Nickel-catalyzed one-pot Suzuki-Miyaura cross-coupling of phenols and arylboronic acids mediated by N,N-ditosylaniline

Article abstract of DOI:10.1002/ejoc.201402475

An efficient method for the construction of two distinct C _aryl-C_aryl bonds through the Ni-catalyzed Suzuki-Miyaura cross-coupling of phenols with arylboronic acids has been developed. This reaction proceeds through the in situ tosylation of phenols by using N,N-ditosylaniline as the sulfonylating reagent, which is highly active, markedly stable, and easily prepared. The scope with respect to the coupling partners - phenols and boronic acids - is broad, and sensitive functional groups are tolerated. Phenols, especially those containing an unprotected amino group, which are generally problematic for coupling under conventional one-pot conditions, are also viable substrates in this transformation.

Full text of DOI:10.1002/ejoc.201402475

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201402475
Nickel-Catalyzed One-Pot Suzuki–Miyaura Cross-Coupling of Phenols and
Arylboronic Acids Mediated by N,N-Ditosylaniline
Liangshun Chen,[a] Hongyue Lang,[a] Lei Fang,[a] Mengyun Zhu,[a] Jinqian Liu,[a]
Jianjun Yu,*[a] and Limin Wang*[a]
Keywords: Nickel / Chemoselectivity / Boronic acids / Biaryls / Cross-coupling
An efficient method for the construction of two distinct Caryl
–
prepared. The scope with respect to the coupling partners –
phenols and boronic acids – is broad, and sensitive functional
groups are tolerated. Phenols, especially those containing an
unprotected amino group, which are generally problematic
for coupling under conventional one-pot conditions, are also
viable substrates in this transformation.
Caryl bonds through the Ni-catalyzed Suzuki–Miyaura cross-
coupling of phenols with arylboronic acids has been devel-
oped. This reaction proceeds through the in situ tosylation
of phenols by using N,N-ditosylaniline as the sulfonylating
reagent, which is highly active, markedly stable, and easily
organic phenolic phosphonium salts,[12] pivalate esters,[13]
and aryl nonaflates.[3] These novel strategies merged phenol
activation and subsequent cross-coupling into a single oper-
ation,[14] and therefore, they made the transformation more
practical in terms of efficiency, economy, and environmen-
tal impact,[15] but their limitation to specific substrates, gen-
erality to labile functional groups, in addition to a high
loading of expensive Pd catalysts and ligands[16,17] [e.g.,
2-dicyclohexylphosphino-2Ј,6Ј-dimethoxybiphenyl (SPhos)]
made it imperfect. Consequently, a more general, efficient,
and direct approach for phenol coupling has been long
awaited. In this communication, we successfully obtained
the coupling products by one operation of the phenols in
a single flask by using a novel sulfonylating reagent, N,N-
ditosylaniline (ArNTs2).
Introduction
The transition-metal-catalyzed Suzuki–Miyaura cross-
coupling reaction constitutes one of the most powerful
C
aryl–Caryl bond-forming transformations.[1] This reaction
has been successfully applied to a wide range of academic
areas, but it has also been used in the pharmaceutical, agro-
chemical, and fine-chemical industries.[2] Over the history
of Suzuki–Miyaura cross-coupling, the starting materials
have been limited to aryl iodides, aryl bromides, and certain
activated aryl chlorides until 20 years ago. A broad range
of aryl electrophiles have attracted much interest, and they
are more commonly used in coupling chemistry.[3] These
electrophiles, according to the atoms of the leaving groups,
can be classified as N-,[4] S-,[5] P-,[6] C-,[7] as well as O-
based. Among these leaving groups, O-based electrophiles
are much more appealing owing to the ubiquitous presence
of their starting materials – phenols – both in the natural
world and in synthetic systems.[8]
Results and Discussion
Previous work using phenols as coupling partners in-
volved prior activation of the phenols to more active pre-
cursors.[9] These activation strategies included the formation
of aryl triflates, sulfonates, ethers, esters, carbamates,
sulfamates, and phosphoramides.[10,11] However, such a pre-
paratory process limits their efficiency for overall yields and
step economy. Very recently, several direct coupling reac-
tions have been reported by the in situ activation of phenols
through the formation of inorganic salts (e.g., ArOM),[9]
Recently, an investigation on ArNTs2 drew our attention
to their ability to donate a Ts group in the presence of a
base. This inspired us to test the feasibility of tosylating
phenolic hydroxy groups, and we envisioned the domino
sulfonylation and cross-coupling reaction of phenols
[Scheme 1, Equation (1)].
First, the sulfonylation of phenols worked smoothly with
ArNTs2 (1.1 equiv.) in 1,4-dioxane by using anhydrous
K3PO4 (2 equiv.) as base, and the corresponding sulfonated
products were obtained in quantitative yield. Afterwards,
we were very surprised to find that phenols 1 containing a
naked amino group could undergo tosylation and coupling
to afford desired product 5 with the intact amino group
[Scheme 1, Equation (2)].
[a] Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, East China University of Science and Technology,
130 Meilong Road, Shanghai 200237, P. R. China
E-mail: wanglimin@ecust.edu.cn
yjzsh@163.com
http://hyxy.ecust.edu.cn/new/viewProf.php?id=249
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402475.
Optimization of the critical reaction conditions was con-
ducted on the model reaction of 4Ј-hydroxyacetophenone
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L. Chen, H. Lang, L. Fang, M. Zhu, J. Liu, J. Yu, L. Wang
SHORT COMMUNICATION
products in yields of 87, 90, 90%, respectively. Benzyl-sub-
stituted 2e gave the product in a decreased yield of 72%,
whereas strongly electron-withdrawing 4-nitro-substituted
2c gave the product in a similarly dissatisfying yield of 61%.
Furthermore, we replaced the Ts group with Ms (methyl-
sulfonyl) and Ns (4-nitrobenzenesulfonyl) groups (see 2h
and 2i) (Figure 1), and the yield dropped sharply to 20 and
31%, respectively. By comparison, sulfonated secondary
amines 2j and 2k, TsCl (2l), and Ts2O (2m) failed to give
desired product 5ae, except for TsCl, which provided the
product in only 7% yield.
Scheme 1. Stepwise Suzuki–Miyaura coupling of phenols. dppp =
1,3-bis(diphenylphosphino)propane.
(1a) with phenylboronic acid (4e) (Table 1). Various specific
nickel-catalyzed systems were tested (Table 1, Entries 1–5).
In stark contrast, NiCl2(dppp) proved to be a much more
effective catalyst coordinated with K3PO4 as base in diox-
ane at 110 °C. K3PO4 was irreplaceable, as Cs2CO3,
tBuOK, tBuOLi, and K2CO3 provided much lower yields
of the products (Table 1, Entries 6–9). Furthermore, the use
of 1,2-dichloroethane (DCE), 1,2-dimethoxyethane (DME),
THF, and toluene as solvents for the reaction gave dissatis-
factory results (Table 1, Entries 10–13).
Then, a variety of sulfonylating reagents were examined
for the optimized catalyst system, and the results are sum-
marized in Figure 1. Substituents on the aniline ring slightly
influenced the efficiency of the domino coupling reaction.
Among all the sulfonylating reagents, unsubstituted 2a and
4-methoxy-substituted 2b gave the best results, both of
which provided 5ae in 93% yield. Naphthyl-, n-butyl-, and
3,4-dimethyl-substituted 2f, 2g, and 2d also provided the Figure 1. Screening of sulfonylating reagents from 2a to 2m.
Table 1. Selected results of the screening of the optimal conditions.[a]
Entry
Catalyst (mol-%)
Base
Solvent
T [°C]
Time [h]
Yield[b] [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
Ni(cod)2 (3)/PCy3 (12)
NiCl2(PCy3)2 (3)/PCy3 (12)
Ni(PPh3)2(1-naphthyl)Cl (5)/PPh3 (5)
NiCl2(dppf)/Zn (5)
NiCl2(dppp) (5)
K3PO4
K3PO4
K2CO3
K3PO4
K3PO4
Cs2CO3
tBuOK
tBuOLi
K2CO3
K3PO4
K3PO4
K3PO4
K3PO4
THF
dioxane
toluene
THF
dioxane
dioxane
dioxane
dioxane
dioxane
DCE
70
130
120
70
110
110
110
110
110
90
24
36
24
24
36
36
36
36
36
36
36
36
36
trace
76
n.r.
n.r.
93
10
25
trace
17
34
21
trace
n.r.
NiCl2(dppp) (5)
NiCl2(dppp) (5)
NiCl2(dppp) (5)
NiCl2(dppp) (5)
NiCl2(dppp) (5)
NiCl2(dppp) (5)
NiCl2(dppp) (5)
NiCl2(dppp) (5)
DME
THF
toluene
110
70
120
[a] Reaction conditions: 1a (0.50 mmol), 4e (1.00 mmol), 2a (0.55 mmol), K3PO4 (2.50 mmol), dioxane (6.00 mL), under N2. cod = 1,5-
cyclooctadiene, Cy = cyclohexyl, dppf = 1,1Ј-bis(diphenylphosphino)ferrocene. [b] Yield of isolated product. n.r. = no reaction.
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Ni-Catalyzed One-Pot Suzuki–Miyaura Cross-Coupling of Phenols
Thus, with the optimized reaction conditions available, strongly electron-withdrawing NO2 group failed in the reac-
we first investigated the effects of electronic and structural tion (see 5ge) (Table 3), and substrates with electron-rich
variations of the boronic acids. The results are presented in and electron-neutral groups such as OMe, NH2, Me, tBu,
Table 2. Generally, the products were isolated in excellent and H gave slightly lower yields (see 5ha, 5ia, 5ka, 5pe–se)
yields, and the overall reaction efficiency was not sensitive (Table 3). Most interestingly, F and Cl were retained under
to the electronic properties of the groups on the phenyl ring the coupling conditions with acceptable efficiency, whereas
of the boronic acids, as cross-couplings of boronic acids Br was not immune (see 5ma–oa) (Table 3). ortho-Substi-
modified with an electron-rich OMe group (see 5aa) tuted phenols gave lower yields than para-substituted ana-
(Table 2), electron-neutral Me and H groups (see 5ab and logues (see 5fe, 5ja vs. 5ae, 5ia) (Table 3). If there were two
5ae) (Table 2), and electron-deficient CN and CO2Me substituents in the ortho position, an even lower yield was
groups (see 5ai and 5aj) (Table 2) all proceeded very obtained (see 5la) (Table 3). Notably, the direct arylation
smoothly. The hindrance of the phenyl ring had a slight of 4-hydroxycoumarin was somewhat challenging, and the
effect on the efficiency (see 5ac and 5ad) (Table 2). Notably, desired product was provided in 30% yield (see 5te)
halo groups on the phenyl ring of the boronic acids behaved (Table 3).
very differently: an F substituent survived the procedure
without being affected, whereas Cl and Br were modified in
the presence of the highly active catalyst system, and much
lower yields of the products were obtained (see 5af, 5ag,
and 5ah) (Table 2). Moreover, 2-naphthyl-, 3-thienyl-, and
3-pyridylboronic acids also gave arylation products in mod-
erate yields (see 5ak, 5al, and 5am) (Table 2).
Table 3. Suzuki–Miyaura cross-coupling of phenols 1a–s with
phenylboronic acid (4e) or 4-methoxyphenylboronic acid (4a) me-
diated by 2a.[a,b]
Table 2. Suzuki–Miyaura cross-coupling of 4Ј-hydroxyacetophen-
one (1a) with boronic acids 4a–m mediated by N,N-ditosylaniline
(2a).[a,b]
[a] Reaction conditions: 1a (0.50 mmol), 4a–m (1.00 mmol), 2a
(0.55 mmol), NiCl2(dppp) (5.00 mol-%), K3PO4 (2.50 mmol), diox-
ane (6.00 mL), 110 °C, under N2, 24–60 h. [b] Yields of isolated
products are given. [c] Compound 2b was used instead of 2a.
[a] Reaction conditions: 1a–s (0.50 mmol), 4a or 4e (1.00 mmol),
2a (0.55 mmol), NiCl2(dppp) (5.00 mol-%), K3PO4 (3.00 mmol), di-
oxane (6.00 mL), 110 °C, under N2, 24–60 h. [b] Yields of isolated
products are given. [c] 2a (0.75 mmol) and 4e or 4a (1.50 mmol)
were used.
Next, the reaction efficiency of the direct Suzuki–Mi-
yaura coupling was further inspected by varying the phenol
components (Table 3). As expected, a series of functional
groups on the phenol ring, such as chloro, fluoro, cyano,
ether, ester, formyl, acetyl, and even amino, were compati-
Of particular note is that the coupling of phenols con-
ble under this procedure. Generally, we observed that elec- taining a free amino group (see 5qe–se) (Table 3) occurred
tron-deficient phenol compounds such as those with a CN, exclusively at the hydroxy position, and the unprotected
CO2Me, or C(O)Me substituent, could be transformed NH2 functional group remained intact, which was discussed
smoothly into the corresponding products in 83–95% yield in Scheme 1. To the best of our knowledge, such a highly
(5ae–ee) (Table 3). Comparatively, analogues containing a chemoselective cross-coupling with the use of phenols has
Eur. J. Org. Chem. 2014, 4953–4957
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L. Chen, H. Lang, L. Fang, M. Zhu, J. Liu, J. Yu, L. Wang
SHORT COMMUNICATION
evaporated under vacuum. Purification by column chromatography
(cyclohexane/dichloromethane, 5:1–1:1) afforded the target com-
pound as a white solid.
not been observed for known protocols. In contrast, similar
conventional Pd-catalyzed cross-coupling reactions require
prior protection of the amino group.[18] Thus, one of the
foreseeable advantages with the direct use of unprotected
Supporting Information (see footnote on the first page of this arti-
amino phenols is that it provides a much more straightfor- cle): Experimental details and copies of the 1H NMR and
13C NMR spectra.
ward pathway for the construction of polyaromatic-ring-
containing amines. Besides, the orthogonal use of this pro-
tocol with Buchwald–Hartwig coupling[19] is a good pros-
Acknowledgments
pect for selective C(sp2)–C(sp2) and C(sp2)–N bond forma-
tion through proper control of the catalyst system and the
reaction sequences.[20]
The work was supported by the National Nature Science Founda-
tion of China (NSFC, 21272069, 20672035), the Fundamental Re-
search Funds for the Central Universities, and the Key Laboratory
of Organofluorine Chemistry, Shanghai Institute of Organic Chem-
istry, Chinese Academy of Sciences.
Conclusions
We developed a novel, practical sulfonylating reagent –
ArNTs2 – and successfully applied it to the direct coupling
of phenols with arylboronic acids. This method does not
require the isolation of the intermediary sulfonate, and
therefore, it is much more convenient and useful than
known traditional ways. In addition, the method also shows
broad applicability and high efficiency towards various
functional groups on the aromatic rings, and even a naked
amino group was tolerated. We highlight its use for the con-
struction of polyaromatic-ring-containing amines, which
are core motifs and important building blocks in the syn-
thesis of pharmaceuticals, functional materials, and coordi-
nation compounds by orthogonal use with Buchwald–Hart-
wig coupling.
As a stable source of the Ts group, ArNTs2 exhibits a lot
of incomparable superiority towards conventional reagents
such as TsCl and Ts2O. We described its highly chemical
selectivity between phenols and amines; its stability in
strong base, its high activity in the metal catalyst system,
and the lack of the generation of an acid such as HCl are
impressive as well. Studies on these properties of ArNTs2
are currently underway in our laboratory.
[1] a) N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457–2483;
b) A. Suzuki, J. Organomet. Chem. 1999, 576, 147–168; c) N.
Miyaura, Top. Curr. Chem. 2002, 219, 11–59; d) J. Hassan, M.
Sevignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev. 2002,
102, 1359–1469; e) S. Kotha, K. Lahiri, D. Kashinath, Tetrahe-
dron 2002, 58, 9633–9695; f) A. F. Littke, G. C. Fu, Angew.
Chem. Int. Ed. 2002, 41, 4176–4211; Angew. Chem. 2002, 114,
4350–4386; g) F. Bellina, A. Carpita, R. Rossi, Synthesis 2004,
2419–2440; h) U. Christmann, R. Vilar, Angew. Chem. Int. Ed.
2005, 44, 366–374; Angew. Chem. 2005, 117, 370–378; i) F.
Alonso, I. P. Beletskaya, M. Yus, Tetrahedron 2008, 64, 3047–
3101.
[2] F.-S. Han, Chem. Soc. Rev. 2013, 42, 5270–5298.
[3] T. Ikawa, K. Saito, S. Akai, Synlett 2012, 23, 2241–2246.
[4] a) S. B. Blakey, D. W. C. MacMillan, J. Am. Chem. Soc. 2003,
125, 6046–6047; b) S. Darses, J. P. Jeffrey, J. P. Genet, J. L.
Brayer, J. P. Demoute, Tetrahedron Lett. 1996, 37, 3857–3860;
c) K. Kikukawa, K. Kono, F. Wada, T. Matsuda, J. Org. Chem.
1983, 48, 1333–1336.
[5] a) L. S. Liebeskind, J. Srogl, J. Am. Chem. Soc. 2000, 122,
11260–11261; b) J. M. Villalobos, J. Srogl, L. S. Liebeskind, J.
Am. Chem. Soc. 2007, 129, 15734–15735.
[6] L. K. Hwang, Y. Na, J. Lee, Y. Do, S. Chang, Angew. Chem.
Int. Ed. 2005, 44, 6166–6169; Angew. Chem. 2005, 117, 6322–
6325.
[7] S. M. Bonesi, M. Fagnoni, A. Albini, Angew. Chem. Int. Ed.
2008, 47, 10022–10025; Angew. Chem. 2008, 120, 10172–10175.
[8] A. Corma, S. Iborra, A. Velty, Chem. Rev. 2007, 107, 2411–
2502.
Experimental Section
[9] D.-G. Yu, B.-J. Li, Z.-J. Shi, Angew. Chem. Int. Ed. 2010, 49,
4566–4570; Angew. Chem. 2010, 122, 4670–4674.
General Procedure for the Suzuki–Miyaura Cross-Coupling Reac-
tions of Phenols with Arylboronic Acids: Into a 25 mL Schlenk tube
[10] For recent reviews containing Ni-catalyzed cross-couplings in-
volving C–O bonds, see: a) D.-G. Yu, B.-J. Li, Z.-J. Shi, Acc.
Chem. Res. 2010, 43, 1486–1495; b) B. M. Rosen, K. W. Quas-
dorf, D. A. Wilson, N. Zhang, A.-M. Resmerita, N. K. Garg,
V. Percec, Chem. Rev. 2011, 111, 1346–1416; c) B.-J. Li, D.-G.
Yu, C.-L. Sun, Z.-J. Shi, Chem. Eur. J. 2011, 17, 1728–1759; d)
T. Mesganaw, N. K. Garg, Org. Process Res. Dev. 2013, 17, 29–
39; e) M. Tobisu, N. Chatani, Top. Organomet. Chem. 2013, 44,
35–53.
equipped with
a
magnetic bar were added NiCl2(dppp)
(0.025 mmol, 13.5 mg), the phenol (0.5 mmol), the arylboronic acid
(1.0 mmol), the N,N-ditosylamine (0.55 mmol), and anhydrous
K3PO4 (2.5 mmol). The tube was then evacuated (3ϫ 10 min) and
backfilled with N2. Dried dioxane (5.0 mL) was injected by syringe,
and the mixture was stirred at 110 °C until the phenol had disap-
peared, as monitored by TLC. The mixture was poured into water
(20 mL) and then extracted with CH2Cl2 (3ϫ 20 mL). The com-
bined organic layers were dried with anhydrous Na2SO4, filtered,
and concentrated to dryness. The crude material was purified by
flash chromatography (silica gel; hexane/ethyl acetate or hexane/
CH2Cl2) to give the desired cross-coupled product.
[11] a) Y.-L. Zhao, G.-J. Wu, F.-S. Han, Chem. Commun. 2012, 48,
5868–5870; b) Y.-L. Zhao, G.-J. Wu, Y. Li, L.-X. Gao, F.-S.
Han, Chem. Eur. J. 2012, 18, 9622–9627.
[12] For a review concerning the one-pot Suzuki–Miyaura coupling
of phenols mediated by phosphonium salts, see: a) F.-A. Kang,
Z. Sui, W. V. Murray, Eur. J. Org. Chem. 2009, 4, 461–479; b)
G.-J. Chen, J. Zhang, L.-X. Gao, F.-S. Han, Chem. Eur. J. 2011,
17, 4038–4042.
General Procedure for the Synthesis of the N,N-Ditosylamines: The
amine was treated with sulfonyl chloride (2.1 equiv.) and Et3N
(3 equiv.) in dry CH2Cl2 (3 mLmmol–1 of amine). After 24 h of
stirring at room temperature, the mixture was washed with water;
the final organic layer was dried (Na2SO4), and the solvents were
[13] K. W. Quasdorf, X. Tian, N. K. Garg, J. Am. Chem. Soc. 2008,
130, 14422–14423.
[14] J. D. Revell, A. Ganesan, Chem. Commun. 2004, 17, 1916–
1917.
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Ni-Catalyzed One-Pot Suzuki–Miyaura Cross-Coupling of Phenols
[15]
[18] S. Caron, S. S. Massett, D. E. Bogle, M. J. Castaldi, T. F. Bra-
ish, Org. Process Res. Dev. 2001, 5, 254–256.
a) C. Shi, C. C. Aldrich, Org. Lett. 2010, 12, 2286–2289; b) F.-
A. Kang, J. C. Lanter, C. Cai, Z. Sui, W. V. Murray, Chem.
Commun. 2010, 46, 1347–1349; c) V. P. Mehta, S. G. Modha,
E. V. Van der Eycken, J. Org. Chem. 2010, 75, 976–979; d) F.-
A. Kang, Z. Sui, W. V. Murray, J. Am. Chem. Soc. 2008, 130,
11300–11302.
[19] For examples: a) G. Mann, J. F. Hartwig, M. S. Driver, C.
Fernández-Rivas, J. Am. Chem. Soc. 1998, 120, 827–828; b)
J. P. Wolfe, H. Tomori, J. P. Sadighi, J. Yin, S. L. Buchwald, J.
Org. Chem. 2000, 65, 1158–1174; c) X. Huang, K. W. An-
derson, D. Zim, L. Jiang, A. Klapars, S. L. Buchwald, J. Am.
Chem. Soc. 2003, 125, 6653–6655; d) B. P. Fors, D. A. Watson,
M. R. Biscoe, S. L. Buchwald, J. Am. Chem. Soc. 2008, 130,
13552–13354; e) C. M. So, Z. Zhou, C. P. Lau, F. Y. Kwong,
Angew. Chem. Int. Ed. 2008, 47, 6402–6406; Angew. Chem.
2008, 120, 6502–6506.
[16] a) L. Ackermann, M. Mulzer, Org. Lett. 2008, 10, 5043–5045;
b) D.-G. Yu, Z.-J. Shi, Angew. Chem. Int. Ed. 2011, 50, 7097–
7100; Angew. Chem. 2011, 123, 7235–7238; c) L. Ackermann,
J. Pospiech, H. K. Potukuchi, Org. Lett. 2012, 14, 3146–3149.
[17] For direct couplings of related aromatic compounds with hy-
droxy groups, see: a) Y. Luo, J. Wu, Tetrahedron Lett. 2009, 50,
2103–2105; b) Y. Luo, J. Wu, Tetrahedron 2009, 65, 6810–6814;
[20] H. Gao, Y. Li, Y.-G. Zhou, F.-S. Han, Y.-J. Lin, Adv. Synth.
Catal. 2011, 353, 309–314.
ˇ
c) M. Cernˇová, R. Pohl, B. Klepetárˇová, M. Hocek, Synlett
Received: April 23, 2014
Published Online: July 8, 2014
2012, 23, 1305–1308.
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