CO (π acceptor) to the Pd center.6 As one kind of strong
σ-donor and weak π-acceptor, N-heterocyclic carbenes
(NHCs) represent robust ligands and could accelerate the
oxidation addition of transition metals into aryl halides.8
However, to the best of our knowledge, there is only one
example on the aminocarbonylation catalyzed by Pd-NHCs,
in which up to a 10 mol % catalyst was required to achieve
moderate yields.7e In comparison with imidazole ylidenes,
the less intensively studied ylidenes derived from benzimi-
dazolium and acenaphthoimidazolium salts behave dif-
ferently;9 as stronger σ-donors and weaker π-acceptors they
may further increase the electron density of the catalytic
center and facilitate the oxidative addition even under an
excess of CO atmosphere. Therefore, we are interested in
exploring the possibility of the catalytic activity enhance-
ment by using Pd-NHC complexes based on acenaphthoi-
midazolium salts in the aminocarbonylation reactions.
Following our recent research interests in the synthesis of
metal complexes and their potential applications in catalysis
and soft matter aspects,10À12 robust palladium NHC com-
plexes 2 based on π-extended sterically bulky imidazolium
salts were synthesized and exhibited very high catalytic
activity toward the amination and SuzukiÀMiyaura cross-
coupling of sterically hindered (hetero-) aryl halides in
excellent yields.12a,b In addition, we have successfully devel-
oped a straightforward, efficient, and practical hydration
protocol to access a variety of primary (hetero)aromatic
amides from the corresponding organonitriles in water
catalyzed by K2CO3 under microwave irradiation.13 To
further extend these works, herein, we synthesized the allylic
palladium NHC complex 3 and explored its catalytic po-
tential toward aminocarbonylation of (hetero)aryl iodides
with various amines under atmospheric pressure.
Figure 1. Selected pharmaceuticals containing the amide bonds.
temperature. Yellow needle-shaped crystals were obtained
by slow diffusion of petroleum ether into a dichloromethane
solution of complex 3, which were suitable for single crystal
diffraction analysis. As anticipated, the space around the Pd
center is quite congested in contrast to its imidazol-2-ylidene
analogues 1 (see the Supporting Information (SI)). The two
phenyl rings are almost perpendicular to the plane of the
˚
acenaphtho-ring. The distance of PdÀNCN is 2.048(4) A,
which is similar to what was observed in Pd-NHC 1 in the
literature,14 while the distances of PdÀCallyl are 2.096(5),
˚
2.121(5), and 2.175(5) A, respectively, which are shorter than
what was reported for Pd-NHC 1 due to the trans-effect of
the strong σ-donor property of the acenaphtho-ring.15
Palladium complex 3 was readily accessible in a good
yield from the corresponding acenaphthoimidazolium salt
by stirring with [Pd(allyl)Cl]2 and KOt-Bu in THF at room
€
(8) (a) Wurtz, S.; Glorius, F. Acc. Chem. Res. 2008, 41, 1523. (b) Dıez-
ꢀ
Gonzalez, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612. (c)
Valente, C.; C-alimsiz, S.; Hoi, K. H.; Mallik, D.; Sayah, M.; Organ,
M. G. Angew. Chem., Int. Ed. 2012, 51, 3314. (d) Chartoire, A.; Lesieur,
M.; Falivene, L.; Slawin, A. M. Z.; Cavallo, L.; Cazin, C. S. J.; Nolan,
S. P. Chem.;Eur. J. 2012, 18, 4517.
Figure 2. A selection of phosphine ligands and PdÀNHC com-
plexes for the Pd-catalyzed aminocarbonylation reactions.
€
(9) (a) Hahn, F. E.; Wittenbecher, L.; Boese, R.; Blaser, D. Chem.;
Eur. J. 1999, 5, 1931. (b) Hahn, F. E.; Wittenbecher, L.; Van, D. L.;
€
Frohlich, R. Angew. Chem., Int. Ed. 2000, 39, 541. (c) Grasa, G. A.;
Viciu, M. S.; Huang, J.; Zhang, C.; Trudell, M. L.; Nolan, S. P.
Organometallics 2002, 21, 2866. (d) Hahn, F. E.; Jahnke, M. C. Angew.
Chem., Int. Ed. 2008, 47, 3122.
To evaluate the efficiency of Pd-NHC complex 3,
p-iodotoluene and morpholine were selected as model
substrates to optimize the reaction conditions (Table 1).
Delightedly, with 5 mol % catalyst, N-(4-methylbenzoyl)
morpholine 4a was formed in an 87% yield when the
reaction was carried out with K3PO4 and toluene at
90 °C within 19 h under atmospheric CO pressure
(balloon, entry 1, Table 1). However, both increasing the
reaction temperature to 100 °C and decreasing it to 80 °C
led to inferior yields (46% and 79% entries 2À3, Table 1).
(10) (a) Tu, T.; Assenmacher, W.; Peterlik, H.; Weisbarth, R.; Nieger,
€
M.; Dotz, K. H. Angew. Chem., Int. Ed. 2007, 46, 6368. (b) Tu, T.;
€
Assenmacher, W.; Peterlik, H.; Schnakenburg, G.; Dotz, K. H. Angew.
Chem., Int. Ed. 2008, 47, 7127. (c) Tu, T.; Bao, X.; Assenmacher, W.;
€
Peterlik, H.; Daniels, J.; Dotz, K. H. Chem.;Eur. J. 2009, 15, 1853.
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(d) Tu, T.; Fang, W.; Bao, X.; Li, X.; Dotz, K. H. Angew. Chem., Int. Ed.
2011, 50, 6601.
€
(11) (a) Tu, T.; Malineni, J.; Dotz, K. H. Adv. Synth. Catal. 2008, 350,
€
1791. (b) Tu, T.; Malineni, J.; Bao, X.; Dotz, K. H. Adv. Synth. Catal.
€
2009, 351, 1029. (c) Tu, T.; Mao, H.; Herbert, C.; Xu, M.; Dotz, K. H.
Chem. Commun. 2010, 46, 7796. (d) Tu, T.; Feng, X.; Wang, Z.; Liu, X.
Dalton Trans. 2010, 10598. (e) Wang, Z.; Feng, X.; Fang, W.; Tu, T.
Synlett 2011, 951.
(12) (a) Tu, T.; Fang, W.; Jiang, J. Chem. Commun. 2011, 47, 12358.
(b) Tu, T.; Sun, Z.; Fang, W.; Xu, M.; Zhou, Y. Org. Lett. 2012, 14, 4250.
(c) Fang, W.; Jiang, J.; Xu, Y.; Zhou, J.; Tu, T. Tetrahedron 2013, 69, 673.
(13) Tu, T.; Wang, Z.; Liu, Z.; Feng, X.; Wang, Q. Green Chem. 2012,
14, 921.
(14) Viciu, M. S.; Navarro, O.; Germaneau, R. F.; Kelly, R. A.;
Sommer, W.; Marion, N.; Stevens, E. D.; Cavallo, L.; Nolan, S. P.
Organometallics 2004, 23, 1629.
(15) Tu, T.; Zhou, Y.-G.; Hou, X.-L.; Dai, L.-X.; Dong, X.-C.; Yu,
Y.-H.; Sun, J. Organometallics 2003, 22, 1255.
B
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