Experimental and theoretical studies on nickel-zinc-catalyzed cross-coupling of gem-dibromoalkenes with P(O)-H compounds

Article abstract of DOI:10.1039/c3ra45212c

A new stereoselective one-pot protocol for the preparation of E-alkenyl-phosphorus compounds under catalysis of an inexpensive nickel-zinc catalyst system has been developed, which provides a potential useful method for C-P bond formation. ³¹P NMR spectrum and density functional theory calculations were performed to study the reaction mechanism.

Full text of DOI:10.1039/c3ra45212c

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Experimental and theoretical studies on
nickel–zinc-catalyzed cross-coupling of
Cite this: RSC Adv., 2014, 4, 2322
gem-dibromoalkenes with P(O)–H compounds†
Received 18th September 2013
Accepted 14th November 2013
a
a
a
b
b
a
a
Liu Liu, Ye Lv, Yile Wu, Xiang Gao, Zhiping Zeng, Yuxing Gao,* Guo Tang*
a
and Yufen Zhao
DOI: 10.1039/c3ra45212c
www.rsc.org/advances
A new stereoselective one-pot protocol for the preparation of Djakovitch and co-workers reported a Pd-catalyzed Heck
11a
E-alkenyl-phosphorus compounds under catalysis of an inexpensive reaction for synthesis of diethyl 2-(aryl)vinylphosphonates.
nickel–zinc catalyst system has been developed, which provides a Very recently, Yang, Liang and co-workers described a copper-
31
11b
potential useful method for C–P bond formation. P NMR spectrum catalyzed C–P coupling through decarboxylation. However,
and density functional theory calculations were performed to study
the reaction mechanism.
The rapid development of new methods for carbon–carbon
(
C–C) and carbon-heteroatom (C–X) bond formation is a critical
Scheme 1 Ni(II)–Zn-catalyzed one-pot synthesis of alkenyl-phos-
phorus compounds.
challenge in modern organic chemistry owing to the synthesis
of functional molecules, such as biologically active compounds
and natural products. In this regard, the transition-metal-
1
catalyzed cross-coupling reactions are very useful protocols for
the construction of structurally sophisticated compounds over
the past years. Organophosphorus compounds are an
a
Table 1 Optimization of reaction condition
2
important class of chemicals, which are extensively used in
3
pharmaceuticals and material sciences. Particularly, alkenyl-
phosphorus compounds have attracted more attention due to
their prime properties. For example, in addition to their
4
eminent metal-complexing abilities and ame-retardants or
5
copolymers in polymer science, several nucleophiles, such as
6
7
8
9
alcohols, thiols, amines, phosphines and carbanion
1
0
species, can readily add to the olenic bond to provide
potential useful bifunctional adducts, which could be further
synthetically elaborated.
b
Entry
Catalyst
Ligand
Solvent
Base
Yield (%)
1
2
3
NiCl
NiCl
NiCl
NiCl
NiCl
NiCl
NiCl
NiCl
NiCl
NiCl
NiCl
—
2
2
2
2
2
2
2
2
2
2
2
L1
L1
L1
L1
L2
L3
L4
L3
L3
L3
L3
—
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Toluene
THF
K
K
NEt
3
PO
PO
4
18
60
n.d.
38
n.d.
88
n.d.
0
70
10
0
/Zn
/Zn
/Zn
/Zn
/Zn
/Zn
/Zn
/Zn
/Zn
/Zn
3
4
Recently, a number of transition-metal mediated methods
for the synthesis of alkenyl-phosphorus compounds have
c
3
4
5
K
2
K
3
K
3
K
3
K
3
K
3
K
3
K
3
K
3
CO
3
11
been continuously emerged.
