Phosphines with tethered electron-withdrawing olefins as ligands for efficient Pd-catalyzed aryl-alkyl coupling

Article abstract of DOI:10.1021/om4004303

A group of phosphine/alkene ligands L = Ph₂P(2-RC ₆F₄) (R = CH=CHMe, CH=CHPh, CH=CHCOPh) or Ph ₂P(CH₂)₂CF=CF₂ and their [PdCl ₂L] complexes have been prepared. These phosphines are easy to prepare and fairly stable toward oxidation. Their palladium complexes feature chelated L structures with the double bond coordinated as the Z isomer, except for Ph₂P(CH₂)₂CF=CF₂, where the double bond is not coordinated and the complex is a dimer with Cl bridges. The ligands have been tested for their activity in the Negishi palladium-catalyzed aryl-alkyl coupling, where there is a competition of coupling and reduction products. Only the ligands forming chelated Pd^II complexes rise the coupling/reduction ratio to values 42/58 or higher. Of these, it is the ligand bearing the more electron-withdrawing substituent (R = CH=CHCOPh) that is the one producing remarkably high selectivity toward coupling: 90/10 under the usual Negishi conditions, and noticeably higher (97/3) if the proportion of ZnEt ₂ in the reaction is lowered. These results fit well with the hypothesis that chelating phosphines with tethered electron-acceptor olefins improve the selectivity toward coupling products mostly because they reduce the activation barrier for C-C coupling, and not because they protect the complex from β-H elimination.

Full text of DOI:10.1021/om4004303

Article
pubs.acs.org/Organometallics
Phosphines with Tethered Electron-Withdrawing Olefins as Ligands
for Efficient Pd-Catalyzed Aryl-Alkyl Coupling
Estefanía Gioria, Jesus M. Martínez-Ilarduya, Domingo García-Cuadrado, Jesus A. Miguel,
́
́
Miroslav Genov,† and Pablo Espinet*
́
IU CINQUIMA/Química Inorganica, Facultad de Ciencias, Universidad de Valladolid, 47071 Valladolid, Spain
S
* Supporting Information
ABSTRACT: A group of phosphine/alkene ligands L = Ph2P(2-
RC6F4) (R = CHCHMe, CHCHPh, CHCHCOPh) or
Ph2P(CH2)2CFCF2 and their [PdCl2L] complexes have been
prepared. These phosphines are easy to prepare and fairly stable
toward oxidation. Their palladium complexes feature chelated L
structures with the double bond coordinated as the Z isomer, except
for Ph2P(CH2)2CFCF2, where the double bond is not coordinated
and the complex is a dimer with Cl bridges. The ligands have been
tested for their activity in the Negishi palladium-catalyzed aryl-alkyl
coupling, where there is a competition of coupling and reduction products. Only the ligands forming chelated PdII complexes rise
the coupling/reduction ratio to values 42/58 or higher. Of these, it is the ligand bearing the more electron-withdrawing
substituent (R = CHCHCOPh) that is the one producing remarkably high selectivity toward coupling: 90/10 under the usual
Negishi conditions, and noticeably higher (97/3) if the proportion of ZnEt2 in the reaction is lowered. These results fit well with
the hypothesis that chelating phosphines with tethered electron-acceptor olefins improve the selectivity toward coupling products
mostly because they reduce the activation barrier for C−C coupling, and not because they protect the complex from β-H
elimination.
A few years ago we showed experimentally and calculated
theoretically why and how much olefins can reduce the
activation energy of the C−C coupling step in Pd-catalyzed
processes.4 For instance, it was calculated that the Me−Me
coupling activation barriers are about 20−23 kcal/mol in
regular square-planar tetracoordinated complexes cis-
[PdMe2(PMe3)L] (L = conventional ligand). This value goes
down to 13 kcal/mol in a three-coordinated intermediate cis-
[PdMe2(PMe3)]. Impressively, it further falls down to values as
low as 6 kcal/mol in tetracoordinated complexes cis-
[PdMe2(PMe3)(EWO)], increasing several orders of magni-
tude the coupling rate. It is not easy to conceive that an
electron-withdrawing olefin will reduce the electron density on
the metal more than the lack of one ligand, particularly in a PdII
system, where back-donation is moderate because of the high
stabilization of the d orbitals.5 In effect, our theoretical analysis
showed that the extraordinary effect of EWO, at least in Pd,
goes beyond a simple variation in electron density on PdII and
has its origin in how the EWO−Pd orbital interactions change
as the coupling rearrangement progresses, thus leading to a
large reduction of the activation energy of the process.4,6
The suggested protecting effect of EWO against β-H
elimination by blocking the coordination vacant is also an
unsatisfactory explanation for improvement of the coupling/
reduction ratio in a conflicting synthesis, because often the
INTRODUCTION
■
In the field of metal-catalyzed reactions, olefins are looked at as
reagents more often than as ligands. However, the presence of a
double bond function in the reaction mixture, whether in an
additive, as a part of the structure of a reagent, or included in
the catalyst molecule, can dramatically influence the outcome of
the process and determine the rate and selectivity of the
transformation.1,2 Olefins with electron-withdrawing groups
(electron-withdrawing olefins, EWO) are particularly important
additives in metal-catalyzed cross-coupling reactions. Although
their importance is well-known and they are present in the
panoply of the synthetic chemist, it is acknowledged that “there
is a current lack of understanding of their mode of action in
transition metal catalysis”.1 A particularly useful effect of EWO
is that they facilitate the C−C coupling (so-called reductive
elimination) step. Moreover, they do this when alkyl groups are
involved, thus preventing or diminishing the undesired β-
hydride elimination processes that lead to C−H rather than C−
C coupled products (this process is often confusingly named
“reduction” because of the product formed). The EWO effect
has been traditionally attributed to two plausible but not fully
demonstrated reasons: (i) coordination of an electron-with-
drawing olefin results in reduced electron density at the metal
center, which facilitates the reductive elimination step; and (ii)
coordination of the olefin fills the vacant coordination sites
required for β-H elimination to occur, thus inhibiting this
process.3
Received: May 15, 2013
© XXXX American Chemical Society
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addition of better coordinating ligands than olefins does not
suppress nor reduce β-H elimination in systems where EWO
do produce that effect.
