Zhaoyun Wang, Cheng Cheng, Zhengxin Kang, Wei Miao, Qingyun Liu, Hua Wang, and Erhong Hao J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b03145 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 1, 2019

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The Journal of Organic Chemistry

Organotrifluoroborate Salts as Complexation Reagents for Synthesizing BODIPY Dyes Containing Both Fluoride and an Organo Substituent at the Boron Center Zhaoyun Wang,a Cheng Cheng,a Zhengxin Kang,a Wei Miao,a Qingyun Liu,b Hua Wang,a and Erhong Hao*a

a

Laboratory of Functional Molecular Solids, Ministry of Education; School of Chemistry and

Materials Science, Anhui Normal University, Wuhu, 241000, China;

b

College of Chemistry and Environmental Engineering, Shandong University of Science and

Technology, Qingdao, China.

*To whom correspondence should be addressed. E-mail: [email protected]

Abstract Graphic R

R-BF3K VS BF3.OEt2

R

1) DDQ NH HN

2) R-BF3K

 one-pot reaction

N

B

N

R F

 broad substrate scope  stable and available organotrifluoroborate salts

ACS Paragon Plus Environment

The Journal of Organic Chemistry 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

Abstract: A convenient procedure for the preparation of functionalized BODIPYs bearing both F and an organo substituent at the boron center, using one-pot reactions between in situ formed dipyrromethenes and organotrifluoroborate salts, has been reported. The complexation reaction utilizes stable and commercial accessible organotrifluoroborate potassium salts and provides a facile access to a variety of novel B-functionalized BODIPYs, which are hard to access through current synthetic methods.


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INTRODUCTION Boron dipyrromethene (BODIPY, Scheme 1a) dyes1 have excellent photophysical properties and have found widely applications in diverse research fields.2-4 The key to their success is closely associated with their rich functionalization chemistry,5-7 and every position on BODIPY core can be modified to tune the photophysical properties and to add tethering groups for further conjugation. Among them, substitutions on the boron center8 (4-position) open up elegant methods to modulate solubility, (photo)stability, Stokes shift, and optoelectronic properties of resultant dyes. Pioneered by Ziessel et. al,9 modifications on the boron center by activation of B-F bond generate a series of ethynyl, alkyl or aryl derivatives (Scheme 1b, formed C-BODIPYs9,10), and alkoxy, aroxy or acetoxy derivatives (formed O-BODIPYs).11-13 However, these reactions normally require strong nucleophiles, such as Grignard or organolithium reagents,9,10 and alkoxides,11 or strong Lewis acid (usually AlCl3).12 Careful control of the reaction conditions are needed to afford either the 4-mono- or 4,4’-disubstituted BODIPYs.9b However, pyrrolic unsubstituted F-BODIPYs were not suitable for these nucleophilic reactions due to possible side reactions at their 3,5-positions of BODIPY core.10a On the other hand, direct complexation of in situ formed dipyrromethene with suitable haloboranes is a straight one-pot procedure and is therefore more appealing. However, since originally synthesized by complexation of dipyrromethene with BF3.Et2O (F-BODIPY, Scheme 1a),1 there are very limited other haloboranes reported to complex with dipyrromethenes.14,15 More importantly, these lately reported haloboranes are also far less useable than BF3.Et2O. The limited stability and accessibility of organohaloboranes are two main reasons for inhibiting their wide



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usage. Indeed, by using this method, only a few examples of FC-BODIPYs were synthesized in multiple steps by treatment of 4-hydroxy-4-phenyl-BODIPY C with potassium bifluoride, where C was prepared from the corresponding dipyrromethene with unstable dichlorophenylborane in the presence of sodium hydride (giving unstable moisture-sensitive 4-chloro-4-phenyl-BODIPY D), followed by hydrolysis. Scheme 1. Synthetic methods of FC-BODIPYs containing both Fluoride and an organo substituent at the boron center: a) one-pot synthesis of BODIPYs from dipyrromethane reported in this work; b) reported postfunctionalizations of F-BODIPY (B) with Grignard reagents; c) multiple-step synthesis of FC-BODIPYs from unstable ClC-BODIPY (D). traditional BF2 complexation

(a)

N 4 N B F F F-BODIPY (b)

this work DDQ

DDQ .

