Selective dehydrocoupling of phosphines by lithium chloride carbenoids

Article abstract of DOI:10.1021/ja509381w

The development of a simple, transition-metal-free approach for the formation of phosphorus-phosphorus bonds through dehydrocoupling of phosphines is presented. The reaction is mediated by electronically stabilized lithium chloride carbenoids and affords a variety of different diphosphines under mild reaction conditions. The developed protocol is simple and highly efficient and allows the isolation of novel functionalized diphosphines in high yields.

Full text of DOI:10.1021/ja509381w

Communication
pubs.acs.org/JACS
Selective Dehydrocoupling of Phosphines by Lithium Chloride
Carbenoids
Sebastian Molitor, Julia Becker, and Viktoria H. Gessner*
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany
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̈
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S
* Supporting Information
formation reactions.9,10 In the case of P−P coupling reactions,
however, only the tin complex Cp*2SnCl2 (D) reported by
Wright and co-workers has been active in P−P bond formation
reactions directly from phosphines.11 Although this bond
formation proceeds in a catalytic manner (10 mol %), high
reaction temperatures and extended reaction times were
necessary to allow for sufficient conversions. To the best of our
knowledge, no other practicable or high-yielding synthesis of
diphosphines directly from R2PH precursors using catalytic or
stoichiometric amounts of a main-group metal species has been
reported.12 Instead, 1,1-addition of P−H bonds has been
observed for free carbenes13 and silylenes (e.g., E → F).14 In
this work, we focused on the reactivity of carbenoids toward the
P−H bond in phosphines, which led to the development of a
highly efficient protocol for the synthesis of diphosphines from
the corresponding phosphine precursors.
In previous reports on the ambiphilic nature of carbenoids, we
have focused on the influence of stabilization effects on the
reactivity.15,16 Carbenoid 1 was found to be sufficiently stable for
controlled handling while keeping its ambiphilic nature.17 This
resulted, for example, in activation of the B−H bond in borane
Lewis base adducts.15 Thus, 1 was chosen as starting point for
reactivity studies toward P−H bonds. Treatment of a cooled
solution of the carbenoid in tetrahydrofuran (THF) with an
excess of diphenylphosphine instantly resulted in decoloration of
the yellow solution upon warming to room temperature
(Scheme 1). 31P{1H} NMR spectroscopy after reaction for 1 h
ABSTRACT: The development of a simple, transition-
metal-free approach for the formation of phosphorus−
phosphorus bonds through dehydrocoupling of phos-
phines is presented. The reaction is mediated by
electronically stabilized lithium chloride carbenoids and
affords a variety of different diphosphines under mild
reaction conditions. The developed protocol is simple and
highly efficient and allows the isolation of novel function-
alized diphosphines in high yields.
ehydrocoupling has become a powerful synthetic method-
D
ology for the formation of a variety of homo- and
heteroatomic main-group element bonds. The development of
this reaction has particularly been fueled by the synthetic
potential and the applicability of the obtained products, e.g., as
polymeric and molecular materials.1 However, the formation of
an E−E bond directly from E−H precursors usually requires the
use of transition-metal catalysts. One of the first reports goes
back to the early 1980s, when Sneddon reported Pt-catalyzed B−
B bond formation between borane clusters.2 Since then, a variety
of E−E coupling reactions have been realized using early- and
late-transition-metal complexes as catalysts.3,4 In the case of P−P
coupling reactions, the first reports by Stephan’s group focused
on the use of an anionic zirconocene complex (A in Figure 1).5,6
a
Scheme 1. Dehydrocoupling of Ph2PH with Carbenoid 1
a
Conditions: (i) −78 °C → RT, THF, <1 h, −LiCl.
showed the formation of only two new compounds characterized
by singlets at δP = 36.6 and −14.8 ppm (C6D6). The first signal
was assigned to the protonated species 1-H2, while the species
resonating at higher field was identified as Ph4P2. Thus,
dehydrocoupling of the phosphine by simultaneous protonation
of the carbenoid had taken place. This observation is remarkable
not only with respect to the fast and clean conversion but also
because it is unprecedented in carbenoid chemistry. Typically,
secondary phosphines are lithiated by lithium bases (e.g., Ph2PH
Figure 1. Catalysts for the dehydrocoupling of phosphines (A−D) and
P−H bond activation by silylene E.
These studies were followed by reports on the use of other
group-4 and late-transition-metal complexes (e.g., B and C) by
the groups of Tilley, Brookhart, Waterman, and others.7,8
Despite the advances in transition-metal catalysis, the past two
decades have revealed a series of compounds containing main-
group elements that are applicable in bond activation and
Received: September 17, 2014
Published: October 16, 2014
© 2014 American Chemical Society
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Communication
to Ph2PLi) and react with alkyl halides to form P−C bonds. In
the reaction of 1 with Ph2PH, however, no comparable reactivity
was observed.
Table 1. Results of the Carbenoid-Mediated Dehydrocoupling
of Phosphines
We thus turned our attention to an evaluation of the scope of
the reaction in terms of applicable carbenoids and phosphine
precursors. Thereby, the electronically stabilized carbenoids 1−
3, with the sulfonyl-substituted derivative 3 being a room-
temperature-stable representative, as well as the highly reactive
compounds 4 and 5 were tested (Figure 2). In a typical
a
entry R2PH carbenoid time [h]
product
yield [%]
1
2
6
11
15
8
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
3
3
3
3
3
3
1
1
1
Ph4P2
88
91
69
(p-Tol)4P2
Figure 2. Carbenoids applied in the P−P coupling reaction.
3
1
(o-Tol)4P2
b
4
24
Mes4P2
−
experiment, a cooled solution of the carbenoid was treated with 2
equiv of Ph2PH, and the obtained reaction mixture was studied
by 31P{1H} NMR spectroscopy. While carbenoids 1−3 cleanly
formed their corresponding protonated derivatives and
diphosphines [see the Supporting Information (SI)], the more
reactive carbenoids 4 and 5 delivered complex product mixtures
with the diphosphine in less than 10% yield. Hence, electronic
stabilization of the carbenoid is required for selective
dehydrocoupling (vide infra).
5
12
13
16
17
9
1
(p-Me2NC6H4)4P2
(p-MeOC6H4)4P2
(o-MeOC6H4)4P2
(3,5-(CF3)2C6H4)4P2
tBu4P2
92
91
80
32
6
1
7
1
8
1
b
9
1
−
−
b
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
10
11
15
12
13
16
14
18
8
1
Cy4P2
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
24
(p-Tol)4P2
98 [82]
99 [74]
99 [98]
99 [90]
99 [91]
67 [61]
98
(o-Tol)4P2
(p-Me2NC6H4)4P2
(p-MeOC6H4)4P2
(o-MeOC6H4)4P2
(4-ClC6H5)2P2
(3,5-Cl2C6H4)4P2
Mes4P2
Next, stabilized carbenoids 1−3 were used for evaluation of
the substrate scope (Table 1; for experimental details, see the SI).
Coupling to the diphosphines was accomplished with a huge
variety of different aryl-substituted phosphines. However, no
selective conversion to the diphosphines was observed with
aliphatic compounds such as tBu2PH (9) and Cy2PH (10)
(entries 9 and 10), suggesting that electronic effects and the P−H
polarity might play a role in the reaction mechanism. The
reactions were found to be fast, in general being complete within
less than 1 h. In most of the cases, decoloring of the yellow
carbenoid solution occurred instantly after phosphine addition.
Even in the case of room-temperature-stable carbenoid 3, only a
couple of minutes were needed for the reactions to reach
completion. Coupling was accomplished using phosphines with
electron-rich (entries 5−7 and 13−15) and electron-poor
aromatics (entries 16, 17, and 22−24), showing high conversions
in all cases. Steric effects were found to influence the efficiency of
the coupling reaction. While (o-Tol)2PH was still efficiently
coupled to give (o-Tol)4P2 (entries 3 and 12), the more bulky
mesityl substituent in Mes2PH limited its dehydrocoupling
(entry 4). However, in case of the more reactive and sterically less
demanding carbenoid 2, conversion to the diphosphine was
observed in 16% yield (entry 18), suggesting that smaller
carbenoids should also efficiently couple more bulky phosphines.
It is interesting to note that the synthetic protocol also allows
the presence of functional groups, even chloro and trifluor-
omethyl groups. This is probably due to the efficient electronic
stabilization of the used carbenoids. For example, in the case of
the methoxy- or dimethylamino-functionalized systems 12, 13,
and 16 (Table 1, entries 5−7 and 13−15), no competing side
reactions such as ortho metalations were observed. The same was
true for chloro-substituted phosphines 14 and 18 (entries 16, 17,
23, and 24), allowing for further functionalizations after the
coupling reaction. Besides the secondary phosphines, even the
16
6
1
Ph4P2
78
12
13
17
14
18
7
<0.5
<0.5
<0.5
24
(p-Me2NC6H4)4P2
(p-MeOC6H4)4P2
(3,5-(CF3)2C6H4)4P2
(4-ClC6H5)2P2
(3,5-Cl2C6H4)4P2
c-Ph5P5
82
93
93
95
<0.5
3
87
70
c-Ph6P6
5
Ph(H)PP(H)Ph
17
a
Determined by 31P{1H} and 1H NMR spectroscopy; values in
brackets are isolated yields. The reaction was unselective because of
decomposition of the carbenoid; multiple products were formed.
b
primary phosphine PhPH2 was found to undergo P−P bond
formation, with a preference for the five-membered cyclo-
phosphane [Ph5P5] (70%) over the six-membered analogue
[Ph6P6] (17%) (entry 25).
Overall, the carbenoid-mediated P−P coupling of phosphines
offers a facile protocol for the synthesis of a variety of aryl-
substituted diphosphines. Contrary to other methods available
for the formation of P−P bonds, the “carbenoid route” does not
require laborious workup, transition-metal catalysis, long
reaction times, elevated temperatures, or different P-containing
starting materials.8 High conversions to the diphosphines were
achieved with all three carbenoids 1−3. However, trimethylsilyl-
substituted carbenoid 2 turned out to be the reagent of choice for
convenient isolation of the formed diphosphines. Here the
protonated species could easily be removed from the reaction
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Communication
mixture by washing with pentane and recycled for the carbenoid
synthesis. Hence, this strategy allowed the clean isolation of the
diphosphines in good to excellent yields (Table 1, entries 11−
16). Also, novel diphosphines such as the amino-and methoxy-
functionalized compounds (p-Me2NC6H4)4P2, (p-
MeOC6H4)4P2, and (o-MeOC6H4)4P2 could be isolated as
colorless solids in high yields of 98, 90, and 91%, respectively.
The diphosphines (p-Me2NC6H4)4P2, (p-Tol)4P2, and (p-
MeOC6H4)4P2 were additionally characterized by X-ray
diffraction analysis, which confirmed the nature of the coupling
products (see the SI). The P−P bond lengths in these
compounds are 2.214(1), 2.238(1), and 2.252(1) Å, respectively,
and are thus comparable to the one in Ph4P2.8f
In view of the efficiency of the P−P bond formation reaction
mediated by Li/Cl carbenoids and the broad substrate scope, it
was of interest to explore the mechanism of the reaction and the
reason for the observed selectivity. At first, the possible
involvement of radical species was addressed. In general, no
coloring of the reaction mixtures indicative of radical
intermediates was observed, except for the reactions with the
electron-poor phosphines 14, 17, and 18. In these cases, the
mixtures turned to orange (14, 18) and purple (17) for a couple
of minutes (<5 min) before complete decoloration occurred.
However, no radical species could be trapped with the radical
scavenger 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO). Even
in this case, selective formation of the diphosphine was observed.
The same holds true when the reaction was performed in the
dark. This suggests that no radical mechanism but rather a
substitution mechanism including (de)protonation steps is
operative. In fact, the observed colors can also be attributed to
the corresponding lithium phosphides (Ar2PLi),18 which are
formed by a stepwise mechanism in which deprotonation of the
phosphine is the initial step. This hypothesis was confirmed by a
trapping experiment. Carbenoid 2 was treated with only 1 equiv
of 13 at −78 °C, followed by immediate addition of Ph2PCl.
31P{1H} NMR spectroscopy of the reaction mixture showed the
Figure 3. Reaction pathways and calculated free energies for the
reactions of the carbenoids TMSC(LiCl)P(S)Me2 and CHCl2Li with
Ph2PH [M06-2X/6-311+g(d)].
experiments. Hence, the electronic stabilization of the carbenoid
is necessary to favor the initial deprotonation step over the LiCl
elimination and to allow for the selective formation of the
diphosphines.19
In conclusion, we have shown that Li/Cl carbenoids can be
applied for the coupling of phosphines to give the corresponding
diphosphines. This observation led to the development of a
simple, highly efficient, and versatile synthetic protocol that
furnishes the diphosphines in high yields, usually within less than
1 h of reaction. Despite the fast reaction process, the coupling
conditions also allow the presence of functional groups (−Cl,
−NMe2, −OMe) and thus the isolation of novel diphosphines.
Electronic stabilization of the Li/Cl carbenoids was found to be
necessary for the selective transformation. The presented
protocol allows facile access to a variety of diphosphines, which
may now give rise to the development of novel applications of
this still scarcely explored class of compounds.
formation of the heterocoupled diphosphine (p-MeOC6H4)2P−
1
PPh2 as the main product (δP = −16.6 and −19.1 ppm; JPP
=
158.4 Hz) together with (p-MeOC6H4)4P2 and several
compounds arising from Li/Cl exchange reactions (see the SI).
Thus, these findings suggest the formation of a phosphide species
as an intermediate of the P−P bond formation. The next step of
the P−P coupling probably involves hydride−chloride exchange,
as found in the case of the B−H bond activation.15
ASSOCIATED CONTENT
* Supporting Information
Experimental section, spectroscopic and crystallographic data
(CIF), and details on the computational studies. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
The origin of the observed selectivity was finally addressed by
computational methods (M06-2X/6-311+g(d); see the SI for
details) using a monomeric model system for carbenoid 2 with
methyl groups at phosphorus and Me2O for completion of the
coordination sphere of lithium. Deprotonation of the phosphine
as the initial step of the reaction mechanism was found to require
only 69 kJ mol−1. This activation barrier is lower in energy than
those of alternative pathways (Figure 3, left). Hydride transfer
(ΔG⧧ = 104 kJ mol−1) or carbene formation via LiCl elimination
(ΔG⧧ = 158 kJ·mol−1) require considerably more energy. This
confirms the progress of the reaction at low reaction temper-
atures and the selective formation of the phosphide as a reaction
intermediate of the P−P bond formation. A different picture was
obtained for the more labile carbenoid CHCl2Li (5) (Figure 3,
right). Here carbene formation possesses a lower activation
barrier (ΔG⧧ = 50 kJ mol−1) than deprotonation of the
phosphine (ΔG⧧ = 88 kJ mol−1). The liberated carbene is highly
reactive and thus results in the formation of multiple products
due to various decomposition reactions, as observed in the
AUTHOR INFORMATION
Corresponding Author
vgessner@uni-wuerzburg.de
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the DFG (Emmy Noether Grant to V.H.G.) and the
Fonds der Chemischen Industrie for financial support and
Rockwood Lithium GmbH for the supply of chemicals.
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