Scheme 1. Synthesis of the Phosphines
Besides the above-mentioned synthetic advantages toward
coupling, there are some drawbacks when using EWO as
external coupling additives. Thus, if a substitution reaction of a
stronger conventional ligand for an external EWO is needed,
the latter has to be used in fairly high concentration in order to
produce small but kinetically significant amounts of the EWO
complex undergoing coupling. On the other hand, EWO can
produce, after coupling, highly stable Pd0 intermediates
[PdLn(EWO)], which could slow down or even preclude the
oxidative addition needed for Pd to reenter the catalytic
cycle.7,8 The use of hybrid PR2(EWO) ligands, favoring the
intramolecular chelating coordination of the olefin group,
should skip the need of a large amount of EWO for its
coordination to PdII, and perhaps the structural constrains of an
appropriate ligand might contribute to diminish the stability of
the Pd0 intermediate, thus reducing its undesired reluctance to
oxidation. Thus, a PR2(EWO) ligand might offer a good trade-
off between the pros and cons of external EWO additives. In
fact, PR2(EWO) ligands in Chart 1 have led to dramatic
Chart 1
improvement in the efficiency of C(sp2)−C(sp2) and C(sp3)−
C(sp2) couplings in Suzuki,9 and Negishi reactions,10
enhancing the selectivity of processes involving C(sp3) atoms
toward C−C coupling instead of β-hydride elimination and C−
H coupling (reduction). It was argued that coordination of the
olefin slowed down the undesired β-H elimination.
commercially available 1, followed by reaction with ClPPh2
gave phosphine 2. A second lithiation, followed by quenching
with DMF, afforded the crude phosphine-aldehyde 3, which
could be used without further purification. Wittig olefination of
3 with Ph3PCH2R afforded ligands 4−6 (with R = Me, Ph,
and COPh, respectively). Phosphine 4 is formed as a mixture of
E- and Z-isomers, which can be separated by column
chromatography. Additionally, a phosphine without the alkene
functionalization but containing the tetrafluorinated ring (7)
was synthesized from 2 by lithiation and quenching with a
water solution saturated with NH4Cl. Finally, a related
phosphine (8), with an aliphatic chain with the same number
of carbons linking P to the double bond, as the others, and a
fluorinated double bond, was synthesized by alkylation of
PPh2H.
In this study we are trying to get additional information
about the behavior of PR2(olefin) systems as experimental
support for future mechanistic and catalytic studies. The
structure of the efficient ligands in Chart 1 with R1 = Ph was
chosen, but we designed a set of related phosphines,
polyfluorinated in the functionalyzed aryl ring, which have
some practical advantages for our purposes. First, the
phosphines in this work are easier to synthesize and handle,
as the fluorinated ring makes them less sensitive to oxidation
than those in Chart 1. Second, they provide improved
solubility, which has allowed us to obtain and characterize
1
the corresponding PdII complexes by H, 19F and 31P NMR,
Synthesis and Characterization of the Palladium(II)
Complexes. Addition of phosphine 4−6 to a solution of
[PdCl2(NCMe)2] in THF (phosphine:Pd = 1:1), at room
temperature, yielded the corresponding complexes 9−11
(Scheme 2). Their NMR spectra suggest that the ligand is
and by X-ray diffraction.11 Third, the presence of F atoms in the
molecule facilitates monitoring of the reactions by 19F NMR. A
potential disadvantage of our phosphines for the stabilization of
PdII, and at the Pd0/PdII oxidation step, is that they are
expectedly somewhat less donor and more π-acceptor at the
phosphorus, but in practice this has turned out not to be a
problem, and some of them have shown to be very efficient in
catalysis. It is interesting to note that the fluorinated aryl ring is
already an electron-withdrawing olefin substituent itself.
Scheme 2. Synthesis of the PdII Complexes
RESULTS AND DISCUSSION
Synthesis of the Phosphines. The syntheses of the
phosphines are summarized in Scheme 1. Monolithiation of the
■
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bonded to PdII as a chelate:12 upon coordination, the 31P{1H}
NMR signal (multiplet, due to coupling to 19F) shows a clear
downfield shift in all cases, and the 1H NMR spectra also show
a clear shift of the olefinic protons. The IR spectra also display a
clear shift of the CC frequency (from ca. 1600 cm−1 in the
free ligands to ca. 1500 cm−1 in the complexes),13 revealing a
weakening of the double bond that is consistent with alkene
coordination. Interestingly, for R = Me, both isomers 4(Z) and
1
4(E) formed the same palladium complex 9, as seen by H
NMR. The X-ray structure of 9, discussed below, reveals that
the olefin group coordinates in the Z conformation, hence 4(Z)
coordinates directly but 4(E) needs isomerization to afford 9.
The isomerization of olefins is known to be catalyzed by PdII
and by Pd0 complexes.14,15 Interestingly, when the coordinated
phosphine 4(Z) is displaced from 9 by addition of excess of
PPh3, 4 is liberated as a mixture of E and Z isomers that, in the
presence of Pd, slowly isomerize to the most stable E isomer.
The proposed structures, containing chelated PPh2(olefin)
ligands, were unambiguously confirmed by X-ray diffraction for
9 (Figure 1) and 11 (Figure 2). Both structures feature square-
Figure 2. ORTEP of the crystal structure of complex 11. The
ellipsoids are shown at 30% probability (H atoms are omitted for
clarity). Selected distances (Å): Pd(1)−C(7) 2.189(3); Pd(1)−C(8)
2.178(3); Pd(1)−P(1) 2.2489(8); Pd(1)−Cl(1) 2.3857(8); Pd(1)−
Cl(2) 2.3107(9); C(7)−C(8) 1.381(5); C(9)−O(1) 1.222(4).
