Stable fluorophosphines: Predicted and realized ligands for catalysis

Article abstract of DOI:10.1002/anie.201105954

Ligand maps lead to treasure! The activity of complexes of fluorophosphines (R₂PF) in catalytic hydroformylation and hydrocyanation is predicted from a ligand map. However, the instability of R₂PF to disproportionation is well-documented. Examples of R₂PF ligands (see scheme) are described that are stabilized to such an extent that they can be used in catalysis and are shown to be highly effective.

Full text of DOI:10.1002/anie.201105954

.
Angewandte
Communications
DOI: 10.1002/anie.201105954
Fluorophosphines
Stable Fluorophosphines: Predicted and Realized Ligands for
Catalysis**
Natalie Fey,* Michael Garland, Jonathan P. Hopewell, Claire L. McMullin, Sergio Mastroianni,
A. Guy Orpen, and Paul G. Pringle*
Phosphorus(III) compounds are the ancillary ligands of
[
1,2]
choice in many areas of modern homogeneous catalysis,
from bulk chemical processes (e.g. hydroformylation, hydro-
cyanation, carbonylation) to fine-chemicals production (e.g.
asymmetric hydrogenation, CÀC coupling, CÀN coupling).
The field is dominated by ligands containing PÀC and PÀO
bonds with a few examples featuring PÀN bonds. This fact is
not surprising, because these PÀX bonds are generally
thermally robust and inert to cleavage by transition metals
under the catalysis conditions; these ligand properties are
essential for the maintenance of catalyst integrity.
Triaryl phosphites are used as ligands in the commodity
[1]
chemical processes hydroformylation and hydrocyanation.
However, the susceptibility of P(OAr) ligands to hydrolysis
phosphines (green triangles) are clearly distinguished from
aryl phosphites (blue triangles) and therefore, on this
measure, would not be expected to resemble each other as
ligands for catalysis. It was of interest to explore whether this
map could be used to discover non-labile ligands with
activities similar to bulky aryl phosphites.
With the limited experimental data available for predic-
tive modeling (see Ref. [8] for a discussion of multivariate
regression models), proximity in ligand space was used as a
first indicator of catalytic potential. It is evident in Figure 1
3
has necessitated the installation of large hydrophobic groups
in diphosphites such as I to provide sufficient protection of
the PÀO functionality to make their commercial deployment
[
3]
feasible. Furthermore, the lability of phosphites makes their
application as ligands problematic for catalytic processes in
[4]
aqueous or alcoholic media and with substrates containing
protic functional groups. Therefore, there is a need for robust,
[5]
p-acceptor ligands as alternatives to phosphites. Fluoroaryl
phosphines such as II were considered candidates, since they
feature non-labile PÀC bonds and their Tolman electronic and
that fluorophosphines of the type R PF (red squares) lie in the
2
[
6]
steric parameters are similar to those for bulky phosphites.
Disappointingly, we found that ligand II gives catalysts with
essentially zero activity in hydroformylation and hydrocya-
vicinity of triaryl phosphites.
Fluorophosphines R PF are readily made from the
2
corresponding compound R PCl and have been known for
2
[
7]
[9]
III
nation. Thus the fact that ligands have similar Tolman
parameters is not a reliable predictor of catalyst activity.
over 50 years.
545 kJmol ,
The P ÀF bond is very strong at
À1 [10]
and metal complexes have been reported
[8]
[11]
Recently we reported a map (Figure 1) derived from a
broader range of calculated ligand parameters collected in the
ligand knowledge base. On this ligand map, fluoroaryl
for some R PF ligands.
However, no applications of
2
[12]
fluorophosphines in catalysis have been described, perhaps
because of the instability of R PF with respect to the
2
[
13]
disproportionation reaction shown in Equation (1).