BF3 OEt2

NH HN A

R-MgBr

N

N

R1

B

R1 N

N

N

R Cl D

B

N

F F Ar-MgBr R1

R1 KHF2

H 2O N

N

B

R F FC-BODIPY

B R R C-BODIPY

B F F B

(c)

N

R1

R1 N

R-BF3K

N

N

N B R OH C

B

N

R F FC-BODIPY

Herein we developed a convenient procedure for the preparation of functionalized BODIPYs bearing both fluoride and an organo substituent at the boron center (FC-BODIPY, Scheme 1a), using one-pot reactions between in situ formed dipyrromethenes

and

organotrifluoroborate

salts.


We

were

attracted

to

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organotrifluoroborate potassium salts because they have excellent stability and easy accessibility (hundreds of commercially available reagents).16 This would render the desired structural versatility of the resultant FC-BODIPYs.17

RESULTS AND DISCUSSION Table 1. Optimization of the Reaction Conditions for the One-pot Synthesis of FC-BODIPY 2a.a

1) DDQ (1.1 equiv) solvent NH HN 1a

2) base PhBF3K (3 equiv) temperature

N

B

N

Ph F 2a

entry

solvents

base/equiv

temp./Cb

yield (%)c

1

dichloromethane

-

40

0d

2

DCEe

-

80

0d

3

acetonitrile

-

80

10

4

acetonitrile

-

110

41

5

acetonitrile

Et3N, 1.0

110

trace

6

acetonitrile

K2CO3, 1.0

110

trace

7

acetonitrile

NaHCO3, 1.0

110

47

8

acetonitrile

Na2CO3, 1.0

110

82

9

acetonitrile

Na2CO3, 0.5

110

46

10

acetonitrile

Na2CO3, 1.5

110

74

11

acetonitrile

Na2CO3, 1.0

90

71

12

acetonitrile

Na2CO3, 1.0

120

81

13f

acetonitrile

Na2CO3, 1.0

110

72

14

THF

Na2CO3, 1.0

110

48

15

ethanol

Na2CO3, 1.0

110

trace

16

methanol

Na2CO3, 1.0

110

trace

aReaction

condition: 1) 1a (0.2 mmol), 8 mL of solvent, 1.1 equiv of DDQ at room temperature

for 0.5 h. 2) 3 equiv of PhBF3K in the presence of base at indicated temperature in sealed reaction



tube for 8 h. bOil bath temperature. cIsolated yields based on 1a. dNo reaction. e1,2-dichloroethane. f2.2

equiv of PhBF3K was used.

Syntheses Initially, we applied dipyrromethane 1a with DDQ oxidation in dichloromethane to give dipyrromethene intermediate which was attempted to reaction with potassium phenyltrifluoroborate under typical condition of BF2 complexation (entry 1, Table 1). No reaction was observed. Raising the temperature of the later complexation reaction to 80 C using 1,2-dichloroethane as solvent still did not give any reaction (entry 2). Considering the solubility of potassium phenyltrifluoroborate, we then used acetonitrile as the reaction solvent and a new highly fluorescent product 2a was formed at refluxing condition (entry 3). The formation of 2a was confirmed by HRMS with a desired ion peak at m/z 349.1872 (calcd. 349.1871 for [M-F]+). The structure of 2a was further supported by 1H, 13C and 19F NMR spectra. Further raising the reaction temperature to 110 C in sealed reaction tube improved the yield to 41% (entry 4). We then added various bases to this complexation reaction to further optimized this reaction condition (entries 5-10). Surprisingly, adding organic base triethylamine, which is commonly used for BF2 complexation, quenched this reaction and gave extremely low yield, while Na2CO3 was found to be an efficient base for this complexation reaction. With further optimization of the amount base, potassium phenyltrifluoroborate, the reaction temperature and solvent, we found the optimized reaction conditions to be 1 equiv of Na2CO3, 3 equiv of PhBF3K in acetonitrile at 110°C, from which FC-BODIPY 2a was isolated in 82% yield (entry 8). To test the versatility of this reaction, we further applied dipyrromethane 1a with different organotrifluoroborate potassium salts under the optimized condition


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(Scheme 2). Aryltrifluoroborates, vinyltrifluoroborate and alkytrifluoroborates all showed good reactivities in this one-pot reaction. The desired FC-BODIPYs 2b-f were obtained in 73-85% yields.

