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Iron-Catalyzed Borylation of Aryl Chlorides in the Presence of Potassium t-Butoxide Takumi Yoshida, Laurean Ilies, and Eiichi Nakamura ACS Catal., Just Accepted Manuscript • Publication Date (Web): 23 Mar 2017 Downloaded from http://pubs.acs.org on March 24, 2017
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ACS Catalysis
Iron-Catalyzed Borylation of Aryl Chlorides in the Presence of Potassium t -Butoxide Takumi Yoshida, Laurean Ilies,* and Eiichi Nakamura* Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 KEYWORDS: iron catalysis, aryl chloride, borylation, alkoxide, aryl boron compound ABSTRACT: A catalytic amount of an inorganic iron salt such as Fe(acac)3 catalyzes borylation of various aryl and heteroaryl chlorides with bis(pinacolato)diboron, where the presence of potassium t-butoxide is crucially important. The alkoxide is considered to produce in situ an electron-rich iron alkoxide complex as the active species. The reaction requires only an iron salt and potassium t-butoxide as promoters and is easily scalable. The arylboron compound prepared by this reaction can be further coupled in situ with an aryl halide under the Suzuki–Miyaura conditions.
INTRODUCTION
Cl
Aryl- and heteroarylboronic acid compounds have long been a focus of interest because of their utility as synthetic building blocks, for example, in Suzuki–Miyaura coupling.1 Chemists, therefore, have focused on transition-metal-catalyzed borylation reactions of aryl halides,2 among which iodides and bromides are frequently used. Although widely available and inexpensive, aryl chlorides have been found to be rather unreactive under palladium, 3 nickel,4 and cobalt 5 catalysis. Ironcatalyzed borylation6 of alkyl halides in the presence of an organometallic activator, t-BuLi7 or EtMgBr,8 which reduces iron to a low-valent state, has been known for some time.9 However, such catalytic systems are ineffective for aryl chlorides (e.g., 5% yield for chlorobenzene).7 We report here that a catalytic amount of an inorganic iron salt such as Fe(acac)310,11 and two equivalents of potassium tert-butoxide 12 enables bis(pinacolato)diboron to react with aryl chlorides in hot toluene to produce a pinacol arylboronate (eq. 1). The reaction needs tert-butoxide as the only ligand on the iron atom and does not require any additional ligand molecules, and represents a rare example13 of the use of an iron alkoxide ate for catalytic organic synthesis.
O
Fe(acac) 3 (5 mol %) KOt-Bu (2.1 equiv)
O
Toluene, 130 ºC, 16 h
B B
+ O
1 (10 mmol)
O
2 (2.0 equiv)
O
B
O
(1)
3: 71%
Several reaction parameters have been examined and summarized in Table 1. In the absence of the iron catalyst, the reaction did not proceed at all, and 1 was recovered (entry 1). Interestingly, the nature of the iron precatalyst is unimportant, iron(II) and iron(III) salts giving the desired product in similar yields (entries 2–4). Ni(acac)2 and Co(acac)3 gave the product in low yield (ca. 20%), and Cu(OAc)2, and Mn(acac)2 were entirely ineffective (entries 5–9). The reaction is sensitive to the nature of the alkoxide: KOt-Bu and LiOt-Bu gave the product in good yield (entries 2 and 10), KOMe in low yield (entry 9), and NaOt-Bu, KF, and K2CO3 gave no desired product (entries 11–13). The amount of base can be reduced to 1.5 equiv without a significant decrease in yield (entry 14). Table 1. Various Parameters Affecting the Borylation of 1Chloronaphthalene (1) with Bis(pinacolato)diboron (2)a
RESULTS AND DISCUSSION In eq. 1, we show a typical example, a 10 mmol scale borylation of 1-chloronaphthalene. A mixture of Fe(acac)3 (175 mg, 0.49 mmol), potassium tert-butoxide (2.3 g, 20.5 mmol), bis(pinacolato)diboron (2, 5.1 g, 20.1 mmol), and 1chloronaphthalene (1, 1.59 g, 9.8 mmol) in toluene was stirred at 130 °C for 16 h under argon to obtain 1-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)naphthalene (3) in 71% yield (1.76 g) after aqueous workup and silica gel chromatography. The starting material was completely consumed, and we also obtained a dechlorinated product, naphthalene, in 20% yield.
