A study on ligand-coupling reactions of imidoyl palladium complexes

Article abstract of DOI:10.1016/j.jorganchem.2006.04.055

A new ligand-coupling reaction of imidoylpalladium(II) complexes is described. Heating a toluene solution of imidoylpalladium complexes gave rise to the α-diimines (1,4-diazabutadienes).

Full text of DOI:10.1016/j.jorganchem.2006.04.055

Journal of Organometallic Chemistry 692 (2007) 620–624
www.elsevier.com/locate/jorganchem
A study on ligand-coupling reactions of imidoyl palladium complexes
*
Masashi Morishita, Hideki Amii
Department of Chemistry, Faculty of Science, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
Received 28 February 2006; received in revised form 25 March 2006; accepted 3 April 2006
Available online 9 September 2006
Abstract
A new ligand-coupling reaction of imidoylpalladium(II) complexes is described. Heating a toluene solution of imidoylpalladium com-
plexes gave rise to the a-diimines (1,4-diazabutadienes).
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Palladium; Oxidative addition; Isocyanide; Insertion; Ligand coupling; Diimine
1. Introduction
stepwise insertion mechanism involving (i) initial insertion
of an isocyanide into an alkylsamarium species to generate
an imidoylsamarium(III) intermediate and (ii) the subse-
quent insertion of the second equivalent of isocyanide into
a C–Sm bond in the imidoylsamarium species.
Herein, we report a new example of Pd-mediated 2:2
coupling reactions of aryl iodides and isocyanides to pro-
vide a-diimines 1 via ligand-coupling reactions of the imi-
doylpalladium(II) species 2 (Eq. (1)).
In various catalytic transformations, a-diimines 1 are
useful catalyst ligands for transition-metal catalyzed reac-
tions, because they are characteristic of good r-donor
and p-acceptor properties which stabilize organometallic
complexes [1,2]. The diimine complexes have been utilized
as catalysts for a wide repertoire of reactions, such as
alkene polymerization [3a,3b], alkene–CO copolymeriza-
tion [3c], aziridination [3d], allylic amination [3e], C–H
bond activation [3f], Si–H bond activation [3g], Suzuki–
Miyaura cross-coupling [3h], alkyne hydrogenation [3i],
alkyne coupling reactions [3j,3k] and many others.
N-Xy
Xy
Xy-N
Ar
N-Xy
Ar
Pd(0)
CNXy
I
N
C
+
Ar-I
Ar
Pd
XyNC
(Xy = 2,6-Me2C6H3)
2
1
The preparative methods for a-diimines 1 have involved
(a) condensation of a-diketones with primary amines [4,5],
(b) homologation of bis(imidoyl chlorides) [6], and (c)
dimerization of iminoacyl compounds such as aldimines
[7], iminoethers [8], imidoyl iodides [9], nitriles [10] (Scheme
1). One of the effective methodologies for the preparation
of a-diimines is (d) multi-component coupling of organic
halides (or alkynes) and isonitriles [11,12]. In 1991, Mura-
kami and Ito reported SmI2-mediated double insertion
reactions of aryl isocyanides into organic halides to afford
vicinal di- and tricarbonyl compounds [11]. They proposed
ð1Þ
2. Results and discussion
Heating a mixture of tris(dibenzylideneacetone)dipalla-
dium (Pd2(dba)3CHCl3), iodobenzene (1.5 equiv to Pd)
and 2,6-dimethylphenylisocyanide (XyNC, 3.0 equiv to
Pd) in toluene at 100 °C led to complete consumption of
the starting materials. A 2:2 coupling reaction of iodoben-
zene and isonitrile occurred to afford a-diimine 1a in 34%
yield (Scheme 2).
*
The reaction to provide a-diimine 1a would proceed via
the following pathway; (i) oxidative addition of Ph–I into
Corresponding author. Tel./fax: +81 78 803 5799.
E-mail address: amii@kobe-u.ac.jp (H. Amii).
0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.04.055

M. Morishita, H. Amii / Journal of Organometallic Chemistry 692 (2007) 620–624
621
N-R2
with either electron-donating or -withdrawing substituents
on the aryl rings provided the corresponding palladium(II)
isocynaide complexes 2 (entries 2, 4–6). Obviously, aryl
iodides possessing electron-withdrawing substituents (Cl,
CO2Et and Ac) on the aryl ring provided 2d–f in high yields
(entries 4–6) due to their great ability of oxidative addition
to Pd(0).
