Free-amine directed arylation of biaryl-2-amines with aryl iodides by palladium catalysis

Article abstract of DOI:10.1021/ol402207g

A new palladium-catalyzed free-amine directed arylation of C(sp ²)-H bonds in the presence of AgOAc and TFA is described. Biaryl-2-amines react with various aryl iodides to give the corresponding mono- or diarylated products with exclusive regioselectivity.

Full text of DOI:10.1021/ol402207g

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Free-Amine Directed Arylation
of Biaryl-2-amines with Aryl Iodides
by Palladium Catalysis
Zunjun Liang,† Ruokun Feng,† Hong Yin,† and Yuhong Zhang*,†,‡
ZJU-NHU United R&D Center, Department of Chemistry, Zhejiang University,
Hangzhou 310027, China, and State Key Laboratory of Applied Organic Chemistry,
Lanzhou University, Lanzhou 730000, China
yhzhang@zju.edu.cn
Received July 24, 2013
ABSTRACT
A new palladium-catalyzed free-amine directed arylation of C(sp2)ꢀH bonds in the presence of AgOAc and TFA is described. Biaryl-2-amines react
with various aryl iodides to give the corresponding mono- or diarylated products with exclusive regioselectivity.
The biaryl linkage is an important structural motif that
is prevalent in natural products, pharmaceutically active
reagents, and conjugated materials.1 Transition-metal-
catalyzed direct CꢀH arylation reactions of arenes are
increasingly viable alternatives to traditional cross-coupling
reactions, which provide powerful and straightforward tools
for the preparation of arylꢀaryl scaffolds. For example,
significant progress has been achieved in the development
of a C(sp2)ꢀH activation reaction by palladium, ruthenium,
and rhodium catalysis.2 Among these transformations,
chelation assisted CꢀH arylation reactions are particularly
attractive due to their excellent regioselectivity and reactivity.
A variety of nitrogen-containing directing groups (DGs),
such as amides,3 imines,4 oximes,5 and N-heterocycles,6 have
been shown to be efficient in these reactions. In view of the
general applicability and atom economy of the directing
† Zhejiang University.
‡ Lanzhou University.
(1) (a) Torsell, K. B. G. Natural Product Chemistry; Wiley: Chichester,
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C. J. Am. Chem. Soc. 2006, 128, 13326. (g) Kozlowski, M. C.; Morgan,
B. J.; Linton, E. C. Chem. Soc. Rev. 2009, 38, 3193.
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O.; Zaitsev, V. G. Angew. Chem., Int. Ed. 2005, 44, 4046. (b) Kalyani, D.;
Deprez, N. R.; Desai, L. V.; Sanford, M. S. J. Am. Chem. Soc. 2005, 127,
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(2) For select reviews that discuss CꢀH arylation reactions, see: (a)
Miura, M.; Satoh, T. Top. Organomet. Chem. 2005, 14, 55. (b) Godula,
K.; Sames, D. Science 2006, 312, 67. (c) Campeau, L. C.; Fagnou, K.
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Chem. Rev. 2007, 107, 174. (f) Catellani, M.; Motti, E.; Della Ca, N. Acc.
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r
10.1021/ol402207g
XXXX American Chemical Society

groups, expanding the scope to include simple and commonly
encountered DGs is highly desirable. The amine group is one
of the most prevalent nitrogen-containing groups, which has
a long history of acting as a chelation group in cyclometala-
tion reactions.7 However, there are few reports of transition-
metal-catalyzed CꢀH bond functionalization reactions by
the use of amines as DGs,8 especially in the case of using free
amines in the direct CꢀH bond functionalization.9 In 2006,
Daugulis and co-workers disclosed that unsubstituted ben-
zylamines and N-methylbenzylamine can be ortho-arylated
under palladium catalysis at 130 °C.10 In 2011, Gaunt and
co-workers developed a Pd(II)-catalyzed C(sp2)ꢀH aryla-
tion of β-arylethylamines directed by N-aryl substituted
amine.11 Very recently, You and our group reported that
N,N-dimethylaminomethyl could be used successfully as
the directing group for Pd-catalyzed C(sp2)ꢀH arylation.12
Our ongoing research program on free-amine directed CꢀH
activation prompted us to explore the arylation of biaryl-2-
amines.13 Herein, we report a Pd(OAc)2-catalyzed C(sp2)ꢀH
arylation of arenes with aryl iodides directed by a free-amine
group in the presence of trifluoroacetic acid.
