NCN pincer palladium complexes based on 1,3-dipicolyl-3,4,5,6-tetrahydropyrimidin-2-ylidenes: Synthesis, characterization and catalytic activities

Article abstract of DOI:10.1039/c5ra01706h

The synthesis of novel pincer palladium complexes containing ring expanded six-membered N-heterocyclic carbenes (NHCs) via direct metallation of the precursors tetrahydropyrimidin-1-ium hexafluorophosphates in the presence of NaN(SiMe₃)₂ is presented. The structure has been characterized unambiguously by X-ray single crystal analysis. Catalytic activity investigation showed that the complexes catalyzed the Heck reaction of aryl bromides with acrylate/styrene efficiently when using Et₃N as base and DMA as solvent.

Full text of DOI:10.1039/c5ra01706h

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PAPER
NCN pincer palladium complexes based on 1,3-
dipicolyl-3,4,5,6-tetrahydropyrimidin-2-ylidenes:
synthesis, characterization and catalytic activities†
Cite this: RSC Adv., 2015, 5, 25723
Liangru Yang,* Xinchi Zhang, Pu Mao,* Yongmei Xiao, Huanyu Bian, Jinwei Yuan,
Wenpeng Mai and Lingbo Qu
The synthesis of novel pincer palladium complexes containing ring expanded six-membered
N-heterocyclic carbenes (NHCs) via direct metallation of the precursors tetrahydropyrimidin-1-ium
Received 28th January 2015
Accepted 5th March 2015
hexafluorophosphates in the presence of NaN(SiMe
3
)
2
is presented. The structure has been
characterized unambiguously by X-ray single crystal analysis. Catalytic activity investigation showed that
DOI: 10.1039/c5ra01706h
the complexes catalyzed the Heck reaction of aryl bromides with acrylate/styrene efficiently when using
www.rsc.org/advances
3
Et N as base and DMA as solvent.
been synthesized, characterized, and applied to organic trans-
formations successfully. For example, the ring expanded six-, or
Introduction
Since the successful isolation and characterization of the rst seven-membered NHC copper(I) complexes have been proved to
stable N-heterocyclic carbene (NHC) by Arduengo et al. in be effective catalysts for the 1,3-dipolar cycloaddition of alkynes
1
10a
1
991, these molecules have been widely used as ancillary and azide.
A ring-expanded six-membered NHC Nickel(I)
2
ligands for the preparation of transition-metal-based catalysts.
complex has been reported to be a useful precursor for catalytic
9
Today, NHC metal complexes rank among the most powerful hydrodehalogenation. Palladium(II) complexes bearing ring
tools in organic chemistry, with numerous publications related expanded NHCs have been demonstrated to be effective cata-
to their coordination chemistry and catalytic properties being lysts for the intramolecular aerobic oxidative amination of
reported. NHC palladium complexes are attracting a consid- alkenes, the Heck reaction of aliphatic and aromatic vinyl
erable amount of interest because of their easy accessibility, compounds with aryl bromides and chlorides, Suzuki–
3
8a
8c
high thermal stability, and remarkable catalytic activities in Miyaura cross-coupling of aryl bromide and chloride, and the
5
a
various C–C coupling reactions. Numerous functionalized NHC catalytic dehalogenation of aryl chloride.
palladium complexes have been synthesized and applied to
The pincer architecture, which provides a preorganised
catalytic hydrogenation, hydrophosphination, C–H function- backbone featuring unique properties of high stability and
alization, Suzuki–Miyaura reactions, Mizoroki–Heck reactions, modular variability has also been extensively studied in a wide
4
14
Buchwald–Hartwig reactions, etc. successfully. However, most range of elds. Therefore, the inclusion of NHC donors within
of the reported NHC ligands are based on ve-membered pincer systems has attracted an increasing level of interest, with
15
heterocyclic rings (imidazol-2-ylidenes, imidazolin-2-ylidenes, a particular emphasis on their catalytic potential. Multiple
or benzimidazolin-2-ylidene, etc.).
pincer-type NHC palladium complexes carrying different donor
In recently years, the ring expanded NHCs based on six-, moieties have been prepared and employed as catalysts for a
16
seven-, or eight-membered heterocyclic rings began to attract number of catalytic organic transformations (Fig. 1). For
extensive attention due to the enhanced s-donor ability and example, the palladium complexes of phosphine/NHC-based
NHC
easy tunability of the electronic property and steric effect of the pincer ligand PC
P (B) have been reported to be active cata-
5
16b,16c
ligands. Various kinds of ring expanded NHC metal complexes, lysts for Suzuki coupling and Heck coupling reactions.
including Ag, Au(I), Pd, Ni, Cu, Ru, Rh, and Ir, etc. have these systems, however, contain NHC scaffolds based on ve-
membered heterocyclic rings.
