TCDA: Practical Synthesis and Application in the Trifluoromethylation

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TCDA: practical synthesis and application in trifluoromethylation of arenes and heteroarenes Jian Wang, Xiaomin Zhang, Zehong Wan, and Feng Ren Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.6b00079 • Publication Date (Web): 29 Mar 2016 Downloaded from http://pubs.acs.org on March 29, 2016

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Organic Process Research & Development

TCDA: practical synthesis and application in trifluoromethylation of arenes and heteroarenes Jian Wang, Xiaomin Zhang*, Zehong Wan, Feng Ren Neurodegeneration DPU, GlaxoSmithKline, 898 Halei Road, Zhangjiang Hi-Tech Park, Pudong, Shanghai, 201203, China

ACS Paragon Plus Environment


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Table of contents graphic


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Abstract A practical synthesis of the reagent trimethylsilyl chloro-difluoroacetate (TCDA) is reported on 50 g scale. The trifluoromethylation with TCDA was optimized and the reaction shows very broad scope with respect to electron-deficient, -neutral, rich aryl / heteroaryl iodides, as well as excellent functional group tolerability, such as ester, amide, aldehyde, hydroxyl, and carboxylic acid. The reagent was also applied to the late-stage trifluoromethylation of three medicinally relevant compounds. Additionally, the building block trifluoromethyl pyridine and one drug related molecule Boc-Fluoxetin were synthesized in 10 g scale by this method, demonstrating its practical applications in process chemistry. Key word: TCDA, trifluoromethylation, aryl iodide, process chemistry



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Introduction Trifluoromethylation is a hot topic in recent years due to its wide applications in pharmaceutics, agrochemicals and material science. 1,2 The introduction of a CF3 group to a molecule can often improve its bioactivity, permeability, and metabolic stability. Therefore, many CF3 transformations have been reported, basically via electrophilic (+CF3), nucleophilic (-CF3), and radical (·CF3) reactions. 3 A novel reagent, TCDA designed by our group, was demonstrated to efficiently introduce a CF3 group via cooperative interaction of AgF and CuI. 4,5,6 The reaction was proposed to form difluoro-carbene intermediate then CF3 anion, which has been well described by Burton and Chen’s group. 7 Although the reaction of TCDA has shown very good functional group tolerance, the drawback for the reaction is also obvious due to the high loading of expensive AgF, which could limit its practical application. Additionally, the synthesis of TCDA especially in large scale is yet to be improved, and its trifluoromethylation reaction in gram scale needs to be evaluated from process chemistry aspect. Herein, we report the practical synthesis of TCDA in 50 g scale, as well as further optimization of the reaction condition either reducing equivalent of AgF or replacing it with KF to introduce the trifluoromethyl group. The scope of this transformation and its application to make drug molecule and building block in decagram scale are also explored.

Scheme 1. Synthesis of TCDA


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As shown in Scheme 1, TCDA is prepared in a one-step process by mixing sodium

chlorodifluoroacetate

and

chlorotrimethylsilane

without

solvent.

Comparing with our previous report, the reaction time was reduced from 6 h to 2 h with decreased temperature from 60 to 50 °C, and the yield was increased from 43% to 58%. The reaction was executed on 50 g scale, followed by one distillation to afford TCDA with purity above 95%. In addition, the starting materials are commercially available and inexpensive. As such, TCDA is very convenient to scale up, and no obvious decomposition was observed when storing under nitrogen at 5 °C for two weeks.

Table 1. Optimization of the molar ratio of reagents.

yield [%]

ClF2CO2TMS/ KF/ CuI/TMEDA

temp

1

1.5: 3: 1.5: 1.5

100 oC

22

2

2: 4: 1.5: 1.5

90 oC

34

3 4

2: 4: 1.5: 1.5 2: 4: 1.5: (TEA)

100 oC 90 oC

37 52

53

2: 5: 1.5: 1.5

90 C

2: 5: 1.5: (TEA)

entry

product

5

6 entry

product

7

9

4h

o

43

90 C

o

50

64

ClF2CO2TMS/ KF [a] (AgF [b] )/CuI/TMEDA

temp

yield [%] 4h 6h

2: 5: 1.5: 1.5 [a] (TEA)

90 oC

24

30

2: 4: 1.5: 1.5

90 oC

37

43

2: 4: 1.5: 1.5 [b]

100 oC

46

53

[b]

8

2h

Reaction conditions: [a] 1a-1b (1 mmol), CuI, KF and 2 in DMF (1 mL) and TEA (1 mL); [b] 1a-1b (1 mmol), CuI, AgF, o 19 TMEDA and 2 in DMF (2 mL) were stirred at 100 C under nitrogen in a sealed vial for 1 h. Yields were determined by F NMR spetroscopy using PhCF3 as an internal standard.



