Highly efficient palladium-catalyzed cross-coupling of diarylborinic acids with arenediazoniums for practical diaryl synthesis

Journal Pre-proofs Digest paper Highly efficient palladium-catalyzed cross-coupling of diarylborinic acids with arenediazoniums for practical diaryl synthesis Fengze Wang, Chen Wang, Guoping Sun, Gang Zou PII: DOI: Reference:

S0040-4039(19)31290-0 https://doi.org/10.1016/j.tetlet.2019.151491 TETL 151491

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Tetrahedron Letters

Received Date: Revised Date: Accepted Date:

20 October 2019 1 December 2019 4 December 2019

Please cite this article as: Wang, F., Wang, C., Sun, G., Zou, G., Highly efficient palladium-catalyzed cross-coupling of diarylborinic acids with arenediazoniums for practical diaryl synthesis, Tetrahedron Letters (2019), doi: https:// doi.org/10.1016/j.tetlet.2019.151491

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Graphical Abstract

Highly efficient palladium-catalyzed crosscoupling of diarylborinic acids with arenediazoniums for practical diaryl synthesis

Leave this area blank for abstract info.

Fengze Wang,a Chen Wang,a Guoping Sunb and Gang Zou*a aSchool

of Chemistry & Molecular Engineering, East China University of Science & Technology, 130 Meilong Rd, Shanghai, China. bJiangsu Danxia New Material Co. Ltd., Jingsi Road, Ecological Chemical Technology Industrial Park, Suqian, Jiangsu Province, China R1

N 2L +

Ar

Ar R L = BF , NMe 2 4

B OH

0.3mol% Pd(OAc)2

R1

sol, r.t., 12h, open flask R up to 99%

Ar

1

Tetrahedron Letters journal homepage: www.elsevier.com

Highly efficient palladium-catalyzed cross-coupling of diarylborinic acids with arenediazoniums for practical diaryl synthesis Fengze Wang,a Chen Wang,a Guoping Sunb and Gang Zou*a School of Chemistry & Molecular Engineering, East China University of Science & Technology, 130 Meilong Rd, Shanghai, China. Jiangsu Danxia New Material Co. Ltd., Jingsi Road, Ecological Chemical Technology Industrial Park, Suqian, Jiangsu Province, China

a b

ARTICLE INFO

ABSTRACT

Article history: Received Received in revised form Accepted Available online

A highly efficient cross-coupling of cost-effective diarylborinic acids with both isolatable and latent arenediazoniums, i.e. tetrafluoroborates and aryltriazenes, respectively, has been developed with a practical palladium catalyst system under base-free conditions in open flask at room temperature. A variety of electronically and sterically various biaryls, in particular, those bearing a coordinative ortho-substituent, could be obtained in good to excellent yields by using 0.3mol% palladium acetate as catalyst. Features of the protocol including cost-effectiveness of diarylborinic acids, efficacy to heteroatom ortho-substituted substrates and high chemoselectivity to aryl chlorides have been clearly demonstrated in practical synthesis of fungicide Boscalid.

Keywords: Suzuki coupling diarylborinic acid arenediazonium aryltriazene

2009 Elsevier Ltd. All rights reserved.

1. Introduction Suzuki coupling of arenediazonium salts has increasingly attracted interests1 as a complement to the conventional aryl halide and phenol derived electrophiles since the seminal publications by Genêt2 and Sengupta groups.3 independently. Several advantages of using arenediazoniums as aryl sources in Suzuki-coupling have come to be recognized.1f Firstly, the superior reactivity of arenediazoniums allows high chemoselectivity. Secondly, the reaction could even work efficiently in open flask under mild conditions without basic additives. Thirdly, a variety of arenediazoniums could be readily prepared from cheap commercial anilines, promising a high potential in large-scale production. Moreover, some doubly or multiply substituted aryl halides, the conventional electrophiles in Suzuki coupling, are in fact prepared from anilines via Sandmeyer reaction, a diazotization-halogenation procedure, taking advantage of the strong electronic effects of amino or its precursor nitro group. To further improve the protocol, many efforts have been recently devoted towards development of sustainable procedures by in situ generation of arenediazoniums1d, 4 and/or use of heterogeneous palladium catalysts.1b, 1e, 5 However, the high cost of arylboronic acids that generally have to be used as the nucleophile counterpart still remains to be addressed, in particular, in potential application of the protocol in industrial production of fine chemicals. Zarei and co-workers have reported Suzuki coupling of sodium tetraarylboronates with arenediazoniums grafted on SiO2 under basic aqueous conditions.6 However, only two aryl groups in tetraarylboronates could be utilized. The use of support-