Scheme 2. One-pot Synthesis of FC-BODIPYs 2a-g from Dipyrromethane 1a with Various Organotrifluoroborate Salts. Ar

1a Ar = mesityl

2) Na2CO3 (1.0 equiv) RBF3K (3.0 equiv) 110 oC

Ar

Ar

NH HN

N

N

B

N

F

MeO

B

Ph F 2a, 82%

N

B

2d, 83% Ar

Ar N

N

F

B

N

N

B

N F

Me F

2e, 85%

N F

2c, 75%

Ar

N

B

Ar N

B

N

F

2b, 82%

N

Ar

1) DDQ (1.1 equiv) acetonitrile

2f, 73%

2g, 78%

To explore the limits of this new synthetic route for FC-BODIPYs with various meso-substituents, the reaction was next carried out with different dippyromethanes18 (Scheme 3a). FC-BODIPYs 3-7 with electron-rich or -deficient meso-aryl substituents were all obtained in moderate to good yields. Among those, slightly lower yields (51%) were observed for FC-BODIPYs 6 and 7 containing electron withdrawing (-CN or -Cl) substituents. We

next

examined

the

1,3,5,7-tetramethyl-FC-BODIPY

three-step 8

one-flask

from

procedure

to

synthesize

2,4,6-trimethylbenzaldehyde


and


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2,4-dimethylpyrrole (Scheme 3b). Trimethylbenzaldehyde and 2,4-dimethylpyrrole were first dissolved in anhydrous dichloromethane and treated with catalytic amount of trifluoroacetic acid (TFA). To this solution were added 1.1 equiv of oxidant DDQ to give dipyrromethene intermediate, which was redissolved in acetonitrile. Na2CO3 and potassium phenyltrifluoroborate were added to the acetonitrile solution and the mixture was stirred at 110 C for 12 h in a sealed reaction tube. The target 1,3,5,7-tetramethyl-FC-BODIPY 8 was isolated in 37 % overall yield as shown in Scheme 3b, which is compatible with the yield for the synthesis of corresponding F-BODIPY using BF3.Et2O. Scheme

3.

One-pot

Synthesis

of

FC-BODIPYs

3-8

from

Various

Dipyrromethanes 1b-e or Mesitaldehyde with Potassium Phenyltrifluoroborate. OMe a)

1) DDQ (1.1 equiv) acetonitrile

Ar

2) Na2CO3 (1.0 equiv) PhBF3K (3.0 equiv) 110 oC

NH HN 1b-e

NO2

N

CN

B

N

Ph F 3, 61%

Cl N

B

N

N

B

N

Ph F 5, 69%

Ph F 4, 81%

N

B

N

Ph F 6, 67%

Cl N

N

B

Ph F 7, 51%

b) 1) TFA (0.1 equiv), CH2Cl2 + CHO

2) DDQ (1.1 equiv) HN

3) Na2CO3 (1.0 equiv) PhBF3K (3.0 equiv) acetonitrile, 110 oC 37%


N

B

N

Ph F 8

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Finally, we demonstrated post-modifications of the FC-BODIPY 2a through activation B-F bond (Scheme 4). The activation of F-BODIPY with TMSOTf followed by reaction with an alcohol has been recently investigated for the synthesis of mono-4-alkoxy-BODIPY.13a By adapting this reaction to our FC-BODIPY 2a with N,N-dimethylethanolamine in the presence of TMSOTf, OC-BODIPY 9 was smoothly obtained in 71% yield. Next, the activation of B-F bond with TMSCN was studied to synthesize B-CN derived BODIPY dyes. The reaction of FC-BODIPY 2a with 1.5 equiv TMSCN in the presence of catalytic amount BF3.Et2O at room temperature quickly gave BODIPY 10 in 86% yield. Surprisingly, direct reaction between 5 equiv TMSCN and 2a in refluxing toluene (without Lewis acid) gave 4,4’-dicyano-BODIPY 11 in 83% yield,19 indicating that the B-Ph moiety was also cleaved in this condition. Scheme 4. Post-modification of FC-BODIPY 2a to OC-BODIPY 9 ， FC-BODIPY 10 and C-BODIPY 11. Ar