For instance, in 2009,
c
PO
PO
PO
PO
PO
PO
PO
PO
4
4
4
4
4
4
4
4
6
7
c
a
8
9
10
1
1
Department of Chemistry, College of Chemistry and Chemical Engineering, Key
Laboratory for Chemical Biology of Fujian Province, Collaborative Innovation Center
of Chemistry for Energy Materials (iChEM), Xiamen University, Xiamen 361005,
Fujian, China. E-mail: t12g21@xmu.edu.cn; gaoxingchem@xmu.edu.cn; Web: http://
chem.xmu.edu.cn/group/yfzhao/zhao-home.html
dioxane
1
2
H
2
O
c
DMF
or NEt
3
n.d.
b
School of Pharmaceutical Sciences, Xiamen University, Xiamen 361102, Fujian, China
a
Reaction conditions: 1a (0.2 mmol), 2b (0.44 mmol), NiCl
2
(0.02 mmol),
†
Electronic supplementary information (ESI) available: General experimental
procedure, characterization and computational details. See DOI:
0.1039/c3ra45212c
Zn powder (0.4 mmol), ligand (0.04 mmol), base (1.0 mmol), solvent
ꢀ
b
c
(2.0 mL), 80 C, 20 h. Isolated yield. Only reduced product was
1
detected. n.d. ¼ no detected.
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most of these methods lack satisfactory yield, high stereo-
In our initial study, 2,2-dibromovinyl benzene (1a, 0.2 mmol)
selectivity or relatively mild reaction conditions. Herein, our was treated with diisopropyl phosphonate (2b, 0.44 mmol) in
ꢀ
1
2
ongoing interests in nickel and organophosphorus chemis- DMF at 80 C under catalysis of NiCl2 (0.02 mmol), N ,N -
try
12,9c,11g
led us to investigate a new procedure for the dimethylethane-1,2-diamine (L1, 0.04 mmol) and K PO4
3
synthesis of 1-alkenylphosphonates, 1-alkenylphosphinates or (1.0 mmol) system (Table 1). Unfortunately, the desired product
1
-alkenylphosphine oxides both experimentally and theoreti- 3b, was obtained in only 18% yield (Table 1, entry 1). Drawing
9
c,12d
cally (Scheme 1).
inspiration from our experience in nickel(II) catalysis,
we
a
Table 2 Reaction of gem-dibromoalkenes with P(O)H compounds
b
c
Entry
Substrate
Product
Yield (%)
E/Z
1
2
1a
1a
3a
3b
82
88
96/4
96/4
3
1a
3c
88
95/5
4
5
6
1a
1a
1a
3d
86
0
98/2
—
—
3e
3f
81
95/5
7
8
1a
1b
72
78
94/6
97/3
3g
3h
3i
9
1b
1b
72
74
91/9
94/6
1
0
1
1
1
2
1c
1c
3j
80
77
97/3
94/6
3k
1
3
1d
3l
54
90/10
1
1
4
5
1e
1f
3m
3n
56
66
89/11
88/12
2g
a
Reaction conditions: 1 (0.2 mmol), 2 (0.44 mmol), NiCl
0 C, 20 h. Isolated yield. Dened by H NMR. n.d. ¼ no detected.
2
(0.02 mmol), Zn powder (0.4 mmol), L3 (0.04 mmol), K
3 4
PO (1.0 mmol), DMF (2.0 mL),
ꢀ
b
c
1
8
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chose Zn powder (0.4 mmol) as the reducing agent to test this
reaction. We were pleased to nd that NiCl /Zn greatly
2
13
improved the yield to 60%. Base screening showed that K PO
3
4
was the best choice (Table 1, entries 2–4). The type of ligands
was very crucial to the present reaction (Table 1, entries 2 and
1
1
2
2
5
–7). For example, when N ,N ,N ,N -tetramethylethane-1,2-
diamine (L2) or N,N-dimethylpyridin-4-amine (L4) was chosen
as the ligand, no expected product could be detected in the
0
crude reaction mixture. Gratifyingly, 2,2 -bipyridine (L3)
exhibited an excellent catalytic performance and gave the
desired product 3b in 88% yield. Next, a screening of the solvent
showed that besides DMF (88%) and THF (70%), other solvents
provided only a trace amount of 3b (Table 1, entries 6 and 8–11).