[*] Dr. N. Fey, M. Garland, Dr. J. P. Hopewell, C. L. McMullin,
Prof. A. G. Orpen, Prof. P. G. Pringle
School of Chemistry, University of Bristol
Cantock’s Close, Bristol BS8 1TS (UK)
E-mail: natalie.fey@bristol.ac.uk
paul.pringle@bristol.ac.uk
Homepage:
[13-17]
Schmutzler, Riesel, and others
have thoroughly
http://www.inchm.bris.ac.uk/people/fey/group_top.html
http://www.inchm.bris.ac.uk/people/pringle/welcome.html
investigated this disproportionation and found that 1) fluo-
rophosphines such as Ph PF, Me PF, and nBu PF dispropor-
2
2
2
Dr. S. Mastroianni
Rhodia Centre de Recherches et Technologies de Lyon
[13,14]
tionate readily,
tion in catalytic processes that involve ligand dissociation;
) fluorophosphines with bulky or electron-withdrawing sub-
which essentially precludes their applica-
85 Rue des Frꢀres Perrets, 69192 Saint-Fons Cedex (France)
2
[
**] We whould like to thank the EPSRC (Grant No. EP/E059376/1 to
N.F.), COST Network CM0802, and Rhodia for support and
Johnson–Matthey for the loan of precious metals.
[
15]
[9,16]
[17]
stituents such as (C F ) PF, (CF ) PF,
and tBu PF are
2
6
5
2
3
2
thermally stable. Whether the source of the stability is
thermodynamic or kinetic has not been determined (see
below).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105954.
1
18
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Chemie
solutions of 1 in aqueous MeCN (1%)
remained unchanged after 60 days. The
stability of CgPF is consistent with the
bulk and electronegativity of the CgP
moiety. For the following reasons, we sug-
gest that a third component of the stability
of CgPF is the constraint that the cage puts
on the C-P-C angle. The higher apicophilic-
ity of F than of alkyl ligands explains the
observation that the isomer formed by
[
19]
R PF has the R groups diequatorial. If
2
3
CgPF were to disproportionate according to
Equation (1), the same apicophilicity argu-
ments would lead to the prediction that the
favored isomer of CgPF would have the
3
cage occupying diequatorial sites (isomer
A). However, the cage imposes a C-P-C
angle close to 908 that would destabilize
isomer A and relatively favor the apical–
[
20]
equatorial isomer B.
Either way, the
F
Figure 1. Map of ligand space, showing PAr (black diamonds), P(Ar ) (green triangles),
energy of CgPF3 will be raised, and this
increase would contribute to the stability of
CgPF to disproportionation.
3
3
P(OAr) (blue triangles), and R PF (red squares). Other ligand classes are represented as
3
2
F
gray dots. Ar =fluoroaryl, Tol=toluene, PC=Principal Component.
The phosphatrioxa-adamantane cage moiety [denoted
CgP in Eq. (2)] behaves like a bulky and electron-withdraw-
These ideas led us to consider whether the thermody-
namic stability of R PF to disproportionation could be
2
predicted from computational studies. Some very simple
density functional theory (DFT) calculations were carried out
[
18]
ing R P group,
and therefore we reasoned that the
2
compound CgPF (1) may be a particularly stable fluorophos-
phine. Indeed, treatment of CgPBr with CsF in THF gave 1
quantitatively as an air-stable, white solid, which has been
fully characterized. Single crystals of 1 were grown from
on R PF compounds to assess their stability with respect to
disproportionation [Eq. (1)] (see the Supporting Information
2
for details). It is observed experimentally that 1 and tBu PF
2
[
17]
are stable with respect to disproportionation, while Ph PF
2
[13,14]
CHCl , and its structure is shown in Figure 2. This is the first
and Me PF are unstable.
Consistent with this finding, the
3
2
reported crystal structure of a fluorophosphine. The small C-
P-C angle of 94.58 is a result of the constraints of the cage.
calculated DFT energies (Table 1) for the disproportionation
[18]
were calculated to be greater than zero for 1 and tBu PF and
2
Compound 1 is remarkably thermally and hydrolytically
less than zero for Ph PF and Me PF. Calculations also
2
2
stable. As a solid it can be stored in air indefinitely, and
Table 1: DFT calculated relative energy differences (DE) for the dispro-
portionation shown in Equation (1) and relative isomer stabilities for A
and B.