Next, the reactions of isolated palladium complexes 2
leading to a-diimines 1 have been investigated. Heating a
toluene solution of bis(xylylisocyanide)imidoylpalla-
dium(II) complex 2a at 100 °C for 1 h afforded 1a, a bis-
imine of benzil in 43% yield (Table 2, entry 1).
O
O
R1
X
2
2
(a)
(b)
(c)
(d)
R1
R1
or
R2-N
R1
N-R2
R1
R1 C N
R2-N
Cl
N-R2
Cl
1
R2
R1–X
N
C
+
2
2
Scheme 1.
Under the similar conditions, the ligand homocoupling
reactions of imidoylpalladium complexes 2b and 2d which
possess p-tolyl and p-chlorophenyl groups in the iminoacyl
groups, provided a-diimines 1b and 1d, respectively (entries
2 and 3) [14]. However, the imidoylpalladium complexes
having electron-withdrawing substituents, such as ethoxy-
carbonyl and acetyl groups on the aryl rings of 2 did not
undergo the desired ligand homocoupling reactions (entries
4 and 5).
+
+
Pd2(dba)3·CHCl3
Ph-I
NC
(1.5 eq)
(3.0 eq)
Xy-N
Ph
N-Xy
Ph
toluene
100˚C, 1h
1a (34%)
The formation of a-diimines 1 from imidoylpalladium
complexes 2 can be explained by assuming a disproportion-
ation mechanism pictured in Scheme 3. The ligand-
exchange reactions of imidoylpalladium complexes 2 pro-
vide diiodobis(isocyanide)palladium 3 and bis(imidoyl)pal-
ladium(II) intermediates 4, which yield a-diimines 1 via
reductive elimination.
Scheme 2.
Pd(0), (ii) insertion of the isocyanide into the carbon–metal
r-bond in the phenylpalladium complex to give the imi-
doylpalladium(II) intermediate [13], and (iii) ligand-cou-
pling of the imidoylpalladium complex to yield a-diimine
1a. In order to elucidate the reaction mechanism, isolation
of imidoylpalladium complexes and investigation of their
reactivities are considered to be significant.
When a toluene solution of a 1:1 mixture of imidoylpal-
ladium complexes 2b and 2d was heated to 100 °C, the
As shown in Table 1, Pd2(dba)3 Æ CHCl3 was treated
with iodobenzene (1.5 equiv to Pd) and XyNC (3.0 equiv
to Pd) in toluene at 26 °C for 24 h, oxidative addition
and isocyanide-insertion took place to give imidoylpalla-
dium(II) complex 2a in 48% isolated yield (entry 1). Con-
ducting the reaction at 26 °C led to selective formation of
imidoylpalladium complex 2a, whereas heating the reaction
mixture of Pd2(dba)3CHCl3, Ph–I and XyNC at 100 °C
gave a-diimine 1a (Scheme 2). Other examples of the for-
mation of 2 are given in Table 1. Under the same experi-
mental conditions, the reactions of aryl iodides endowed
Table 2
N-Xy
Xy-N
N-Xy
toluene
CNXy
I
p
-R-C6H4
XyNC
Pd
100˚C,1h
p
-R-C6H4
C6H4-R-p
2a-f
1a-f
Entry
R
1
Yielda (%)
1
2
3
4
5
a
H
Me
Cl
CO2Et
Ac
1a
1b
1d
1e
1f
43
32
32
b
b
Isolated yield.
Not detected.
Table 1
b
R
I (1.5 eq)
N-Xy
Pd
Xy-NC (3.0 eq)
CNXy
I
p
-R-C6H4
Pd2(dba)3·CHCl3
N-Xy
Xy-N
N-Xy
toluene
XyNC
I
I
CNXy
I
2
Pd
+
26 ˚C, 24h
Ar
Ar
XyNC
Ar
CNXy
Pd
Pd
2
XyNC
CNXy
XyNC
Entry
R
2 (Yield (%))
2
3
4
1
2
3
4
5
6
H
Me
OMe
Cl
CO2Et
Ac
2a (48)
2b (65)
2c (0)
2d (83)
2e (79)
2f (84)
Xy-N
Ar
N-Xy
Ar
1
Scheme 3.