We chose 30-methylbiphenyl-2-amine 1a and iodoben-
zene 2a as the model substrates at 100 °C for 12 h to study
the arylation reaction. Initially, we used the same reaction
conditions that we have reported for the alkenylation
of biaryl-2-amine: 2.5 mol % Pd(OAc)2 and 1.5 equiv of
AgOAc in the presence of acetic acid (HOAc) (Table 1,
entry 1). However, the desired CꢀC cross-coupling pro-
duct was not detected. To our delight, such a transforma-
tion was highly promoted when trifluoroacetic acid (TFA)
was utilized in place of HOAc to give the C(sp2)ꢀH
arylation product in 86% yield (Table 1, entry 2). It should
be noted that neither the BuchwaldꢀHartwig N-arylation
product nor amide from TFA and amine 1a were detected
by GC-MS analysis, showing the excellent chemoselectiv-
ity for C(sp2)ꢀH arylation.14 Palladium catalysts such
as PdCl2, Pd(CH3CN)2Cl2, Pd(PPh3)2Cl2, Pd(OTFA)2,
Pd(PPh3)4, and Pd(dba)2 were also screened, and the
(5) For oxime group directed CꢀH bond arylation, see: (a)
Thirunavukkarasu, V. S.; Parthasarathy, K.; Cheng, C.-H. Angew.
Chem., Int. Ed. 2008, 47, 9462. (b) Sun, C.-L.; Liu, N.; Li, B.-J.; Yu,
D.-G.; Wang, Y.; Shi, Z.-J. Org. Lett. 2010, 12, 184.
(6) For N-heterocycle directed CꢀH bond arylation, see: (a) Oi, S.;
Fukita, S.; Inoue, Y. Chem. Commun. 1998, 2439. (b) Shabashov, D.;
Daugulis, O. Org. Lett. 2005, 7, 3657. (c) Ackermann, L.; Althammer,
A.; Born, R. Angew. Chem., Int. Ed. 2006, 45, 2619. (d) Hull, K. L.;
Sanford, M. S. J. Am. Chem. Soc. 2007, 129, 11904. (e) Ozdemir, I.;
Demir, S.; Cetinkaya, B.; Gourlaouen, C.; Maseras, F.; Bruneau, C.;
Dixneuf, P. H. J. Am. Chem. Soc. 2008, 130, 1156. (f) Lakshman, M. K.;
Deb, A. C.; Ram Chamala, R.; Pradhan, P.; Pratap, R. Angew. Chem.,
Int. Ed. 2011, 50, 11400. (g) Guo, H.-M.; Jiang, L.-L.; Niu, H.-Y.; Rao,
W.-H.; Liang, L.; Mao, R.-Z.; Li, D.-Y.; Qu, G.-R. Org. Lett. 2011, 13,
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4987. (i) Kwak, J.; Kim, M.; Chang, S. J. Am. Chem. Soc. 2011, 133,
3780.