All
6
7
8
9
10
11
12
13
Based on the reports mentioned above, it would be of interest
School of Chemistry and Chemical Engineering, Henan University of Technology, to explore whether the introduction of ring expanded NHCs to a
Zhengzhou 450001, P. R. China. E-mail: lryang@haut.edu.cn; pumao@haut.edu.cn
pincer framework will result in new pincer complexes displaying
novel reactivity and enhanced catalytic activities. While to the
best of our knowledge, the only report on the pincer ring
expanded NHC complexes was about the phosphine function-
alized dihydroperimidine-based NHC Rh, Ir complexes
1
13
†
Electronic supplementary information (ESI) available: H NMR and C NMR
spectra of compounds 1–4, DEPT 135 of 4b, COSY of 4b, HSQC of 4b, and
characterization data of the products of the catalytic Heck reaction. CCDC
1
038971 and 1038972. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5ra01706h
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hexauorophosphates (3a–b). The ligands 3a–b were obtained
analytically pure in high yields aer recrystallization. Their
structures were fully characterized by various NMR techniques
and mass spectra measurement, and in the case of complex 3b, by
X-ray crystallographic determination. In hexahydropyrimidines
1
a–b, the proton resonances of the methylene groups on picolyl
occurred as singlet signals at 3.61 and 3.51, respectively. The
resonances of NCH N protons appeared at 3.19 and 3.06 ppm,
2
Fig. 1 NHC-based pincer ligands.
respectively. While in the corresponding tetrahydropyridin-1-ium
salts (2a–b, 3a–b), the proton resonances of the methylene groups
generated via chelate-assisted double C–H activation of on picolyl downelded within the range of 4.86–4.84 ppm, and
17
substituted 2,3-dihydroperimidine proligands. In view of the the resonances of NCHN protons downelded within the range of
strong donating properties of the ring expanded NHCs and 9.07–8.91 ppm, respectively. Single crystals of 3b suitable for X-ray
obvious rare of precedent studies on their pincer metal diffraction analysis were obtained by slow diffusion of diethyl
complexes, here we report the synthesis, characterization and ether to a dichloromethane solution of 3b.
catalytic activity of picolyl functionalized pincer six-membered
NHC palladium complexes based on tetrahydropyrimidin- lengths and bond angles listed in the caption. The bonding
-ylidenes. As expected, the pincer six-membered NHC palla- within the pyrimidinyl ring indicates a pattern of delocalization
dium complexes proved remarkably stable toward air and that extends from N(1) to N(2) through C(1), with N(1)–C(1)
Fig. 2 shows the molecular structure of 3b with selected bond
2
˚
˚
moisture, and showed high catalytic activity toward Heck reac- [1.305(3) A] and N(2)–C(1) [1.305(4) A] being signicantly shorter
˚
tion. These results underline the high potential of this class of than those between N(1)–C(2) [1.463(4) A] and N(2)–C(4) [1.465(4)
A˚ ]. The nitrogen donors of the picolyl groups are rotated away
from C(1), showing that the donor arms rotating freely to take up
positions suitable for chelation to a metal ligated at C(1).
carbene ligands in catalysis.
Results and discussion
Synthesis of tetrahydropyrimidin-1-ium derivatives
Synthesis of pincer NHC palladium complexes
19
In literature report, dehydrogenation of hexahydropyrimidine Following a similar literature report, the pincer NHC–Pd
by NBS have been used quite oen to synthesis complexes (4a–b) were prepared by heating the corresponding
tetrahydropyrimidin-1-iums salts, along with other procedures, tetrahydropyridin-1-ium hexauorophosphates (3a or 3b) and
including quarterisation of tetrahydropyrimidine, and direct PdCl
2
ꢀ
in the presence of NaN(SiMe
3
)
2
as a base in pyridine at
0
cyclization of N,N -dialkylpropan-1,3-diamines with methyl 140 C (Scheme 1). The formation of the pincer NHC palladium
18
orthoformate catalyzed by acid. It features the obvious advan- complexes was observed from the studies of NMR spectra,
tages of high yield and repeatability, and easy purication of the showing the conspicuous absence of the NCHN resonances of
product. Here we synthesized the pincer six-membered NHC the reacting cationic tetrahydropyrimidin-1-ium hexa-
precursors 1,3-dipicolyl-3,4,5,6-tetrahydropyrimidin-1-ium salts uorophosphates and the appearance of the highly downeld
1
3
through the dehydrogenation of 1,3-disubstituted hexahydropyr- shied at 174.9 (4a) and 174.0 (4b) ppm in the C NMR spectra,
imidine by NBS. The synthetic procedure is shown in Scheme 1. which should be attributed to the new Pd–Ccarbene resonance. In
Condensation of 1,3-propandiamines with pyridine-2- addition, the proton resonances of the methylene groups on
formaldehyde in methanol produced Schiff bases in high picolyl occurred as a doublet of doublets at 5.30 and 4.80 (4a),
yields. Reduction of the resulting Schiff bases with NaBH lead to and 5.38 and 4.78 (4b) ppm, respectively. This conrms the
4
0
the formation of N,N -dialkylpropan-1,3-diamines. The formation of the chelate ring, which makes the two protons of
following reaction with aqueous formaldehyde in methanol the methylene group on the picolyl magnetic unequal. At the
afforded the 1,3-dipicolyl-hexahydropyrimidines (1a–b), which same time, the pyridine proton resonances appeared within the
were then treated by NBS to obtain the tetrahydropyridin-1-ium ranges of 9.00–7.63 (4a) and 9.01–7.65 (4b) ppm, obviously
bromides (2a–b). Anion exchange with NH
produced the corresponding
4
PF
6
in ethanol/H
2
O
downelded compared to those of the tetrahydropyrimidin-
tetrahydropyridin-1-ium 1-ium hexauorophosphates 3a (8.63–7.41 ppm) and 3b
ꢀ
Scheme 1 Synthesis of the ligands and pincer ring expanded NHC–Pd complexes. (a) (i) MeOH; (ii) NaBH
4
, MeOH, 0–70 C; (iii) HCHO, MeOH;
ꢀ
(
b) (i) NBS, DME; (ii) NH
4
PF
6
, EtOH/H
2
O; (c) PdCl
2
, NaN(SiMe
3
2
) , pyridine, 140 C.
25724 | RSC Adv., 2015, 5, 25723–25729
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Fig.
2 Molecular structure of 3b (counter ion omitted, 50%
displacement ellipsoids). Selected bond lengths ( A˚ ) and angles (deg):
N(1)–C(1) 1.305(3), N(1)–C(2) 1.463(4), N(1)–C(7) 1.458(4), N(2)–C(1)
1
1
.305(4), N(2)–C(4) 1.465(4), N(2)–C(13) 1.458(3), N(1)–C(1)–N(2)
24.1(2), C(1)–N(1)–C(2) 120.3(2), C(1)–N(2)–C(4) 121.3(2), C(1)–N(1)–
C(7) 120.1(2), C(2)–N(1)–C(7) 119.2(2), C(1)–N(2)–C(13) 119.9 (2), C(4)–
N(2)–C(13) 118.6 (2).
Fig.
3
Molecular structure of 4b (counter ion omitted, 50%
displacement ellipsoids). Selected bond lengths ( A˚ ) and angles (deg):
Pd(1)–C(1) 1.975(5), Pd(1)–N(3) 2.037(4), Pd(1)–N(4) 2.035(4), Pd(1)–
Cl(1) 2.3619(14), N(1)–C(1) 1.334(7), N(1)–C(2) 1.452(7), N(1)–C(7)
(
8.94–7.41 ppm), supporting the coordination of pyridine to the
palladium center. The complex 4b has been further character-
ized unambiguously by the single-crystal X-ray diffraction N(3) 87.8(2), C(1)–Pd(1)–N(4) 88.08(19), N(3)–Pd(1)–Cl(1) 91.65(14),
studies.
N(4)–Pd(1)–Cl(1) 92.51(13), N(1)–C(1)–N(2) 120.6(5), C(1)6N(1)–C(2)
22.0(5), C(1)–N(2)–C(4) 122.8(5), C(1)–N(1)–C(7) 120.1(2), C(2)–N(1)–
1
.468(7), N(2)–C(1) 1.316(7), N(2)–C(4) 1.467(8), N(2)–C(13) 1.453(7),
C(1)–Pd(1)–Cl(1) 176.89(16), N(3)–Pd(1)–N(4) 175.83(18), C(1)–Pd(1)–
1
Slow diffusion of diethyl ether to a concentrated acetonitrile
C(7) 119.5(5), C(1)–N(2)–C(13) 119.0(5), C(4)–N(2)–C(13) 117.9(5).
solution of 4b produced single crystals suitable for X-ray
diffraction analysis. The molecular structure is shown in
Fig. 3, selected bond lengths and bond angles listed in the
caption.
As shown in Fig. 3, the palladium atom in 4b adopts a
slightly distorted square-planar coordination bonded to car-
bene and two pyridinyl nitrogen donors, with the two fused six-
membered chelate rings of the ‘pincer’ both exist in boat
conformation. As complex 4b represents the only example of a
structurally characterized pincer six-membered NHC palladium
complex that we are aware of, we compare the structure with
pincer ve-member NHC palladium complexes, and non-
chelating six-membered NHC palladium complex, respec-
tively. The Pd–Ccarbene bond distance of 1.975(5) A is slightly
longer than those found in the dipicolyl functionalized pincer
NHC palladium bearing imidazol-2-ylidene (1.929(4) A) or
imidazolin-2-ylidene (1.936(2) A), while very similar to those
found in the pincer NHC palladium complexes bearing
diphosphine-substituted benzimidazolin-2-ylidene (1.974(5) A˚
entries 1–5). Tests of different solvents proved DMA to be the
proper solvent. Reaction in DMF and toluene produced the
target product in low yields, and the reaction in DME or dioxane
did not occur at all (Table 1, entries 6–9). Increasing the catalyst
loading to 2.5% raised yield to 98%.
Under the standard conditions, using Et N as base and DMA
3
as solvent, the catalytic activity of complexes 4a and 4b towards
Heck reaction of a series of aryl bromides with olens (acrylate
and styrene) were investigated and the results are summarized
in Table 2. The results showed that complexes 4a and 4b pre-
sented almost equally catalytic efficiency, producing the target
products in moderate to high yields. The substituents on the
phenyl ring of the aryl bromides did not show any obvious
˚
˚
˚
20
Table 1 Screening of the base and solvent effect
˚
˚
and 1.980(6) A) or imidazol-2-ylidene (1.983(7) A) ligands
16b,c
reported by Hahn and Lee,
and non-chelating six-membered
˚
NHC palladium complex reported by Cavell (1.983(3) A) and
5
a,19
˚
Ghosh (1.9812(19) A).
a
b
Entry
Solvent
Base
Cat. (mol%)
Yield (%)
˚
˚
The Pd–N distances (2.037(4) A and 2.035(4) A) and Pd–Cl
˚
distance (2.3619(14) A), are very similar to those of the picolyl
1
2
3
4
5
6
7
8
9
DMA
DMA
DMA
DMA
DMA
DMF
DME
Toluene
Dioxane
DMA
Et N
t-BuOK
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.5
2.5
54
<5
<5
<5
<5
40
0
36
0
70
98
3
functionalized NHC palladium carrying imidazol-2-ylidene
2.073(3) A and 2.062(3) A, 2.3737(3) A) or imidazolin-2-ylidene
2.066(2) A and 2.054(4) A, 2.3688(5) A).
˚
˚
˚
K
2
CO
3
(
(
Na
Cs
2
CO
3
CO
N
N
N
N
N
N
˚
˚
˚
20
2
3
Et
Et
Et
Et
Et
Et
3
3
3
3
3
3
Catalytic studies
Initially, running the Heck reaction of bromobenzene and
n-butyl acrylate catalyzed by 1.0% complex 4a in DMA as a
model reaction, a brief screening of the base and solvent was
conducted (Table 1). Among the bases tested, organic base NEt3
1
1
0
1
DMA
a
Reaction condition: 0.5 mmol bromobenzene, 0.75 mmol n-butyl-
ꢀ
b
acrylate, 0.75 mmol base, 2 mL solvent, 135 C, 12 h. Yields
determined by HPLC.
afforded the moderate yield. Inorganic bases t-BuOK, K
2 3
CO ,
Na CO , and Cs CO all produced very low yields (Table 1,
2
3
2
3
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electronic effect, while the steric effect was obvious. In some Elemental analyses were obtained from a thermo Flash 2000.
cases of ortho-substituted phenyl bromide and 1-naphthyl ESI-MS spectra were recorded on a Bruker Esquire 3000.
bromide, comparatively low yields were obtained (Table 2,
entries 10–15, 23 and 24).