stabilized diazoniums, requirement of 2equiv. basic additives and loss of half amount of aryl groups in boronates almost nullified the benefits of the high reactivity of arenediazoniums and atom economy of tetraarylboronates. Similar to tetraarylboronates, diarylborinic acids could be economically prepared via Barbier-type one-pot procedure from aryl bromides, magnesium and trialkylboronates under non-cryogenic conditions.7 We have shown that diarylborinic acids could be efficiently used as a cost-effective alternative to arylboronic acids in palladium and nickel catalyzed cross-coupling for biaryl synthesis.8 In particular, the aerobic copper-catalyzed Chan-Lam coupling, which appeared to be incompatible with reducing ability of high-order arylborons, could also well accommodate diarylborinic acids as aryl source with proper catalyst systems.9 We anticipate that Suzuki coupling of arenediazoniums with diarylborinic acids should be also feasible. Herein, we report a highly efficient cross-coupling of diarylborinic acids with isolatable or latent arenediazoniums catalyzed by simple ligandless palladium catalyst under base-free conditions in open flask at room temperature, in which both aryl groups of diarylborinic acids could be utilized at low catalyst loading. 2. Results and Discussion Cross-coupling of 2-nitrobezenediazonium tetrafluoroborate (1a) with bis(4-chlorophenyl)borinic acid (2a) was taken as the model for condition optimization considering the desired product 4'-chloro-2-nitro-biphenyl (3aa) could be used for synthesis of fungicide Boscalid (Table 1).

Tetrahedron Letters

2

Table 1. Optimization of palladium-catalyzed cross-coupling of diarylborinic acids with arenediazoniums a NO2 L

NO2

Cat. +

Cl

1a

2

B(OH)

Cl

base. sol. r.t. 3aa

2a

L = N2+BF4- (1a), N2NMe2 (1a')

Entry

N /B (mol ratio)

Cat. (mol%)

Solvent

Additive (equiv)

Time (h)

Yield (%)b

1

1a/2a (1.0)

PdCl2 (1)

MeOH

/

12

49

2

1a/2a (1.2)

PdCl2 (1)

MeOH

/

12

59

3

1a/2a (0.8)

PdCl2 (1)

MeOH

/

12

37

4

1a/2a (1.2)

Pd(OAc)2 (1)

MeOH

/

12

78

5

1a/2a (1.2)

Pd(OAc)2 (1)

MeOH

/

12

trace

6

1a/2a (1.2)

Pd(OAc)2 (1)

H2O

/

12

10

7

1a/2a (1.2)

Pd(OAc)2 (1)

THF

/

12

86

8

1a/2a (1.2)

Pd(OAc)2 (1)

Dioxane

/

12

92

9

1a/2a (1.2)

Pd(OAc)2 (1)

CH2Cl2

/

12

trace

10

1a/2a (1.2)

Pd(OAc)2 (1)

DMF

/

12

92

11

1a/2a (1.2)

Pd(OAc)2 (1)

EtOH

/

12

90

12

1a/2a (1.2)

Pd(OAc)2 (1)

i-PrOH

/

6

96

13

1a/2a (1.2)

Pd(OAc)2 (1)

t-BuOH

/

6

99

14

1a/2a (1.2)

Pd(OAc)2 (0.5)

t-BuOH

/

6

97

15

1a/2a (1.2)