It is important to note that only the reduced product (2-bro-
movinyl)benzene could be detected without the presence of
Fig. 2 Proposed mechanism.
14
2
NiCl /Zn (Table 1, entry 12).
To test the scope of the reaction, we next explored the one-
pot protocol of various P(O)H compounds with 1a as shown in
Table 2. In addition to H-phosphonate diesters (Table 2, entries
14
To investigate the mechanism of the Hirao reduction, we
3
1
decided to trace the reaction by P NMR spectroscopy, which
has been successfully applied in studying various phosphorus-
containing reactions. A mixture of 1a (0.15 mmol), diethyl
phosphonate (2a, 0.30 mmol) and triethylamine (0.30 mmol) in
dry DMF was followed by P{H} NMR spectroscopy (Fig. 1).
Initially, only one signal was assigned to diethyl phosphonate
1–4), H-phosphinate esters and diphenylphosphine oxides
16
(Table 2, entries 6 and 7) all could be efficiently coupled with 2a
3
1
to afford the corresponding alkenyl-phosphorus compounds in
good isolated yields in favor of the E-isomer. Note that diphenyl
phosphonate 2e was found to be unsuitable reaction partner
since it might be hydrolyzed under the alkaline condition
(
d
a
¼ 7.8 ppm). Sixty minutes later, two new phosphorus-con-
taining species, intermediate b and byproduct c, were detected
¼ 19.0 ppm and d ¼ ꢁ12.5 ppm), which indicated the
15
(
Table 2, entry 5). gem-Dibromoalkenes with the electron-rich
(d
b
c
substituent (p-Me) or electron-decient substituent (p-CF ) all
10d,17
3
formation of the P–C and P–Br bonds,
respectively. Then,
gave good yields with high stereoselectivity (Table 2, entries
the peak of b was decreasing while that of c was increasing over
time. The reaction reached the thermodynamic equilibrium in
8–12). For instance, 1-(2,2-dibromovinyl)-4-(triuoromethyl)-
benzene (1b) and 1-(2,2-dibromovinyl)-4-methylbenzene (1c)
coupled with dipropyl phosphonate (2c), giving the desired
products in 78% and 80% isolated yields in favor of the
E-isomer, respectively. Gratifyingly, aliphatic gem-dibro-
moalkenes participated in the reaction, providing the corre-
sponding E-alkenyl-phosphorus compounds with moderate
yields (Table 2, entries 13 and 14). 4-(2,2-Dibromovinyl)phenyl
acetate (1f) reacted with 2g, concomitant with hydrolyzation,
providing 3n in 66% yield (Table 2, entry 15). Therefore,
undoubtedly, this Ni–Zn-catalyzed coupling is a general and
practically useful method for the synthesis of E-1-alkenyl-
phosphonates, E-1-alkenylphosphinates or E-1-alkenylphos-
phine oxides.
18
approximately 180 minutes.
3
1
A possible mechanism is proposed based on above P NMR
spectrum analysis and previous reports (Fig. 2). Firstly, the
reaction of tetracoordinated R P(O)H with a base generates a
three coordinated R PO , which would add to the olenic
bond of gem-dibromoalkenes and followed by protonation to
give the intermediate b. Next, two possible pathways are
considered (Fig. 2A and B). Both processes involves elimination
2
of R P(O)Br (c) species, leading to the product of the Hirao-type
reduction. In order to explain why Hirao-type reduction fav-
19
2
ꢁ
19c
2
oured E-isomer, density functional theory (DFT) calculations
23
Fig. 3 Transition state structures. For clarity, some C–H hydrogen
atoms are not shown. Bond lengths are in A˚ .
31
Fig. 1 The stack P{H} NMR spectra.
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(0.44 mmol), gem-dibromoalkenes (0.2 mmol) and K
3 4
PO
(1.0 mmol) was evacuated and purged with argon three times.