N OH (100 equiv) TMSOTf CH2Cl2, 0 oC

Ar N

B

N

Ph F 2a Ar = mesityl

N

Ph O 9

71% N

B

N

Ar

TMSCN (1.5 equiv) BF3.Et2O (0.1 equiv) CH2Cl2, rt 86%

N

B

N

Ph CN 10 Ar

TMSCN (5 equiv) N

toluene, reflux 83%

All BODIPYs 2-11 were characterized by 1H,

B

N

NC CN 11 13C,

and

19F

NMR, HRMS

(ESI-TOF), and in the case of BODIPYs 2f, 3, 8 and 9 also by X-ray crystallography



(Figure 1). The 1H and

13C

NMR spectra indicated that the introduction of boron

substituents has negligible effect on the chemical shifts of the BODIPY core. All these FC-BODIPYs exhibited singlet signals centered at -150  -170 ppm in their 19F NMR spectra, which are corresponding to the boron-bound fluorine atom and are also in agreement with typically observed spectra for four coordinate fluoroborate moieties.15 In contrast, the corresponding F-BODIPYs all showed typical quartet signals at around -145 ppm with B-F coupling constants of 28 Hz (pages S43-45 in SI).20a,b X-ray Structure X-ray diffraction structure analysis of crystals 2f, 3, 8 and 9 indicates that, in all cases, the boron atom adopts a slightly distorted tetrahedral geometry with dihedral angles N1-B-N2 of 103 or 104°, which in good agreement with what has been observed for those F-BODIPY derivatives.18 In all cases, the central BN2C3 ring is fairly planar, with mean deviation less than 0.03 Å for all four compounds. The phenyl group on B is oriented approximately perpendicular to the BODIPY core plane, to avoid steric hindrance, the dihedral angles being 74 for 3, 83 for 8, and 86 for 9. The average B-N bond lengths being 1.57 Å for 2f, 1.56 Å for 3, 1.58 Å for 8, and 1.59 Å for 9, are slightly longer than that observed in F-BODIPY (around 1.54 Å). The B–F bond lengths of 1.41-1.43 Å found 2f, 3 and 8 in are comparable to the B–F bond distance of 1.41 Å measured in previously reported FC-BODIPY,15 while the B-C bond lengths of 1.58 Å for 2f, 1.61 Å for 3, 1.63 Å for 8, and 1.59 Å for 9, are also similar to those reported C-BODIPY (average 1.62 Å).9


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Figure 1. ORTEP views of compounds 2f, 3, 8 and 9 at 50% probability ellipsoids. Hydrogen atoms were omitted for clarity. Spectroscopic properties Fundamental spectroscopic/photophysical data of these dyes in dichloromethane are shown in Table 2 and Figs. S2-16 (ESI). Overall typical F-BODIPY features were also obtained for these FC-BODIPY dyes, which showed a strong π-π* transition with a clear maximum between 498 and 512 nm. Similarly, high fluorescence quantum yields were also obtained for dyes 2a-g, 7 and 8, while weak fluorescence was



Page 12 of 33

Table 2: Photophysical properties of dyes 2-10 in dichloromethane at room temperature.a dyes

λabsmax (nm)

λemmax (nm)

logεmaxb

c

Stokes Shift (cm-1)

2a

502

515

4.70

0.95

500

2b

502

515

4.68

0.90

500

2c

502

515

4.56

0.10

500

2d

502

515

4.65

0.80

500

2e

499

515

4.63

0.92

620

2f

498

514

4.79

0.90

630

2g

498

514

4.62

0.93

630

3

500

516

4.57

0.08

620

4

502

521

4.62

0.05

730

5

511

545

4.54

0.003

1220

6

509

536

4.53

0.003

990

7

512

528

4.68

0.83

590

8

501

512

4.74

0.94

430

9d

500

514

4.70

0.03

550

9e

503

517

4.63

0.95

540

10

505

515

4.81

0.88

540

11

504

517

4.66

0.96

500

aAll Φ values are corrected for changes in refractive indexes of solvents. bMolar extinction coefficients are in the maximum of the highest absorption peak. cFluorescence quantum yields. The standard errors are less than 10%. d1% Et N was 3 added. e1% TFA was added.

