Metal-free Lewis acid mediated dehydrocoupling of phosphines and concurrent hydrogenation

Article abstract of DOI:10.1039/c4cc09526j

The stoichiometric reaction of trityl cation with two equivalents of Ph₂PH affords the phosphine stabilized phosphenium salt [Ph₂(H)PPPh₂][B(C₆F₅)₄] via hydride abstraction, while catalytic amounts of B(p-HC₆F₄)₃ effects catalytic phosphine dehydrocoupling with the liberation of H₂. This reaction is accelerated by the presence of olefin or imine, effecting concurrent hydrogenation.

Full text of DOI:10.1039/c4cc09526j

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Metal-free Lewis Acid Mediated Dehydrocoupling of
Phosphines and Concurrent Hydrogenation†
Cite this: DOI: 10.1039/x0xx00000x
a
a
a,b
Roman Dobrovetsky , Katsuhiko Takeuchi and Douglas W. Stephan
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract: The stoichiometric reaction of trityl cation with two phosphines and even concurrent transfer hydrogenation using the
equivalents of Ph PH affords the phosphine stabilized Lewis acid as a catalyst.
2
phosphenium salt [Ph (H)PPPh ][B(C F ) ] via hydride
The reaction of Ph
PH with [Ph C][B(C F ) ] in 2:1 ratio in
2 3 6 5 3
2
2
6 5 4
C H Br for 3 hours at 130 °C afforded Ph CH and [Ph (H)PPPh ]
abstraction, while catalytic amounts of B(
p
-HC F ) effects
6
5
3
2
2
6
4 3
[
B(C F ) ] (1) [Eq. (1)]. The concurrent formation of Ph CH was
6 5 4 3
catalytic phosphine dehydrocoupling with the liberation of
1
clearly indicated by the H NMR signal at 5.4 ppm. Compound 1
was separated and isolated as oil and its formulation confirmed by
H . This reaction is accelerated by the presence of olefin or
2
imine, effecting concurrent hydrogenation.
3
1
the observation of the two signals in P NMR spectrum, a doublet at
.5 ppm (J(PH) = 417 Hz) and singlet at δ = ꢀ25.4 ppm.
Interestingly, this stands in contrast to a previous report of a 1:1
reaction of Ph PH with [Ph C][B(C F ) ] which afforded the adduct
4
Phosphorus coordination chemistry is dominated by the donor
behaviour of trivalent, tricoordinate phosphines. However, very
recently the Lewis acidity of P(V) phosphonium cations has
garnered much attention as these species can be employed in a
2
3
6 5 4
9
[Ph (H)PCPh ][B(C F ) ].
2 3 6 5 4
1
variety of catalytic reactions. In a related sense, the donorꢀacceptor
+
properties of lowꢀcoordinate phosphenium cations, R P: has also
2
2
drawn attention in recent years. These species provide an interesting
3
isolobal analogy to carbenes. Phosphenium cations are typically
readily prepared by halide abstraction or displacement from a
suitable precursor using either weaklyꢀcoordinating anion or a
suitable Lewis base.
This prompted further efforts to employ Lewis acids to effect Pꢀ
1
0
2
b, 4, 5
P dehydrocoupling. To that end, the Lewis acid B(pꢀC F H) 2 was
6
4
3
employed as the common electrophilic borane. B(C F ) is known to
PꢀH bond activation typically involves treatment with a strong
base resulting in proton abstraction and generation of a phosphide
anion although Wright and coworkers have described the
6 5 3
undergo reaction with phosphines to give paraꢀattack products of the
11
6
form R P(H)(C F )BF(C F ) . In an initial stoichiometric reaction
2
6
4
6 5 2
2
was added to two equivalents of Ph PH and heated to 130 °C. This
2
stannocene, mediated dehydrocoupling of a range of primary
3
1
resulted in slow formation of (Ph P) as evidenced by the P NMR
phosphines.However,
the
similarities
of
the
Pauling
2
2
signal ꢀ15 ppm. The concurrent formation of H was evident from
the H NMR peak at 4.5 ppm. Under catalytic conditions, reaction of
electronegativities of hydrogen and phosphorus (2.20 and 2.19,
respectively), suggest that it should also be possible to generate a
2
1
7
1
0 mol % of the borane 2 with Ph PH was heated to 130 °C for 12 h
2
hydride and phosphenium cation from a phosphine. Nonetheless,
8
in a closed vessel. This afforded a 38% conversion to (Ph P) 3.