Selected angles (°): C(8)−Pd(1)−C(7) 36.87(12); C(8)−Pd(1)−
P(1) 94.72(9); C(7)−Pd(1)−P(1) 85.82(9); C(8)−Pd(1)−Cl(2)
154.81(9); C(7)−Pd(1)−Cl(2) 167.57(9); P(1)−Pd(1)−Cl(2)
87.87(3); C(8)−Pd(1)−Cl(1) 84.80(9); C(7)−Pd(1)−Cl(1)
93.93(9); P(1)−Pd(1)−Cl(1) 179.46(3); Cl(2)−Pd(1)−Cl(1)
92.46(3).
allowed us to prove unambiguously that the PdII complexes are
monomeric and the double bond is coordinated to palladium.
Some structural differences between 9 (R = Me) and 11 (R =
COPh) are noteworthy. In complex 9 the double bond shows a
nonsymmetrical coordination (Pd−C7 = 2.193(3) Å; Pd−C8 =
2.263(3) Å), whereas in complex 11, with a second electron-
withdrawing substituent in the olefin, the alkene coordination is
almost symmetrical (Pd−C7 = 2.189(3) Å; Pd−C8 = 2.178(3)
Å). Curiously the bond lengths of the coordinated double bond
are almost identical in both complexes (C7−C8 = 1.383(4) Å
for 9; C7−C8 = 1.381(5) Å for 11), in spite of their expectedly
different electronic properties. Both distances are longer than
the typical bond length of CC bonds not involved in
coordination (1.34 Å), as expected.16 A theoretical study of
different olefins coordinated to Pd suggested that nonsym-
metrical coordination,17 typically found in PdII complexes,
indicates olefins behaving on coordination as a mostly σ-
electron donor ligand from its π olefin full orbital, with
negligible back-donation from the metal to the π* olefin empty
orbital; in contrast, symmetric coordination is found in systems
involving more π-back-donation, typically in Pd0 complexes.18
Using this criterion our data would suggest that the double
bond of ligand 4 in complex 9 is a somewhat better σ donor
and worse π acceptor than the double bond of ligand 6 in
complex 11. Both components of the olefin-Pd bond, σ-
donation and π-back-donation, contribute to elongate the C
C bond and, by chance, have led in this case to similar double
bond distances in 9 and 11. It is worth reminding that electron
Figure 1. ORTEP of the crystal structure of complex 9. The ellipsoids
are shown at 30% probability (H atoms are omitted for clarity).
Selected distances (Å): Pd(1)−C(7) 2.193(3); Pd(1)−C(8) 2.263(3);
Pd(1)−P(1) 2.2370(7); Pd(1)−Cl(1) 2.3587(7); Pd(1)−Cl(2)
2.2951(7); C(7)−C(8) 1.383(4). Selected angles (°): C(7)−Pd(1)−
P(1) 85.62(8); C(7)−Pd(1)−C(8) 36.11(9); P(1)−Pd(1)−C(8)
97.23(8); C(7)−Pd(1)−Cl(2) 164.57(7); P(1)−Pd(1)−Cl(2)
87.12(3); C(8)−Pd(1)−Cl(2) 158.99(8); C(7)−Pd(1)−Cl(1)
95.05(8); P(1)−Pd(1)−Cl(1) 174.07(2); C(8)−Pd(1)−Cl(1)
86.68(8); Cl(2)−Pd(1)−Cl(1) 90.80(3).
planar coordination of Pd. Taking the midpoint of the double
bond for the olefin as the coordination position, the angles
defined by adjacent ligands in the plane are close to 90° in the
two complexes. The C7−C8 double bond lengths are longer
than the typical distances in free olefins, as expected from the
effect of coordination of the olefin to the metal. The most
remarkable feature is that the Z isomer is found in both cases,
1
confirming the structure suggested by their H NMR spectra.
These structures are important because in the previous work
with the phosphines in Chart 1, the structure of this kind of
catalyst could not be demonstrated, because of the poor
solubility of those dichloropalladium complexes. In fact, the
structure of the PdII complexes was proposed as a polymeric
one, with monodentate phosphines and uncoordinated double
bonds.10b Here, the higher solubility of our complexes has
5
π-back-donation is expectedly small for PdII compared to Pd0
complexes.19 This predicts that the coupling accelerating effects
arising from π-back-donation, which become more important as
the system is evolving from PdII toward Pd0, will show at that
C
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point stronger for 11. This is consistent with the results of
catalysis discussed below.
In contrast with ligands 4−6, which form chelates when
coordinated to PdII in 1:1 ratio, 8 coordinates only through the
phosphorus atom, affording a dimeric complex (12) with
bridging Cl atoms, whether in THF or in the crystal (eq 1).
This is supported in solution by the only very small
modification of the 19F NMR chemical shifts of the olefinic F
atoms upon coordination of the phosphine. The structure in
the solid state could be confirmed by X-ray diffraction (Figure
3). The molecule is centrosymmetric, so the atoms generated
by inversion are labeled with A.
Figure 4. ORTEP of the crystal structures of complex 13. The
ellipsoids are shown at 30% probability (H atoms are omitted for
clarity). Selected distances (Å): Pd(1)−Cl(1) 2.2903(7); Pd(1)−P(1)
2.3231(7); C(7)−C(8) 1.314(4); C(9)−O(1) 1.212(4). Selected
angles (°): Cl(1)−Pd(1)−P(1) 87.89(3); Cl(1A)−Pd(1)−P(1)
92.11(3).
prominent feature, the two phosphine ligands are acting as
monodentate, and the double bond of the EWO is not bonded
to Pd and shows an E conformation, as in the free phosphine.