The Heck reaction of phenyl bromide or para-substituted Synthesis of 1,3-dipicolyl hexahydropyrimidines (1a–b)
phenyl bromide with olenes was achieved in less time or lower
A
methanol solution (50 mL) containing pyridine-
catalyst loading, using pincer diphosphine-substituted imidazol-
2-formaldehyde (30 mmol, 3.21 g) and 1,3-propanediamine
16c
2-ylidene palladium complex, than the results reported here
(15 mmol, 1.11 g) were stirred at room temperature for 5 h.
for complexes 4a and 4b. While for the Heck reaction of ortho-
substituted phenyl bromide or 1-naphthyl bromide, moderate to
good yields were obtained using complexes 4a and 4b. This
suggests that there is considerable potential to improve the
activity of these complexes, with the possibility of developing
these complexes as catalysts for the Heck reaction of more
substituted, sterically demanding substrates.
Infrared detection showed the disappearance of the carbonyl
group. Additional 20 mL methanol was added and the mixture
was put into an ice-bath, then NaBH (120 mmol, 4.54 g) was
4
added portion-wise for 1 h. The mixture was then warmed up to
ꢀ
room temperature and then heated to 70 C overnight. The
solvent was then evaporated and the residue was poured into a
mixture of water (40 mL) and CH Cl (40 mL). The resulting
2 2
suspension liquid was ltered and the ltrate was extracted by
CH Cl (20 mL) for 3 times. The combined organic phase was
2
2
Experimental section
General consideration
evaporated and the residue obtained was dissolved in methanol
10 mL) for the following reaction directly. The solution was
(
All solvents and chemicals were used as received or dried with then treated with aqueous HCHO solution (36.5%, 15 mmol).
standard methods and freshly distilled prior to use if needed. The mixture was stirred at room temperature for 6 h before
ꢀ
NMR spectra were recorded at 25 C on a 400 MHz Bruker being evaporated. Purication of the residue by ash chroma-
spectrometer. Chemical shis (d in ppm, coupling constants tography (silica, acetone/CH
J in Hz) were referenced to the residual solvent resonances. the pure products.
2
Cl
2
/Et
3
N ¼ 2/8/1, v/v/v) afforded
Table 2 Heck reaction of aryl bromides with olefines
b
Yields (%)
a
Entry
Ar–Br
Olenes
Products
Cat. 4a
Cat. 4b
1
2
3
4
5
6
7
8
9
C
C
C
6
6
6
H
H
H
5
5
5
Br
Br
Br
Methyl acrylate
n-Butyl acrylate
Styrene
Methyl acrylate
n-Butyl acrylate
Styrene
Methyl acrylate
n-Butyl acrylate
Styrene
Methyl acrylate
n-Butyl acrylate
Styrene
Methyl acrylate
n-Butyl acrylate
Styrene
Methyl acrylate
n-Butyl acrylate
Styrene
Methyl acrylate
n-Butyl acrylate
Styrene
Methyl acrylate
n-Butyl acrylate
Styrene
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
5p
5q
5r
95
94
88
89
89
76
93
91
78
82
78
70
81
78
68
91
88
76
95
89
79
87
65
70
93
90
86
88
87
80
90
89
75
80
79
68
83
80
72
88
85
80
94
87
76
84
60
68
2-Me–C
2-Me–C
2-Me–C
4-Me–C
4-Me–C
4-Me–C
2,4-Me
2,4-Me
2,4-Me
2-OCH
2-OCH
2-OCH
4-OCH
4-OCH
4-OCH
6
6
6
6
6
6
H
H
H
H
H
H
–C
–C
–C
–C
–C
–C
–C
–C
–C
4
4
4
4
4
4
6
6
6
6
6
6
6
6
6
–Br
–Br
–Br
–Br
–Br
–Br
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2
2
2
3
3
3
3
3
3
3
H –Br
3
H –Br
3
H –Br
4
H –Br
4
H –Br
4
H –Br
4
H –Br
4
H –Br
4
H –Br
4-COCH
4-COCH
4-COCH
1-C10
1-C10
1-C10
3
3
3
–C
–C
–C
6
6
6
H
H
H
4
4
4
–Br
–Br
–Br
5s
5t
5u
5v
5w
5x
H
7
H
7
H
7
–Br
–Br
–Br
a
ꢀ
b
Reaction condition: 0.5 mmol aryl bromide 0.75 mmol olene, 0.75 mmol Et
3
N, 0.0125 mmol 4a or 4b, 2 mL solvent, 135 C, 12 h. Isolated yields.
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1
,3-Dipicolyl hexahydropyrimidine (1a). Yellow oil (3.50 g, white powder from CH
2
Cl
2
/diethyl ether produced the pure
1
8
(
7%, based on the starting pyridine-2-formaldehyde). H NMR products.
400 MHz, CDCl ): d 8.37–8.35 (m, 2H, Py–H), 7.48–7.44 (m, 2H,
1,3-Dipicolyl
Py–H), 7.32 (d, J ¼ 7.6 Hz, 2H, Py–H), 6.99–6.96 (m, 2H, Py–H), phosphate (3a). Colorless crystal (0.78 g, 95%). Mp: 107–
tetrahydropyrimidin-1-ium
hexauoro-
3
ꢀ
1
3
(
.60 (s, 4H, picolyl–CH
2 2
), 3.19 (s, 2H, pyrimidine–CH
6
), 2.51 108 C. H NMR (400 MHz, DMSO-d ): d 8.84 (s, 1H, pyrimidine–
1
3
s, 4H, pyrimidine–CH ), 1.61 (s, 2H, pyrimidine–CH ) ppm.
C
CH), 8.63 (d, J ¼ 4.2 Hz, 2H, Py–H), 7.93–7.89 (m, 2H, Py–H),
2
2
NMR (100 MHz, CDCl ): d 158.7, 148.8, 136.2, 122.8, 121.8, 75.2, 7.52 (d, J ¼ 7.8 Hz, 2H, Py–H), 7.44–7.41 (m, 2H, Py–H), 4.85 (s,
3
+
6
0.7, 52.2, 22.6 ppm. ESI-MS (m/z): 269.0 [M + H] .
4H, picolyl–CH
2
), 3.37–3.32 (m, 4H, pyrimidine–CH
2
), 2.02–1.95
): d
oil (3.96 g, 89%, based on the starting pyridine- 155.6, 154.3, 150.1, 137.9, 124.0, 123.1, 59.0, 43.5, 18.8 ppm.
1
3
5
2 6
,5-Dimethyl-1,3-dipicolyl hexahydropyrimidine (1b). Yellow (m, 2H, pyrimidine–CH ) ppm. C NMR (100 MHz, DMSO-d
1
+
2
(
3
(
-formaldehyde). H NMR (400 MHz, CDCl
m, 2H, Py–H), 7.50–7.40 (m, 4H, Py–H), 6.99–6.95 (m, 2H, Py–H),
.51 (s, 4H, picolyl–CH ), 3.06 (bs, 2H, pyrimidine–CH
), 2.03 uorophosphate (3b). Colorless crystal (0.83 g, 94%). Mp:
bs, 4H, pyrimidine–CH ), 0.87 (s, 6H, CH ) ppm. C NMR (100 109 C. H NMR (400 MHz, DMSO-d
3
): d 8.35–8.34 ESI-MS (m/z): 267.2 [M ꢁ PF
6
] .
5,5-Dimethyl-1,3-dipicolyl tetrahydropyrimidin-1-ium hexa-
2
2
1
3
ꢀ
1
6
): d 8.94 (s, 1H, pyrimidine–
2
3
MHz, CDCl ): d 158.9, 148.7, 136.3, 122.5, 121.8, 76.7, 64.6, 61.5, CH), 8.64 (d, J ¼ 4.2 Hz, 2H, Py–H), 7.93–7.89 (m, 2H, Py–H),
3
+
+
3
1.2, 25.9 ppm. ESI-MS (m/z): 297.1 [M + H] , 319.2 [M + Na] .