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In order to make the trifluromethylation more practical for process chemistry, the expensive AgF should be replaced by other fluoride reagent, or at least its equiv needs to be reduced. This time, 1.5 equiv CuI was used instead of 2.5 equiv comparing with our previous condition. We firstly tried KF to replace AgF for the trifluoromethylation. The optimization was started with 1.5 equiv TCDA to react with eletron-poor 4-iodobenzoate and the yield was only 22% (entry 1, Table 1) after 2 h at 100 °C. Then the TCDA was increased to 2 equiv, similar yields were obtained for the reaction either at 90 or 100 °C (entries 2, 3, Table 1). As the temperature has marginal influence on the reaction yield, the lower temperature 90 °C was selected for further optimization. The yield was increased to 52% when 5 equiv KF was used (entry 4, Table 1). To our surprise, the higher yield 64% was observed in 4 h when TMEDA was replaced by TEA (entry 6, Table 1). However, the same reaction condition worked much slower and gave poor yield of 24% in 4 h and 30% in 6 h for electron-rich 4-methoxybenzene iodide (entry 7, Table 1). We subsquently chose AgF to optimize the trifluoromethylation of electron-rich substrate. In our earlier report, 2.5 equiv TCDA and 5 equiv AgF were found to be optimal conditions. Here, AgF was reduced to 4 equiv to obtain the higher yield at 100 °C than 90 °C. In addition, longer reaction time gave better yields (entries 7, 8, 9, Table 1). Based on the optimized results, the molar ratios of TCDA/CuI/KF 2:1.5:5 together with TEA and TCDA/CuI/AgF/TMEDA 2:1.5:4:1.5 were used for electron-poor and -rich substrates, respectively.


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Table 2. Substrate scope of the trifluoromethylation. entry

product

yield [%]

entry

1

64 [a]

2

3

product

yield [%]

entry

product

8

50 [a]

15

26 [b] 12 [c]

99 [a]

9

60 [b]

16

68

42 [b]

10

25 [b]

17

Cl 3q

yield [%]

[b]

25 [c]

N

75 [a] 38 [c] CF3

4

75 [b]

11

27 [b]

18

92 [a]

5

87 [a]

12

53 [b]

19

52 [a]

6

48 [a]

13

34 [b]

20

100 [a] 72 [c]

7

46 [a]

14

72 [b] 42 [c]

[a] Reaction conditions: 1a-t (1.0 mmol), CuI (1.5 mmol), KF (5.0 mmol) and 2 (2.0 mmol) in DMF (1 mL) and Triethylamine (1 mL) were stirred at 90 °C for 4 h unless specifized, under nitrogen in a sealed tube. Yields were 19 determined by F NMR spetroscopy using PhCF3 as an internal standard. [b] 1a-t (1.0 mmol), CuI (1.5 mmol), AgF (4.0 mmol), TMEDA (1.5 mmol) and 2 (2.0 mmol) in DMF (2 mL) were stirred at 100 °C for 6 h. [c] Isolated yields.

Having the two sets of optimal reaction conditions, we switched our effort to explore the scope of this transformation. A broad range of aryl iodides were selected based on the electron density, substituted functional group diversity, and some commonly used heterocycles. The condition of KF and TEA combination was applied for electron-deficient aryl, heteroaryl and brominated aryl iodides. The optimized reaction conditions with AgF was used for the trifluoromethylation of

aldyhyde/ketone, electron-rich aryl iodides, and aryl iodides having protic

functional groups. The reactions were analyzed at different time points at 4 h or 6



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h depending on the reaction conditions. As shown in Table 2, it is clear to see that ring electronics has no obvious effect on the reaction yield. In most cases we found it was difficult to isolate the trifluoromethylated products either in high yield or in pure form, due to their high volatility or similar polarity to the iodoarene starting materials. All the yields in Table 2 were determined by

19

F NMR

spectroscopy with an internal standard. The isolated yields were also obtained for 3n, 3o, 3p, 3q and 3t. The reaction was well tolerated with different functional groups, such as ester, formyl, nitro, cyano, amide groups (42-99% yield, entries 1-7, Table 2). However, the observed low yield with 4-iodophenyl acetate (25%, entry 10, Table 2) is likely due to its decomposition during reaction. The trifluoromethylation of electron-rich aryl iodides gave good yields (34-60%, entries 9, 12 and 13, Table 2). Remarkbly, the iodoarenes containing carboxylic acid, hydroxyl groups (3n, 3o, and 3p) could also be efficiently trifluoromethylated, demonstrating excellent tolerance of protic functionality. As expected, the isolated yields of 3n 3o, and 3p were lower than the yield determined by

19

F NMR spectroscopy due to difficult separation.

