Sequential Addition of Phosphine to Alkynes for the Selective Synthesis of 1,2-Diphosphinoethanes under Catalysis. Well-Defined NHC-Copper Phosphides vs in Situ CuCl2/NHC Catalyst

Article abstract of DOI:10.1021/acs.organomet.6b00854

The well-defined NHC-copper phosphides [(NHC)CuPPh₂]₃ (1, NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IⁱPr); 2, NHC = N,N-di-tert-butylimidazol-2-ylidene (I^tBu)) have been prepared by the reaction of simple copper halides with HPPh₂ in the presence of N-heterocyclic carbenes (NHCs). Complexes 1 and 2 enabled catalytic double hydrophosphination of alkyl and aryl terminal alkynes to yield 1,2-diphosphinoethanes selectively in good yields. On the basis of these results, the most efficient and pratical in situ CuCl₂/NHC catalyst has been developed. It catalyzes the selective double hydrophosphination of the alkynes with high efficiency and a wide substrate scope and exhibits even better performance than the well-defined NHC-Cu phosphides. The mechanistic studies disclosed that the formation of a copper acetylide in the catalytic cycle played an important role in the acceleration of the catalytic process.

Full text of DOI:10.1021/acs.organomet.6b00854

Article
pubs.acs.org/Organometallics
Sequential Addition of Phosphine to Alkynes for the Selective
Synthesis of 1,2-Diphosphinoethanes under Catalysis. Well-Defined
NHC-Copper Phosphides vs in Situ CuCl2/NHC Catalyst
Jia Yuan,† Lizhao Zhu,† Jianying Zhang,† Jianfeng Li,† and Chunming Cui*,†,‡
†State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
‡Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, People’s Republic of
China
S
* Supporting Information
ABSTRACT: The well-defined NHC-copper phosphides
[(NHC)CuPPh2]3 (1, NHC = 1,3-diisopropyl-4,5-dimethyli-
midazol-2-ylidene (IiPr); 2, NHC = N,N-di-tert-butylimidazol-
2-ylidene (ItBu)) have been prepared by the reaction of simple
copper halides with HPPh2 in the presence of N-heterocyclic
carbenes (NHCs). Complexes 1 and 2 enabled catalytic
double hydrophosphination of alkyl and aryl terminal alkynes to yield 1,2-diphosphinoethanes selectively in good yields. On the
basis of these results, the most efficient and pratical in situ CuCl2/NHC catalyst has been developed. It catalyzes the selective
double hydrophosphination of the alkynes with high efficiency and a wide substrate scope and exhibits even better performance
than the well-defined NHC-Cu phosphides. The mechanistic studies disclosed that the formation of a copper acetylide in the
catalytic cycle played an important role in the acceleration of the catalytic process.
decades, NHC-copper complexes have been extensively studied
as catalysts for diverse reactions due to the strongly electron
donating character and structural diversity of NHC ligands.9
With regard to hydrophosphination catalysts, metal phosphido
complexes have been postulated as the key intermediates.6
However, there are only very limited reports on the synthesis of
well-defined copper phosphido complexes, since they tend to
form large aggregates in the solid state in the absence of
suitable supporting ligands.10,11 Nolan and co-workers reported
the synthesis of an NHC-Cu(I) phosphide by the reaction of an
NHC-copper hydroxide with HPPh2, but its structure and
reactions have not been studied.12
Herein, we report the straightforward synthesis of NHC-
copper phosphides by reaction of a simple copper halide with
HPPh2 in the presence of NHC ligands (Scheme 1). These
phosphides enabled the catalytic double hydrophosphination of
alkynes to yield 1,2-diphosphinoethanes selectively. On the
INTRODUCTION
■
Bidentate organophosphines are immensely important ligands
in coordination chemistry and transition-metal catalysis because
of their relatively rigid coordination environments and
collaborative electronic effects.1 However, the modification of
a desired backbone with different substituents is impeded by
the use of strong bases and multistep synthesis.2−4 Thus, the
efficient and concise synthesis of various bidentate phosphines
is of crucial importance for their wide and practical applications
in catalysis and related fields.