Ligand
Disproportionation DE
Relative energy
À1
À1
[kcalmol
]
[kcalmol
]
A
B
[
[
[
a]
a]
a]
Me PF
À15.8
5.4
0.0
0.0
0.0
1.3
0.8
not found
12.7
37.1
18.6
0.0
2
tBu PF
2
Ph PF
À15.9
2
1
2
3
3.2
4.2
À2.5
0.0
0.0
Figure 2. Molecular structure of 1. Thermal ellipsoids are set at the
[
b]
5
0% probability level. Selected bond lengths [ꢁ] and angles [8]: P1A–
F1A 1.574(7), P1A–C2 1.887(6), P1A–C9 1.873(8); F1A-P1A-C2 99.0(4),
F1A-P1A-C9 100.9(4), C2-P1A-C9 92.7(3).
[a] Optimized with frozen C-P-C angle. [b] Optimized to axial–equatorial
isomer B.
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119

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Angewandte
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[
a]
suggested that the new bicyclic fluorophosphines 2 and 3 (see
Figure 1) should be tenable, and so these then became
synthetic targets (see below). In agreement with our ideas
on the relative destabilization of the diequatorial isomer A
for ligands with constrained C-P-C angles, ligands 1–3 no
Table 2: Catalytic hydroformylation of 1-heptene.
À1
[b]
Ligand
nCO [cm
]
Conversion [%]
Aldehyde [%]
n/iso
1
2
3
4
5
1
2
3
2011
1991
1995
1992
95
73
98
84
99
89
95
95
85
90
3.9
2.9
2.4
1.9
2.2
longer favor diequatorial isomer A for R PF , whereas the
2
3
tBu PF
PPh3
2
[
18]
others showed a substantial preference for A. The source of
1965
the stability of tBu PF with respect to disproportionation lies
2
[
a] Reactions carried out in toluene except for entry 4, which was carried
in the high energy of the very hindered tBu PÀPtBu product.
2
2
out in MeCN. At the end of a catalytic run, the autoclave was cooled
rapidly and the catalyst quenched by addition of an excess of P(OMe)3
See experimental details in the Supporting Information for catalysis
conditions. IR spectra measured in CH Cl . [b] The remaining product is
a mixture of 2- and 3-heptene.
[
25]
(
Similarly, the congestion in CgPÀPCg destabilizes this
[21]
product. ) The energies of all trigonal-bipyramidal R PF
2
3
isomers and key structural parameters have been included in
the Supporting Information. In cognizance of the ligand-map
information (Figure 1) and the stability predictions, we
decided to focus the catalyst testing on ligands 1–3 and
2
2
tBu PF.
the presence of rotamers, as has been observed in complexes
featuring {M(CgPH) } moieties.
2
2
[
22]
The synthesis of sym-PhobPF (2) and asym-PhobPF (3)
was straightforward from the corresponding chlorophobanes
and CsF in MeCN, and both compounds showed no tendency
towards disproportionation. In aqueous MeCN (1%) 50% of
The hydroformylation of 1-heptene [Eq. (3)] is the first
[23]
step in the Sasol process for its homologation to 1-octene.
The hydroformylation was carried out under conditions that
2
or 3 was hydrolyzed in approximately ten hours, which
makes them less hydrolytically stable than 1 but much more
stable than tBu PF (100% hydrolyzed under the same
2
conditions in less than four minutes).