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entry
base
catalyst
3 (%) b
1 (%) b Naphthalene (%) b
1
KOt-Bu
none
0
96
0
2
KOt-Bu
Fe(acac) 3
80
0
20
3
KOt-Bu
Fe(acac) 2
78
0
21
4
KOt-Bu
FeCl 2
75
0
30
5
KOt-Bu
Mn(acac) 2
0
95
0
6
KOt-Bu
Co(acac) 3
15
36
26
7
KOt-Bu
Ni(acac) 2
18
57
15
8
KOt-Bu
Cu(OAc)2
0
96
0
9
KOMe
Fe(acac) 3
13
65
21
10
LiOt-Bu
Fe(acac) 3
77
0
16
11
NaOt-Bu
Fe(acac) 3
0
97
0
12
KF
Fe(acac) 3
0
87
0
13
K 2CO 3 KOt-Bu
Fe(acac) 3
0 75
93 0
0
14 c
Fe(acac) 3
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better than an electron-deficient chloride (16). For these reactions, the dechlorinated product (such as discussed for eq. 1) was the main byproduct; efforts to suppress the competing hydrodehalogenation have been unsuccessful. In some cases, recovery of the starting material was observed (7, 10, 11, 12, 15, 16, 17). Unreactive substrates are listed in Figure 1. Chlorinated heterocycles such as 3-chloropyridine, 2-chloro-4,6diphenyl-1,3,5-triazine, or 6-chloroflavone gave no product, and the starting material was largely recovered. 5-Chloro-3methylbenzothiophene gave the corresponding borylated product in 10% yield. Substrates containing reactive functional group such as nitro, nitrile, and carboxylic acid gave no product. Table 2. Iron-Catalyzed Borylation of Various Aryl and Heteroaryl Chloridesa
22
a
Reaction conditions: 1-chloronaphthalene (1, 0.2 mmol), bis(pinacolato)diboron (2, 0.4 mmol), catalyst (0.01 mmol), base (0.42 mmol) in toluene (1.0 mL) at 130 °C for 16 h. bYield determined by GC in the presence of tridecane as an internal standard. c1.5 equiv of KOt-Bu was used.
O
X
BPin +
O
O B
B
O
O 5, 82%c
4, 70%
N
N O
B O
X +
toluene, 130 ºC, 16 h
O
3, 75% (71%) b
O
B 2Pin 2 (2.0 equiv) Fe(acac) 3 (5 mol%) KOt-Bu (2.1 equiv)
B
O B
B O
O
7, 38%
6, 57%
68%d
8,
(2) O B
MeO
X = Br
41%
54%
0%
X=I
20%
67%
0%
X = OTs
12%
1%
80%
O
B O
B
F O
O
9, 65%d
10, 52%d
11, 49%d
O
Aryl bromide and iodide were found to be less suitable substrates than aryl chloride (1-bromonaphthalene: 41%, 1iodonaphthalene: 20%), and the dehalogenated product (naphthalene) accounted for the rest of the product (eq. 2). Efforts to identify the hydrogen source have been unsuccessful. An aryl tosylate reacted poorly (12%), and the starting material was recovered. With the optimized conditions in hand, we investigated the scope of the aryl chlorides (Table 2). 1-Chloro- and 2chloronaphthalene gave the corresponding borylated compounds (3 and 4) regioselectively, which excludes the possibility of a benzyne intermediate. Bis(hexylenepinacolato)diboron gave the desired product (5) in similar yield, but other diboron reagents were ineffective (Figure 1). Nitrogen-containing heteroaromatic chlorides gave the product (6 and 7) in moderate yield together with recovery of the starting material. Biphenyl chlorides (8–12) reacted well at a higher temperature (150 °C), and electron-rich substrates (9) reacted with slightly higher yield than electron-deficient substrates (10). Meta- and orthosubstituted biphenyl chlorides gave the corresponding products (11 and 12) in lower yield together with recovery of the starting material, suggesting the sensitivity of the iron reactive species to steric hindrance. Polycyclic aromatic chlorides such as 2-chloroanthracene (13) and 1-chloropyrene (14) reacted well to produce polyaromatic boronates of interest for the synthesis of conjugated molecules for materials science.14 Dechlorination accounted for the rest of the product of the present borylation reaction. Simple chlorobenzenes (15–17) reacted in a rather low yield, and an electron-rich chloride (15) reacted
O B
O B O
12, 28%d O
O B
OMe
B
F
O
O
16, 21%e
15, 46%
O
14, 53%
O B
B
O
13, 57%
O
O
17, 27%f
a Reaction conditions: aryl chloride (0.4 mmol), bis(pinacolato)diboron (2, 0.8 mmol), Fe(acac)3 (5–10 mol %), KOt-Bu (0.84 mmol) in toluene (2.0 mL) at 130 °C for 16 h, unless mentioned otherwise. See the SI for details. bReaction with 10 mmol of the substrate. cBis(hexylenepinacolato)diboron was used instead of bis(pinacolato)diboron. dReaction temperature was 150 °C. eYield was determined by 19F NMR using trifluorotoluene as an internal standard. fYield was determined by GC by calibration against tridecane as an internal standard.