attempts to abstract hydride with trityl borate to generate a
2
2
9
Prolonged heating of the reaction mixture did not increase
conversion, however, removal of H2 from the reaction vessel,
furthered conversion to 54%. Subsequent and continuous removal of
H led to quantitative formation of 3. This observation infers that the
Lewis acid mediated hydrogenation of the biphosphine regenerates
phosphenium cation failed. Herein, we report the first hydride
abstraction from secondary and primary arylphosphines by the
concurrent action of a Lewis acid and excess phosphine, affording a
route to a phosphineꢀstabilized phosphenium salt. Moreover, this
reactivity is extended to effect the catalytic dehydrocoupling of
2
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4Crn
Ph PH. While the reduction of PꢀP bonds has been previously
reported, the present result is the first to describe the reverse exploited to effect transfer hydrogenation of organic unsaturates.
reaction, namely the Lewis acid mediated dehydrocoupling of Moreover, the presence of a hydrogen atom acceptor serves to
Interestingly, the present dehydrocoupling reactions can also be
2
12
phosphines.
accelerate the dehydrocoupling reactions. Thus, reaction of Ph PH in
2
the presence of
a
stoichiometric amount of 1ꢀphenylꢀ1ꢀ
trimethylsiloxyethylene or Nꢀbenzylideneꢀtertꢀbutylamine and a
B(p-C F H)
3
6
4
catalytic amount of B(pꢀC F H) at 130 °C results in the complete
6
4
3
(
2)
0 mol %
transformation to (Ph P) and hydrogenation of the organic species
1
2
2
R PH
R P-PR + H
2
after 30 hours and 38 hours, respectively [Eq. (2 and 3)].
2
2
2
C H Br
6
5
1
30°C
R = Ph 3: 88%, 120 h;
p-tol 4:80%, 120 h
B(C F H)
6
4
3
(
R P) + H
R PH
2
2
2
2
B(p-C F H)
3
Ph
P
6
4
R
(
2)
Ph
Ph
R
P
P
B(C F H)
6 4 3
1
0 mol %
+ H2
P
P
PhPH2
H
B(C F H)
3
6
4
P P
C H Br
P
6
5
R
H
Ph
Ph
R
H
1
30°C
R
R
5
98%, 120 h
B(C F H)
3
6
4
R PH
2
R
Scheme 1. Catalytic dehydrocoupling of phosphines
P
P
H
R
H
R
R
Analogous dehydrocoupling of (p-tol) PH at 130 °C for 120 h
2
proceeds in a similar fashion to give ((p-tol) P) 4 in 80% while
2
2
2
Scheme 2. Proposed mechanism of R PH dehydrocoupling.
PhPH2 undergo dehydrocoupling to give (PhP)5 5 in 98% yield
Scheme 1). Interestingly, the sterically demanding phosphines (oꢀ
tol) PH or Mes PH groups did not lead to PꢀP coupling, rather only
(
2
2
A radical mechanism for this dehydrocoupling was excluded as
performance of the reaction in the presence of the radical trap
the phosphineꢀborane adducts were observed. Similarly efforts to
dehydrocouple secondary alkylphosphines gave only the phosphineꢀ
borane adducts. These latter observations indicated the steric and
electronic limits for this Lewis acid mediated PꢀP dehydrocoupling.