expected for dyes 3-6 due to the free rotation of the meso-aryl groups as previously reported in meso-phenyl F-BODIPY.14a These dyes also exhibit similar small Stokes shifts (430-730 cm−1 ), comparable to that F-BODIPY (normaly around 500 cm−1).14 Dyes 5 and 6


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with nitro and cyano subsitutted meso-aryl groups showed very weak fluorescence and large Stokes shifts due to the possible photoinduced electron transfer from

1.0

1.0

0.5

0.5

0.0 400

500

600

0.0

Normalized Fluorescence

BODIPY core to the meso-aryl groups.

Normalized Absorbance

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


wavelength (nm)

Figure 2. Normalized absorption spectra (black), fluorescence spectra (red solid line, excited at 475 nm) in dichloromethane (with 1% Et3N) and fluorescence spectra (red dashed line, excited at 475 nm) in dichloromethane (with 1% TFA) of OC-BODIPY 9. Interestingly, OC-BODIPY 9 also showed very week fluorescence (Φ = 0.09 in dichloromethane, Φ = 0.03 in dichloromethane in the presence of 1% Et3N) due to the possible photoinduced electron transfer from dimethylamine moiety to BODIPY core. Strong fluorescence enhancement (Φ = 0.95, Table 2) was observed in the presence of 1% TFA in dichloromethane (Figure 2), indicating its possible application as pH indicator.

Similar

to

previous

results,19

4-cyano-BODIPY

10

and

4,4’-dicyano-BODIPY 11 both exhibit intense fluorescence, which are also comparable to those F-BODIPY. Finally, using 2a as an example, the solubility and photostability of 2a in acetonitrile were studied. Due to the presence of bulky aryl group at B atom and the unique tetrahedron comformation,2d,2e 2a showed good solubility upto 40 uM in



Page 14 of 33

acetonitrile (Figure S17, SI). FC-BODIPY 2a also showed similar photostability with the coresponding F-BODIPY (4,4-difluoro-8-mesityl-BODIPY) under strong continuous irradiation with a 50 W white light LED lamp (Figure S18, SI).20c

Conclusion In conclusion, we developed a general and facile one-pot synthesis of functionalized FC-BODIPYs bearing both F and an organo substituent on boron, from dipyrromethanes and organotrifluoroborate salts. The method utilizes stable and commercial accessible organotrifluoroborate potassium salts, and thus provides a direct access to a variety of novel B-functionalized BODIPYs, which are previously difficult to access. This method will thus stimulate the modulation of boron position to generate novel dyes with desired properties including solubility, (photo)stability, Stokes shift, and optoelectronic properties for advance applications.

Experimental Section General. Reagents and solvents were used as received from commercial suppliers unless noted otherwise. All reactions were performed in oven-dried or flame-dried glassware unless otherwise stated, and were monitored by TLC using 0.25 mm silica gel plates with UV indicator (60F-254). Nuclear magnetic resonance (NMR) spectra were recorded on a 500 MHz spectrometer (500 MHz for 1H NMR, 125 MHz for 13C{1H}

NMR and 470 MHz for

19F

NMR). Chemical shifts (δ) are given in ppm

relative to CDCl3 (7.26 ppm for 1H and 77.1 ppm for Chemical shifts of

19F

13C{1H})

or to internal TMS.

NMR spectra are quoted in ppm relative to external CFCl3


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(19F). High-resolution mass spectra (HRMS) were obtained using APCI-TOF or ESI-TOF in positive mode. Melting points were determined on a X-4 micro melting point apparatus. Photophysical measurements. UV-visible absorption and fluorescence emission spectra were recorded on commercial spectrophotometers (190-900 nm scan range) at room temperature (10 mm quartz cuvette). Relative fluorescence quantum efficiencies of BODIPY derivatives were obtained by comparing the areas under the corrected emission spectrum of the test sample in dichloromethane with Fluorescein (Φ = 0.90 in 0.1 M NaOH solution)21. Spectroscopic grade solvents and a 10 mm quartz cuvette were used. Dilute solutions (0.01

Organotrifluoroborate Salts as Complexation Reagents for

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