Figure 1. Examples of unreactive substrates. Ph N
N Cl
Ph
0%
N
Cl
O
Cl
O
0% O O B B O O ~20%
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Ph
0% O
0%
Cl
Cl
10% O
HO
O
HO
NO 2
CN
COOH
0%
0%
0%
OH
O
OH
O
B Si
B B
B B O
Cl
S
Cl
N
0%
0%
9,10-Dichloroanthracene (18) underwent twofold borylation with 3.3 equiv of diboron reagent in the presence of lithium tert-butoxide at 130 °C to give a diborylated compound (19) in 45% yield (eq. 3). The aryl boronic acid pinacol esters can be used without isolation for a subsequent cross-coupling reaction under the standard palladium-catalyzed Suzuki–Miyaura conditions. 15 Thus, after the iron-catalyzed borylation of 1 was over, a catalytic amount of Pd(PPh3)4, potassium carbonate, water, and 9bromoanthracence or 2-bromothiophene were added to the reaction mixture, which was further stirred at 110 °C for 16 h. After column chromatography, the desired products 20 and 21 were obtained in 73% and 62% yield (based on the aryl halide), respectively (eq. 4). Cl +
O
Fe(acac) 3 (10 mol %) LiOt-Bu (3.3 equiv)
O O B B O O
O
2 (3.0 equiv)
18
B
2000 1000 0
–1000 –2000 70
170
270
370
470
570
470
570
magnetic field (mT) (b)
O
2000
1000 (3)
Toluene/THF 130 ºC, 16 h
Cl
B
(a)
O
intensity
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ACS Catalysis
intensity
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0
–1000 19: 45%
Cl
toluene, 130 ºC, 16 h
1
–2000 70
2 (2.0 equiv) aryl halide (0.8 equiv) Fe(acac) 3 (5 mol %) Pd(PPh 3)4 (3 mol %) KOt-Bu (2.1 equiv) K 2CO 3 (1.5 equiv)
Ar
Ar = 9-anthracenyl (20) 2-thienyl (21)
73% 62%
In order to gain an insight into the reaction mechanism, we performed the reaction in the presence of radical scavengers (eq. 5). Although all the radical scavengers tested retarded the desired reaction, this result does not unambiguously demonstrate the involvement of a radical intermediate, because TEMPO may promote decomposition of B2pin2.16 Moreover, we could not identify any TEMPO adducts at the end of the reaction. When the reaction was performed in the presence of TEMPO, the recovery of B2pin2 was not observed, whereas under the standard conditions without a radical scavenger (eq. 1), a small amount of B2pin2 was observed at the end of the reaction; this also suggests the reaction of the boron compound with TEMPO. An attempted radical clock experiment failed because the reaction itself did not occur. Cl +
B 2Pin 2 (2.0 equiv)
Fe(acac) 3 (5 mol%) KOt-Bu (2.1 equiv) additive (1.0 equiv)
BPin (5)
Toluene, 130 ºC, 16 h TEMPO
13%
galvinoxyl
7%
BHT
25%
270
370
magnetic field (mT) (4)
toluene/H 2O (2:1), 110 ºC, 16 h
170
In order to gain insight into the nature of the iron catalyst and rationalize why both iron(II) and iron(III) salts reacted equally well, we performed EPR experiments (Figure 2). The reaction of 1 with 2 was monitored to show that a similar iron species is generated from both Fe(acac)2 and Fe(acac)3 precatalysts. However, this experiment alone cannot distinguish if the observed species is the active species or a resting state.