622
M. Morishita, H. Amii / Journal of Organometallic Chemistry 692 (2007) 620–624
N-Xy
Pd
N-Xy
Pd
CNXy
I
CNXy
I
p-Me-C6H4
p-Cl-C6H4
+
XyNC
XyNC
2b
2d
(1:1 mixture)
Xy-N
N-Xy
Xy-N
N-Xy
Xy-N
N-Xy
toluene
100˚C ,1h
+
+
p-Me-C6H4
C6H4-Me-p p-Cl-C6H4
C6H4-Cl-p p-Me-C6H4
C6H4-Cl-p
1b (32%)
1d (21%)
1g (17%)
Scheme 4.
homocoupling products 1b and 1d were obtained in 32%
and 21% yields, respectively (Scheme 4). Beside symmetri-
cal bis-imines 1b and 1d, cross-coupling product 1g, an
unsymmetrical a-diimine containing both iminoacyl groups
of 2b and 2d was formed in 17% yield. The present cross-
over experiment suggests that the ligand-exchange reac-
tions of imidoylpalladium complexes 2 proceed in an
intermolecular fashion [15].
In conclusion, we have demonstrated the 2:2 coupling
reactions of aryl halides and isocyanides. The key interme-
diates are imidoylpalladium complexes 2, which undergo
intermolecular ligand-exchange reactions to provide a-dii-
mines 1. At present, the use of a stoichiometric amount
of a Pd(0) complex is required for the present transforma-
tion. Fine-tuning of the catalyst system to make a stoichi-
ometric transformation catalytic is one of the practical
subjects worthy of further investigation.
24 h at 26 °C, the solvent was evaporated. Then, the resi-
due was purified by silica gel column chromatography
(hexane:AcOEt = 5:1) to afford imidoylpalladium complex
2a (24 mg, 48%) as a yellow solid. Mp 118.0–120.0 °C
(dec.). IR (KBr): 2178, 1617, 1581 cmꢀ1
.
1H NMR
(400 MHz, CDCl3): d 8.20 (dd, J = 9.2, 1.6, 2H), 7.47–
7.43 (m, 3H), 7.22 (t, J = 7.6, 2H), 7.05 (d, J = 7.6, 4H),
6.94–6.91 (m, 3H), 2.20 (s, 12H), 2.13 (s, 6H). 13C NMR
(100 MHz, CDCl3): d 176.9, 145.0, 135.9, 130.0 128.1,
128.0, 126.6, 123.4, 18.8, 18.7. Anal. Calc. for C33H32NPdI:
C, 56.30; H, 4.58; N, 5.97. Found: C, 56.15; H, 4.59; N,
5.97%.
3.3. [Pd{C(@NXy)C6H4Me-p}I(XyNC)2] (2b)
By a procedure similar to that for 2a, the title compound
was obtained (18 mg, 65%) from Pd2(dba)3 Æ CHCl3
(20 mg, 19 lmol), XyNC (15 mg, 0.19 mmol), and p-iodo-
toluene (13 mg, 57 lmol) as a yellow solid. Mp 118.0–
3. Experimental
119.0 °C (dec.). IR (KBr): 2171, 1621, 1588 cmꢀ1 1H
.
3.1. General remarks
NMR (400 MHz, CDCl3): d 8.20 (d, J = 8.4, 2H), 7.26–
7.24 (m, 2H), 7.22 (t, J = 7.6, 2H), 7.04 (d, J = 7.6, 4H),
6.96–6.90 (m, 3H), 2.41 (s, 3H), 2.20 (s, 12H), 2.12 (s,
6H). 13C NMR (100 MHz, CDCl3): d 176.4, 149.6, 140.1,
136.7, 135.9, 129.9, 129.8, 128.7, 128.0, 126.6, 123.2,
110.9, 110.8, 21.3, 18.8, 18.6. Anal. Calc. for C34H34N3PdI:
C, 56.88; H, 4.77; N, 5.85. Found: C, 56.62; H, 6.62; N,
5.78%.