The moderate to high yield (52-100%) for reactions with quinoline, pyridazine, and pyridine, is very encouraging (entries 17-20, Table 2). Moreover, it is noteworthy to see that trifluoromethylation of bromo- and chloro-substituted iodoarenes 3h and 3q selectively took place at the iodide selectively. Additionally, we also compared the reactivity of TCDA with ClCF2CO2Na under the two sets of optimized conditions. The condition of KF and TEA combination


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was applied to the reaction of 1a and 1t with ClCF2CO2Na. Unfortunately, both reaction didn’t afford any desired trifluoromethylated compound 3a and 3t. Then we tested the reaction using the optimized reaction condition of AgF/TMEDA, moderate yields were obtained for 3a (45%) and 3t (40%) after 4 h, which is still much lower than the optimal condition of KF/TEA (64% for 3a, 100% for 3t, Table 2). The new reagent TCDA shows superior reactivity to that of ClCF2CO2Na for trifluoromethylation reaction. Given the simplicity and efficiency of this method, we expect the incorporation of a CF3 group at the late stage of a multi-step synthesis should be possible, even with highly functionalized molecules. To further demonstrate the practical applications of TCDA, the reagent was subsequently evaluated for late-stage trifluoromethylation to make pharmaceutically relevant molecules. As shown in Scheme 2, Boc-Fluoxetin, Fluvoxamine, and Flutamide were prepared in moderate to high yields (35%, 55%, 99%, respectively) by introducing a CF3 group

with

TCDA

in

the

last

step.

In

addition,

the

methyl

6-

(trifluoromethyl)nicotinate and Boc-Fluoxetin were prepared in 10 g scale with isolated yields of 71.9% and 35.6%, demonstrating its practical applications in process chemistry.

Scheme 2. Trifluoromethylation to make medicinal relevant molecules



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Reaction conditions: aryl iodide(1.0 mmol), CuI (1.5 mmol), AgF (4.0 mmol), TMEDA (1.5 mmol) and 2 (2.0 mmol) in DMF (2 mL) were stirred at 100 °C for 6 - 8 h.

Conclusions We have developed the improved synthesis of a readily accessible and relatively inexpensive reagent TCDA and its applications to introduce the trifluoromethyl group to arenes and heteroarenes, including complicated drug molecules at the last step. The reagent provides good conversion for electron-deficient, electronneutral and electron–rich substrates, and its application to process chemistry is also demonstrated through making the pyridine building block and drug related molecule in decagram scale.

Experimental details

General.

1

H NMR and 19F NMR spectrums were recorded on Bruker AM400 and

Varian Mercury Plus 300 MHz spectrometer. Chemical shifts (δ) are reported in ppm, and coupling constants (J) are in Hertz (Hz). The following abbreviations were used to explain the multiplicities: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, br = broad.


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Materials: All reagents were used as received from commercial sources, unless specified otherwise, or prepared as described in the literature. Trimethylsilyl 2-chloro-2,2-difluoroacetate (TCDA). A mixture of sodium 2chloro-2,2-difluoroacetate (50 g, 0.33 mol) and TMSCl (39 g, 0.36 mol) was stirred for 2 h at 50 oC (oil bath temperature) under N2. After cooled to rt, the mixture was distilled under reduced pressure using water pump (0.95 MPa) to afford the title compound (BP: 97-98 oC) as a colorless liquid (38.8 g, yield: 58%). Since TCDA is water sensitive, it should be kept under N2. 1H NMR (300 MHz, CDCl3): 0.35 (s, 9H).

19

F NMR (377MHz, CDCl3) δ (ppm): -64.74. 13C NMR (101

Hz, CDCl3) δ (ppm): 0.3, 117.8 (t, J=302.2 Hz), 159.1 (t, J= 34.3 Hz). Ethyl 4-(trifluoromethyl)-benzoate (3a). To a mixture of 4-Iodo-benzoic acid ethyl ester (276 mg, 1 mmol) in anhydrous triethylamine (1 mL) and anhydrous DMF (1 mL) in a 5 mL vial were added trifluoromethylbenzene (160 mg, 1 mmol), CuI (286 mg, 1.5 mmol), KF (290 mg, 5.0 mmol), and TCDA (373 mg, 2.0 mmol), then sealed. The mixture was stirred for 4 h under N2 at 90 oC. After cooled to rt, the yield was monitored by

19

F NMR spectroscopy.

19

F NMR (377MHz, CDCl3) δ

(ppm): -63.05. Procedure for the synthesis of 3b, 3e, 3f, 3g, 3h, 3q, 3r, 3s, 3t is similar to 3a.