Dry DMF (2.0 mL) was added at room temperature, and the
resulting mixture was stirred at room temperature for 30
ꢀ
minutes. Then the mixture was heated at 80 C for 20 hours. The
mixture was cooled to room temperature and then transferred
to a round-bottom ask. Silica gel (3.0 g) was added, and the
solvent was removed under reduced pressure to afford a free-

owing powder. This powder was then dry-loaded onto a silica
Fig. 4 The HOMOs (isovalue ¼ 0.04) of the located transition states by
gel column and puried by ash chromatography to yield the
DFT calculations.
desired product.
20
were carried out. The model substrate methyl methyl-
phosphinate (Me)(MeO)P(O)H and (2,2-dibromovinyl)benzene
Acknowledgements
(1a) were chosen. Relative free energies in solution are
This work was supported by the National Basic Research
Program of China (2012CB821600), Chinese National Natural
Science Foundation (21305115, 21202135, 21173178, 21232005
and 81301888) and the Program for Changjiang Scholars and
Innovative Research Team in University.
employed to analyze the reaction mechanism. From interme-
diate b, four transition states (TSA1, TSA2, TSB1 and TSB2) were
identied (Fig. 3). TSA1 and TSB1 connect the E-isomer while
TSA2 and TSB2 lead to formation of the Z-isomer. Gratifyingly,
the calculation results indicate that the free energies of TSA1
and TSB1 are relatively lower than those of TSA2 and TSB2 by
ꢁ
1
ꢁ1
7.1 kJ mol and 29.1 kJ mol , respectively, thus suggesting
Notes and references
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proposed cross-coupling process includes three basic steps
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1a,b,12d,20
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ꢁ
1
21
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ꢀ
22
at 80 C. Interestingly, the plotted highest occupied molecular
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3
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In summary, we have developed a mild, operationally
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selective preparation of E-1-alkenylphosphonates, E-1-alkenyl-
phosphinates or E-1-alkenylphosphine oxides under catalysis of
inexpensive and commercially available nickel–zinc catalyst
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2
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Experimental procedure
An oven-dried Schlenk tube containing NiCl
powder (0.4 mmol), 2,2 -bipyridine (0.04 mmol), P(O)H
2
(0.02 mmol), Zn
0
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2
3
1
7
8
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18 When the mixture was added 0.5 mL H
(Et NBr) appeared and we could not detect (EtO)
species.
new signal (ꢁ0.3 ppm) was assigned to
(EtO) P(O)–OH, which was conrmed by MS (ESI).
2
O, the precipitate
1
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2
P(O)–Br
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21 For computational details see the ESI.†
‡
1
2 For selected recent publications, see: (a) Y. Gao, G. Wang, 22 According to the Eyring equation, i.e., k ¼ (k
B
T/h)exp(ꢁDG /
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Chem. Soc., 2009, 131, 7956; (b) W. Miao, Y. Gao, X. Li,
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B
RT), where k is rate constant, k is Boltzmann's constant, T is
‡
the temperature, DG is the activation free energy, R is the
gas constant and h is Planck's constant, we evaluate the
barrier. Assuming the half life of this reaction is 24 h and
2
659; (c) X. Li, G. Hu, P. Luo, G. Tang, Y. Gao, P. Xu and
ꢁ
1
Y. Zhao, Adv. Synth. Catal., 2012, 354, 2427; (d) X. Zhang,
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the concentration of each reactant is 1 mol L , we obtain
ꢁ5
ꢁ1 ꢁ1
the second-order rate constant k is 1.2 ꢂ 10 L mol
s .
‡
ꢁ1
ꢀ
1
3, 3478; (e) H. Chen, Z. Huang, X. Hu, G. Tang, P. Xu,
Y. Zhao and C.-H. Cheng, J. Org. Chem., 2011, 76, 2338; (f) 23 C. Y. Legault, CYLview, 1.0b, Universit ´e de Sherbrooke, 2009,
Z. Zhao, W. Xue, Y. Gao, G. Tang and Y. Zhao, Chem.–Asian http://www.cylview.org.
Therefore, DG is 120.4 kJ mol at 80 C.
2326 | RSC Adv., 2014, 4, 2322–2326
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