IR spectra were measured on Perkin–Elmer FT-IR
Spectrometer SPECTRUM 1000 apparatus. 1H NMR
(400 MHz) and 13C NMR (100 MHz) spectra were
recorded by JEOL JNM-LA apparatus and the chemical
shift are reported in d (ppm) values relative to TMS (d
0 ppm). Coupling constants (J) are reported in hertz. Mass
spectra were measured on Shimadzu GCMS-QP 2000 A
instrument. Elemental analysis was performed on Yanaco
CHN CORDER MT-5. All reactions were carried out
under an argon atmosphere. Toluene was dried over
CaH2 and then distilled. All other reagents and solvents
were employed without further purification. Fuji-Silysia
BW-820MH silica gel was employed for column chroma-
tography and Merck silica gel 60 PF254 was used for pre-
parative thin layer chromatography (PTLC).
3.4. [Pd{C(@NXy)C6H4Cl-p}I(XyNC)2] (2d)
By a procedure similar to that for 2a, the title compound
was obtained (23 mg, 83%) from Pd2(dba)3 Æ CHCl3
(20 mg, 19 lmol), XyNC (15 mg, 0.19 mmol), and p-chlo-
roiodobenzene (14 mg, 57 lmol) as a yellow solid. Mp
128.0–129.0 °C (dec.). IR (KBr): 2181, 1619, 1585 cmꢀ1
.
1H NMR (400 MHz, CDCl3): d 8.27 (d, J = 8.8, 2H),
7.43 (d, J = 8.8, 2H), 7.22 (t, J = 7.6, 2H), 7.06 (d,
J = 7.6, 4H), 6.95–6.89 (m, 3H), 2.21 (s, 12H), 2.11 (s,
6H). 13C NMR (100 MHz, CDCl3): d 175.8, 143.4, 136.7,
135.9, 135.8, 130.9, 130.1, 128.8, 128.3, 128.0, 126.5,
123.4, 18.7, 18.6. Anal. Calc. for C33H31N3ClPdI: C,
53.68; H, 4.23; N, 5.69. Found: C, 54.03; H, 4.53; N, 6.06%.
3.2. [Pd{C(@NXy)Ph}I(XyNC)2] (2a)
To a solution of Pd2(dba)3 Æ CHCl3 (20 mg, 19 lmol)
and XyNC (15 mg, 0.19 mmol) in 1 mL of toluene was
added iodobenzene (12 mg, 57 lmol). After stirring for

M. Morishita, H. Amii / Journal of Organometallic Chemistry 692 (2007) 620–624
623
3.5. [Pd{C(@NXy)C6H4(CO2Et)-p}I(XyNC)2] (2e)
103 (20.7), 79 (34.4). Anal. Calc. for C32H32N2: C, 86.44;
H, 7.25; N, 6.30. Found: C, 85.88; H, 7.50; N, 5.98%.
By a procedure similar to that for 2a, the title compound
was obtained (23 mg, 79%) from Pd2(dba)3 Æ CHCl3
(20 mg, 19 lmol), XyNC (15 mg, 0.19 mmol), and ethyl
p-iodobenzoate (16 mg, 57 lmol) as a yellow solid. Mp
129.0–130.0 °C (dec.). IR (KBr): 2177, 1706, 1612,
3.9. 1,2-Bis[N-(2,6-dimethylphenyl)imino]-1,2-di(4-
chlorophenyl)ethane (1d)
A viscous yellow oil. IR (neat): 1633, 1591 cmꢀ1. H
1
1584 cmꢀ1
.
1H NMR (400 MHz, CDCl3): d 8.34 (d,
NMR (400 MHz, CDCl3) [as a mixture of two geometrical
isomers, an isomer ratio of 4:1]: d 8.18–6.84 (m, 14H),
2.03–1.78 (m, 12H). GC–MS: m/z (%) 488 (0.4) 486 (0.8),
484 (1.2) [M+], 244 (33.4), 242 (100), 105 (3.8), 77 (51.4).
Anal. Calc. for C30H26N2Cl2: C, 74.22; H, 5.40; N, 5.77.
Found: C, 72.43; H, 5.87; N, 5.39%.
J = 8.8, 2H), 8.13 (d, J = 8.8, 2H), 7.21 (t, J = 7.6, 2H),
7.05 (d, J = 7.6, 4H), 6.96–6.90 (m, 3H), 4.42 (q, J = 7.2,
2H), 2.20 (s, 12H), 2.12 (s, 6H), 1.43 (t, J = 7.2, 3H). 13C
NMR (100 MHz, CDCl3): d 177.0, 166.3, 149.6, 148.6,
148.5, 143.0, 135.8, 131.3, 130.1, 129.3, 128.0, 126.3
126.3, 123.5, 61.1 18.7, 18.6, 14.3. Anal. Calc. for
C36H36N3O2PdI: C, 55.72; H, 4.68; N, 5.41. Found: C,
55.81; H, 4.69; N, 5.54%.
Acknowledgement
This work has been supported by the Ministry of Educa-
tion, Culture, Sports, Science and Technology of Japan
(Grant-in-Aid for Young Scientists (B), No. 16750081
and Grant-in-Aid for Scientific Research on Priority Areas
(‘‘Reaction Control of Dynamic Complexes’’), No.
16033621). We also thank Professor Masahiko Hayashi
(Kobe University) for helpful discussion.
3.6. [Pd{C(@NXy)C6H4(COCH3)-p}I(XyNC)2] (2f)
By a procedure similar to that for 2a, the title compound
was obtained (24 mg, 84%) from Pd2(dba)3 Æ CHCl3
(20 mg, 19 lmol), XyNC (15 mg, 0.19 mmol), and p-iodo-
acetophenone (14 mg, 57 lmol) as a yellow solid. Mp
139.0–140.0 °C (dec.). IR (KBr): 2186, 1689, 1617, 1604,
1583 cmꢀ1
.
1H NMR (400 MHz, CDCl3): d 8.37 (d,
References
J = 8.4, 2H), 8.05 (d, J = 8.4, 2H), 7.21 (t, J = 7.6, 2H),
7.05 (d, J = 7.6, 4H), 6.96–6.90 (m, 3H), 2.67 (s, 3H),
2.20 (s, 12H), 2.12 (s, 6H). 13C NMR (100 MHz, CDCl3):
d 197.8, 177.1, 149.7, 148.6, 137.6, 135.8, 130.1, 129.6,
128.2, 128.0, 126.4, 123.6, 26.8, 18.8, 18.7, 18.6. Anal. Calc.
for C35H34N3OPdI: C, 56.35; H, 4.59; N, 5.63. Found: C,
56.57; H, 4.74; N, 5.40%.
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of imidoylpalladium complex 2a (7.9 mg, 1.1 lmol) was
heated at 100 °C for 1 h. Then, the reaction mixture was
concentrated under reduce pressure, purified by PTLC
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m/z (%) 444 (1.5) [M+], 429 (6.3), 222 (100), 207 (27.9)

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compounds endowed with imidoyl moieties were not detected. After
heating of the imidoylpalladium complexes 2 at 100 °C, we observed
the formation of [PdI(CNXy)]n oligomers [Ref. [13h]] besides a-
diimines.
[12] N. Chatani, S. Murai, T. Hanafusa, Chem. Exp. 6 (1991) 339–342.
[13] (a) S. Otsuka, A. Nakamura, T. Yoshida, J. Am. Chem. Soc. 91
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[15] There exists a possibility of other reaction pathway such as
scrambling of the aryl groups in the Pd complexes 2. When a
mixture of the imidoylpalladium complex 2b and p-Cl–C6H4–I
(1 equiv) was heated at 100 °C for 1 h, homocoupling product
1b was obtained selectively in 60% yield. And also heating a
mixture of Pd complex 2d and p-Me–C6H4–I (1 equiv) under
the same conditions gave a-diimine 1d in 38% yield. In each
experiment, the formation of the cross-coupling product 1g was
not observed. These results suggest that the imidoyl–imidoyl
coupling reactions would proceed via bimolecular ligand-
exchange pathway.
(b) Y. Yamamoto, H. Yamazaki, Bull. Chem. Soc. Jpn. 43 (1970)
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(c) S. Otsuka, M. Naruto, T. Yoshida, A. Nakamura, J. Chem. Soc.,
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´
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(d) R. Uson, J. Fornies, P. Espinet, E. Lalinde, P.G. Jones, G.M.
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(e) R. Uson, J. Fornies, P. Espinet, A. Garcia, C.F. Foces, F.H.
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