Pd(OAc)2 (0.3)

t-BuOH

/

12

99

16

1a/2a (1.2)

Pd(OAc)2 (0.1)

t-BuOH

/

12

88

17

1a/2a (1.2)

5%Pd/C (1)

t-BuOH

/

6

9

18

1a/2a (1.2)

5%Pd/C (1)

MeOH

/

12

83

19

1a/2a (1.2)

5%Pd/C (1)

MeOH

/

6

82d

20

1a/2a (1.2)

5%Pd/C (1)

EtOH

/

12

75

21

1a/2a (1.2)

5%Pd/C (1)

THF

/

12

20

22

1a/2a (1.2)

5%Pd/C (1)

i-PrOH

/

12

48

23

1a/2a (1.2)

5%Pd/C (1)

Dioxane

/

12

14

24

1a/2a (1.2)

5%Pd/C (1)

H2O

/

6

trace

25

1a/2a (1.2)

5%Pd/C (1)

MeOH

K2CO3 (1.0)

12

14

26

1a/2a (1.2)

5%Pd/C (1)

MeOH

K3PO4·3H2O (1.0)

6

trace

27

1a/2a (1.2)

5%Pd/C (1)

MeOH

NaF (1.0)

12

43

28

2a/1a’ (0.65)

Pd(OAc)2 (0.3)

t-BuOH

BF3·Et2O (1.5）

12

67

29

2a/1a’ (0.65)

Pd(OAc)2 (0.3)

MeOH

BF3·Et2O (1.5）

12

75

30

2a/1a’ (0.65)

Pd(OAc)2 (0.3)

Dioxane

BF3·Et2O (1.5）

12

65

31

2a/1a’ (0.65)

Pd(OAc)2 (0.3)

MeOH

BF3·2MeOH (1.5）

12

84

32

2a/1a’ (0.65)

Pd(OAc)2 (0.3)

Dioxane

BF3·dioxane (1.5）

12

96

33

2a/1a’ (0.65)

Pd(OAc)2 (0.3)

Dioxane

BF3·dioxane (1.2）

12

77

c

Reaction conditions: reaction was carried out at 0.5mmol scale with respect to 2a for arenediazoniums or 1.0mmol scale with respect to 1a’, solvent (4 mL), open flask, at room temperature. bIsolated yields. c0.3mmol tetraarylboroate used. dAt 50oC. a

The product 3aa was obtained in 37-59% yields with 0.81.2equiv. 1a using 1mol% PdCl2 as catalyst in methanol at room temperature (Table 1, entries 1-3). The yield could be increased to 78% by using 1mol% Pd(OAc)2 (Table 1, entry 4). When sodium tetra(4-chlorophenyl) boronate was used only trace amount of 3aa was detected along with nitrobenzene as the major product formed via protodediazotization of arenediazonium 1a (Table 1, entry 5). Solvent screening implied that homogenous catalysis should be crucial for the crosscoupling. Therefore, solvents that could dissolve both substrates

1a and 2a, e.g. THF (86%, 12h), dioxane (92%, 12h), DMF (90%, 12h), EtOH (90%, 12h), i-PrOH (96%, 6h) and t-BuOH (98%, 6h), gave 3aa in excellent yields while poor yields were obtained in CH2Cl2 and H2O (Table 1, entries 6-13). In fact, the catalyst loading could be reduced to 0.3mol% without significant decrease in 3aa yields (Table 1, entries 13-15). Use of heterogeneous palladium catalyst allows readily isolation, reuse or recovery of the noble metal component, therefore having attracted extensive interests.10 Several reports have been published on use of solid supported palladium catalysts in the