7.54 (d, J ¼ 7.7 Hz, 2H, Py–H), 7.44–7.41 (m, 2H, Py–H), 4.84 (s,
H, picolyl–CH ), 3.07 (s, 4H, pyrimidine–CH ), 0.82 (s, 6H,
CH ) ppm. C NMR (100 MHz, DMSO-d ): d 154.5, 154.0, 150.1,
4
2
2
1
3
3
6
Synthesis of 1,3-dipicolyl tetrahydropyrimidin-1-ium
bromides (2a–b)
1
2
37.9, 124.1, 123.6, 58.9, 54.0, 27.2, 23.7 ppm. ESI-MS (m/z):
+
95.1 [M ꢁ PF ] .
6
Hexahydropyrimidine (1a or 1b, 5 mmol) was dissolved in DME
(
20 mL). NBS (5 mmol, 0.89 g) was added portion-wise and the
Synthesis of 1,3-dipicolyl-3,4,5,6-tetrahydropyrimidin-
-ylidenes palladium complexes (4a–b)
A mixture of tetrahydropyrimidin-1-ium hexauorophosphate
resulting mixture was stirred at room temperature for 3 h,
during which time a white precipitate formed. The precipitate
was ltered and washed with DME. Crystallization of the
precipitate from CH Cl /diethyl ether produced the pure
products.
,3-Dipicolyl tetrahydropyrimidin-1-ium bromides (2a).
2
(
(
3a or 3b, 2.0 mmol), PdCl (2.0 mmol, 0.35 g) and NaN(SiMe )
2 3 2
1.2 mL, 2.0 M in THF) in pyridine (6 mL) was heated at 140 C
2
2
ꢀ
for 12 h. The reaction mixture was then evaporated and puri-
cation of the residue by column chromatography (silica,
1
ꢀ
1
Colorless crystal (1.40 g, 81%). Mp: 158–159 C. H NMR
400 MHz, DMSO-d
CHCl
palladium complexes.
1,3-Dipicolyl-3,4,5,6-tetrahydropyrimidin-2-ylidenes) PdCl$PF6
3
/diethyl ether, gradient elution) produced the pure
(
6
): d 8.91 (s, 1H, pyrimidine–CH), 8.63 (d, J ¼
.8 Hz, 2H, Py–H), 7.93–7.89 (m, 2H, Py–H), 7.52 (d, J ¼ 7.7 Hz,
H, Py–H), 7.44–7.41 (m, 2H, Py–H), 4.85 (s, 4H, picolyl–CH ),
.33 (t, J ¼ 5.8 Hz, 4H, pyrimidine–CH ), 1.96 (t, J ¼ 5.6 Hz, 2H,
4
2
3
(
2
1
(
4a). Colorless crystal (0.64 g, 58%). H NMR (400 MHz, DMSO-
2
d6): d 9.00–8.98 (m, 2H, Py–H), 8.22–8.18 (m, 2H, Py–H), 7.88 (d,
J ¼ 7.1 Hz, 2H, Py–H), 7.67–7.63 (m, 2H, Py–H), 5.30 (d, J ¼ 15.1
1
3
pyrimidine–CH ) ppm. C NMR (100 MHz, CDCl ): d 154.9,
1
2
3
53.0, 149.4, 137.2, 123.4, 123.2, 58.8, 53.7, 43.1, 18.7 ppm. ESI-
Hz, 2H, picolyl–CH
2
), 4.80 (d, J ¼ 15.3 Hz, 2H, picolyl–CH
2
),
+
MS (m/z): 267.1 [M ꢁ Br] .
,5-Dimethyl-1,3-dipicolyl
bromides (2b). Colorless crystal (1.45 g, 77%). Mp: 239 C. H
NMR (400 MHz, DMSO-d ): d 9.07 (s, 1H, pyrimidine–CH), 8.63
t, J ¼ 2.4 Hz, 2H, Py–H), 7.93–7.89 (m, 2H, Py–H), 7.57 (d, J ¼ 7.8
Hz, 2H, Py–H), 7.44–7.41 (m, 2H, Py–H), 4.86 (s, 4H, picolyl–
3.58–3.48 (m, 4H, pyrimidine–CH
2
), 1.92 (t, J ¼ 5.8 Hz, 2H,
5
tetrahydropyrimidin-1-ium
1
3
pyrimidine–CH
2
) ppm. C NMR (100 MHz, DMSO-d ): d 174.9,
6
ꢀ
1
154.8, 154.0, 141.4, 125.6, 125.2, 61.6, 47.5, 20.8 ppm. Anal.