Figure 2. EPR spectra for the reaction of 1 with 2 under the standard reaction conditions (eq 1) using catalysts of different valence: (a) Fe(acac)2 as the precatalyst; (b) Fe(acac)3 as the precatalyst.
We propose a possible catalytic cycle in Figure 3. First, a ligand exchange between Fe(acac)3 (or an iron(II) precatalyst, see above) and excess KOt-Bu produces an iron ate complex A. A previous report by Uchiyama on iron alkoxide complexes13a suggests that the catalytically active species A may be an iron(II) butoxide ate complex. Complex A transmetalates with diboron17 to produce a boryliron ate complex B, together with alkoxy-Bpin. This complex B next reacts with aryl chloride (1) in a similar manner to iron-catalyzed cross-couplings.11g,h,18 Thus, after an initial one-electron transfer,12 an iron(IV) chloride ate complex (C) is formed,13b,18 which is stabilized by the alkoxy groups and possibly by the potassium countercation, and is in resonance with an iron(III) formalism Cʹ.19,20 The following reductive elimination gives the product 3 and regenerates the iron(II) active intermediate through D. The reactivity difference among aryl chloride, bromide, iodide, and tosylate can be explained by this mechanism. Thus, an aryl tosylate is simply unreactive toward iron alkoxide ate species A and B, as indicated by the recovery of a large amount of starting material. Aryl bromide and iodide, which gave a large amount of dehalogenated byproduct (eq 2), has a lower reduction potential and may directly react with the iron alkoxide A rather than with boryl iron B. The reactivity difference between KOt-Bu and NaOt-Bu may stem from the difference in the iron alkoxide species involved.13 A mechanism where the iron ate complex A directly initiates electron transfer, followed by attack of an aryl radical to alkoxide-activated sp2–sp3 diboronate species21,22,23 is also possible.
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KOt-Bu KCl FeII(Ot-Bu)3 A
KOt-Bu
pinB Bpin pinB Ot-Bu
pinB FeII(Ot-Bu)2
FeII Cl(Ot-Bu)2 D
B Cl
Cl Bpin
pinB FeIV(Ot-Bu)2
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In an oven-dried Schlenk tube, Fe(acac)3 (0.02–0.04 mmol), potassium tert-butoxide (0.84 mmol), and bis(pinacolato)diboron (0.80 mmol) were added. The reaction vessel was evacuated and refilled with argon, then aromatic chloride (0.40 mmol) and toluene (2.0 mL) were added successively. The resulting mixture was stirred at 130 °C–150 ºC for 16 h. After cooling to room temperature, saturated ammonium chloride aqueous solution (3.0 mL) was added. The organic phase was extracted with ethyl acetate (3×5 ml), and the organic layer was passed over a pad of Florisil. The volatiles were removed in vacuo to obtain an oily residue. The crude mixture was purified by column chromatography on silica gel (typically hexane/ethyl acetate = 97:3) to afford the desired compound.
1 3
Iron-Catalyzed Twofold Dichloroanthracene (eq. 3)
C
Cl pinB
FeIII(Ot-Bu)2
C'
Figure 3. A possible catalytic cycle, where Fe(II) is assumed for A.