The C7−C8 distance is 1.314(4) Å, within the typical range of
CC bonds, in contrast with the longer distances (ca. 1.38 Å)
observed in the structures of 9 and 11, where the double bond
is coordinated to PdII. This structure shows that, in the
presence of excess phosphine, the formation of a second Pd−P
bond is preferred to the Pd-(EWO) chelation, in spite of the
favorable entropic contribution for the latter.
Catalytic Activity. The catalytic performance of the ligands
was tested in the model reaction in eq 2. The reactions were
Figure 3. ORTEP of the crystal structure of complex 12. The
ellipsoids are shown at 30% probability (H atoms are omitted for
clarity). Selected distances (Å): Pd(1)−P(1) 2.2202(15); Pd(1)−
Cl(1) 2.4149(15); Pd(1)− Cl(1A) 2.3098(15); Pd(1)−Cl(2)
2.2636(16); Pd(1)−Cl(2) 2.3107(9); C(3)−C(4) 1.257(11). Selected
angles (°): P(1)−Pd(1)−Cl(2) 90.61(6); P(1)−Pd(1)−Cl(1A)
93.53(6); P(1)−Pd(1)−Cl(1) 174.30(7); Cl(2)−Pd(1)−Cl(1A)
175.85(6).
carried out in THF for 2 h affording, after hydrolysis, coupling
(14) and reduction (15) products, with only traces of other
compounds. All the conversions were quantified by GC
analysis. The results are given in Table 1.
By far the best performance was found for ligand 6, and in
general, the ligands able to coordinate to PdII in a chelating
mode (entries 1−5) gave better selectivity toward the coupling
product than the ligands that do not coordinate as chelates
(entries 6−9). This suggests that coordination of the double
The structure of 12 features, as expected, a dinuclear Pd
complex with monodentate phosphines and chloro bridges.
The bond distances and angles are within normal values. No
other interactions with the metal are observed, so the double
bonds of the fluorinated olefin are uncoordinated. Since ligand
8 could form, upon coordination of the double bond, a
palladacycle with the same number of links as 4−6, its different
behavior is probably due not to geometrical restrictions, but to
the fact that the double bond in 8 is a much worse σ donor than
those in 4−6, and this is not compensated by its higher π*-
acceptor nature because of the poor back-donation from PdII in
all cases. In other words, it is the σ donation that determines
the coordination ability of the double bond in PdII complexes.
This is in keeping with previous observations by Brookhart and
calculations by Ziegler.20,21
Table 1. Catalytic Results for the Et-Ar Coupling
entry
Et2Zn:ArI
phosphine
14:15
1
2
3
4
5
2.5
2.5
2.5
2.5
0.65
2.5
2.5
2.5
2.5
4(Z) (0.05)
4(E) (0.05)
5 (0.05)
6 (0.05)
6 (0.05)
7 (0.05)
7 (0.10)
7 (0.05)
8 (0.05)
63:37
49:51
42:58
90:10
97:3
a
6
0:100
15:85
29:71
15:85
7
b
Finally, the reaction of phosphine 6 with [PdCl2(NCMe)2]
in 2:1 ratio in chloroform produces trans-[PdCl2((E)-6)2] (13).
The X-ray diffraction structure of 13 is shown in Figure 4. The
Pd atom displays a trans square-planar geometry defined by two
P and two Cl atoms, with bond angles close to 90°. As the most
8
9
a
b
20% of starting material. 5 equiv of chalcone were added as external
olefin.
D
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bond has to occur at the stage of PdII in order to favor coupling
versus reduction. In fact ligands 7 (lacking a double bond
substituent) and 8 (with a poor donor double bond, as
discussed) gave only very modest enhancement of the 14/15
ratio.
coupling step. There is no indication in favor of the proposal
that PR2(EWO) protects the complexes against β-H elimi-
nation followed by reduction. In fact, one should expect higher
protection against β-H elimination in palladium complexes with
two strong PR3 ligands (such as 13) than with one hemilabile
PR2(EWO) ligand (such as 11), but the opposite effect (more
reduction) is observed. It looks more logic to think that, from a
common intermediate, when the coupling pathway becomes
faster with the help of a coordinated EWO the β-hydride
elimination pathway simply becomes kinetically less compet-
itive.
The addition of chalcone (large excess) to the monodentate
phosphine 7 was tested (chalcone would provide the olefinic
fragment missing in ligand 7 as compared to 6, but as external
cocatalyst). The use of chalcone produced some improvement
(entry 8 vs. 6 and 7), but as expected, the 14/15 ratio was still
far from the results obtained with ligand 6, or even with other
ligands containing more conventional double bonds.4,5 The
catalytic results with 4(Z) and 4(E) (entries 1 and 2) are
significantly different, in spite of the fact that the complex they
eventually form is the same, 9. This is probably due to the need
of isomerization when 4(E) is used for catalysis. It seems that
the presence of the fluorinated aryl core of the phosphines is
enough to provide some EWO character to the double bonds in
4 and 5. In fact, the selectivity toward coupling of 4 and 5 is
much better than with a simple phosphine 7 lacking olefin
(entries 5 and 6).
EXPERIMENTAL SECTION
■
General Methods. All the manipulations were performed under
an atmosphere of nitrogen using standard Schlenk techniques unless
otherwise stated. Solvents were dried using a solvent purification
system SPS PS-MD-5 or distilled from appropriate drying agents
under nitrogen, prior to use. The compounds [PdCl2(NCMe)2]23 and
2-BrC6F4PPh2 (2)24 were prepared by literature methods; all other
reagents were commercially available and used as received.
1H, 13C{1H}, 19F, and 31P{1H} spectra were recorded on a Bruker
AV-400 or a Varian Inova 500 spectrometer. Chemicals shifts (in δ
units, parts per million) were referenced to the residual solvent signal,
to CFCl3 and to 85% H3PO4, respectively. The spectral data were
recorded at 293 K unless otherwise noted. GC−mass spectra were
recorded on a Thermo Scientific Focus DSQII system. Elemental
analyses were performed on a Perkin-Elmer 2400B CHN analyzer.
2-(Diphenylphosphino)-3,4,5,6-tetrafluorobenzaldehyde
In addition to the results in Table 1, if the catalysis is carried
out using complex trans-[PdCl2((E)-6)2] (13), the 14/15 ratio
obtained is 19/81 (cf. the result reported by Lei for PPh3, 39/
61).10a These bad results confirm that chelation via double
bond is not operating in solution when the Pd center has two P
donor atoms available to coordination.
n
(3). BuLi 1.6 M in hexanes (1.51 mL, 2.42 mmol) was added to a
Although in detail interpretation of the catalytic activity of
the different ligands is not possible in these systems, as it
depends on a combination of electronic and steric factors
further complicated by possible double bond isomerizations, in
general it seems that a prerequisite for a ligand to induce good
coupling/reduction ratios is a sufficiently efficient coordination
of the olefin to PdII, which mostly depends on the σ donor
ability of the double bond and on sterics. On the other hand, as
the molecule evolves toward the coupling transition state
(hence toward the precursor of the Pd0 and R−R′ coupling
products), it is the growing π-back-donation from Pd that
matters and takes the main control of the coupling barrier.4
Finally, a synthetically interesting finding was that diminish-
ing the proportion of zinc reagent from 2.5 to 0.65 (Table 1,
entry 5) produced a notable increase of selectivity toward the
coupling product. Total conversion of the reagents was
observed, indicating that the second ethyl group of ZnEt2
had also been transferred to Pd and used in the reaction.
Besides that, the coupling/reduction ratio went up to 97/3,
instead of 90/10, although with formation of a small amount of
the homocoupling biaryl product.22 This is, no doubt, an
interesting improvement in atom economy of the process, with
a bonus in the selectivity of the reaction.
In conclusion, we have synthesized a group of phosphine/
alkene ligands and have tested their activity in the Negishi Pd-
catalyzed aryl-alkyl coupling. The ligands acting as mono-
dentate P-donors give poor coupling/reduction ratios, about
15/85. Only the ligands able to coordinate in a chelating mode
to PdII increase this ratio to values 42/58 or higher. Among the
latter, it is the ligand bearing the more electron-withdrawing
substituent, 6, the one that produces the best coupling/
reduction selectivity: 90/10, or higher (97/3) if the proportion
of ZnEt2 in the reaction is lowered. These results fit well in the
hypothesis that the selectivity toward the coupling product
increases using PR2(EWO) ligands because the coordination of
the EWO fragment reduces the activation energy of the
solution of 2 (1.00 g, 2.42 mmol) in dry ether (35 mL) at −78 °C.
The solution was stirred for 15 min, and then dry DMF (376 μL, 4.84
mmol) was added, and the resulting solution was further stirred for 1 h
at the same temperature. The solution was allowed to warm up to −50
°C, and a deoxygenated saturated NH4Cl solution in water was added
(50 mL). The mixture was extracted with ether (2 × 50 mL), and the
organic layer was dried over MgSO4. The volatiles were removed, and
the resulting oily yellow residue 3 (0.72 g, 82%) was used in the next
steps without further purification. 1H NMR (400.13 MHz, δ, CDCl3):
10.69 (d, J = 6.0 Hz, 1H), 7.46−7.31 (m, 10H). 19F NMR (376.46
MHz, δ, CDCl3): −121.42 (m, 1F), −143.22 (m, 1F), −145.13 (m,
1F), −150.89 (m, 1F). 31P{1H} NMR (161.97 MHz, δ, CDCl3):
−17.02 (ddd, J = 12.1, 3.6, 1.7 Hz, 1P). Anal. Calcd for C19H11F4OP:
C, 62.99; H, 3.06. Found: C, 61.65; H, 3.11.
(E)- and (Z)-Diphenyl(2,3,4,5-tetrafluoro-6-(prop-1-enyl)-
n
phenyl)phosphine (4(E) and 4(Z)). BuLi 1.6 M in hexanes (0.45
mL, 0.72 mmol) was added to a suspension of ethyltriphenylphos-
phonium iodide (0.30 g, 0.72 mmol) in dry THF (20 mL) at 0 °C and
was allowed to warm up to room temperature. The resulting solution
was added over a solution of 3 (0.24 g, 0.66 mmol) in dry THF (10
mL), and stirred for 2 h. A deoxygenated NH4Cl saturated aqueous
solution was added (20 mL), and the mixture was extracted with ether
(2 × 20 mL). The organic layer was dried over MgSO4, and the
volatiles were removed. The residue was purified by chromatography
(SiO2, hexane) giving two fractions as white solids. The first fraction
was 4(E): 39.4 mg, 16% overall yield from 3. 1H NMR (400.13 MHz,
δ, CDCl3): 7.40−7.33 (m, 10H), 6.80 (dm, J = 16.1 Hz, 1H), 6.12
(dqm, J = 16.1, 6.8 Hz, 1H), 1.89 (d, J = 6.8 Hz, 3H). 19F NMR
(376.46 MHz, δ, CDCl3): −122.81 (m, 1F), −141.12 (m, 1F),
−152.83 (m, 1F), −157.16 (m, 1F). 31P{1H} NMR (161.97 MHz, δ,
CDCl3): −17.86 (ddd, J = 14.3, 6.7, 3.8 Hz, 1P). Anal. Calcd for
C21H15F4P: C, 67.38; H, 4.04. Found: C, 67.12; H, 4.13. The second
fraction was 4(Z): 49.6 mg, 20% overall yield from 3. 1H NMR
(400.13 MHz, δ, CDCl3): 7.42−7.32 (m, 10H), 6.41 (dm, J = 11.2 Hz,
1H), 6.01 (dqt, J = 11.2, 7.0, 1.8 Hz, 1H), 1.44 (dm, J = 7.0 Hz, 3H).