13
reagent cyclohexadiene, showed no formation of benzene; rather
only 3 was formed. Thus, an ionicꢀpolar mechanism is proposed in
which both hydride and proton originate from R PH. Coordination
2
of a secondary phosphine to borane as in the adduct (Ph PH)B(pꢀ
2
7
7
10%
10%
OSiMe3
OSiMe3
Ph
C F H) presumably generates an electrophilic P center due to low
6
4
3
2
2
Ph PH +
Ph
2
PPPh
3
2
+
+
(2)
2
lying but unoccupied molecular orbital (LUMO) formed at P
Ph
1
b, 1e
centre
supported by DFT calculations of the molecular orbitals of the
adduct (Ph PH)B(pꢀC H) performed at WB97XD/def2TZV level
prompting nucleophilic attack by free Ph PH. This view is
2
H
N
N
Ph
2
Ph PH +
Ph
2
PPPh
2
Ph
2
14
6
F
4
3
(3)
tBu
tBu
3
of theory. The LUMO is concentrated on the B and P centers.
Attack by Ph PH at the boron center would result in replacement of
2
one phosphine by the other. On the other hand, attack at phosphorus
center generates the proposed pentacoordinate P center. The
transient five coordinate phosphorus atom transfers hydride to
borane generating [Ph (H)PPPh ][HB(C F H) ]. This salt can either
2
2
6
4
3
evolve H or sequentially deliver proton and hydride to an organic
2
unsaturate (Scheme 2). Hydrogenation of olefin or imine is thought
to proceed in a manner similar to FLP reductions however in the
+
present case [Ph PꢀP(H)Ph ] is the proton source and the anion
2
2
ꢀ
[
(HC F ) BH] is the source of hydride. Calculations employing the
6 4 3
conductorꢀlike polarizable continuum solvation model (CPCM)
in bromobenzene were carried out. The reaction of 1 with 6 is
slightly exothermic with ꢁH = ꢀ5.2 kcal mol and ꢁG = 0.6 kcal
mol . The subsequent generation of the intermediate salt
Ph (H)PPPh ][HB(C F H) ] is slightly endothermic and endergonic
1
5
ꢀ
1
ꢀ
1
[
2
2
6
4
3
ꢀ
1
ꢀ1
with ꢁH = 1.3 kcal mol and ꢁG = 15.4 kcal mol consistent with
the thermal conditions required for dehydrocoupling.
To put this reactivity in context PꢀP dehydrocoupling is typically
achieved by either stoichiometric or catalytic processes. Würtz
Figure 2. LUMO of (p-C
6
F
4 3 2
H) B(PHR ) (isovalue = 0.033)
16
2
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type reduction of the phosphine halides, or dehydrohalogenation of
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R PX and R PH are well established. A variety of other
2
2
1
8
stoichiometric methods have also been described. Metal catalyzed
dehydrocoupling of phosphines has also been demonstrated
employing Ti,
19
20
21
Zr
and Rh ꢀbased catalysts. Wright and
have described the stannocene, mediated
1
6c, 16f
coworkers
dehydrocoupling of a range of primary phosphines, providing the
first mainꢀgroup mediated PꢀP dehydrocoupling. Very recently a
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®
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reaction that is accelerated in the presence of a hydrogen acceptor. In
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2013ꢀ2014.
a
Department
of
Chemistry,
University
of
Toronto
8
0
St. George Street, Toronto, Ontario, M5S 3H6 (Canada)
8
9
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.
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Eꢀmail:dstephan@chem.utoronto.ca
Homepage: http://www.chem.utoronto.ca/staff/DSTEPHAN
.
b
Department of Chemistry, Faculty of Science, King Abdulaziz
University, Jeddah, Saudi Arabia
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†
The authors gratefully acknowledges the financial support of the
NSERC of Canada and DWS acknowledges the award of a Canada
Research Chair.
1
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