CONCLUSIONS In summary, we have developed an iron-catalyzed borylation of aryl chlorides in the presence of KOt-Bu, which produces aryl boronic acid esters. The procedure is simple and easily scalable. The choice of the metal alkoxide is crucial for promoting the reaction, presumably because of the reactivity of alkoxy iron ate species and possible stabilization of a highvalent boryliron intermediate. Exploration of alkoxy iron ate species, largely unexplored catalytic intermediates13a in organic synthesis, is underway in our laboratory. EXPERIMENTAL SECTION Iron-Catalyzed Borylation of Aryl Chlorides on 10 mmol scale (eq. 1). In an oven-dried 100 mL two-neck flask fitted with a reflux condenser, Fe(acac)3 (175 mg, 0.49 mmol), potassium tertbutoxide (2.3 g, 20.5 mmol), and bis(pinacolato)diboron (5.1 g, 20.1 mmol) were added. The reaction vessel was evacuated and refilled with argon, then 1-chloronaphthalene (1.59 g, 9.8 mmol) and toluene (50 mL) were added successively. The resulting mixture was stirred under reflux at 130 °C for 16 h. After cooling the reaction mixture to room temperature, saturated ammonium chloride aqueous solution (50 mL) was added. The organic phase was extracted with ethyl acetate (3×50 ml), and the organic layer was dried over magnesium sulfate. After filtering off the solids, the volatiles were removed in vacuo to obtain an oily residue. The crude mixture was purified by column chromatography on silica gel (hexane/ethyl acetate = 97:3) to afford the desired compound as a colorless solid in 71% yield (1.76 g). General Procedure for Borylation of Aryl Chlorides (Table 2)
Borylation
of
9,10-
In an oven-dried Schlenk tube, Fe(acac)3 (14.0 mg, 0.04 mmol), bis(pinacolato)diboron (302 mg, 1.20 mmol) and 9,10dichloroanthracene (100 mg, 0.40 mmol) were added. The reaction vessel was evacuated and refilled with argon, then lithium tert-butoxide in THF (1.0M) (1.3 mL, 1.3 mmol) and toluene (2.0 mL) were added successively. The resulting mixture was stirred at 130 °C for a 16 hours. After cooling to room temperature, saturated ammonium chloride aqueous solution (3.0 mL) was added. The organic phase was extracted with toluene (3×5 ml), and the organic layer was passed over a pad of Florisil. The volatiles were removed in vacuo to obtain an oily residue. The crude mixture was purified by column chromatography on silica gel (hexane/ethyl acetate = 95:5) to afford the desired compound as a white granular solid in 45% yield (76 mg). General Procedure for One-pot Suzuki-Coupling using Pd Catalysis (eq. 4) In an oven-dried Schlenk tube, Fe(acac)3 (0.02 mmol), potassium tert-butoxide (0.84 mmol), and bis(pinacolato)diboron (0.80 mmol) were added. The reaction vessel was evacuated and refilled with argon, then 1-chloronaphthalene (0.40 mmol) and toluene (2.0 mL) were added successively. The resulting mixture was stirred at 130 °C for a 16 hours. After cooling the reaction mixture to room temperature, Pd(PPh3)4 (0.011 mmol), potassium carbonate (0.6 mmol), water (1 mL) and 9bromoanthracene or 2-bromothiophene (0.31 mmol) were added. The resulting mixture was stirred at 110 °C for 16 h. After cooling the reaction mixture to room temperature, saturated NH4Cl aqueous solution was added. The organic phase was extracted with ethyl acetate (3×5 ml), and the organic layer was passed over a pad of Florisil. The volatiles were removed in vacuo to obtain an oily residue. The crude mixture was purified by column chromatography on silica gel (hexane 100%) to afford the desired compound. Preparation of the EPR Sample (Figure 2) In an oven-dried Schlenk tube, Fe(acac)3 or Fe(acac)2 (0.01 mmol), potassium tert-butoxide (0.21 mmol), and bis(pinacolato)diboron (0.20 mmol) were added. The reaction vessel was evacuated and refilled with argon, then 1chloronaphthalene (0.10 mmol) and toluene (0.5 mL) were added successively. The resulting mixture was stirred at 130 °C for 1 hour. After cooling to room temperature, the re-
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action mixture was transferred to an EPR tube and the measurement was performed.
ASSOCIATED CONTENT
AUTHOR INFORMATION Corresponding Author *
[email protected];
[email protected] SUPPORTING INFORMATION Experimental procedures and physical properties of the compounds. The Supporting Information is available free of charge on the ACS Publications website.
ACKNOWLEDGMENTS We thank MEXT for financial support KAKENHI (15H05754 for E.N. and JP26708011 to L.I.). Y.T. thanks the University of Tokyo’s “Evonik Scholars Fund” Scholarship. This work was partially supported by CREST, JST (Molecular Technology).
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(a) Mfuh, A. M.; Doyle, J. D.; Chhetri, B.; Arman, H. D.; Larionov, O. V. J. Am. Chem. Soc. 2016, 138, 2985–2988. (b) Mfuh, A. M.; Nguyen, V. T.; Chhetri, B.; Burch, J. E.; Doyle, J. D.;
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Nesterov, V. N.; Arman, H. D.; Larionov, O. V. J. Am. Chem. Soc. 2016, 138, 8408–8411. (c) Li, Z.; Jiao, L. J. Am. Chem. Soc. 2017, 139, 607–610.
O B B
Cl
O
O
O
B
Fe(acac) 3 (5 mol %)
1.59 g
KOt-Bu (2.1 equiv) toluene, 130 ºC
71%
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O