1-Methoxy-4-(trifluoromethyl)benzene

(3l).

To

a

mixture

of

1-iodo-4-

methoxybenzene (234 mg, 1 mmol) in anhydrous DMF (2 mL) in a 5 mL vial were added trifluoromethylbenzene (160 mg, 1 mmol), CuI (286 mg, 1.5 mmol), AgF (508 mg, 4.0 mmol), TMEDA (174 mg, 1.5 mmol), and TCDA (373 mg, 2.0 mmol),



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then sealed. The mixture was stirred for 6 h under N2 at 100 oC. After cooled to rt, the yield was monitored by

19

F NMR spectroscopy.

19

F NMR (377MHz, CDCl3) δ

(ppm): -61.44. The Procedure for the synthesis of 3c, 3d, 3i, 3j, 3k, 3m, 3n, 30, 3p, Flutamide, Boc-Fluoxectin, N-Acetyl Fluvoxamine is similar to 3l.

Methyl 6-(trifluoromethyl)nicotinate. To a mixture of (trifluoromethyl)benzene (4.67 ml, 38.0 mmol), methyl 6iodonicotinate (10 g, 38.0 mmol), TCDA (15.41 g, 76 mmol), CuI (10.86 g, 57.0 mmol) and KF (11.04 g, 190 mmol) in N,N-Dimethylformamide (DMF) (40 mL) and TEA (40ml, 287 mmol), was added trifluoromethylbenzene (4.67 ml, 38.0 mmol). The reaction mixture was stirred and heated under N2 at 90 °C for 8 h, then cooled to rt and diluted with 500 mL ethyl acetate, filtrated and concentrated. Purification via silica column (PE:EA=4:1), afforded the title compound as a white solid (5.7 g, 27.3 mmol, 71.9 % yield). 1H NMR (400 MHz, CDCl3) δ (ppm): d 4.01 (s, 3H), 7.79 (d, J=8.19 Hz, 1H), 8.50 (d, J=8.07 Hz, 1H), 9.31 (s, 1H). NMR (377MHz, CDCl3) (ppm): -68.3.

13

19

F

C NMR (101MHz, CDCl3) δ (ppm): 52.9,

120.2 (q, J=3.03 Hz), 121.1 (q, J=275.7 Hz), 128.5, 138.8, 151.0, 151.3 (q, J=35.4 Hz),164.5. Tert-butylmethyl(3-phenyl-3-(4-(trifluoromethyl)phenoxy)propyl) carbamate (Boc-Fluoxetin).

To

a

mixture

of

tert-butyl

(3-(4-iodophenoxy)-3-

phenylpropyl)(methyl) carbamate (10.0 g, 21.40 mmol), TCDA (8.67 g, 42.8 mmol), AgF (10.86 g, 86 mmol), CuI (6.11 g, 32.1 mmol) and TMEDA (4.84 mL,


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32.1

mmol)

in

N,N-Dimethylformamide

(DMF)

(50

mL)

was

added

trifluoromethylbenzene (2.63 mL, 21.40 mmol). The reaction mixture was stirred and heated under N2 at 100 °C for 8 h, then cooled to rt, diluted with 500 mL ethyl acetate, filtrated, and concentrated. Purification via short silica column (PE: EA= 4:1) then Pre-HPLC, afforded the title compound as a clear oil (3.15 g, 7.62 mmol, 35.6 % yield). 1H NMR (400 MHz, DMSO-d6) δ (ppm): 1.26 (br. s., 5H), 1.34 (br. s., 4H), 2.02 (br. s., 1H), 2.11 (dt, J=13.54 Hz, 6.74 Hz, 1H), 2.78 (s, 3H), 3.28 (br. s., 1H), 3.32-3.58 (m, 1H), 5.42 (d, J=3.79 Hz, 1H), 7.06 (d, J=8.31 Hz, 2H), 7.22-7.31 (m, 1H), 7.36 (t, J=7.52 Hz, 2H), 7.39-7.45 (m, 2H), 7.55 (br. s., 1H), 7.57 (br. s., 1H).

19

F NMR (377MHz, DMSO-d6) (ppm): -59.97.

13

C NMR

(101MHz, DMSO-d6) δ (ppm): 28.3, 34.1, 36.5, 45.3, 77.4, 78.8, 116.5, 121.7 ( q, J=32.3 Hz), 124.9 (q, J=272.7 Hz), 126.4, 127.2 (d, J=3.03 Hz), 128.2, 129.1, 141.2, 155.1, 160.9. ASSOCIATED CONTENT Supporting Information Experimental procedures and compound characterization. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] Notes The authors declare no competing financial interest.



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TCDA: Practical Synthesis and Application in the Trifluoromethylation

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