Table 1. Screening of the Reaction Conditionsa
(7) For amine group directed cyclometalation reactions, see: (a)
Cope, A. C.; Friedrich, E. C. J. Am. Chem. Soc. 1968, 90, 909. (b)
Ryabov, A. D. Chem. Rev. 1990, 90, 403. (c) Vicente, J.; Saura-Llamas,
I.; Palin, M. J.; Jones, P. G.; Ramırez de Arellano, M. C. Organome-
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Organometallics 2003, 22, 5513. (e) Dupont, J.; Consorti, C. S.; Spencer,
J. Chem. Rev. 2005, 105, 2527. (f) Albert, J.; Granell, J.; Zafrilla, J.; Font-
entry
catalyst
Pd(OAc)2
additive
acid
(%)b
1
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOTFA
Ag2O
HOAc
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
0
Bardia, M.; Solans, X. J. Organomet. Chem. 2005, 690, 422. (g) Vicente,
2
Pd(OAc)2
PdCl2
86
75
72
76
78
82
77
80c
82
51
40
0
ꢀ
J.; Saura-Llamas, I.; Garcı
´
a-Lοpez, J.-A.; Calmuschi-Cula, B. Organo-
3
metallics 2007, 26, 2768. (h) Albert, J.; D’Andrea, L.; Granell, J.;
Zafrilla, J.; Font-Bardia, M.; Solans, X. J. Organomet. Chem. 2007,
4
Pd(CH3CN)2Cl2
Pd(PPh3)2Cl2
Pd(OTFA)2
Pd(PPh3)4
Pd(dba)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
ꢀ
ꢀ
692, 4895. (i) Vicente, J.; Saura-Llamas, I.; Garcı
´
a-Lοpez, J.-A. Orga-
5
ꢀ
nometallics 2009, 28, 448. (j) Oliva-Madrid, M.-J.; Garcıa-Lοpez, J.-A.;
´
6
Saura-Llamas, I.; Bautista, D.; Vicente, J. Organometallics 2012, 31,
3647.
7
8
(8) For amine group directed CꢀH activation reactions, see: (a)
Kakiuchi, F.; Igi, K.; Matsumoto, M.; Hayamizu, T.; Chatani, N.;
Murai, S. Chem. Lett. 2002, 396. (b) Orito, K.; Horibata, A.; Nakamura,
T.; Ushito, H.; Nagasaki, H.; Yuguchi, M.; Yamashita, S.; Tokuda, M.
J. Am. Chem. Soc. 2004, 126, 14342. (c) Cai, G.; Fu, Y.; Li, Y.; Wan, X.;
Shi, Z.-J. J. Am. Chem. Soc. 2007, 129, 7666. (d) Li, H.; Cai, G.-X.; Shi,
Z.-J. Dalton Trans. 2010, 39, 10442. (e) Zhang, H.; Cui, X.; Yao, X.;
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Hale, L. V. A.; Squier, P. A.; Ringgold, M. A.; Wiederspan, E. R.; Clark,
T. B. Org. Lett. 2012, 14, 3558.
(9) For free-amine group directed CꢀH activation reactions, see: (a)
Yi, C. S.; Yun, S. Y. J. Am. Chem. Soc. 2005, 127, 17000. (b) He, H.; Liu,
W.-B.; Dai, L.-X.; You, S.-L. J. Am. Chem. Soc. 2009, 131, 8346. (c) Ye,
K.-Y.; He, H.; Liu, W.-B.; Dai, L.-X.; Helmchen, G.; You, S.-L. J. Am.
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173.
9
10
11
12
13
14
15
16
Ag2CO3
Cu(OAc)2
AgOAc
ꢀ
0
Pd(OAc)2
Pd(OAc)2
trd
tre
ꢀ
a Reaction conditions: 1a (0.5 mmol), 2a (3.0 mmol), Pd catalyst
(0.0125 mmol, 2.5 mol %), additives (0.75 mmol, 1.5 equiv), acid (1 mL),
TFEtOH (1 mL), 10 min at room temperature (rt), then heated at 100 °C
with stirring for 12 h. b Isolated yields based on biphenylamine 1a.
c Pd(OAc)2 (1 mol %). d Under a O2 atmosphere. e Under a N2 atmosphere.
(10) Lazareva, A.; Daugulis, O. Org. Lett. 2006, 8, 5211.
(11) Haffemayer, B.; Gulias, M.; Gaunt, M. J. Chem. Sci. 2011, 2,
312.
