Evaluation of catalytic properties of aminocarbene species derived from the integration between 3-iminoisoindolin-1-ones and palladium-bound isonitriles in Suzuki-Miyaura cross-coupling

Article abstract of DOI:10.1016/j.ica.2011.09.047

A series of palladium aminocarbene complexes [PdCl{C(NC(C₆R ²R³R⁴R⁵CON))N(H)R ¹}(CNR¹)] (9-23, 80-85% isolated yield) were obtained via the metal-mediated integration of one ison

Full text of DOI:10.1016/j.ica.2011.09.047

Inorganica Chimica Acta 380 (2012) 322–327
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Evaluation of catalytic properties of aminocarbene species derived from the
integration between 3-iminoisoindolin-1-ones and palladium-bound
isonitriles in Suzuki–Miyaura cross-coupling
a
a,b,
⇑
Rogério S. Chay , Konstantin V. Luzyanin
a
Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
St. Petersburg State University, 198504 Stary Petergof, Russian Federation
b
a r t i c l e i n f o
a b s t r a c t
2
3
4
5
1
1
Article history:
Available online 2 October 2011
6
A series of palladium aminocarbene complexes [PdCl{C(N@C(C R R R R CON))@N(H)R }(C„NR )] (9–23,
8
[
0–85% isolated yield) were obtained via the metal-mediated integration of one isonitrile in cis-
PdCl (C„NR ) ] [R = Cy 1, t-Bu 2, Xyl 3, 2-Cl,6-Me-C H 4] and various 3-iminoisoindolin-1-ones [R –
2 2 6 3
R = H 5; R , R , R = H, R = Me/R , R , R = H, R = Me 6 (isomeric mixture); R , R = H, R , R = Cl 7;
1
1
2
Young Investigator Award Special Issue
5
2
4
5
3
2
3
5
4
2
5
3
4
2
5
+
R –R = F 8]. New compounds 18–23 were completely characterized using (C, H, N), ESI –MS, IR, 1D
Keywords:
1
H, C{ H}) and 2D ( H, H COSY, H, C HMQC/ H, C HSQC, 1H, C HMBC) NMR spectroscopies.
13
1
1
1
1
13
1
13
13
(
Acyclic aminocarbenes
Palladium complexes
Suzuki–Miyaura coupling
Mild catalytic conditions
The efficiency of all prepared aminocarbene species 9–23 in Suzuki–Miyaura cross-coupling reaction of
aryl-bromides and iodides was evaluated, showing that complexes 9, 14 and 21 manifest the highest
activity furnishing the coupling product in 81–99% yield, and giving maximum turnover numbers (TONs)
up to 7.6 Â 10 . Catalytic system employs non-dried EtOH as an environmentally benign solvent, K
4
2 3
CO
as a base, and runs under air.
Ó 2011 Elsevier B.V. All rights reserved.
1
. Introduction
Acyclic aminocarbenes resemble N-heterocyclic species in terms
of net electron-donor properties and steric flexibility [16], being,
however, much less explored as catalysts. Complexes with acyclic
aminocarbene ligands can be generated via an atom-efficient and
modular protocol based on a stoichiometric nucleophilic addition
to metal-bound isonitriles (Scheme 1) [16,17]. The vast majority
of reported examples of the latter reaction concern the coupling
Development of efficient cross-coupling catalytic systems de-
pends predominantly upon design of appropriate metal catalysts
and corresponding ligands [1,2]. At this point, the emergence of
N-heterocyclic carbenes (heterocylic aminocarbenes, NHCs) as
ancillary ligands in the place of commonly used phosphines, allows
for better thermal and chemical stability of the catalysts, reduce
the sensitivity of the system to air and humidity, preserving high
catalytic activity [3–13]. Despite a widespread application of NHCs,
a major disadvantage of these compounds regards their prepara-
tion, in particular, rather challenging synthesis of unsymmetrically
substituted or chiral carbenes [14,15]. Furthermore, taking into ac-
count that the fine-tuning of the prepared metal–carbenes toward
selected catalytic application is mostly achieved via the empiric
variation of carbene structure, that, consequently, leads to a vast
amplification of the synthetic protocol towards modified NHCs,
the search for the alternative catalytic systems is being of high
importance.
3
of M-CNR species with amines/hydrazines (sp -N type nucleo-
philes) leading to complexes with acyclic diaminocarbene ligands
(M-ADCs) [16]. Only recently, we have reported on first examples
2
of the coupling between various imines (sp -N type nucleophile),
such as benzophenone imine [18], 3-iminoisoindolin-1-one [19],
or N-phenylbenzamidine [20], and metal-bound isonitriles,
affording complexes containing novel types of aminocarbene li-
gands. For instance, when 3-iminoisoindolin-1-one (a stable het-
erocyclic imine) was used as a nucleophile, the formation of
rare chelating acyclic aminocarbene species was observed
(Scheme 2) [19]. Obtained compounds exhibit a superior chemical
(towards air and humidity) and thermal (in a 20–120 °C range)
stability as compared to the corresponding monodentate amino-
carbenes [19], thus, being of potential interest for the catalytic
applications.
Following our ongoing project on development of novel mild
and efficient cross-coupling catalytic systems employing metal
catalysts of novel types [20,21], we aimed to expand the series
of palladium aminocarbene complexes by use of other
⇑
Corresponding author at: Centro de Química Estrutural, Complexo I, Instituto
Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon,
Portugal.
E-mail address: kluzyanin@ist.utl.pt (K.V. Luzyanin).
0
020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.09.047

R.S. Chay, K.V. Luzyanin / Inorganica Chimica Acta 380 (2012) 322–327
323
R3
R4
R3
R4
R3
N
H
C
N
R3
R4
R4
H
H
C
H
N
N
[
M]
C
[M]
C
N
R1
[M]
C
N
N
R1
R1
Addition of amine
Addition of imine
Scheme 1. Generation of M-ADC species via the addition of nucleophiles to metal-bound isonitriles.
3
-iminoisoindolin-1-ones and isonitriles, and, to evaluate the cata-
197.4 (Ccarbene@N), 167.2 (C@O), 161.5 (C@N), 144.9, 143.9, and
141.3 (C–F aryls), 117.8, 116.1, (aryls), 56.0 and 53.9 (HCCy),
32.3, 31.9, 31.7, 24.7, 24.6, and 24.1 (H C–Cy).
2
lytic efficiency of the thus prepared palladium–aminocarbenes in
Suzuki–Miyaura coupling. The results of our studies are disclosed
below.
2
.2.3. [PdCl{C(N@C(C
6
F
4
CON))@N(H)Xyl}(C„NXyl)] (19)
2
. Experimental
.1. Materials and instrumentation
All solvents, PdCl , and all isonitriles were obtained from com-
F
F
2
O
Xyl
F
2
mercial sources and used as received apart from chloroform that
was purified by conventional distillation over calcium chloride.
The starting palladium-isonitrile complexes cis-[PdCl
N
N
C
F
1
2
(C„NR )
2
]
Pd
H
N
1
[
6 3
R = Cy 1, t-Bu 2, Xyl 3, 2-Cl,6-Me-C H 4] [22–26] and various
C
Cl
2
5
2
4
5
3
2
3
-iminoisoindolin-1-ones [R –R = H 5; R , R , R = H, R = Me/R ,
3
5
4
2
5
3
4
N
R , R = H, R = Me 6 (isomeric mixture); R , R = H, R , R = Cl 7;
Xyl
ClF
2
5
R –R = F 8] [27], were prepared in accord with the published pro-
cedures. Known aminocarbene complexes 9–17 (Scheme 2) de-
rived from the integration of 1–3 with 3-iminoisoindolin-1-ones
Anal. Calc. for C26
C, 50.13; H, 3.12; N, 9.00%. ESI –MS, m/z: 620 [M] . IR (KBr, selected
H
19
N
4
4
OPd: C, 50.26; H, 3.08; N, 9.02. Found:
+
+
À1
5–7, were prepared as previously reported [19].
bands, cm ): 3246 m
m(N–H); 2931 s, 2857 m
m
(C–H); 2229 s
(C@N); 1554 vs
(Ccarbene@N); 730 s d(C–H from aryls). H NMR (CDCl , d): 10.38
m
m
(C„N); 1725 s, 1721 m, 1680 mw
m
(C@O) + m
1
2.2. Preparation and analytical data for the palladium–aminocarbene
3
complexes
(s, br, 1H, NH), 7.34–7.13 (m, 6H, aryls) 2.56, 2.49 and 2.28 (s,
1
(C@O), 167.2 (C@N), 136.6, 136.5, 136.3, and 135.2 (C–F aryls),
133.6, 131.0, 130.7, 130.1, 128.6, 128.4, 128.2, and 128.0 (aryls),
18.9 and 18.7 (Me).
1
3
1
2H, Me).
3
C{ H} NMR (CDCl , d): 200.0 (Ccarbene@N), 181.3
2.2.1. Preparation of complexes 18–23
A slightly modified known procedure was employed [19]. Thus,
a finely ground solid 3-iminoisoindolin-1-one (5–8) (0.1 mmol)
1
was added to a solution of cis-[PdCl
CHCl (10 mL), and the resulting mixture was further refluxed for
h. After 4 h, the reaction mixture was filtered, and the filtrate
2 2
(C„NR ) ] (0.1 mmol) in
3
2
6 4 6 3
.2.4. [PdCl{C(N@C(C H CON))@N(H)(2-Cl,6-Me-C H )}(C„N(2-Cl,6-
4
Me-C ))] (20)
6 3
H
was evaporated to dryness under a stream of nitrogen at room
temperature, and washed with five 1-mL portions of cold (5 °C)
2
Et O, and dried in vacuo at 20–25 °C. Yields of 18–23 were 80–85%.
R1
O
2
.2.2. [PdCl{C(N@C(C
6
F
4
CON))@N(H)Cy}(C„NCy)] (18)
N
N
C
F
F
N
Pd
H
C
Cl
O
Cy
F
N
N
R1
N
C
F
R1 = 2-Cl,6-Me-C
6 3
H
N
Pd
H
17 4 3
Anal. Calc. for C24H N Cl OPd: C, 48.84; H, 2.90; N, 9.49. Found: C,
C
Cl
+
+
4
8.74; H, 2.82; N, 9.52%. ESI –MS, m/z: 553 [MÀCl] . IR (KBr,
N
À1
selected bands, cm ): 3201 mw
m(N–H); 2966 mw, 2926 mw
Cy
m
(C–H); 2199 s
m(C„N); 1735 s, 1673 s, 1607 s m(C@O) + m(C@N);
1
Anal. Calc. for C22
H
23
N
4
ClF
4
OPd: C, 45.77; H, 4.02; N, 9.70. Found:
1522 vs (Ccarbene@N); 711 s d(C–H from aryls). H NMR (CDCl ,
m
3
+
+
C, 45.82; H, 4.00; N, 9.65%. ESI –MS, m/z: 595 [M+H
3
O] . IR (KBr, se-
(C–H);
(C@N); 1553 s
(Ccarbene@N); 730 s d(C–H from aryls). H NMR (CDCl , d): 10.56
s, br, 1H, NH), 4.50 (m, br, 1H, HCCy), 3.91 (m, br, 1H, HCCy),
d): 10.27 (s, br, 1H, NH), 7.77 and 7.70 (m, 4H) (aryls from the iso-
À1
lected bands, cm ): 3245 w
m
(N–H); 2933 m, 2858 w
m
indoline moiety), 7.60–7.48 (m, 6H, aryls), 2.59 and 2.32
1
3
1
2
229 s
m
(C„N); 1723 s, 1601 m
m
(C@O) +
m
3
(s, 6H, Me). C{ H} NMR (CDCl , d): 202.5 (Ccarbene@N), 188.6
1
m
(
1
3
(C@O), 173.6 (C@N), 139.5, 138.8, 136.9, 135.5, 134.4, 133.9,
132.3, 131.9, 130.8, 130.6, 129.3, 129.1, 128.8, 127.7, 127.3, and
124.4 (aryls), 22.9 and 19.3 (Me).
.94–1.79 and 1.47–1.44 (m, 20H, Cy). 13C{ H} NMR (CDCl
1
, d):
3

3
24
R.S. Chay, K.V. Luzyanin / Inorganica Chimica Acta 380 (2012) 322–327
R4
R5
R1
O
R3
R5
O
N
R4
N
CHCl , reflux
R2
C
3
Cl
C
Cl
C
Pd
NH
Pd
N
R3
−HCl
C
Cl
N
N
R2
NH
N
R1
R1
R1
H
R –R = H 5; R ,R ,R = H, R3 = Me /
2
5
2
4
5
R1 = Cy 1, t-Bu 2, Xyl 3,
-Cl,6-Me-C
2
3
5
4
R ,R ,R = H, R = Me 6 (isomeric mixture);
9–23
2
6 3
H 4
2
5
3
4
2
5
R ,R = H, R ,R = Cl 7; R –R = F 8
Scheme 2. Reaction of 3-iminoisoindolin-1-ones (5–8) with an isonitrile in cis-[PdCl
2
(C„NR1)
] (1–4) affording palladium–aminocarbene complexes (9–23).
2
2
.2.5. [PdCl{C(N@C(C
H
6 3
(4-Me)CON))@N(H)(2-Cl,6-Me-C
6
3
H )}(C„N-
(C@N), 138.8, 138.3, 136.8, 136.6, 134.8 (aryls), 133.8, 133.1 (CH
(
2-Cl,6-Me-C ))] (21)
6
H
3
aryls from isoindoline moiety), 131.8, 130.9, 130.6, 129.3, 129.1,
1
28.8, 127.6, 127.4, and 126.0 (aryls), 19.3 and 19.1 (Me).
R1
O
2
.2.7. [PdCl{C(N@C(C CON))@N(H)(2-Cl,6-Me-C )}(C„N(2-Cl,6-
6
F
4
6 3
H
N
Me-C H ))] (23)
6
3
N
C
N
Pd
H
F
F
C
Cl
R1
O
N
F
R1
N
N
R1 = 2-Cl,6-Me-C
C
6
H
3
F
N
Pd
H
Anal. Calc. for C25
H
19
N
4
Cl
3
OPd: C, 49.69; H, 3.17; N, 9.27. Found: C,
C
+
+
Cl
4
9.72; H, 3.02; N, 9.26%. ESI –MS, m/z: 569 [MÀCl] . IR (KBr, se-
À1
lected bands, cm ): 3233 mw
H); 2197 s
521 vs (Ccarbene@N); 734 s d(C–H from aryls). H NMR (CDCl
d): 10.23 (s, br, 1H, NH), 7.87–7.37 (m, 9H, aryls), 2.64 (Me from
m
(N–H); 2968 mw, 2922 mw
m(C–
N
R1
m(C„N); 1751 s, 1720 s, 1674 s m(C@O) + m(C@N);
1
1
m
3
,
R1 = 2-Cl,6-Me-C
6 3
H
isoindoline moiety), 2.59, 2.32 (s, 9H, Me). 13C{ H} NMR (CDCl
1
4 3 4
Anal. Calc. for C24H13N Cl F OPd: C, 43.53; H, 1.98; N, 8.46. Found:
3
,
+
+
C, 43.73; H, 1.89; N, 8.36%. ESI –MS, m/z: 626 [MÀCl+H] . IR (KBr,
d): 202.4 (Ccarbene@N), 188.7 (C@O), 173.5 (C@N), 144.9, 143.4,
À1
selected bands, cm ): 3200 mw
m
(N–H); 3065 w, 2972 w
m
(C–H);
1
1
39.5, 138.7, 136.8, 134.9, 134.0, 132.8, 131.8, 131.5, 130.7,
29.3, 129.1, 128.8, 127.7, 127.5, 127.4, and 127.3 (aryls), 22.9
2
7
7
196 s
41 s d(C–H from aryls). H NMR (CDCl
.38–7.23 (m, 6H, aryls) 2.62 and 2.30 (s, 6H, Me). C{ H} NMR
, d): 201.5 (Ccarbene@N), 182.4 (C@O), 167.1 (C@N), 139.5,
m
(C„N); 1730 s
m
(C@O) +
m(C@N); 1533 s
m(Ccarbene@N);
1
3
, d): 10.43 (s, br, 1H, NH),
(
Me from isoindoline moiety), 21.7 and 19.4 (Me).
1
3
1
(
CDCl
3
2
.2.6. [PdCl{C(N@C(C
6
H
2
(3,4-Cl
2
)CON))@N(H)(2-Cl,6-Me-C
6
H
3
)}-
138.7, and 136.4 (C–F aryls), 133.4, 131.9, 131.8, 131.6, 130.9,
130.2, 129.6, 129.3, 129.1, 128.9, 127.7, 127.6, and 127.3 (aryls),
(
C„N(2-Cl,6-Me-C
6
H
3
))] (22)
2
2.8 and 19.4 (Me).
Cl
2
.3. General procedure for the Suzuki–Miyaura coupling
R1
O
Cl
À4
Selected base (2.0 Â 10 mol, 1.5 equiv), aryl bromide (1.0 Â
N
À4
À4
1
0
mol, 1.0 equiv) and phenylboronic acid (1.2 Â 10 mol, 1.2
N
C
equiv) were mixed in a 5-mL round-bottom tube. Consequently,
À8
N
Pd
H
a solution of the precatalyst (1 Â 10 mol) in non-dried EtOH
C
Cl
(2 mL) was added, and the tube was closed with a septum and
an aluminum crimp seal. The tube was placed in a preheated oil
bath at 80 °C, stirred for 2.5 h, and then cooled down to 25 °C.
The reaction mixture was evaporated to dryness under a stream
of dinitrogen followed by addition of 1.0 equiv of 1,2-dimethoxy-
ethane (NMR internal standard), and extraction of the reaction
N
R1
R1 = 2-Cl,6-Me-C
H
6 3
Anal. Calc. for C24
4
H
15
N
4
Cl
5
OPd: C, 43.64; H, 2.29; N, 8.50. Found: C,
+
+
3.34; H, 2.19; N, 8.28%. ESI –MS, m/z: 659 [M+H] . IR (KBr, se-
3
mixture with three 0.20-mL portions of CDCl . All fractions were
À1
1
lected bands, cm ): 3199 w
m
(N–H); 2965 w, 2924 w
m
(C–H);
(C@N);
(Ccarbene@N); 730 s d(C–H from aryls). H NMR (CDCl
combined and subjected to H NMR monitoring. The product peak
assignments were based on authentic samples or on the published
data [28–31], while quantifications were performed upon integra-
tion of the selected peak of the product relatively to the peak of the
standard. In some cases, the products were isolated by extraction
of the residue after evaporation of the reaction mixture with
2
1
196 s m(C„N); 1775 m, 1721 m, 1686 m m(C@O) + m
1
527 vs
m
3
,
d): 10.35 (s, br, 1H, NH), 8.04, 7.96 (s, 2H, aryls from isoindoline
moiety), 7.38–7.26 (m, 6H, aryls) 2.65 and 2.33 (s, 6H, Me).
C{ H} NMR (CDCl , d): 202.3 (Ccarbene@N), 186.3 (C@O), 171.2
3
1
3
1

R.S. Chay, K.V. Luzyanin / Inorganica Chimica Acta 380 (2012) 322–327
325
2
CH Cl
2
, followed by column chromatography on silica gel (Fluka
due to overlapped
m(C@O) and
m(C@N) vibrations from the imi-
4
0/60; 10:1 hexane/ethyl acetate, v/v).
noisoindolin-1-one moiety are found between 1624 and
À1
1
730 cm [19,27].
The H NMR spectra of the carbene complexes 18–23 display a
1
3
. Results and discussion
broad peak in the range of d 9.03–10.44 assigned to the M–
carbene@N(H)R proton, while the corresponding signal of the
C
3.1. Synthesis and characterization of palladium–aminocarbene
13
1
Ccarbene@NH carbon in the C{ H} spectra was found to resonate
complexes
at ca. 200 ppm, i.e. approximately 80 ppm to lower field in compar-
II
ison with the starting (isonitrile)M complexes (e.g. 115 ppm for
Synthesis of new (18–23) and known (9–17) palladium–amino-
1
13
2 2
C„N in cis-[PdCl (C„NCy) ]). Complete H and C signal assign-
carbene complexes was accomplished under previously described
experimental conditions [19]. Thus, the reaction between a palla-
dium-bound isonitrile in cis-[PdCl
ments were performed by interpretation of the gradient enhanced
two-dimensional H, C HMQC/ H, C HSQC and 1H, C HMBC
1
13
1
13
13
1
2
(C„NR )
2
] (1, 3, and 4) and
NMR spectra.
the 3-iminoisoindolin-1-ones (5–8) proceeds smoothly under re-
flux in CHCl for 4 h (Scheme 2). Subsequent workup provides
the carbene species [PdCl{C(N@C(C
3
2
3
4
5
1
6
R R R R CON))@N(H)R }-
C„NR )] (9–23) in 80–85% isolated yields (Scheme 2, Table 1).
1
(
3.2. Catalytic studies
2 2
The reaction between cis-[PdCl (C„N(t-Bu)) ] (2) and 8 did not af-
ford any isolable aminocarbene species. The authenticity of known
species 9–17 was established upon comparison of the recorded H
NMR spectra with those previously reported by one of us [19],
while new complexes 18–23 were completely characterized using
As a model system for the Suzuki–Miyaura coupling, we have
chosen the commonly used reaction of 4-methoxy-bromobenzene
(4-bromoanisole) with phenylboronic acid accomplishing 4-
methoxybiphenyl (Scheme 3). Taking into account our preliminary
data on optimization of the reaction conditions (performed for
other Pd-aminocarbene species) [21], we opt to use ethanol as an
environmentally benign solvent for our catalytic runs. In refluxing
EtOH, the conversion of 4-bromoanisole is essentially complete
after ca. 2.5 h, furnishing the target 4-methoxy-1,1’-biphenyl in
ca. 84–92% yield (for precatalyst 9). No visible catalyst decomposi-
tion (i.e. formation of the palladium black) was observed under
these conditions.
1
+
1
13
1
1
1
1
13
(
C, H, N), ESI –MS, IR, 1D ( H, C{ H}) and 2D ( H, H COSY, H, C
1
13
1
13
HMQC/ H, C HSQC, H, C HMBC) NMR spectroscopies.
Complexes 18–23 gave C, H, and elemental analyses
consistent with the proposed formulations of the palladium–
N
2
3 4 5
aminocarbene
complexes
6
[PdCl{C(N@C(C R R R R CON))@
1
1
+
N(H)R }(C„NR )], whereas the ESI –MS mass spectra display
molecular ion peaks and/or a fragmentation corresponding to the
+
+
loss of Cl’s, viz. [MÀnCl] or the protonation [M+H
3
O] of the
molecular ion. The IR spectra of 18–23 exhibit one strong
It is known, that a type of base employed directly influences the
catalyst activity in Suzuki–Miyaura cross-coupling reactions. Thus,
different types of bases were screened and the results are summa-
rized in Table 2. As can be inferred from an inspection of the ob-
m
2
(C„N) stretching vibration in the range between 2196 and
230 cm suggesting the presence of one unreacted isonitrile li-
À1
2 2
gand in 18–23, while in the starting cis-[PdCl (C„NR) ] (1–4)
complexes two overlapped stretches in the interval 2150–2250
tained results,
appropriate base for this catalytic system, while when K
(74%, entry 3) and Cs CO (74%, entry 5) are applied, the yields
are significantly decreased; KOH (13%, entry 2) and Et N (4%, entry
1) were not efficient. For further studies, K CO has been chosen.
K
2
CO
3
(Table 2, 92%, entry 4) is the most
À1
cm
are normally detected [23]. In the C@N(H)R carbene
3
PO
4
moiety, very strong bands due to
range between 1521 and 1554 cm
m
(Ccarbene@N) appear in the
as in the previously re-
2
3
À1
3
ported Pd-ADC species [19,21,22], while corresponding bands
2
3
Table 1
Numbering of prepared palladium–aminocarbenes species.
1
R in starting complex and products
R4
R2
R5
Imine
2
-Cl,6-Me-
O
R1
Cy
9
10
11
18
t-Bu
Xyl
R3
C6H3
N
5
6
7
8
12
13
14
–
15
16
17
19
20
21
22
23
N
C
N
Pd
C
Cl
N
R1
H
X
B(OH)2 precatalyst
solvent, base
R6
R6
R6 = OMe, Me, NO ; X = Br, I
Model system: R = OMe, X = Br
2
6
Scheme 3. Suzuki–Miyaura cross-coupling system employing 9–23 as precatalysts.

3
26
R.S. Chay, K.V. Luzyanin / Inorganica Chimica Acta 380 (2012) 322–327
Table 2
Table 5
Screening for an appropriate base for the model Suzuki–Miayura
coupling system.
Effect of catalyst loading (for 9, 14, and 21) in the Suzuki–Miayura cross-coupling
system.
Entry
Base
NEt
KOH
Yield (%)
Entry
Precatalyst
Yield (%)
1
2
3
4
5
3
4
1
2
3
4
–
9
14
21
4
13
74
92
74
76
74
53
K
K
3
PO
CO
CO
4
2
3
Cs
2
3
À4
À4
4
-Bromoanisole (1.0 Â 10 mol, 1.0 equiv), phenylboronic acid (1.2 Â 10 mol,
À4
À4
À9
Base (2.0 Â 10 mol, 2.0 equiv), 4-bromoanisole (1.0 Â
1.2 equiv), K
EtOH (2 ml).
2
CO
3
(2.0 Â 10 mol, 2.0 equiv); selected precatalyst (1 Â 10 mol);
À4
mol, 1.0 equiv), phenylboronic acid (1.2 Â 10À4 mol,
1
1
0
À8
.2 equiv); precatalyst 9 (1 Â 10 mol); EtOH (2 ml).
aminocarbenes [29–31]. Thus, we employ environmentally benign
solvent (EtOH) in a place of toxic THF or toluene traditionally used
for Suzuki–Miyaura coupling, as well as a cheap inorganic base
Table 3
Screening of the catalytic efficiency for 9–23 in Suzuki–Miayura cross-coupling
system.
(K
2
CO
3
). Our system also demonstrates high tolerance to air and
À2
humidity and requires low catalyst loading (less than 1 Â 10
Entry
Precatalyst
Yield (%)
Entry
Precatalyst
Yield (%)
mol% of catalyst).
1
2
3
4
5
6
7
8
9
92
75
73
46
73
64
86
54
9
16
17
19
20
21
22
23
–
63
58
52
62
81
77
60
4
10
11
18
12
13
14
15
10
11
12
13
14
15
16
4. Final remarks
Within our studies we have widen the series of palladium–
aminocarbene species derived from the metal-mediated integra-
tion between cis-[PdCl
twenty-four species, employing, among others, 2,3,4,5-tetra-
fluoro-3-iminoisoindolin-1-one and 2-chloro,6-methyl-phenyliso-
nitrile as the previously unused coupling partners.
À4
À4
1
4
1
-Bromoanisole (1.0 Â 10 mol, 1 equiv), phenylboronic acid (1.2 Â 10 mol,
(C„NR )] and 3-iminoisoindolin-1-one to
2
À4
À8
.2 equiv), K
2
CO
3
(2.0 Â 10 mol, 2.0 equiv); selected precatalysts (1 Â 10 mol);
EtOH (2 ml).
Prepared aminocarbene derivatives were evaluated as catalysts
in Suzuki–Miyaura cross-coupling of aryl- bromides and iodides
with phenylboronic acids, and we found that, catalytic systems
based on 9, 14, and 21 exhibit highest efficiency in terms of
The comparison of the catalytic activity for all prepared Pd-
aminocarbene complexes 9–23 in the model Suzuki–Miyaura
cross-coupling system was undertaken, and the results are sum-
marized in Table 3. Thus, among all palladium species, complexes
4
yields/TONs (yields up to 91%, TONs up to 7.6 Â 10 with
9, 14 and 21 manifest the highest activity furnishing the coupling
4
6 4
-MeOC H Br as a substrate) and run in non-dried EtOH as an
product in 81–92% yield.
environmentally safe solvent and under air. The reasons for the
superior catalytic activity of 9, 14, and 21 as compared to other
aminocarbene complexes from this study are not clear, and, in
our opinion, can be justified by the achieved optimal balance
between a sterical hindrance and an electronic distribution within
the carbene species. Despite the fact that the conversion of aryl
chlorides was not accomplished with the prepared precatalysts
For the most efficient precatalysts 9, 14 and 21, we have verified
the functional group tolerance – scope of the catalytic process. It
6
was found that the various organobromides 4-R C
H
6 4
Br bearing
6
either electron-donor (R = Me, OMe) or electron-withdrawing
NO ) groups and organoiodide 4-MeOC I, react with phenylbo-
(
2
6 4
H
ronic acid to give excellent yields of biphenyl species (Table 4),
thus showing a valuable versatility of our catalytic system.
In order to estimate the catalytic efficiency, we also examined
(
see, for instance, examples for the transformation of those chal-
lenging substrates employing M-NHCs [2,9]), our catalytic scheme
is being one of the mildest and most efficient Suzuki–Miyaura sys-
tems for transformation of aryl bromides based on acyclic
aminocarbenes.
Further studies towards a development of efficient and green
catalytic systems based on the novel types of carbene complexes,
are currently under way in our group.
the effect of catalyst loading in the model catalytic system
4
(
Table 5). Yields up to 76%, and the TONs up to 7.6 Â 10 , were ob-
À5
tained with the catalyst loadings as low as 10 mol per mole of
substrate. Moreover, we also evaluated the air/moisture stability
of the catalytic system by performing some additional runs in dried
EtOH and under dinitrogen, and observed no significant difference
with those performed under air and in non-dried ethanol.
It is worth mentioning that understudied system is being highly
efficient and green in a comparison with the other based on acyclic
Acknowledgements
Table 4
This work has been partially supported by the Fundação para a
Ciência e a Tecnologia (FCT), Portugal (including FCT projects
PTDC/QUI-QUI/098760/2008, PTDC/QUI-QUI/109846/2009 and
PEst-OE/QUI/UI0100/2011). K.V.L. express gratitude to the FCT
and the Instituto Superior Técnico (IST) for a research contract
Evaluation of the scope of the catalytic system with the selected precatalysts.
Entries
Aryl halide
Yields with the selected precatalyst (%)
9
14
21
1
4
7
1
–3
–6
–9
0–12
4-MeOC
4-MeC
4-NO
4-MeOC
6
H
H
6 4
4
Br
Br
Br
92
86
91
98
86
83
98
99
81
94
95
99
(
Ciência 2008 program), and R.S.C. to the FCT for an early-stage re-
2
C
6
H
4
search grant (within FCT project PTDC/QUI-QUI/098760/2008). The
authors thank the Portuguese NMR Network (IST-UTL Center) for
providing access to the NMR facilities and the Portuguese MS Net-
work (IST Node) for the ESI–MS measurements. The authors are
grateful to Prof. Nadezhda Bokach for valuable discussions.
6 4
H I
À4
À4
Aryl halide (1.0 Â 10 mol, 1 equiv), phenylboronic acid (1.2 Â 10 mol,
À4
À8
1
.2 equiv), K
2
CO
3
(2.0 Â 10 mol, 2.0 equiv); selected precatalysts (1 Â 10 mol);
EtOH (2 ml).

R.S. Chay, K.V. Luzyanin / Inorganica Chimica Acta 380 (2012) 322–327
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