Transition Met Chem
derivatives as supporting ligands and diphosphine ligands
as linkers [39]. During preparation of this manuscript, Lu
et al. [40] have reported a new type of N-heterocyclic
carbene–PdCl2–(iso)quinoline complex, which proved to
be an efficient catalyst for C–N coupling of primary and
secondary amines with aryl chlorides at low catalyst
loadings. Although these species have been found to be
particularly useful as catalysts in the Buchwald–Hartwig
amination of aryl chlorides, the development of easily
prepared, highly reactive, and stable NHC–Pd(II) com-
plexes still constitutes a challenging endeavor in current
organometallic chemistry. Very recently, we reported the
first synthesis of N-heterocyclic carbene–palladium(II)
complexes 1–4 with benzoxazole or benzothiazole coli-
gands (Scheme 1) [41]. The potential of these complexes in
the catalytic Suzuki–Miyaura coupling of aryl chlorides
and benzyl chlorides with arylboronic acids was evaluated.
Under the optimal reaction conditions, the expected biaryl
products were obtained in high yields of up to 99 %.
Encouraged by these results, we have extended our inves-
tigations of the catalytic potential of these complexes.
Herein, we report their application in the Buchwald–
Hartwig amination of aryl chlorides with amines.
and toluene (0.5 mL). The mixture was stirred at 110 °C
for 15 h under N2. After cooling, the mixture was evapo-
rated and the product was isolated by preparative TLC on
silica gel plates. The purified products were identified by
1H NMR spectra, and their analytical data are given in the
Supporting Information.
Results and discussion
In order to test the catalytic activities of these N-hetero-
cyclic carbene–palladium(II) complexes 1–4, initial
experiments were carried out using chlorobenzene
(0.25 mmol) and morpholine (0.30 mmol) as the reactants,
plus complex 1 (2.0 mol%) as the catalyst, in toluene
(0.5 mL) at 110 °C, with the results shown in Table 1. The
choice of base proved to have an important influence on the
reaction (Table 1, entries 1–6) [42]. The reaction pro-
ceeded efficiently only when KOtBu was used as base,
giving the corresponding coupling product 5a in 91 %
yield in this case (Table 1, entry 1). When 3.0 mol% of
complex 1 was tested, the yield was not enhanced further
(Table 1, entry 7). However, by prolonging the reaction
time to 15 h the product 5a was obtained in a 99 % yield
(Table 1, entry 8). Complex 3 also showed good catalytic
activity (Table 1, entry 10), although slightly lower than
that of complex 1, indicating that bulkier NHC ligands
make a more reactive catalyst. Interestingly, complexes 2
and 4 were much less efficient than 1 and 3 under identical
conditions (entry 8 vs. entry 9; entry 10 vs. entry 11) which
Experimental
General remarks
Solvents were dried by standard methods and freshly dis-
tilled prior to use if needed. All other chemicals were used
as purchased. The N-heterocyclic carbene–palladium(II)
complexes were synthesized according to our previous
report [41]. Preparations of the complexes as well as all the
catalytic reactions were carried out under a nitrogen
atmosphere. NMR spectra were recorded on a Bruker DPX
400 instrument using TMS as an internal standard.
Table 1 Optimization of reaction conditions for the coupling of
chlorobenzene with morpholine catalyzed by the NHC–Pd(II)
complexes
Entry
Cat.
Base
Yield (%)a
1
1
1
1
1
1
1
1
1
2
3
4
KOtBu
K2CO3
Cs2CO3
NaOH
NaOtBu
KOH
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
91
53
37
43
67
9
2
3
General procedure for the catalytic amination
4
5
A Schlenk flask was charged with the required aryl chlo-
ride (0.25 mmol), amine (0.30 mmol), N-heterocyclic car-
bene–palladium(II) complex (2 mol%), KOtBu (1.3 equiv),
6
7b
8c
9c
10c
11c
92
99
47
83
37
Scheme 1 N-Heterocyclic
carbene–palladium(II)
R
complexes. 1: R = o,o0-iPr2,
All reactions were carried out using chlorobenzene (0.25 mmol),
morpholine (0.30 mmol), base (1.3 equiv), plus catalyst (2.0 mol%)
in toluene (0.5 mL) at 110 °C for 12 h
X = O; 2: R = o,o0-iPr2,
X = S; 3: R = o,p,o0-Me3,
X = O; 4: R = o,p,o0-Me3,
X = S
l
C
N
N
X
Pd
N
a
Isolated yields
l
C
b
Cat. (3.0 mol%)
c
R
Reaction time was 15 h
123