Treatment
of
[Rh Cl (CO) ]
with
1
gave
2
2
4
trans-[RhCl(CO)(CgPF) ] as a mixture of rac and meso
2
[
18]
isomers (associated with the C symmetry of the CgPF); the
two well-resolved AA’MM’X patterns in the P NMR
1
3
1
spectrum of the product are evident in Figure 3. Fluorophos-
allowed comparison to be made between the performance of
phines 2, 3, and tBu PF also gave complexes of the type
the fluorophosphines and the commercial PPh analogue (see
2
3
experimental details in the Supporting Information). The
results given in Table 2 show that the catalysts derived from
1
–3 and tBu PF have comparable activities to the PPh
2
3
analogue. The n/iso ratio for 1 is a considerable improvement
3
1
on the PPh analogue. The P NMR spectra of the reaction
3
mixtures after the catalytic runs with 1 showed the presence of
a rhodium-fluorophosphine complex, and thus it is clear that
[
24]
the PÀF bond has remained intact during the catalysis.
The hydrocyanation of 3-pentenenitrile (3-PN) to give
adiponitrile (ADN, Scheme 1) is the most challenging step in
the nickel(0)-catalyzed hydrocyanation of butadiene.
[
26]
It
requires isomerization of 3-PN to the terminal isomer 4-PN
and subsequent regioselective hydrocyanation. The results
given in Table 3 show that 1 is an excellent ligand for the
isomerization–hydrocyanation catalysis, comparing favorably
with the commercial tritolyl phosphite nickel catalyst
3
1
Figure 3. P NMR spectrum of trans-[RhCl(CO)(CgPF)2].
[
27]
trans-[RhCl(CO)(R PF) ], and the n
values for these
(Table 3, entry 7) in terms of activity (with ZnCl cocata-
2
2
CO
2
complexes (see Table 2) reflect the stronger s-donor proper-
ties of ligands 2, 3, and tBu PF compared to 1. Treatment of
2
[
Ni(cod) ] (cod = 1,5-cyclooctadiene) with one to four equiv-
2
alents of 1 in toluene gave solutions that, according to the
3
1
0
P NMR spectrum, contained mixtures of Ni complexes. In
the presence of two eqivalents of 1 (as in the catalysis
described below) the product consists predominantly of two
3
1
species in a 3:1 ratio. The P NMR spectrum at ambient
temperature showed broad signals which at higher temper-
atures resolved into two characteristic AA’XX’ patterns.
These signals have been assigned to the rac and meso isomers
Scheme 1. Hydrocyanation of 3-PN. ESN=ethylsuccinonitrile,
2-MGN=2-methylglutaronitrile.
of [Ni(cod)(CgPF) ]; the fluxionality is tentatively assigned to
2
1
20
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Chemie
[
a]
Table 3: Catalytic hydrocyanation of 3-pentenenitrile.
Grubbs), Wiley-VCH, Weinheim, 2003; d) Metal-Catalyzed
Cross-Coupling Reactions (Eds.: A. de Meijere, F. Diederich),
Wiley-VCH, Weinheim, 2004.
Entry
Ligand
Lewis acid
Yield [%]
Linearity [%]
1
2
3
4
5
6
1
1
2
2
ZnCl2
83
21
13
28
4
66
85
76
52
50
52
82
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Ph BOBPh2
2
ZnCl2
2
Ph BOBPh2
tBu PF
tBu PF
ZnCl2
2
Ph BOBPh2
10
60
2
2
[
b]
7
P(O-o-Tol)3
ZnCl2
[a] See experimental details in the Supporting Information for condi-
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total dinitrile (ADN/ESN/2-MGN) product. The results with ligand 3
showed that only traces (less than 1%) of hydrocyanation products were
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[
[
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lyst, Table 3, entry 1) and selectivity (with Ph BOBPh2
2
cocatalyst, Table 3, entry 2). The fluorine substituent is
decisive for the success of ligand CgPF (1) in this catalysis
since, under the same conditions, the catalyst derived from
CgPBr or CgPPh gave negligible (lower than 3%) yields of
dinitriles.
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[
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2
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(
Scheme 1).
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5
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Published online: November 11, 2011
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Keywords: homogeneous catalysis · hydrocyanation ·
hydroformylation · phosphacycles · phosphane ligands
.
1
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121

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1
[24] The P NMR spectrum of the reaction mixture after the
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41.7 ppm, (JPF = 1144 Hz).
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