6
cacld for C H ClF N PPd (551.99): C, 34.74; H, 3.28; N, 10.13.
1
6
18
6 4
(
Found: C, 34.56; H, 3.02; N, 9.89%.
5,5-Dimethyl-1,3-dipicolyl-3,4,5,6-tetrahydropyrimidin-2-yli-
(
1
3
CH ), 3.07 (s, 4H, pyrimidine–CH ), 0.82 (s, 6H, CH ) ppm.
NMR (100 MHz, DMSO-d ): d 154.5, 154.1, 150.1, 137.9, 124.1,
1
C
2
2
3
1
denes)PdCl$PF
6
(4b). Light yellow solid (0.70 g, 60%). H NMR
400 MHz, DMSO-d
): d 9.01 (d, J ¼ 4.9 Hz, 2H, Py–H), 8.23–8.19
m, 2H, Py–H), 7.86 (d, J ¼ 7.4 Hz, 2H, Py–H), 7.68–7.65 (m, 2H,
), 4.77 (d, J ¼ 15.2
), 3.28 (d, J ¼ 3.2 Hz, 4H, pyrimidine–CH ),
.79 (s, 6H, CH ) ppm. C NMR (100 MHz, DMSO-d ): d 174.0,
6
(
(
6
+
23.6, 58.9, 54.0, 27.2, 23.7 ppm. ESI-MS (m/z): 295.1 [M ꢁ Br] .
Py–H), 5.38 (d, J ¼ 15.1 Hz, 2H, picolyl–CH
2
Hz, 2H, picolyl–CH
2
2
Synthesis of 1,3-dipicolyl tetrahydropyrimidin-1-ium
hexauorophosphates (3a–b)
1
3
0
1
3
6
54.9, 154.0, 141.5, 125.5, 125.2, 61.6, 58.7, 28.1, 23.5 ppm.
4 6
NH PF (3 mmol, 0.49 g) was dissolved in water (5 mL). The
water solution was then combined with an ethanol (25 mL)
solution of tetrahydropyrimidin-1-ium bromides (2a or 2b,
Anal. cacld for C H ClF N PPd (580.02): C, 37.20; H, 3.82; N,
1
8
22
6 4
9.64. Found: C, 36.93; H, 3.87; N, 9.49%.
2
mmol) and the mixture was then stirred at room temperature
for 5 h. The solvent was then evaporated and the residue was
poured into a mixture of water (10 mL) and CH Cl (10 mL). The The Heck reaction was conducted in a parallel reactor. In a
General procedure for the Heck reaction
2
2
resulting solution was then extracted by CH Cl (10 mL) for typical reaction, A Schlenk tube was charged with aryl bromides
2
2
3
times. The combined organic phase was then washed by (0.5 mmol), acrylate or styrene (0.75 mmol), base (0.75 mmol),
water, dried and evaporated. Crystallization of the resulting pincer NHC–Pd complex 4a or 4b, and solvent (2 mL). The
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ꢀ
mixture was stirred at 135 C for 12 h under Ar. Aer cooling,
the reaction mixture was evaporated. Purication of the residue
by ash chromatography on silica gel (hexanes/CH Cl ¼ 20 : 1)
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afforded the pure products, which were characterized by
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H
1
3
shown in the ESI.†
X-ray diffraction studies
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˚
Ka radiation (l ¼ 0.71073 A). The crystals were kept at 291.15 K
21
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22
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22
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Conclusions
Two novel NCN pincer ring expanded six-membered NHC
palladium
complexes
based
on
1,3-dipicolyl-3,4,5,6-
tetrahydropyrimidin-2-ylidenes have been synthesized via the
direct metallation of the corresponding tetrahydropyridin-1-ium
salts. The structure were characterized unambiguously X-ray
structure analysis, showing that the complex adopts a slightly
distorted square-planar coordination with the two fused six-
membered chelate rings of the ‘pincer’ both exist in boat confor-
mation. Catalytic activity investigation showed that the complexes
catalyzed the Heck reaction of aryl bromides with acrylate/styrene
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The authors are grateful to the National Natural Science Foun-
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Innovative Research Team from Zhengzhou (131PCXTD605),
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