In the past several years, metal-catalyzed addition of
phosphines to unsaturated organic compounds has emerged
as the most straightforward and atom-economical strategy for
the construction of organophosphines.5,6 To access bidentate
phosphines, double addition of organophosphines to con-
jugated dienes and alkynes presents the most powerful and
concise approach. However, double hydrophosphination of
alkynes is very difficult, partially because of the steric hindrance
of the resulting single hydrophosphination product. On the
other hand, the chelating effects of the formed diphosphino-
ethanes may result in their strong bonding to the active metals,
leading to catalyst poisoning.4−7 As such, only a few well-
defined catalysts based on iron, rhodium, and zirconium have
been reported. However, these catalysts suffered from long
reaction times (3−7 days) and limited substrate scopes or side
reactions.7,8
Scheme 1. Synthesis of the Copper(I) Phosphides 1 and 2
To develop more efficient and practical catalysts, strongly
electron donating ligands that are compatible with base metals
may be considered to overcome the aforementioned chelating
effects of the resulting bidentate phosphines. In the past several
Received: November 13, 2016
© XXXX American Chemical Society
A
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basis of these results, the most practical and efficient in situ
CuCl2/NHC catalytic system for the sequential addition of a
phosphine to aryl- and alkyl-substituted terminal alkynes has
been rationally developed. This system enabled stepwise
addition of a phosphine to alkynes and exhibited noticeably
better activity in comparison to the well-defined copper
phosphides. In addition, the mechanistic studies indicated
that the formation of a copper acetylide from a terminal alkyne
plays an important role in the acceleration of the catalytic
process.
Hydrophosphinaiton of Alkynes with NHC-Cu(I)
Phosphide Complexes. With complexes 1 and 2 in hand,
their potentials for the hydrophosphination of alkynes with
HPPh2 have been examined. As shown in Scheme 2, with 0.43
Scheme 2. Hydrophosphination of Alkynes with HPPh2
Catalyzed by 2
RESULTS AND DISCUSSION
■
Synthesis of NHC-Cu(I) Phosphide Complexes. The
synthesis of the NHC-Cu(I) phosphides 1 and 2 was furnished
in good yield by reaction of CuCl2 with HPPh2 in the presence
of NHCs (NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-
ylidene (IiPr), 1,3-di-tert-butylimidazol-2-ylidene (ItBu)) at
110 °C in toluene, along with the formation of (Ph2P)2 and
the imidazolium chloride (Scheme 1).11−13 Alternatively,
complexes 1 and 2 could be similarly prepared by the reaction
of CuCl with HPPh2. However, a similar reaction in the
presence of the more bulky NHC IDipp (1,3-bis(2,6-
ispropylphenyl)imidazol-2-ylidene) did not occur, indicating
the pronounced steric effects on the reaction.
