P. Yao
Table 3. Pd-catalyzed homocoupling of various substratesa
hydrazide to form palladiaziridine intermediate C via metathesis. Af-
ter another oxidative addition with Pd(0)L2, intermediate species D
undergoes similar protonolysis to generate the diphenyl-palladium
complex E. Reductive elimination of intermediate E affords the
desired product and the Pd(0) catalyst is reoxidized to Pd(II) by O2,
thus closing the catalytic cycle. DBU might be an important ligand
in facilitating the catalysis.
Conclusion
Entry
R2
Yield (%)b
We described here a new homocoupling pattern using aryl
hydrazides as coupling partners with high catalytic activity.
Importantly, this transformation is very practical as it does not
require the use of expensive pro-oxidant, and binding of dioxygen
and catalytic activity proceeded well. The transformation was
promoted by component solvent and carry-through via the Pd
(0)/Pd(II) catalytic cycle. The detailed mechanism is undergoing
further study in our lab.
1
2
4-OCH3
4-SCH3
4-F
94
91
79
86
84
88
77
93
90
81
86
65
89
76
3
4
4-Cl
5
4-Br
6
4-I
7
4-CN
8
4-CH3
9
3-CH3
10
11
12
13
14
2-CH3
Experimental
3,5-OCH3
2,6-OCH3
2-Naphthyl
1-Naphthyl
Typical Procedure for the Products
A mixture of aryl hydrazides (0.5 mmol), Pd(OAc)2 (3 mol%) and
DBU (0.5 mmol) was stirred in toluene–acetone = 3:1 (1 ml) in a
sealed tube and the air in the system was exchanged for oxygen.
The system was protected by an O2 balloon at 50°C for 3 h.
Afterwards, 1 ml water was added to the reaction solution and
then filtered through a filter paper, and the solution was extracted
by Et2O (1 ml) three times. The organic phase was combined and
evaporated under reduced pressure. The residue was purified on
an SiO2 column to afford the desired product.
aReaction conditions: phenyl hydrazides (0.5 mmol), Pd(OAc)2 (3
mol%), DBU (0.05 mmol), solvent (1.0 ml, toluene–acetone, 3:1)
at 50°C for 3 h under O2, unless otherwise indicated.
bIsolated homocoupling yield.
hydrazides. The scope of the reaction is presented in Table 3. As
expected, a series of functional groups on the phenyl ring of phenyl
hydrazides, such as methoxy, methylthio, fluoro, bromo, chloro,
iodo and cyano, were all compatible under this procedure, and
the products were isolated in good yields (Table 3, entries 1–7).
Halogen functional groups are tolerant to the palladium catalyzed
homocoupling of the aryl hydrazides (Table 3, entries 3–6),
suggesting that arylsulfonyl hydrazides may have higher coupling
reactivities than the related aromatic halides. Generally, the
reaction efficiency was sensitive to the electronic property of the
groups on the phenyl ring of hydrazides and electron-donating
groups gave a slightly higher yield than electron-withdrawing groups
(Table 3, entries 1–7). The hindrance of the phenyl ring of aryl
hydrazides had a slight effect on the yield (Table 3, entries 8–10).
Although the reaction with meta-disubstituted groups proceeded
well, the reaction with aryl hydrazides bearing two ortho-methyl
groups proceeded sluggishly and afforded biphenyl in 65% yield
(Table 3, entries 11 and12). It was interesting that polycyclic aromatic
hydrocarbon (PAH) substrates still work under this procedure, and
even the product with the high steric hindrance of 1,1′-binaphthyl
was formed in moderate yield (Table 3, entries 13 and 14).
References
[1] J. Hassan, M. Sévignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev.
2002, 102, 1359.
[2] D. A. Horton, G. T. Bourne, M. L. Smythe, Chem. Rev. 2003, 103, 893.
[3] P. Lloyd-Williams, E. Giralt, Chem. Soc. Rev. 2001, 30, 145.
[4] H. Meier, Angew. Chem. Int. Ed. 2005, 44, 2482.
[5] T. D. Nelson, R. D. Crouch, Org. React. 2004, 63, 2655.
[6] A. Monopoli, V. Calò, F. Ciminale, P. Cotugno, C. Angelici, N. Cioffi,
A. Nacci, J. Org. Chem. 2010, 75, 3908.
[7] A. Monopoli, P. Cotugno, G. Palazzo, N. Ditaranto, B. Mariano, N.
Cioffi, F. Ciminale, A. Nacci, Adv. Synth. Catal. 2012, 354, 2777.
[8] N. Iranpoor, H. Firouzabadi, Y. Ahmadi, Eur. J. Org. Chem. 2012, 305.
[9] J. Huang, J. Yin, W. Chai, C. Liang, J. Shen, F. Zhang, New J. Chem.
2012, 36, 1378.
[10] R. N. Dhital, C. Kamonsatikul, E. Somsook, K. Bobuatong, M. Ehara, S.
Karanjit, H. Sakurai, J. Am. Chem. Soc. 2012, 134, 20250.
[11] S. Carrettin, A. Corma, M. Iglesias, F. Sánchez, Appl. Catal. A: Gen.
2005, 291, 247.
[12] N. G. Willis, J. Guzman, Appl. Catal. A: Gen. 2008, 339, 68.
[13] L. Wang, H. Wang, W. Zhang, J. Zhang, J. P. Lewis, X. Meng, F.-S. Xiao,
J. Catal. 2013, 298, 186.
[14] T. Matsuda, T. Asai, S. Shiose, K. Kato, Tetrahedron Lett. 2011, 52, 4779.
[15] B. Mu, T. Li, Z. Fu, Y. Wu, Catal. Commun. 2009, 10, 1497.
[16] J. S. Yadav, K. U. Gayathri, H. Ather, H. Rehman, A. R. Prasad, J. Mol.
Catal. A: Chem. 2007, 271, 25.
Based on the research from Chen’s group,[61] a plausible mecha-
nism for realizing this transformation is illustrated in Scheme 1.
First, the metathesis of phenyl hydrazide with Pd(II)X2 (X= OAc)
forms palladiaziridine intermediate A which, as shown by Muñiz,[62]
undergoes subsequent oxidative addition with Pd(0)L2 to afford
intermediate B. This two palladium(II)-centered complexes were
generated by C―N cleavage, according to a proposal by Loh.[60]
Accompanied by the protonolysis of intermediate B, Ph―Pd(II)
L2―X was generated and then displaced by another phenyl
[17] Z. Jin, S.-X. Guo, X.-P. Gu, L.-L. Qiu, H.-B. Song, J.-X. Fang, Adv. Synth.
Catal. 2009, 351, 1575.
[18] J. Zheng, S. Lin, X. Zhu, B. Jiang, Z. Yang, Z. Pan, Chem. Commun.
2012, 48, 6235.
[19] R. N. Dhital, A. Murugadoss, H. Sakurai, Chem. Asian J. 2012, 7, 55.
[20] H. Sakurai, H. Tsunoyama, T. Tsukuda, J. Organomet. Chem. 2007,
692, 368.
[21] C. Amatore, C. Cammoun, A. Jutand, Eur. J. Org. Chem. 2008, 4567.
wileyonlinelibrary.com/journal/aoc
Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 194–197