Copper-catalyzed hydroboration of arylalkenes at room temperature

Tetrahedron Letters 56 (2015) 2297–2302

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Copper-catalyzed hydroboration of arylalkenes at room temperature Shibin Hong a, Mengyan Liu a, Wei Zhang a, Qiang Zeng a,⇑, Wei Deng a,b,⇑ a b

Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, China School of Chemical and Engineering, Shanghai Institute of Technology, Shanghai 201418, China

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 29 December 2014 Revised 13 February 2015 Accepted 9 March 2015 Available online 20 March 2015

A mild method has been developed for the hydroboration of arylalkenes using Cu2O as catalyst with PPh3 as ligand in methanol at room temperature. A gram-scale reaction under this condition was also achieved with high yield. Ó 2015 Elsevier Ltd. All rights reserved.

Keywords: Copper-catalyzed Hydroboration Arylalkenes Organoboron

Boron chemistry is one of the most important subjects in organic chemistry, and it attracted increasing attention because of their intrinsic scientific importance and industrial applications. Much attention has been focused on looking for easy and efficient ways to prepare organoboron reagents, which is an important reagent family in organic synthesis.1 The traditional C-B formation via addition reaction of borane with alkenes or alkynes was conducted under relatively harsh conditions. Recently, transition metal catalyzed hydroboration of unsaturated compound has been regarded as a useful strategy for the synthesis of alkyl boronic acid derivatives.2 Several transition metals have been used to catalyze hydroboration reactions of unsaturated compounds, including platinum,3 gold,4 palladium,5 rhodium,6 iron7 and nickel.8 Due to the low price and toxicity, Cu salts were widely reported as catalysts for cross-coupling reactions, which are valuable reactions for synthetic organic chemistry.9 Copper-catalyzed reaction showed many advantages, but suffered from several limitations such as low conversion efficiency.10 Previously, we reported our development of mild conditions for copper-catalyzed borylation of primary and secondary alkyl halides.11 Meanwhile, copper catalyzed hydroboration of styrenes activated with electronwithdrawing groups have also been described.12 Herein, we reported an efficient copper-catalyzed borylation reaction of arylalkenes at room temperature in methanol, which is featured with low cost, low toxicity, and high conversion efficiency.

⇑ Corresponding authors. E-mail addresses: (W. Deng).

[email protected]

(Q.

http://dx.doi.org/10.1016/j.tetlet.2015.03.033 0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

Zeng),

[email protected]

The initial experiment was focused on styrene and bis(pinacolato)diboron (B2(pin)2) as model substrates. We examined the effects of various copper salts, bases, ligands, and solvents on the yields of the hydroboration. The detailed results are listed in Table 1. As shown in Table 1, we found that various Cu salts can catalyze the reaction, including CuI, Cu, CuAc2, and Cu2O. Cu2O showed a better yield (93%) (Entry 4) than others. It is important to be note that Cu powder also gives a good yield of 81%. Fixing Cu2O as the catalyst, we then explored the reaction with different bases, and found that K2HPO4 was the best choice (yield = 98%, entry 9). The organic bases of Et3N and DIEA gave relatively low yields. The effects of the ligands on Cu are summarized in entries 12–17. It was found that ligands played an important role in the reaction, which was confirmed by a control experiment without ligand (entry 12). 2-(Di-tert-butylphosphino)biphenyl (DTBB) gave a yield of 75%. The bidentate phosphine ligands of bis(diphenylphosphino) methane (BDPM), 1,4-bis(diphenyl-phosphino)butane (BDPB), and 1,2-bis(diphenylphosphino)-ethane (DPPE) also gave low yields of 72%, 45%, and 77%, respectively. Although amino acids were reported as excellent ligands for Cu13, there was no product observed in the case of L-proline (entry 17). When toluene instead of methanol was used as solvent, no target compound was obtained, indicating that methanol in the reaction acted as not only solvent but also a hydrogen donor. Therefore, the effect of alcohol on the reaction was tested. In glycol, i-propanol and t-butanol, the yields dramatically decreased to 67%, 16%, and 8%, respectively (entries 19–21). Methanol was the best solvent, possibly due to the better solubility of salts and better ability of the hydrogen donor. Furthermore, it was found that in the presence of t-BuOLi,

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Table 1 Optimization studies for ‘Cu’ catalyze the reaction of styrene and B2(pin)2a

O O

O

O

[Cu], Ligand

O

Base, Solvent

B B

+

B O

1a

2a

Entry

Catalyst

Ligand

Solvent

Base

Yieldb (%)

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

CuI Cu CuAc2 Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O Cu2O

PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 none DTBB BDPM BDPB DPPE L-Proline PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3d PPh3e

Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Toluene Glycol i-Propanol t-Butanol DMF Toluene Methanol Methanol

t-BuOLi t-BuOLi t-BuOLi t-BuOLi K2CO3 Na2CO3 Li2CO3 K3PO4 K2HPO Et3N DIEA K2HPO4 K2HPO4 K2HPO4 K2HPO4 K2HPO4 K2HPO4 K2HPO4 K2HPO4 K2HPO4 K2HPO4 t-BuOLi t-BuOLi K2HPO4 K2HPO4

90 81 85 93 94 91 88 93 98(92c) 88 86 0 75 72 45 77 0 0 67 16 8 78 49 93 98

a Reaction conditions: styrene 1a (0.5 mmol), B2(pin)2 (0.75 mmol), catalyst(10 mol %), ligand (13 mol %), and base (1.0 mmol) in 2 mL solvent at room temperature for 18 h. b Yield determined by GC-MS, based on styrene, with an internal standard. c Isolated yield. d Ligand 10 mol %. e Ligand 20 mol %.

Table 2 Copper-catalyzed hydroboration of arylalkenes with B2pin2

O R

O

1b-1m Entry

Alkyl halide

Cu 2O (10 mol%) PPh3 (13 mol%)

O

K 2HPO4 (2 equiv) CH3OH(2 ml), RT,18h

B B

+

O

O

B O

R

2b-2m Yielda (%)

Product

O B 1

O

F

1b

88

F

2b O B 2

O

Br

1c

Br

92

2c O

3

B

F

O

1d

F

90

2d O B

4

O

Br

1e

Br

2e

94

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S. Hong et al. / Tetrahedron Letters 56 (2015) 2297–2302 Table 2 (continued) Entry

Alkyl halide

Yielda (%)

Product

F

F F

F

F

F

O B O

5

F F

95

F F

2f

1f

O B 6

HOOC

O

1g

67

HOOC

2g O B 7

O

O

58

O

1h

2h O B O

8

57

1i 2i O B 9

O

HO

1j

63

HO

2j O B

HO 10

O

B HO

O

60

B O

1k

2k O B

N 11

1l

b

95

O

N

2l O B O

12

95

1m 2m

13

O B

1n

O

85

2n

14

O B

1o

O

95

2o

15

—

ND

—

ND

1p

16

1q a b

Isolated yields. Diethenyl-benzene is a mixture of isomers with ortho-, meta-, and para-diethenyl-benzene.

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Table 3 Copper-catalyzed hydroboration of arylalkenes with B2(neop)2

O R

+

1b-k Entry

Alkyl halide

Cu2O (10 mol%) PPh 3 (13 mol%)

O

K2HPO 4 (2 equiv) CH3OH(2 ml), RT,18h

B B O

O

O

B O

R

3a-k Yielda (%)

Product

O B O

1

72

1a 3a

2

O B O

F

1b

76

F

3b

3

O B O

Br

1c

Br

3c O B O

F

4

84

1d

79

F

3d O B O

5

1e

Br

F

3e O B O

F

F 6

83

Br

F

F

F

F

68

F

F

F

3f

1f

O B O

7

93

1gb 3g O B O

O O

O

8

1j

O

79

3j O B

HO 9

B HO

O

O

1k

59

B O

3k

10

O B O

81

1n 3n O B

11

O

1o 3o

a b

Isolated yields. Diethenyl-benzene is a mixture of isomers with ortho-, meta-, and para- diethenyl-benzene.

82

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O

O B B

+ O 1a (0.52g, 5.0mmol)

O

1.5 equiv

O

Cu 2O (5 mol%) PPh3 (7 mol%) K 2HPO4 (2 equiv) CH3OH(20 ml), RT,18h yield: 90%

B O 2a (1.04g, 4.5mmol)

Scheme 1. Scale-up experiment of hydroboration of styrene with B2(pin)2.

LCu-Bpin 1

(pin)B-OMe

R

I (pin)B-B(pin) CuL Bpin R

L Cu OMe 4

2 II

H Bpin

A possible catalytic cycle for the hydroboration of alkenes promoted by methanol was proposed in Scheme 2. It was assumed that the hydroboration was initiated by Ligand-Cu-Bpin catalyst, and copper–boryl complex added to the alkenes. Then, the addiction product was immediately protonated by MeOH to give the desired product, along with regeneration of the copper-ligand complex, which reacted with B2pin2 to participate in the cycle. In conclusion, an efficient and mild method has been developed for copper-catalyzed hydroboration of arylalkenes at room temperature. The reaction tolerates a variety of functional groups and the procedure is easily achieved gram-scale reaction under 5 mol % catalyst. More applications of this protocol are under exploration.

R 3

MeOH

Scheme 2. A proposed mechanism for the copper-catalyzed hydroboration.

DMF, and toluene gave yields of 78% and 49%. On the basis of the above results, we concluded that Cu2O (10 mol %)/PPh3 (13 mol %)/K2HPO4/methanol was a good catalyst system for the hydroboration of arylalkenes at room temperature. With the optimized conditions established (entry 9, Table 1), we evaluated the hydroboration of various arylalkenes substrates with B2pin2 (Table 2). Different from Pd-catalyzed system, aryl halides remained stable under the conditions. Both of fluoro and bromo substituents afforded desired products in excellent yields without any steric hindrance problem (entries 1–5). It is noteworthy that the tolerance of aryl halides might provide an opportunity for regioselective synthesis of functional compounds. A wide range of substituents on styrene para-position were tolerated under these conditions, and electron-withdrawing and electron-donating groups on phenyl groups led to similar yields (entries 6–9). Interestingly, 4-vinylphenylboronic acid could react with a good yield (entries 10). A heteroarene such as 4-vinylpyridine reacted smoothly to afford the product in 95% yield (entry 11). Notably, the mixture of isomers with ortho-, meta-, para diethenyl-benzene could also be hydroborated to afford the mono borated product in high yield (95%) under the optimized conditions (entry 12). Next, the reactions between bis(neopentyl glycolato)diboron (B2(neop)2) with arylalkenes substrates were investigated under identical conditions (Table 3). To our delight, most substituted alkyl bromides were well tolerated under the optimized conditions, leading to the desired products in moderate to good yields. Many substituents could be tolerated, including fluoro, bromo, carboxylic, and acetic groups (entries 1–8). Similarly, the boronic acid group of 4-vinylphenylboronic acid was fine in the transformation (entry 9). Compared to B2pin2, B2(neop)2 gave a slightly decreased yield for most reactions, possibly due to the poor stability of B2(neop)2 under the conditions. Furthermore, to show the practicality of the reaction in organic synthesis, we confirmed that the process was amenable to a 10-fold scale-up without compromising reactivity. In addition, it was found that the Cu2O catalyst loading can be reduced from 10% to 5 mol % with comparable efficiency (Scheme 1).

Acknowledgments We gratefully acknowledge financial support from the Eastern Scholar, STCSM No. 12nm0503802, Key Subject of Shanghai Municipal Education Commission, and NSFC (Nos. 21102088, 51202138, 51272154 and 21174081). Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.03. 033. References and notes 1. Atack, T. C.; Lecker, R. M.; Cook, S. P. J. Am. Chem. Soc. 2014, 136, 9521–9523. 2. (a) Murphy, J. M.; Liao, X.; Hartwig, J. F. J. Am. Chem. Soc. 2007, 129, 15434– 15435; (b) Tzschucke, C. C.; Murphy, J. M.; Hartwig, J. F. Org. Lett. 2007, 9, 761– 764; (c) Liskey, C. W.; Liao, X.; Hartwig, J. F. J. Am. Chem. Soc. 2010, 132, 11389– 11391; (d) Hartwig, J. F. Acc. Chem. Res. 2011, 45, 864–873; (e) Liu, T.; Shao, X.; Wu, Y.; Shen, Q. Angew. Chem., Int. Ed. 2012, 51, 540–543; (f) Partridge, B. M.; Hartwig, J. F. Org. Lett. 2012, 15, 140–143; (g) Sridhar, T.; Berrée, F.; Sharma, G. V. M.; Carboni, B. J. Org. Chem. 2013, 79, 783–789; (h) Jiao, J.; Hyodo, K.; Hu, H.; Nakajima, K.; Nishihara, Y. J. Org. Chem. 2013, 79, 285–295; (i) Wang, Y.; Dai, W.-M. Eur. J. Org. Chem. 2014, 2014, 323–330; (j) Yu, Z.; Ely, R. J.; Morken, J. P. Angew. Chem., Int. Ed. 2014, 53, 9632–9636; (k) Cai, M.; Daniel, S. L.; Lavigne, J. J. Chem. Commun. 2013, 6504–6506; (l) Chang, Y.; Lee, H. H.; Kim, S. H.; Jo, T. S.; Bae, C. Macromolecules 2013, 46, 1754–1764; (m) Li, H.; Wang, H.; Liu, Y.; Liu, Z. Chem. Commun. 2012, 4115–4117; (n) Yang, C.-T.; Zhang, Z.-Q.; Tajuddin, H.; Wu, C.-C.; Liang, J.; Liu, J.-H.; Fu, Y.; Czyzewska, M.; Steel, P. G.; Marder, T. B.; Liu, L. Angew. Chem., Int. Ed. 2012, 51, 528–532. 3. (a) Baker, R. T.; Nguyen, P.; Marder, T. B.; Westcott, S. A. Angew. Chem., Int. Ed. 1995, 34, 1336–1338; (b) Ishiyama, T.; Yamamoto, M.; Miyaura, N. Chem. Commun. 1997, 689–690. 4. Chen, Q.; Zhao, J.; Ishikawa, Y.; Asao, N.; Yamamoto, Y.; Jin, T. Org. Lett. 2013, 15, 5766–5769. 5. (a) Kirai, N.; Iguchi, S.; Ito, T.; Takaya, J.; Iwasawa, N. Bull. Chem. Soc. Jpn. 2013, 86, 784–799; (b) Lopez-Duran, R.; Martos-Redruejo, A.; Bunuel, E.; PardoRodriguez, V.; Cardenas, D. J. Chem. Commun. 2013, 10691–10693; (c) PardoRodriguez, V.; Bunuel, E.; Collado-Sanz, D.; Cardenas, D. J. Chem. Commun. 2012, 10517–10519; (d) Takaya, J.; Kirai, N.; Iwasawa, N. J. Am. Chem. Soc. 2011, 133, 12980–12983. 6. (a) Toribatake, K.; Nishiyama, H. Angew. Chem., Int. Ed. 2013, 52, 11011–11015; (b) Toribatake, K.; Zhou, L.; Tsuruta, A.; Nishiyama, H. Tetrahedron 2013, 69, 3551–3560. 7. Bonet, A.; Sole, C.; Gulyás, H.; Fernández, E. Chem. Asian J. 2011, 6, 1011–1014. 8. Cho, H. Y.; Morken, J. P. J. Am. Chem. Soc. 2010, 132, 7576–7577. 9. (a) Yang, C.-T.; Zhang, Z.-Q.; Liang, J.; Liu, J.-H.; Lu, X.-Y.; Chen, H.-H.; Liu, L. J. Am. Chem. Soc. 2012, 134, 11124–11127; (b) Li, X.; Li, B.; You, J.; Lan, J. Org. Biomol. Chem. 2013, 11, 1925–1928; (c) Zhang, X.; Yi, H.; Liao, Z.; Zhang, G.; Fan, C.; Qin, C.; Liu, J.; Lei, A. Org. Biomol. Chem. 2014, 12, 6790–6793. 10. Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457–2483.

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