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J. 2012, 18, 15816. (b) Liang, Z.; Zhang, J.; Liu, Z.; Wang, K.; Zhang, Y.
Tetrahedron 2013, 69, 6519.
(12) (a) Gao, D.-W.; Shi, Y.-C.; Gu, Q.; Zhao, Z.-L.; You, S.-L.
J. Am. Chem. Soc. 2013, 135, 86. (b) Feng, R.; Yao, J.; Liang, Z.; Liu, Z.;
Zhang, Y. J. Org. Chem. 2013, 78, 3688.
(14) (a) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805. (b) Hartwig, J. F. Acc. Chem. Res. 1998, 31,
852.
B
Org. Lett., Vol. XX, No. XX, XXXX

results indicated that Pd(OAc)2 was the best catalyst
(Table 1, entries 3ꢀ8). The reaction still show high effi-
ciency when the loading of Pd(OAc)2 was decreased to
1 mol % (Table 1, entry 9). Among the additives tested, the
best result was obtained by the use of AgOAc. Other silver
salts, such as AgOTFA, Ag2O, and Ag2CO3, were found to
beinferiorforthis transformation (Table1, entries10ꢀ12).
Cu(OAc)2 was completely inactive (Table 1, entry 13).
No product was detected without a catalyst (Table 1,
entries 14). We performed the experiments in the absence
of AgOAc under an oxygen or a nitrogen atmosphere.
Both of the reactions gave trace amounts of the products
(Table 1, entries 15 and 16), implicating that dioxygen
plays little role in the transformation. Further investiga-
tion of the reaction temperature led us to establish the
optimized reaction conditions as follows: 2.5 mol % of
Pd(OAc)2, 1.5 equiv of AgOAc, TFA (1 mL), TFEtOH
(1 mL) as solvent at 100 °C for 12 h.
The scope and generality of this direct arylation reaction
under the optimized conditions were investigated as shown
in Figure 1. The reaction displayed good functional group
tolerance toward diverse aryl iodides. Aryliodides with
both electron-withdrawing and -donating groups pre-
sented excellent compatibility with the reaction conditions.
In most cases, electron-deficient aryl iodides afforded the
arylation products in slightly higher yields (3d, 3e, 3f, 3g,
and 3i) than electron-rich aryl iodides (3a, 3b, and 3c). Due
to the poor solubility, 1-iodo-4-(trifluoromethyl)benzene
gave the arylation product in relatively lower yield (3h).
Notably, the tolerance for bromide on the aromatic ring
(3f) in this arylation offers an opportunity for subsequent
cross-coupling, facilitating the synthesis of complex biaryl
molecules. The steric hindrance played a key role in the
reaction. Meta-substituted aryliodides generated the cor-
responding products in lower yields (3j, 3k, and 3l). How-
ever, an attempt to employ ortho-substituted aryliodide
failed to afford the products.
Figure 1. Monoarylation of biphenylamine. Reaction condi-
tions: (a) biphenylamine 1 (0.5 mmol), aryl iodide 2 (3.0 mmol),
Pd(OAc)2 (0.0125 mmol, 2.5 mol %), AgOAc (0.75 mmol, 1.5
equiv), TFA (1 mL), TFEtOH (1 mL), 10 min at rt, then heated
at 100 °C with stirring for 12 h; isolated yields based on
biphenylamine 1; (b) 1a (10 mmol), iodobenzene 2a (60 mmol),
Pd(OAc)2 (0.25 mmol), AgOAc (15 mmol), TFA (10 mL),
TFEtOH (10 mL), 10 min at rt, then heated at 100 °C with
stirring for 12 h.
A wide range of biphenylamine substrates were found to
be compatible with this protocol (Figure 1). Both electron-
rich and -poor biphenylamines participated smoothly in
this transformation to afford the corresponding products
in good yields (3mꢀ3s). A variety of useful functional
groups were tolerated, including methoxy (3m), hydroxyl
(3n), fluoride (3o and 3s), chloride (3p), and methyl
(3q and 3r). The arylation reaction was compatible with
2-(naphthalen-2-yl)aniline to give the desired product in
81% yield (3t). It is noteworthy that the reaction could be
scaled up to 10 mmol and the arylated product 3a can be
obtained in 78% isolated yield.
When we extend the substrate scope to unsubstituted
or 40-substituted biphenylamine, a mixture of mono- and
diarylated products were obtained. After intensive inves-
tigation of the reaction conditions, we obtained the
optimal reaction conditions for clean diarylated products:
5 mol % Pd(OAc)2, 3 equiv of AgOAc, and 10 equiv of aryl
iodides. We explored the generality of the diarylation
reaction, and the results were summarized in Figure 2. It
is similar to the monoarylation transformation; aryl iodides
with electron-donating groups (4aꢀ4c) gave relatively
lower yields than those obtained with electron-withdrawing
groups (4dꢀ4h). Meta-substituted aryl iodides participated
smoothly in the reaction to give the diarylated products in
moderate yields (4i and 4j). This diarylation reaction also
displayed good functional group tolerance toward 5- and
40-substituted biphenylamines (4kꢀ4o and 4pꢀ4q). Again,
electron-rich biphenylamines (4k and 4l) show higher
efficiency than electron-deficient ones (4m, 4n, and 4o).
The six-membered palladacycle complex A was pre-
pared according to the method previously described in
the literature.7e Subjecting the palladacycle complex A to
the reaction with iodobenzene (2a) resulted in the arylated
product with excellent yield under standard reaction con-
ditions in the absence of AgOAc (Scheme 1).
A plausible reaction mechanism has been proposed in
Scheme 2. The intermediate B is formed through the
oxidative addition of Pd(0) to aryl iodide. The electrophilic
palladation of arylamine generates the palladacycle inter-
mediate A, which undergoes the transmetalation with
palladium intermediate B to give intermediate C15and
Pd(II). The subsequent reductive elimination of C affords
Org. Lett., Vol. XX, No. XX, XXXX
C

Scheme 1. Direct Arylation of Palladacycle Complex A
Scheme 2. A Plausible Mechanism
Figure 2. Diarylation of biphenylamine. Reaction conditions:
biphenylamine 1 (0.5 mmol), aryl iodide 2 (5.0 mmol), Pd(OAc)2
(0.025 mmol, 5 mol %), AgOAc (1.5 mmol, 3.0 equiv), TFA
(1 mL), TFEtOH (1 mL), 10 min at rt, then heated at 100 °C with
stirring for 12 h; isolated yields based on biphenylamine 1.
by a free-amine group. Biaryl-2-amines react with various
aryl iodides to give the corresponding ortho-monoarylated
or diarylated products with exclusive regioselectivity.
Detailed mechanistic studies of this arylation reaction are
ongoing in our laboratory.
the arylation product and liberates Pd(0). Silver salt may
act as a halide scavenger to improve the reaction.5a,12b,16
Another possible pathway for this transformation is the
oxidative addition of intermediate A to aryl iodide to give
a Pd(IV) intermediate D, which undergoes the reductive
elimination to afford the arylated product and Pd(II)
(a Pd(II)/Pd(IV) mechanism).
Acknowledgment. Funding from Zhejiang Province (No.
2011C11097) and NSFC (Nos. 2107216 and 1272205) is
highly acknowledged. The work was also supported by the
Program for Zhejiang Leading Team of S&T Innovation.
Inconclusion, wehavedevelopedanefficientmethodfor
Pd(II)-catalyzed arylation of the C(sp2)ꢀH bond directed
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(b) Cardenas, D. J.; Martin-Matute, B.; Echavarren, A. M. J. Am. Chem.
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Supporting Information Available. Experimental pro-
cedures, characterization data (1H NMR, 13C NMR, MS,
HRMS, and IR). This material is available free of charge
via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX