Double hydrophosphination of alkynes promoted by rhodium: The key role of an N-heterocyclic carbene ligand

Article abstract of DOI:10.1039/c5cc09156j

The regioselective double hydrophosphination of alkynes mediated by rhodium catalysts is presented. The distinctive stereoelectronic properties of the NHC ligand prevent the catalyst deactivation by diphosphine coordination thereby allowing for the closing of a productive catalytic cycle.

Full text of DOI:10.1039/c5cc09156j

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Double Hydrophosphination of Alkynes Promoted by Rhodium:
the Key Role of an N-Heterocyclic Carbene Ligand†‡
Andrea Di Giuseppe,*a Roberto De Luca,ab Ricardo Castarlenas,*ab Jesús J. Pérez-Torrente,a
Marcello Crucianelli,b and Luis A. Oro*ac
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The regioselective double hydrophosphination of alkynes communication, we report the first example of a RhI complex
mediated by rhodium catalysts is presented. The distinctive bearing a N-heterocyclic carbene (NHC) ligand able to catalyse
stereoelectronic properties of the NHC ligand prevent the catalyst the regioselective double hydrophosphination of terminal
deactivation by diphosphine coordination thereby allowing for the aromatic or aliphatic alkynes including substrates containing
closing of a productive catalytic cycle.
heteroatoms or a sensitive moiety as cyclopropane.
Our research group has recently reported highly active and
selective RhI-NHC catalysts for hydrofunctionalization
reactions of unsaturated compounds.9 In particular, we have
Phosphines are ubiquitous ligands for transition-metal
catalysts but general synthetic methods are still rather
limited.1 In this context, transition-metal-catalysed P-H bond
addition to unsaturated compounds is probably one of the
most interesting and fast developing areas for the
straightforward atom economical synthesis of phosphines.2
Although several organometallic complexes have been shown
to be efficient catalysts for the monohydrophosphination,3 or
even double hydrophosphorylation4 or hydrophosphinylation,5
the direct double hydrophosphination of alkynes still remains a
challenging reaction.2d 1,2-diphosphinoethane derivatives can
be considered as one of the most significant family of ligands
for the preparation of key homogeneous catalysts in industrial
chemistry, but synthetic methods are restricted to few
processes, including the use of strong bases,6 or two step
procedures.5a-7
shown that complex [Rh(μ-Cl)(IPr)(η2-coe)]2
(1a) (coe =
cyclooctene, IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-
carbene) is a very efficient catalyst for hydrothiolation of
alkynes.9a We have now expanded the application of this
catalytic system to hydrophosphination. As a model reaction
we investigated the addition of diphenylphosphine (2) to
phenylacetylene (3a) (molar ratio 1:1) in C6D6 at 80 °C
catalysed by a 5 mol% of complex 1a. The reaction was
monitored by 31P NMR, which showed a 52% conversion after
24 hours (Scheme 1). It was found that 1,2-
bis(diphenylphosphino)-1-phenylethane (rac-phenphos) (7a),
resulting from the double addition of diphenylphosphine to
the alkyne, was the main product of the reaction (35%)
together with the expected mixture of vinylphosphines 4a
and 6a (1, 23, and 32%, respectively). In addition,
tetraphenylbiphosphine ( ) was also observed (9%), resulting
, 5a,
The biggest hurdle for the development of active
organometallic catalysts in the double hydrophosphination of
alkynes is the reaction product. 1,2-diphosphines are chelating
ligands with strong coordination ability for soft metal centres
thereby inhibiting the organometallic catalyst. This problem
has been elegantly circumvented by Nakazawa and coworkers
that have developed the first transition-metal-based catalyst
by using hard FeII complexes.8 However, the applicability of this
system is limited to aromatic terminal alkynes. In this
8
from the dehydrocoupling of secondary phosphines, a well-
known transformation catalysed by several transition-metals
including Rh.10 Molecular hydrogen was released in the
process that further reacts with phenylacetylene leading to
styrene.
a Departamento de Química Inorgánica, Instituto de Síntesis Química y Catálisis
Homogénea (ISQCH) Universidad de Zaragoza–CSIC, C/ Pedro Cerbuna 12, 50009
Zaragoza (Spain).
bDipartimento di Scienze Fisiche e Chimiche, Università dell’Aquila, Via Vetoio, I-
67100 Coppito (AQ) (Italy).
c Center for Refining and Petrochemicals, King Fahd University of Petroleum and
Minerals, 31261 Dhahran (Saudi Arabia).
† Dedicated to Professor Nazario Martín on the occasion of his 60th birthday.
‡ Electronic Supplementary Information (ESI) available: Synthesis, characterisation,
and experimental details. See DOI: 10.1039/x0xx00000x
Scheme 1. Hydrophosphination of phenylacetylene with diphenylphosphine
catalysed by RhI complexes.
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Table 2. Hydrophosphination of alkynes catalysed by 1a.a
Entry
1
Alkyne
Conv.(%)b
90
4/5/6 (%)c
7 (%)c
8 (%)c
Table 1. Reaction of phenylacetylene with diphenylphosphine catalysed by RhI
complexes.a
(3a)
(3b)
(3c)
(3d)
0/6/4
66
24
Catalystb
T(oC) Conv.(%)c 4a/5a/6a(%)d 7a(%)d
8(%)d
20
2
3
4
90d
0/7/14
0/8/2
47
64
60
31
27
24
Entry
1e
1a
1a
1b
1c
1d
80
60
90
47
45
59
2/20/15
0/6/4
43
66
0
78
2
120
120
120
120
24
82
0/11/5
3
6/51/35
6/25/48
4/21/36
8
0/7/25
53
4
5
16
5
(3e)
66 (83)e
16 (26)e
(0/0/10)e
(64)e
5
19
20
6
7
(3f)
(3g)
94
70
0/4/11
0/2/2
60
47
25
49
a
Reaction conditions: 0.1 mmol of phenylacetylene, 0.2 mmol of Ph2PH, 0.5 mL
b
of toluene-d8, 120 °C, 24 h of reaction in a sealed NMR tube. [Rh]/[PHPh2] =
0.05. c Based on phosphine consumption, quantified by integration of the Inverse
Gated Decoupled-31P NMR spectra. d NMR selectivity in molar ratio. e C6D6
8
9
(3h)
(3i)
54
82
0/14/11
0/4/4
0
75
47
38
10
(3j)
89
-/0/13
0
87
In order to maximize the amount of the diphosphine product
7a, the influence of the different variables of the reaction on
the catalyst performance was studied and the most important
results are reported in Table 1 (see full data in Table S1 of
a
Reaction conditions: 0.1 mmol of alkyne, 0.2 mmol of Ph2PH, 5 mol % of 1a in
b
0.4 mL of toluene + 0.1 mL of C6D6, 120 °C, 24 h of reaction, sealed NMR tube.
Based on phosphine consumption quantified by integration of the Inverse Gated
Decoupled-31P NMR spectra. c NMR selectivity in molar ratio. d 12 h of reaction. e
72 h of reaction.
Supplementary
[Ph2PH]/[PhC CH] ratio from 1:1 to 2:1 (entry 1) led to an
increase of diphosphine 7a (43%) versus monophosphinated
products (37%) together with an increment of . Noteworthy,
Information).
Modification
of
the
≡
The presence of a heteroatom in the aromatic ring, as in 2-
ethynylpyridine (3e), slowed down considerably the reaction
leading to a conversion of 66% in 24 h (entry 5). After 72 h of
reaction, conversion and selectivity reached a value quite
similar to other electron poor alkynes. This can be justified
both by the electron-withdrawing effect of pyridine and the
ability of the nitrogen atom to coordinate to the rhodium
center and inhibit the reaction. As previously noted, the
double hydrophosphination of aliphatic alkynes still remains a
goal to achieve. For this reason we have studied the reactivity
of our catalytic system with a representative group of these
8
a temperature rise from 80 to 120 °C (entry 2, toluene-d8) was
beneficial in terms both of activity and selectivity, resulting in a
90% of conversion with a 66% of selectivity for 7a, along with a
marked reduction of the monophosphinated products (10%).
The catalytic activity of a series of RhI complexes was studied
in order to shed light into the effect of the NHC ligand on the
catalytic performance of 1a (entries 3-5). The presence of the
carbene ligand seems to be fundamental for the preferential
formation of 7a. In fact, [Rh(μ-Cl)(η2-coe)2]2 (1b) was only able
to catalyse the monophosphination of phenylacetylene, albeit
with low conversion and selectivity (entry 3), whereas
Wilkinson’s catalyst RhCl(PPh3)3 (1c) gave only 5% of 7a (entry
4). The introduction of a stronger σ-donor and sterically
encumbered phosphine such as triclyclohexylphosphine (PCy3)
substrates (entry 7-9). 1-Nonyne
(3f) was successfully
hydrophosphinated with good conversion (94%) and a 60%
selectivity in 7f (entry 6). Cyclopropylacetylene (3g) gave a
slightly lower conversion (70%) and selectivity (47%), but the
cyclopropyl group remained intact (entry 7). The double
hydrophosphination reaction was inhibited with the bulkier
tert-butylacetylene (3h), and the main product was the
(
1d) (entry 5) allowed for a better conversion (59%) and
selectivity towards the diphosphine (19%) but lower than the
Rh-NHC system. These results show that the NHC ligand
confers to the catalyst the required stereoelectronic properties
to catalyse efficiently the double hydrophosphination of
phenylacetylene.
dehydrocoupled diphosphine
8 (entry 8). The presence of
heteroatoms in the aliphatic backbone is perfectly tolerated.
Thus, 3-dimethylamino-1-propyne (3i) gave an 82% conversion
with an encouraging 38% of the diphosphine 7i, in contrast to
the results obtained for the ethynylpyridine, probably due to a
lower coordination ability of the dimethylamino group.
Internal alkynes, such as diphenylacetylene (3j), did not
undergo double hydrophosphination (entry 10), instead the
The scope of the reaction was studied for several types of
alkynes including aromatic, aliphatic, or heteroatom containing
substrates (Table 2). The presence of an electron-donating
group in para position such as -OMe (3b) resulted in an
increase of the reaction rate, 90% conversion after 12 h (entry
2), but was deleterious for the diphosphine selectivity (47% of
7b). On the contrary, the presence of an electron-withdrawing
group, para -CF3 (3c) or meta -OMe (3d), led to a decrease of
conversion (78 and 82%, respectively) (entries 3 and 4), while
the selectivity was not significantly affected.
dehydrocoupling reaction predominated (89% of 8). The new
diphosphines 7c,d,f,g,i were derivatized to the corresponding
borane adducts which were isolated by semi-preparative HPLC.
2 | J. Name., 2012, 00, 1-3
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Scheme 2. Stoichiometric reactions on RhI complexes involved in the double
hydrophosphination process.
At this stage of the research, we can postulate that the
catalytic cycle follows the classical oxidative addition of the
secondary phosphine and subsequent migratory insertion and
reductive elimination sequences, firstly for the alkyne and
subsequently for the vinylphosphine intermediates, to give rise
to the diphosphines.10,12e The key point dealing with the
efficiency of the Rh-NHC catalyst is its ability to catalyse the
double hydrophosphination without being poisoned by the
presence of a large excess of the 1,2-diphosphinoethane
products. In the Nakazawa’s catalytic system, the hard-soft
mismatching Fe/diphosphine and/or the bite angle8,13 might
account for enhanced catalytic activity, however, this is not
applicable to our soft RhI catalyst. In order to disclose this
fundamental point, complex RhCl(IPr)(rac-phenphos) (10) was
synthesized by treating 1a with 2 equiv of rac-phenphos 7a in
toluene (Scheme 2). Complex 10 was obtained in 90% yield an
isolated as a mixture of diastereomers (10a and 10b, molar
ratio 98:2). Interestingly, when a solution of 10 in C6D6 was
treated with 2 equiv of Ph2PH at 60°C an equilibrium between
Figure 1. Monitoring of the reaction between phenylacetylene and diphenylphosphine
catalysed by 1a with a molar ratio 20:40:1 in tol-d8 at 100 °C.
In order to get information on the mechanism, the reaction of
phenylacetylene with diphenylphosphine catalysed by 1a (5
mol%) was monitored by 31P NMR. As is shown in Figure 1 the
amount of Ph2PH (
early stage of the reaction (2 h) the monophosphination
derivative E-vinylphosphine 6a ) and tetraphenylbiphosphine
) slowly appeared, in addition with a very small amount of Z-
vinylphosphine 5a ) and traces of gem-vinylphosphine 4a
♦) decrease with a sigmoidal shape. In the
(
●
(■
(
▲
(not showed). After this initial period, the amount of E-
vinylphosphine 6a decreased with the concomitant formation
of the 1,2-diphosphinoethane derivative 7a (▼). This kinetic
profile is compatible with a reaction scheme in which Ph2PH
reacts initially with phenylacetylene giving rise mainly to the
formation of E-vinylphosphine, that is subsequently
transformed into the 1,2-diphosphine product. The steady
increase of the amount of Z-vinylphosphine could be ascribed
to the uncatalysed hydrophosphination of phenylacetylene
10 and
9 was established after 5 h (10:9 ratio of 79:21)
(Scheme 2), which is in agreement with catalytic performance
exhibited by 1a. The lability shown by the diphosphine ligand
in 10 strongly contrasts with the bidentate character of this
ligand. Most likely the understanding of this reactivity pattern
has to be sought in the influence of the IPr ligand. We
hypothesize that the interplay between the steric hindrance
and the high trans effect imparted by the NHC ligand is
responsible for the destabilization of the chelate phosphine.14
In support of this proposal a similar experiment using a related
complex without an NHC ligand has been performed.
that selectively provides 5a 11
dehydrocoupling of Ph2PH takes place but at slower rate.
Interestingly, was not formed in the uncatalysed reaction
. At the same time, the catalysed
8
(see Supplementary Information).
Through the monitoring of the catalytic reaction a steady
increment of a new set of resonances attributable to a new
rhodium species was apparent in the 31P-NMR spectra. In
order to get insight on this species, a stoichiometric reaction
between complex 1a and Ph2PH was carried out leading to the
Treatment
of
RhCl(PPh3)3
with
1,2-
formation of RhCl(IPr)(PHPh2)2 (
the chlorido bridges and the substitution of coe by Ph2PH
(Scheme 2). Complex features two doublet of doublets at δ
9) as a result of the cleavage of
bis(diphenylphosphino)ethane (dppe) leads to the cationic
complex [Rh(dppe)2]Cl (11). However, this compound does not
react with Ph2PH even on heating at 120°C for several weeks.
Furthermore, the hydrophosphination of phenylacetylene
catalysed 11 provides similar results to the uncatalysed
reaction (see Supplementary Information, Table S1). These
experiments confirm that the NHC ligand is fundamental to
prevent the catalyst deactivation by the destabilization of the
9
16.4 (JRh-P = 189, JP-P = 53 Hz) and 9.7 ppm (JRh-P = 118, JP-P = 53
Hz) in the 31P{1H}-NMR spectra which match with those
observed in the catalytic reaction. In addition, the 13C{1H}-NMR
spectrum displays a doublet of doublet of doublets at δ 193.3
ppm (JC-P = 123, JC-Rh = 47, JC-P = 16 Hz) for the carbene carbon
atom. The observation of this complex all along the catalytic
chelate complex with the diphosphine
.
reactions strongly suggests that
the catalyst.
9 could be the resting state of
In conclusion, we have disclosed a Rh-NHC complex able to
promote the double hydrophosphination of alkynes that may
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8
9
M. Kamitani, M. Itazaki, C. Tamiya, H. Nakazawa, J. Am.
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synthesis of bis-phosphorous containing compounds. The
carbene ligand plays a major role in the catalytic system due to
its particular stereoelectronic properties that prevent the
poisoning of catalytic species by strong coordination of the
diphosphine product. The optimization of the catalytic system
by fine tuning of the NHC ligand, including the design of
catalysts able to promote the enantioselective double
hydrophosphination of alkynes, as well as DFT studies on the
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