mol % loading of complex 2, the catalytic reaction of
phenylacetylene with 1 equiv of HPPh2 was complete at 110
°C in 30 min under solvent-free conditions, leading to the
formation of the regioselective products as a mixture of Z and E
isomers (Z:E = 71:29). The reaction in toluene also gave the
products in 93% yield under similar conditions. Remarkably,
the reaction of hex-1-yne with 1 equiv of HPPh2 catalyzed by 2
gave the double-hydrophosphination product as a major
product in 36% yield with the almost complete consumption
of HPPh2, along with the formation of single-addition products
in 12% yield. However, the reactions of 1,2-diphenylethyne and
dec-5-yne only gave the corresponding single hydrophosphi-
nation products in 48% and 17% yields, respectively. The
results indicated that the hydrophosphination reaction is more
efficient for terminal alkenes, and the double hydrophosphina-
tion of alkynes is possible. The successful double hydro-
phosphination of the alkyl-substituted terminal alkyne is
especially noteworthy, since CpFe(CO)2Me-catalyzed double
hydrophosphination is only viable for aryl-substituted terminal
alkynes and required 3 days at 110 °C.7
Complexes 1 and 2 have been fully characterized by NMR
spectroscopy and elemental analysis. The 13C NMR spectra of
the phosphides 1 and 2 exhibit CNHC resonances at δ 188.5 and
188.2 ppm, respectively. The 31P NMR spectra of 1 and 2 show
singlets at δ −35.6 and −30.4 ppm, respectively. Single crystals
of 2 suitable for an X-ray diffraction study were obtained from
toluene/n-hexane at −40 °C. As shown in Figure 1, the
The double hydrophosphination of phenylacetylene with
HPPh2 has been studied as a model reaction for the
optimization of catalysts and reaction conditions. In addition
to complexes 1 and 2, IDippCuPPh2 (3)12 was also prepared by
following the route developed by Nolan for the investigation of
steric effects of NHCs. When 2 molar equiv of HPPh2 was
employed for the reaction of phenylacetylene at 110 °C,
complexes 1−3 resulted in the highly selective formation of
double-addition products in comparable yields under the same
conditions (Scheme 3). Complex 2 exhibits slightly higher
activity and thus was chosen for further studies.
Since complex 2 was prepared by the reaction of CuCl2 with
HPPh2 in the presence of ItBu, we reasoned that the catalytic
double-addition reaction might also proceed with the in situ
CuCl2/ItBu system without the prior isolation of 2. Indeed,
when the reaction was catalyzed by 1.25 mol % of CuCl2 in the
presence of ca. 5 mol % of ItBu under similar conditions, the
double hydrophosphination proceeded more conveniently to
give the expected product in 96% yield. The vinylphosphine
produced by the single addition of phenylacetylene was tested
for the second addition catalyzed by the same catalysts. The
results showed that the vinylphosphine can be converted to the
same double-phosphination product in good yields, indicating
Figure 1. ORTEP drawing of 2 with 30% probability ellipsoids.
Hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å) and angles (deg): Cu1−C1 1.997(3), Cu1−P1 2.3062(9), Cu1−
P3 2.3323(9), Cu2−C24 1.993(3), Cu2−P2 2.3183(9), Cu2−P1
2.3393(8), Cu3−C47 1.990(4), Cu3−P3 2.3211(9), Cu3−P2
2.3225(8); P1−Cu1−P3 116.37(3); P2−Cu2−P1 114.30(3), P3−
Cu3−P2 114.93(3), Cu1−P1−Cu2 116.17(3), Cu2−P2−Cu3
117.23(3), Cu3−P3−Cu1 117.50(4).
phosphide 2 is a trimer with the bridged PPh2 ligand. The
lengths of the six Cu−P bonds range from 2.3062 to 2.3393(8)
Å, much longer than those observed in the tetrameric
phosphides Cu4[P(tBu)2]4 (2.203(5)−2.214(4) Å).11a The
three Cu−CNHC bond lengths of 1.997(3), 1.993(3), and
1.990(4) Å are in the reported range for the NHC-Cu(I)
silylphosphido complexes.10a Complex 2 represents a rare well-
defined NHC-Cu(I) phosphide.10,11
B
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Scheme 3. Comparison of CuCl2/NHC Catalytic System and
NHC-Copper Phosphide Complexes 1−3 for the Double
Hydrophosphination of Phenylacetylene and the Single
Hydrophosphination of Vinylphosphine
Table 1. Catalytic Double Phosphination of Different
a
Alkynes with HPPh2 under the Optimized Conditions
b
entry
R
R′
t (h) yield (%)
TON TOF (h−1
)
1
C6H5
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
Et
8
24
12
24
24
4
96 (95)
91 (88)
91 (89)
76.8
72.8
72.8
68.8
72.8
76.8
72.8
68.8
63.2
52
9.6
3.0
6.1
2.9
3.0
19.2
18.2
5.7
2.6
2.2
1.3
2.6
2.8
2
4-MeC6H4
4-MeOC6H4
2-MeOC6H4
4-tBuC6H4
4-FC6H4
4-ClC6H4
4-BrC6H4
4-Me2NC6H4
2-py
3
c
4
86 (82 )
5
91 (88)
96 (93)
91 (88)
6
7
4
c
8
12
24
24
24
24
24
24
24
24
24
24
24
86 (82 )
9
79 (72)
c
10
11
12
13
14
15
16
18
19
20
65 (58 )
c
2-C4H3S
n-Bu
40 (36 )
32
that the double hydrophosphination occurred in a stepwise
manner. This in situ catalytic system not only is user friendly
but also exhibits an improved catalytic activity in comparison to
complex 2.
It is noteworthy that the transformation of the (Z)-
vinylphosphine to the (E)-vinylphosphine was observed when
the (Z)-vinylphosphine was heated at 110 °C in the presence of
a catalytic amount of 2 (Scheme 4).14 The catalytic
c
79 (75 )
63.2
68
Me2NCH2
Me3SiOCH2
cyclopentyl
ClCH2(CH2)2
tBu
85 (82)
8
<2
0
0
Ph
0
Ph
0
a
Reaction conditions: Ph2PH (2.0 mmol), alkynes (1.0 mmol), ItBu
Scheme 4. Conversion of (Z)-Phenylvinylphosphine to the E
Isomer Catalyzed by 2 and the Hydrophosphination of the E
Isomer
b
(0.05 mmol), and CuCl2 (0.0125 mmol), 110 °C. NMR yield
(isolated yield). Isolated yield after oxidation.
c
product in ca. 5% yield (Table 1, entry 16). The reactions with
the bulky terminal alkyne 3,3-dimethylbut-1-yne (Table 1, entry
18) and internal alkynes 1,2-diphenylethyne (Table 1, entry 19)
and 1-phenyl-1-butyne (Table 1, entry 20) only gave the single
hydrophosphination products in 30%, 12%, and 32% yields,
respectively, indicating that the steric factors of the substrates
have significant effects on the reaction. The double hydro-
phosphination of phenylacetylene with Cy2PH was also tested.
However, the reaction only gave the single-hydrophosphination
product with ca. 12% conversion and the double-addition
product was not observed.
Mechanistic Studies. Monitoring the reaction of 2 with 1
equiv of PhCCH in C6D6 by NMR spectroscopy showed the
generation of HPPh2 and the copper acetylide ItBuCuCCPh
(4) (details are given in the Supporting Information) with the
formation of a small amount of single-hydrophosphination
product as a mixture of Z and E isomers at room temperature in
30 min (Figure S6 in the Supporting Information), indicating
that the copper acetylide is also involved in the catalytic cycle.
Indeed, it was found that complex 4 also exhibited catalytic
activity comparable with that of 2. Monitoring the reaction of 4
with HPPh2 by 1H and 31P NMR spectroscopy in C6D6 (Figure
S7 in the Supporting Information) indicated the formation of
phenylacetylene and 2 with a small amount of single-
hydrophosphination product as a mixture of Z and E isomers
under the same conditions. These results supported that
complexes 2 and 4 are interconvertible in the catalytic cycle.
To understand the interconversion process, the relative
energy profiles for the possible intermediates A−D were
obtained by DFT calculations (Figure 2). The coordination of
HPPh2 to complex 4 and the subsequent hydrogen transfer in
the intermediate A to form B only require a free energy of 6.03
hydrophosphination of the (E)-vinylphosphine with HPPh2 in
the presence of 0.43 mol % of the phosphide 2 for 8 h only led
to the formation of the desired product in 53% yield, suggesting
that the E isomer is less reactive than the Z isomer in the
second addition reaction (Scheme 4).
Double Hydrophosphination of Alkynes. With the
optimized conditions using CuCl2/ItBu as catalyst, a range of
alkynes was examined with 2 molar equiv of HPPh2 (Table 1).
Most of the alkynes were converted into the corresponding
double-hydrophosphination products in high yields (86−96%)
in 4−24 h (Table 1, entries 1−8). The reaction is tolerant to
aromatic halides (F, Cl, Br; Table 1, entries 6−8). Alkynes
substituted with 4-dimethylaminophenyl, pyridinyl, and thio-
phenyl groups were also effective, albeit in modest yields (Table
1, entries 9−11). It is noted that the unactivated 1-hexyne and
N,N-dimethylprop-2-yn-1-amine were also successful in the
double hydrophosphination to give the products in 79% and
85% yields, respectively (Table 1, entries 12 and 13). However,
trimethyl(prop-2-yn-1-yloxy)silane (Table 1, entry 14) and
ethynylcyclopentane (Table 1, entry 15) only gave the expected
double-hydrophosphination products in very low yields. 5-
Chloropent-1-yne only gave the single hydrophosphination
C
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formed vinylphosphine underwent coordination and migra-
tion−insertion reactions with the Cu-PPh2 to yield the
intermediates II and III. Cleavage of the Cu−C bond by
HPPh2 yielded the double-hydrophosphination product with
the regeneration of the catalyst.
CONCLUSIONS
■
In summary, we have developed a straightforward method for
the synthesis of well-defined NHC-copper phosphides by
reactions of copper halides with HPPh2 in combination with N-
heterocyclic carbenes. The NHC-Cu phosphides have been
proven to be highly efficient and selective catalysts for double
hydrophosphination of aryl- and alkyl-subsituted terminal
alkynes. Interestingly, the catalytic reaction can be more
conveniently and practically performed with the in situ
CuCl2/NHC system, which exhibits better performance than
the well-defined copper catalysts. The easy availability of
diverse NHC ligands, including chiral ligands, and copper
halides renders the catalytic system to be the most practical and
tunable. This system might also be amenable to wide
applications in the catalytic addition reactions as well as
stereoselective double hydrophosphinations.
Figure 2. Relative energy profiles for the possible intermediates A−D
obtained by DFT calculations. The values in parentheses are Gibbs
free energies in kcal/mol. The optimized structures of A−D are given
in Figures S1−S5 in the Supporting Information.
kcal/mol. The following migration−insertion of the coordi-
nated alkyne into the Cu−P bond led to the formation of
intermediates C and D, which have a very small energy
difference of 0.67 kcal/mol.
EXPERIMENTAL SECTION
Further details of the synthesis and characterization of complexes 1, 2,
and 4 and typical procedures for catalytic double hydrophosphination
of alkynes are given in the Supporting Information.
■
DFT calculations disclosed the important roles of the copper
acetylide 4 in the acceleration of the single-addition reaction, in
line with the experimental observations. The inability for
double hydrophosphination of internal alkynes is very likely
due to steric effects. This notion is supported by the
experimental observation that the E isomer obtained by the
single addition encounters much more difficulty in undergoing
the second hydrophosphination in comparison to the
corresponding Z isomer. On the basis of the experimental
results and DFT calculations, a tentative mechanism is given in
Scheme 5. Initially, the copper phosphide Cu-PPh2 and
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00854.
Experimental details, characterization data, and crystallo-
graphic data (PDF)
Crystallographic data (CIF)
Scheme 5. Tentative Mechanism for the Double
Hydrophosphination of Alkynes
AUTHOR INFORMATION
Corresponding Author
*E-mail for C.C.: cmcui@nankai.edu.cn.
■
ORCID
Chunming Cui: 0000-0002-1493-7596
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (Grant Nos. 21390401 and 21472098) for financial
support.
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