19F NMR (376.46 MHz, δ, CDCl3): −124.34 (m, 1F), −137.91 (m,
1F), −152.83 (m, 1F), −156.15 (m, 1F). 31P{1H} NMR (161.97 MHz,
δ, CDCl3): −16.88 (ddd, J = 15.0, 7.5, 3.9 Hz, 1P). Anal. Calcd for
C21H15F4P: C, 67.38; H, 4.04. Found: C, 67.21; H, 3.90.
E
dx.doi.org/10.1021/om4004303 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(E)-Diphenyl(2,3,4,5-tetrafluoro-6-styrylphenyl)phosphine
(5). Following the procedure for the Wittig reaction described for 4,
and using benzyltriphenylphosphonium bromide (0.243 g, 0.56 mmol)
and 3 (0.185 g, 0.51 mmol), 5 was obtained, after column
[PdCl2((Z)-4)] (9). [PdCl2(NCMe)2] (7.0 mg, 0.027 mmol) and 4
(Z or E) (10.1 mg, 0.027 mmol) reacted to give 9 as a yellow solid
(10.0 mg, 76%). H NMR (400.13 MHz, δ, CDCl3): 8.06 (m, 2H),
7.72 (m, 1H), 7.62 (m, 2H), 7.58−7.33 (m, 6H), 7.00 (dm, J = 8.6 Hz,
1H), 1.47 (ddd, J = 6.6, 2.5. 1.0 Hz, 3H). 19F NMR (376.46 MHz, δ,
CDCl3): −122.70 (m, 1F), −135.11 (m, 1F), −142.37 (m, 1F),
−147.88 (m, 1F). 31P{1H} NMR (161.97 MHz, δ, CDCl3): 44.13 (m,
1P). Anal. Calcd for C21H15Cl2F4PPd: C, 45.72; H, 2.74. Found: C,
45.89; H, 2.48.
1
1
chromatography (SiO2, hexane), as a white solid (105 mg, 47%). H
NMR (400.13 MHz, δ, CDCl3): 7.74 (dd, J = 16.6, 4.6 Hz, 1H), 7.55−
7.30 (m, 15H), 7.11 (d, J = 16.6 Hz, 1H). 13C{1H} NMR (125.67
MHz, δ, CDCl3): 137.4 (ddd, J = 11.2, 4.2, 1.6 Hz), 119.9 (dm, J =
28.8 Hz) (olefinic carbons). 19F NMR (376.46 MHz, δ, CDCl3):
−122.10 (m, 1F), −139.75 (m, 1F), −152.41 (m, 1F), −155.95 (m,
1F). 31P{1H} NMR (161.97 MHz, δ, CDCl3): −18.06 (ddd, J = 14.0,
5.7, 3.6 Hz, 1P). Anal. Calcd for C26H17F4P: C, 71.56; H, 3.93. Found:
C, 71.70; H, 3.80.
[PdCl2((Z)-5)] (10). [PdCl2(NCMe)2] (8.0 mg, 0.031 mmol) and 5
(13.4 mg, 0.031 mmol) reacted to give 10 as a yellow solid (12.0 mg,
1
72%). H NMR (400.13 MHz, δ, CDCl3): 8.33 (d, J = 9.2 Hz, 1H),
7.51−7.30 (m, 8H), 7.21 (m, 2H), 7.12 (m, 1H), 7.06 (d, J = 9.2 Hz,
1H), 7.02 (m, 2H), 6.94 (m, 2H). 13C{1H} NMR (125.67 MHz, δ,
CDCl3): 115.2 (s), 94.3 (s) (olefinic carbons). 19F NMR (376.46
MHz, δ, CDCl3): −121.78 (m, 1F), −135.20 (m, 1F), −142.49 (m,
1F), −147.39 (m, 1F). 31P{1H} NMR (161.97 MHz, δ, CDCl3): 43.73
(m, 1P). Anal. Calcd for C26H17Cl2F4PPd: C, 50.88; H, 2.79. Found:
C, 50.63; H, 3.32.
(E)-3-(2-(Diphenylphosphino)-3,4,5,6-tetrafluorophenyl)-1-
phenylprop-2-en-1-one (6). (2-Oxo-2-phenylethyl)-
triphenylphosphonium ylide (0.205 g, 0.54 mmol) was added to a
solution of 3 (0.149 g, 0.41 mmol) in dry THF (25 mL), and the
resulting mixture was stirred 2 h. A deoxygenated saturated NH4Cl
aqueous solution was added (20 mL), and the mixture was extracted
with ether (2 × 20 mL). The organic layer was dried over MgSO4, and
the volatiles were removed. The residue was purified by column
chromatography (SiO2, hexane/ether, 9.6/0.4) to give 6 as a white
solid (102 mg, 53%). 1H NMR (400.13 MHz, δ, CDCl3): 8.36 (dd, J =
16.1, 4.8 Hz, 1H), 7.95−7.92 (m, 2H), 7.59 (m, 1H), 7.50−7.40 (m,
3H), 7.38−7.33 (m, 10H). 13C{1H} NMR (125.67 MHz, δ, CDCl3):
134.9 (dm, J = 28.0 Hz), 130.2 (ddd, J = 11.0, 4.4, 1.1 Hz) (olefinic
carbons). 19F NMR (376.46 MHz, δ, CDCl3): −121.31 (m, 1F),
−136.96 (m, 1F), −151.58 (m, 1F), −151.97 (m, 1F). 31P{1H} NMR
(161.97 MHz, δ, CDCl3): −17.42 (ddd, J = 12.4, 5.0, 3.9 Hz, 1P).
Anal. Calcd for C27H17F4OP: C, 69.83; H, 3.69. Found: C, 69.65; H,
3.49.
[PdCl2((Z)-6)] (11). [PdCl2(NCMe)2] (40.0 mg, 0.154 mmol) and
6 (74.3 mg, 0.160 mmol) reacted to give 11 as a yellow solid (80.0 mg,
1
81%). H NMR (400.13 MHz, δ, CDCl3): 7.98 (m, 2H), 7.81 (d, J =
9.6 Hz, 1H), 7.65 (m, 2H), 7.58−7.46 (m, 3H), 7.44−7.18 (m, 9H).
13C{1H} NMR (125.67 MHz, δ, CDCl3): 100.9 (bs), 98.5 (s) (olefinic
carbons). 19F NMR (376.46 MHz, δ, CDCl3): −124.03 (m, 1F),
−135.79 (m, 1F), −143.78 (m, 1F), −148.68 (m, 1F). 31P{1H} NMR
(161.97 MHz, δ, CDCl3): 44.30 (bs, 1P). Anal. Calcd for
C27H17Cl2F4OPPd: C, 50.53; H, 2.67. Found: C, 50.70; H, 2.53.
[PdCl2(8)]2 (12). [PdCl2(NCMe)2] (11.0 mg, 0.042 mmol) and 8
(12.5 mg, 0.042 mmol) reacted to give 12 as an orange solid (15.0 mg,
1
75%). H NMR (400.13 MHz, δ, CDCl3): 7.75 (m, 8H), 7.58 (m,
4H), 7.48 (m, 8H), 2.68−2.42 (m, 8H). 19F NMR (376.46 MHz, δ,
CDCl3): −103.87 (dd, J = 83.1, 32.8 Hz, 1F), −122.13 (ddm, J =
114.4, 83.1 Hz, 1F), −175.73 (ddt, J = 114.4, 32.8, 24.4 Hz, 1F).
31P{1H} NMR (161.97 MHz, δ, CDCl3): 29.28 (bs, 1P). Anal. Calcd
for C32H28Cl4F6P2Pd2: C, 40.75; H, 2.99. Found: C, 40.43; H, 3.32.
trans-[PdCl2((E)-6)2] (13). To a solution of [PdCl2(NCMe)2]
(19.5 mg, 0.075 mmol) in CHCl3 (25 mL), 6 (69.8 mg, 0.15 mmol)
was added, and the mixture was stirred for 2 h. The solvent was
evaporated, and cold MeCN was added. The precipitate was filtered
and washed with ether giving a yellow solid (66.5 mg, 80%). 1H NMR
(400.13 MHz, δ, CDCl3): 7.96 (m, 8H), 7.75 (m, 4H), 7.68 (d, J =
15.9 Hz, 2H), 7.53 (m, 2H), 7.44−7.31 (m, 16H), 7.07 (dd, J = 15.9,
1.9 Hz, 2H). 19F NMR (376.46 MHz, δ, CDCl3): −121.26 (m, 1F),
−136.35 (m, 1F), −149.63 (m, 1F), −152.37 (m, 1F). 31P{1H} NMR
(161.97 MHz, δ, CDCl3): 16.35 (s, 1P). Anal. Calcd for
C54H34Cl2F8O2P2Pd: C, 58.64; H, 3.10. Found: C, 58.75; H, 3.45.
General Procedure for the Catalysis. [PdCl2(NCMe)2] (3.89
mg, 0.015 mmol), the phosphine ligand (0.015 mmol) and dry THF
(1 mL) were introduced in a “flame-dried” Schlenk tube under argon
and stirred for 5 min. Then ethyl-2-iodobenzoate (51.2 μL, 0.300
mmol) and 1 M solution of ZnEt2 in hexane (0.75 mL, 0.75 mmol)
were added, and the resulting mixture was further stirred for 2 h. The
mixture was carefully hydrolyzed with a 2 M HCl solution and
extracted with Et2O. The organic layer was dried over MgSO4, filtered
through a short pad of silica and analyzed with GC. As an example the
isolated yield of 14 in entry 4 was 72%.
n
Diphenyl(2,3,4,5-tetrafluorophenyl)phosphine (7). BuLi 1.6
M in hexanes (185 μL, 0.30 mmol) was added to a solution of 2 (122
mg, 0.30 mmol) in dry ether (8 mL) at −78 °C. The solution was
stirred for 15 min at that temperature, and then a deoxygenated
saturated NH4Cl aqueous solution was added (5 mL). The resulting
mixture was allowed to warm up to room temperature and then was
extracted with ether (2 × 8 mL). The organic layer was dried over
MgSO4. The volatiles were removed to give a white solid (70 mg,
1
71%). H NMR (400.13 MHz, δ, CDCl3): 7.44−7.30 (m, 10H), 6.41
(m, 1H). 19F NMR (376.46 MHz, δ, CDCl3): −129.89 (m, 1F),
−138.63 (m, 1F), −154.06 (m, 1F), −155.21 (m, 1F). 31P{1H} NMR
(161.97 MHz, δ, CDCl3): −17.74 (d, J = 48.0 Hz, 1P). Anal. Calcd for
C18H11F4P: C, 64.68; H, 3.32. Found: C, 64.79; H, 3.53.
Diphenyl(3,4,4-trifluorobut-3-enyl)phosphine (8). nBuLi 1.6
M in hexanes (838 μL, 1.34 mmol) was added to a solution of
diphenylphosphine (232 μL, 1.34 mmol) in dry THF (7 mL) at −35
°C. The resulting solution was stirred for 1 h at that temperature and
then allowed to warm up to 0 °C. Then the reaction mixture was
cooled down to −35 °C, and a solution of 4-bromo-1,1,2-trifluorobut-
1-ene (253 mg, 154 μL, 1.34 mmol) in dry THF (7 mL) was added.
The resulting mixture was allowed to warm up to room temperature. A
deoxygenated saturated NH4Cl aqueous solution was added (10 mL),
and the mixture was extracted with ether (2 × 15 mL). The organic
layer was dried over MgSO4. The volatiles were removed, and the
crude residue was purified by column chromatography (SiO2, hexane/
ether, 9/1) to give 8 as a white solid (177 mg, 45%). 1H NMR (400.13
MHz, δ, CDCl3): 7.46−7.34 (m, 10H), 2.47−2.24 (m, 4H). 19F NMR
(376.46 MHz, δ, CDCl3): −105.66 (dd, J = 87.2, 32.1 Hz, 1F),
−124.03 (dd, J = 114.2, 87.2 Hz, 1F), −175.20 (ddtd, J = 114.2, 32.1,
21.3, 3.5 Hz, 1F). 31P{1H} NMR (161.97 MHz, δ, CDCl3): −16.90 (s,
1P). Anal. Calcd for C16H14F3P: C, 65.31; H, 4.80. Found: C, 65.48;
H, 4.62.
X-ray Crystal Structure Analysis. Single crystals of 9·CH2Cl2, 11
and 12 suitable for X-ray diffraction studies were obtained from slow
diffusion of diethyl ether or hexane into a dichloromethane solution of
the products. Crystals from 13 were grown from slow diffusion of
hexane into a THF solution of the product at −20 °C. Data collection
was performed in an Oxford Diffraction Super Nova diffractometer
with a Mo microfocus source with multilayer optics. Data integration,
scaling and empirical absorption correction were carried out using the
CrysAlisPro program package.25 The structure was solved using direct
methods and refined by Full-Matrix-Least-Squares against F2 with
SHELXTL.26 The non-hydrogen atoms were refined anisotropically,
and hydrogen atoms were placed at idealized positions and refined
using the riding model. Crystallographic data (excluding structure
General Procedure for the Synthesis of the Complexes 9−
11. The corresponding phosphine was added to a 0.035 M solution of
[PdCl2(NCMe)2] in THF (phosphine:Pd molar ratio = 1:1). The
reaction mixture was stirred for 5 h. The volatiles were evaporated, and
hexane was added. The precipitate was filtered and washed with
hexane and pentane.
F
dx.doi.org/10.1021/om4004303 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
factors) for the structures reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publications with the deposition numbers CCDC-937789 for 9,
CCDC-937791 for 11, CCDC-937792 for 12, and CCDC-937792 for
13. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam. A table with the
crystal data and structure refinements is provided in the Supporting
Information.
(8) Espinet, P.; Echavarren, A. Angew. Chem., Int. Ed. 2004, 43,
4704−4734.
(9) Williams, D. B. G.; Shaw, M. L. Tetrahedron 2007, 63, 1624−
1629.
(10) (a) Luo, X.; Zhang, H.; Duan, H.; Liu, Q.; Zhu, L.; Zhang, T.;
Lei, A. Org. Lett. 2007, 9, 4571−4574. (b) Zhang, H.; Luo, X.;
Wongkhan, K.; Duan, H.; Li, Q.; Zhu, L.; Wang, J.; Batsanov, A. S.;
Howard, J. A. K.; Marder, T. B.; Lei, A. Chem.Eur. J. 2009, 15,
3823−3829.
(11) Solubility problems prevented Lei and co-workers from fully
ASSOCIATED CONTENT
* Supporting Information
■
characterizing their PdII catalyst in ref 10b.
S
(12) See, for example: (a) Zehnder, M.; Neuburger, M.; Schaffner, S.;
Jufer, M.; Plattner, D. A. Eur. J. Inorg. Chem. 2002, 1511−1517.
(b) Shintani, R.; Duan, W.-L.; Nagano, T.; Okada, A.; Hayashi, T.
Angew. Chem., Int. Ed. 2005, 44, 4611−4614.
(13) Grogan, M. J.; Nakamoto, K. J. Am. Chem. Soc. 1968, 90, 918−
922.
(14) Sen, A.; Lai, T.-W. Inorg. Chem. 1981, 20, 4036−4038.
(15) Canovese, L.; Santo, C.; Visentin, F. Organometallics 2008, 27,
3577−3581.
(16) In fact, in the published X-ray structure of trans-[PdCl2L2] (L =
ligand in Chart 1 with R1 = Ph and R2 = COPh), where the double
bonds of ligand L are not coordinated, the C7−C8 distance is 1.330 Å
(see ref 10b).
Tables and CIF files giving complete characterization data are
included. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: espinet@qi.uva.es.
■
Present Address
†Sealife Pharma GmbH, Technopark I/Geb.B/EG, 3430 Tulln,
Austria.
Notes
(17) Zhao, H.; Ariafard, A.; Lin, Z. Inorg. Chim. Acta 2006, 359,
3527−3534.
The authors declare no competing financial interest.
(18) Note that the higher electronic similarity of the two olefin
substituents in 11 should contribute to a more symmetrical
coordination, according to the Dewar−Chatt−Duncanson model.
See, for instance: Stahl, S. S.; Thorman, J. L.; de Silva, N.; Guzei, I. A.;
Clark, R. W. J. Am. Chem. Soc. 2003, 125, 12−13.
(19) For instance, in the reported Pd0 structure of [PdL2] (L = ligand
in Chart 1 with R1 = Ph and R2 = COPh), the coordinated CC bond
distance increases to 1.406(4) Å (see ref 10b).
(20) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996,
118, 2436−2448.
(21) (a) Margl, P. M.; Deng, L.; Ziegler, T. Organometallics 1998, 17,
933−946. (b) Margl, P. M.; Deng, L.; Ziegler, T. J. Am. Chem. Soc.
1998, 120, 5517−5525.
ACKNOWLEDGMENTS
Financial support is gratefully acknowledged from the Spanish
MEC (DGI, Grant CTQ2010-18901/BQU; Juan de la Cierva
■
́
contract to D.G.C.), the Junta de Castilla y Leon (Projects
GR169, VA281A11-2 and VA373A11-2) and European
Commission (2010-2401/001-001-EMA2; EU Mobility Pro-
gram − EADIC II Erasmus Mundus Scholarship to E.G.).
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G
dx.doi.org/10.1021/om4004303 | Organometallics XXXX, XXX, XXX−XXX