3 Suzuki coupling of arenediazoniums.1b, 1e, 5, 10c In particular, Felpin and co-workers reported that palladium nanoparticles of low oxidation degree and uniformly dispersed on the charcoal showed very high activity.5c Given the widespread use and readily availability of simple commercial Pd/C as well as to test the homogeneous catalysis speculation (vide supra) we investigated the performance of 5%Pd/C catalyst in the model reaction. Very low yields were obtained in the solvents that worked well for Pd(OAc)2 although a comparable yield could be obtained in MeOH (82-83% vs 78%) (Table 1, entries 17-24). The low efficiency of Pd/C catalyst appeared to indirectly support the speculation of homogenous catalysis. Unlike the conventional Suzuki coupling, the presence of a common base, e.g. K2CO3, K3PO4·3H2O and NaF etc., proved to be deleterious, even completely block the coupling reaction (Table 1, entries 2527). Therefore, the optimal conditions were set as 1.2equiv. arenediazonium tetrafluoroborates catalyzed by 0.3mol% Pd(OAc)2 in t-BuOH at room temperature. Aryltriazenes which release labile arenediazoniums when treated with Brönsted or Lewis acids could serve as protected diazoniums to obtain control of chemoselectivity or improve safety, handling and storing. Aryltriazenes have been shown to be effective thermally stable surrogates of arenediazonium tetrafluoroborates in the presence of BF3 in Suzuki coupling by Tamao et al.11 After establishment of the optimal conditions for cross-coupling of arenediazonium tetrafluoroborates with diarylborinic acids, we further investigated the reactivity of aryltriazene 1a’ in the model reaction. However, unsatisfactory yields were obtained when the catalyst system for 1a was directly adopted with the aid of BF3·Et2O, due to a strongly exothermic ligand exchange between BF3 complexes (Table 1, entries 28-30). To our delight, cross-coupling of 1a’ with a slight excess (1.3equiv.) 2a gave 3aa in 96% yield with 0.3mol% Pd(OAc)2 by using 1.5equiv. pre-formed BF3·dioxane complex in dioxane at room temperature (Table 1, entry 32), remarkably more practical than the coupling of aryltriazenes with excess (2equiv.) arylboronic acids reported in literature, for example, 5mol% Pd(OAc)211c in dioxane or complicated catalyst systems, e.g. 2mol% Pd2(dba)3 / 8mol%P(t-Bu)3) in DME 11a and 2mol% polystyrene-supported N-heterocyclic carbene palladium catalyst in dioxane.11b With the optimal conditions in hands, scope of the crosscoupling of arenediazonium tetrafluoroborates and aryltriazenes with diarylborinic acids was explored (Table 2). Reaction of bis(o-tolyl)borinic acid (2b) with 1a (94%) or 1a’ (94%) also gave 3ab in comparable yields to its para-analog 2a, indicating small steric hindrance could be smoothly overcome. Electronic effects of diarylborinic acids proved to be negligible since excellent yields (93-96%) could be obtained in the reaction with 1a or 1a’ regardless of an electron-donating (Me, or OMe) or withdrawing group (F) on the phenyl rings of borinic acids (Table 2, 3ac-3ah). However, the influence of substituents of arenediazonium tetrafluoroborates or aryltriazenes on crosscoupling is more than simple electronic or steric effects. Generally, arenediazonium tetrafluoroborates bearing weakly electron-withdrawing (Cl) or -donating (Me) groups crosscoupled with 2a affording 3ka, 3la, or 3qa-3ta in excellent yields while those bearing strong electron-withdrawing (m-NO2 1b, p-NO2 1c, p-CO2Et 1d, p-Ac 1e, p-CN 1f, p-MeSO2 1g) or – donating (p-OMe 1n, m-MeS 1o, m-AcNH 1p) para- or metasubstituents gave slightly lower yields (54-86%) for crosscoupling products (3ba-3ga, 3na-3pa). The poor yields for 3fa (55%, p-CN) and 3na (54%, p-OMe) may be attributed to the

competitive reaction of CN group with diazoniums and fast dediazonation due to strong electron-donating ability of MeO group, respectively. However, coordination of an orthosubstituent, e.g. o-NO2 (3ab-3ah) and o-OMe (3ma), at arenediazonium tetrafluoroborates appeared to surpass its electronic and steric effects, remarkably increasing the product yields12 although sterically demanding o-tert-butylbenzene diazonium tetrafluoroborate gave a trace amount of product. In the case of aryltriazenes, lower functional group compatibility was observed than that of the corresponding arenediazonium tetrafluoroborates, possibly due to the presence of strong Lewis acid BF3 in the system. For example, the poorer yields were obtained for aryltriazenes bearing p-CN (1f’, 26%) and p-SO2Me (1g’, trace). An ortho-substituent, e.g. o-OMe (1m’, 75%) and oMe (1q’, 79%), also appeared to slightly decrease the reaction yields although a NO2 group could be tolerated in both cases. The low dependence of the palladium catalyzed cross-coupling of arenediazonium tetrafluoroborates on the electronic activation/deactivation of aryl substituents in both coupling counterparts indicated that neither the oxidative addition (OA) of arenediazoniums nor the transmetalation from diarylborinic acids but the reductive elimination (RE) should be the ratedetermining step in the widely accepted mechanism for Suzuki coupling (Scheme 1). The higher yields obtained from heteroatom ortho-substituted arenediazonium tetrafluoroborates are consistent with the RE rate-determining mechanism although the chelate stabilization should also contribute via preventing protodediazotization of arenediazoniums.12 The lower functional group compatibility and the larger steric effects of aryltriazenes could reasonably be attributed to the use of excess BF3 to in-situ generate arenediazoniums prior to the catalysis. Pd0(sol)n

Ar Ar' rate-determining

RE

Ar N2+LOA

L = BF4, BF3(NMe2)

Ar Pd N 2+ L -

Ar Pd Ar' B(OH)(L)2 Ar'B(OH)L

Ar Pd+L-

Ar'2B(OH)

N2

Scheme 1 A mechanistic explanation To investigate the chemoselectivity of diazonium vs bromide, reactions of bromo (1i, 1i’, 1j and 1j’) and electronic analogs, pchloro (1k and 1k’) diazoniums, with 2a were carried out. In the case of tetrafluoroborates, an excellent yield (96%) was obtained for 3ka while the bromo containing products 3ia (72%) and 3ja (75%) were isolated in significantly lower yields due to the competitive reaction of Caryl-Br bond. These results indicated only modest selectivity of diazoniums vs bromide could be achieved in Suzuki coupling with diarylboronic acids albeit the former was described as super-electrophiles.13 The yields from reaction of the in situ generated arenediazoniums 1i’ (53%) and 1j’ (55%) further decreased because the aryltriazenes have to be converted into diazoniums before the desired cross-coupling could occur while no influence was observed for the chloro analog (1k’, 96%).

Table 2. Scope of the Pd-catalyzed cross-coupling of diarylborinic acids with arenediazoniums

Tetrahedron

4 +

N 2L

R1

R2

L=BF4, 1a-t L=NMe2, 1a'-t'

0.3 mol% Pd(OAc)2 B(OH) Conditions A or B, rt., air, 12h R1 2 2a-h A: t-BuOH for 1a-t B: 1.5equiv. BF3, dioxane for 1a'-t'

NO2

NO2

Me

NO2

NO2

R2 3ab-ta

Me Me

3ac A: 96%, B: 96%

3ab A: 94%, B: 96% NO2

OMe

NO2

NO2 OMe

OMe 3ae A: 95%, B: 95%

3ad A: 93%, B: 95% Cl

F

3af A: 94%, B: 94%

Cl

3ag A: 96%, B: 97% Cl

Cl

O 2N 3ba A: 76%, B: 80%

3ah A: 95%, B: 92%

O 2N

Cl

Cl

O S 3ga Me O A: 63%, B: trace

NC

3fa A: 55%, B: 26% Cl

Cl

Cl

O 3da A: 86%, B: 82%

3ca EtO2C A: 85%, B: 91% Cl

NO2

Cl

Br

3ia A: 72%, B: 53%

3ja A: 75%, B: 55%

Cl

Cl

OMe

Cl

Cl

Br

3ha A: 91%, B: 85%

3ea A: 83%, B: 85%

Cl MeS

Cl

Cl

3la A: 95%, B: 90%

3ka A: 98%, B: 96% Cl

Me

3na A: 54%, B: 71%

Cl

Cl

AcHN 3pa A: 79%, 78%

MeO

3ma A: 90%, B: 75%

3oa A: 75%, B: 78%

Cl

Cl

Me 3qa A: 89%, B: 79%

Me

3ra A: 87%, B: 80%

Me Me

3sa A: 92%, B: 87%

3ta A: 88%, B: 76%

Reaction conditions: 1 (1.2 mmol), 2 (0.5 mmol), or 1’ (1 mmol), 2 (0.65 mmol), 4 mL solvent at rt. in open flask for 12h; isolated yields for products obtained from conditions A and B, respectively. a

NO2 Cl

Application of the palladium catalyzed cross-coupling of arenediazoniums with diarylboronic acids was demonstrated in gram-scale synthesis of Boscalid (Scheme 2), a fungicide developed by BASF company.14 With the above optimal catalyst system, the key intermediate 3aa could be obtained in excellent yields (98-99%), which was chemoselectively converted into aniline intermediate 4 in almost quantitative yield by charcoalassisted, FeCl3 catalyzed nitro reduction with hydrazine although selective (NO2) reduction via palladium catalyzed hydrogenation was hampered by the concurrence of dehalogenation. Boscalid was finally obtained in 93% yield from the condensation of aniline 4 with nicotinoyl chloride under routine amidation conditions. With this three-step process, Boscalid could be obtained in 90% overall yield from readily available and costeffective starting materials under practical conditions, promising a potential in a large scale production.

N2BF4 0.6equiv.

1a

B OH

t

Cl

NO2

0.3 mol% Pd(OAc)2 BuOH, r.t. 12h, 98%

2a

3aa

Cl

Cl 5mol% FeCl3 C*, N2H4.H2O

NH2

MeOH, H2O, 85 oC 2h, 99% Cl N

Cl

N

O Cl

Et3N, THF 25 oC 1 h, 93%

4 O NH

Cl Boscalid (5)

Scheme 2. Synthesis of Boscalid in gram-scale 3. Conclusions In summary, a highly efficient cross-coupling of diarylborinic acids with arenediazonium tetrafluoroborates or aryltriazenes

catalyzed by 0.3mol% Pd(OAc)2 has been developed under basefree conditions in open flask at room temperature to provide a variety of electronically and sterically various biaryls in good to excellent yields. Both aryl groups of diarylborinic acids could be utilized effectively, making the protocol a cost-effective alternative to the conventional version of arylboronic acids. Common alcohol and ether solvents that could efficiently dissolve the substrates worked well, among which t-BuOH performed best for arenediazonium tetrafluoroborates while dioxane appeared to be the choice of solvent in the presence of 1.5equiv, BF3 as activator for aryltriazenes. Features of the protocol, i.e. the cost-effectiveness of diarylborinic acids, high efficacy to heteroatom ortho-substituted substrates, low loading of the simple and ligandless palladium catalyst, good chemoselectivity to halides (Cl) and mild reaction conditions, have been clearly demonstrated in a highly efficient and practical synthesis of fungicide Boscalid, promising a good potential in application in large-scale production of fine chemicals.

6. 7.

8.

Acknowledgments

9. 10.

We are grateful for financial support provided by the National Natural Science Foundation of China (21472041) 11.

References and notes 1.

2. 3. 4.

5.

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Supplementary Material Supplementary material available: experimental procedures, compound characterization and copies of NMR spectra for aryltriazenes and biaryls.

Graphical Abstract

R1

N 2L +

Ar

Ar R L = BF , NMe 4 2

B OH

0.3mol% Pd(OAc)2

sol, r.t., 12h, open flask R up to 99%

High efficacy to heteroatom ortho-substituted diazoniums Highlights Cost-effective arylboron reagent Good functional group compatibility Low loading of simple ligandless catalyst Mild, open-flask and base-free conditions

R1

Ar

6

Tetrahedron Letters

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: