Isomerizing Methoxycarbonylation of Alkenes to Esters Using a Bis(phosphorinone)xylene Palladium Catalyst

Article abstract of DOI:10.1021/acs.organomet.6b00813

The synthesis and characterization of bulky diphosphine 1,2-bis(4-phosphorinone)xylene, BPX, and its palladium complexes [(BPX)PdCl₂] and [(BPX)Pd(O₂CCF₃)₂] are described. BPX was evaluated as a ligand in Pd-catalyzed isomerizing methoxycarbonylation. A broad range of alkenes, including terminal, internal, branched, and functionalized alkenes, can be converted to esters with activities and selectivities matching or surpassing the performance of the state-of-the-art palladium bis(di(tert-butyl)phosphino-o-xylene (Pd-DTBPX) catalyst. A molecular structure of the precatalyst [(BPX)Pd(O₂CCF₃)₂] was obtained showing a square planar geometry and a bite angle of 100.11(3)°. Rhodium carbonyl complexes [(BPX)Rh(CO)Cl] and [(DTBPX)Rh(CO)Cl] were synthesized to compare the relative electronic parameters, revealing a ν(C≡O) of 1956.8 and 1948.3 cm^-1, respectively, suggesting a reduced ability of BPX to donate electron density to the metal relative to DTBPX. Competitive protonation experiments between BPX and DTBPX in the presence of CH₃SO₃H exclusively produce [DTBPX(H)₂]²⁺, providing additional evidence that BPX is a much weaker base than DTBPX. This could be due to either the effect of the electron-withdrawing ketone group in the phosphorinone ring or the compression of the C-P-C bond angle induced by the ring structure. The ³¹P NMR (CDCl₃) chemical shift of BPX is 5.6 ppm, upfield of DTBPX at 27.6 ppm. This anomalous result is attributed to a strong gamma substituent effect of C=O in the BPX ligand. The improved activity of Pd-BPX, relative to Pd-DTBPX, could be attributed to a more electrophilic Pd^II center, which could accelerate the rate-determining methanolysis step.

Full text of DOI:10.1021/acs.organomet.6b00813

Article
pubs.acs.org/Organometallics
Isomerizing Methoxycarbonylation of Alkenes to Esters Using a
Bis(phosphorinone)xylene Palladium Catalyst
James D. Nobbs,† Choon Heng Low,† Ludger P. Stubbs,† Cun Wang,† Eite Drent,‡
and Martin van Meurs*,†
†Institute of Chemical and Engineering Sciences, Jurong Island, Singapore 627833
‡Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands
S
* Supporting Information
ABSTRACT: The synthesis and characterization of bulky
diphosphine 1,2-bis(4-phosphorinone)xylene, BPX, and its palla-
dium complexes [(BPX)PdCl2] and [(BPX)Pd(O2CCF3)2] are
described. BPX was evaluated as a ligand in Pd-catalyzed
isomerizing methoxycarbonylation. A broad range of alkenes,
including terminal, internal, branched, and functionalized alkenes,
can be converted to esters with activities and selectivities matching
or surpassing the performance of the state-of-the-art palladium
bis(di(tert-butyl)phosphino-o-xylene (Pd-DTBPX) catalyst. A mo-
lecular structure of the precatalyst [(BPX)Pd(O2CCF3)2] was
obtained showing a square planar geometry and a bite angle of
100.11(3)°. Rhodium carbonyl complexes [(BPX)Rh(CO)Cl] and
[(DTBPX)Rh(CO)Cl] were synthesized to compare the relative
electronic parameters, revealing a ν(CO) of 1956.8 and 1948.3
cm−1, respectively, suggesting a reduced ability of BPX to donate electron density to the metal relative to DTBPX. Competitive
protonation experiments between BPX and DTBPX in the presence of CH3SO3H exclusively produce [DTBPX(H)2]2+,
providing additional evidence that BPX is a much weaker base than DTBPX. This could be due to either the effect of the
electron-withdrawing ketone group in the phosphorinone ring or the compression of the C−P−C bond angle induced by the
ring structure. The 31P NMR (CDCl3) chemical shift of BPX is 5.6 ppm, upfield of DTBPX at 27.6 ppm. This anomalous result
is attributed to a strong gamma substituent effect of CO in the BPX ligand. The improved activity of Pd-BPX, relative to Pd-
DTBPX, could be attributed to a more electrophilic PdII center, which could accelerate the rate-determining methanolysis step.
successfully employed for the selective alkoxycarbonylation of
internal alkenes (e.g., octenes)1 as well as for functionalized
internal alkenes including natural plant oils (where the double
bond lies deep inside the alkyl chain) such as oleic acid,4
linoleic acid,5 and cardanols.6 It has also been shown to give
adipic acid from the hydroxycarbonylation of butadiene7 or
(bio)-derived pentenoic acids.8,9
It has been two decades since the initial discovery of Pd-
DTBPX as an efficient isomerizing alkoxycarbonylation
catalyst, and yet it still represents the state-of-the-art catalyst
for this reaction. Despite numerous attempts to discover
improved catalysts, examples of Pd-diphosphines that give a
high selectivity to the terminal ester are still extremely rare.
Only a handful of other catalysts are able to match the high
selectivities achieved with Pd-DTBPX, yet they all exhibit lower
activities. For example, the adamantyl analogue of this ligand,
bis[di(1-adamantyl)phosphino]-o-xylene (DAdPX), was re-
ported to give improved selectivity in the methoxycarbonyla-
tion of methyl oleate but a much lower activity compared with
INTRODUCTION
■
The tandem isomerizing alkoxycarbonylation of internal
alkenes to linear esters (or acids) in high selectivity is an
atom economical method of adding functionality to simple
molecules. This reaction has broad applicability since carboxylic
acids and esters are ubiquitous in many commodity chemicals
and active pharmaceutical ingredients. The state-of-the-art
isomerizing alkoxycarbonylation catalyst is palladium bis(di-
tert-butyl)phosphino-o-xylene (Pd-DTBPX).1,2 This catalyst
system is well-known because it is operated industrially in the
production of the important monomer methyl methacrylate.3
In this case Pd-DTBPX is used in the methoxycarbonylation of
ethylene to yield methyl propanoate with exceptionally high
activity (50 000 h−1) and selectivity (>99.9%).
For the methoxycarbonylation of ethylene there is obviously
no isomerization and therefore no regioselectivity issue, with
copolymerization as a possible side reaction. However, for the
methoxycarbonylation of propylene and higher olefins a
number of regioisomers can potentially be formed. However,
for linear alkenes, when Pd-DTBPX is used as the isomerizing
alkoxycarbonylation catalyst, the terminal ester is formed in
remarkably high selectivity. Indeed, DTBPX has been
Received: October 26, 2016
© XXXX American Chemical Society
A
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DTBPX.13 Similarly, Pd-diphosphine catalysts supported by
(dimethylene)cyclohexyl backbones also exhibit high linear
selectivity in the hydroxycarbonylation of pentenoic acid
mixtures to adipic acid, but comparatively low activities
compared to Pd-DTBPX.14
Common features of diphosphines that induce high
selectivity in isomerizing alkoxycarbonylation reactions are a
rigid backbone, a wide P−P bite angle (∼100°), and bulky
tertiary alkyl substituents at phosphorus.14 We now report the
novel palladium 1,2-bis(4-phosphorinone)xylene, Pd-BPX,
which satisfies these criteria (Figure 1). BPX can be viewed
and Day, wherein they noted that the condensation was highly
sensitive to the presence of trace solvent or impurities.15 The
successful condensation was indicated by the significant
downfield shift of the 31P resonance of BPX (δ = 5.6 ppm).
Interestingly, this chemical shift is upfield of the correspond-
ing signal from DTBPX (δ = 27.6 ppm). This might indicate a
higher s-character of the P lone pair, which seems counter-
intuitive in the presence of an electron-withdrawing ketone
functionality in BPX. However, 31P chemical shifts have been
reported to be strongly influenced by beta and gamma
substituent effects.19 In BPX, there is likely to be a significant
shielding effect from the gamma CO, which is gauche to the
phosphorus atom. The diastereotopic nature of the methyl
groups induced by the P lone pair and the ring conformation
gives rise to two sets of doublets in both the 1H and 13C NMR
spectra. Similarly the four methylene protons of the
phosphorinone rings are also diastereotopic. The dihedral
dependence of the two-bond 13C−31P coupling is revealed
starkly in the coupling constant of 25.2 Hz for the CH3 group
syn to the P lone pair (i.e., gauche) and close to zero for the
anti-orientation (broad singlet).20 A similar effect is observed in
1
the H−31P coupling constants for the same methyl groups,
where the values for the coupling constants are 17.0 and 5.4 Hz,
respectively. BPX has a strong absorption at 1700 cm−1 in its IR
spectrum, corresponding to the ketone groups.
Palladium(II) chloride complex [(BPX)PdCl2] was prepared
analogously to the previously reported [(DTBPX)PdCl2]
complex via the aerobic oxidative chlorination of [(BPX)Pd-
(dba)].21,22 The reaction of [(BPX)PdCl2] with two
equivalents of [Ag(O2CCF3)] gave [(BPX)Pd(O2CCF3)2] in
nearly quantitative yield. The bright orange solid was
moderately air-stable and was soluble in chlorinated solvents.
[(BPX)Pd(O2CCF3)2] has a chemical shift of 32.1 ppm in its
31P{1H} NMR spectrum in CDCl3, upfield of the correspond-
ing [(DTBPX)Pd(O2CCF3)2], at 42.3 ppm in CDCl3. Yellow
crystals of [(BPX)PdCl2] suitable for single-crystal X-ray
diffraction (SCXRD) were obtained via slow vapor diffusion
of hexane into a saturated CH2Cl2 solution of the complex. The
molecular structure is shown in the SI. Slow evaporation of
[(BPX)Pd(O2CCF3)2] in chloroform yielded orange crystals
suitable for SCXRD. The molecular structure, along with
pertinent bond lengths and angles, is shown in Figure 2.
Both complexes are four-coordinate in the solid state and
adopt slightly distorted square planar geometries reflected by
dihedral angles between the PdP2 and PdCl2/PdO2 planes of
2.42° and 15.96°, respectively. The six-membered phosphor-
inone rings exist in a chairlike conformation with the keto
group orientated away from the palladium center. In [(BPX)-
Pd(O2CCF3)2] the xylyl backbone lies out of the PdP2 plane at
an angle of 58.79°. The bite angle imposed by the phosphine
ligand in [(BPX)Pd(O2CCF3)2] is 100.11(3)°, which is
comparable to similar complexes such as [(DTBPX)Pd-
(O3SMe)2], with a bite angle of 100.59(6)°.23 A significant
difference between complexes bearing BPX and DTBPX is the
smaller C−P−C angles in the phosphorinone ring compared
with the C(tBu)−P−C(tBu) angle in DTBPX analogues
(∼110°). Thus, for [(BPX)Pd(O2CCF3)2], C18−P1−C22 is
105.99(14)° and C9−P2−C13 is 106.25(13)°. The compres-
sion in the C−P−C angles in the phosphorinone ring causes a
decrease in the σ-donating character of the P lone pair and a
concomitant increase in the π-accepting character of the
phosphorus.24
Figure 1. Diphosphines that give a high selectivity for the linear
product in Pd-catalyzed alkoxycarbonylation of alkenes. DTBPX.10,11
DAdPX.12,13
as broadly isostructural to DTBPX but with the tert-butyl
groups linked together by an electron-withdrawing ketone
group. We show herein that Pd-BPX, as a methoxycarbonyla-
tion catalyst, surpasses the activity of DTBPX, giving reaction
rates up to 6 times higher while maintaining or exceeding the
linear regioselectivity afforded by DTBPX.
RESULTS AND DISCUSSION
■
Ligand and Complex Synthesis. The BPX ligand was
synthesized from the corresponding primary diphosphine and
conjugated dienone based on a reported synthesis of phenyl
phosphorinone (Scheme 1).15 The primary diphosphine, 1,2-
bis(phosphinomethyl)benzene, was first obtained by the
reduction of the corresponding diphosphonate with a
Me3SiCl/LiAlH4 mixture.16,17 The resulting pyrophoric color-
less oil was purified by flash chromatography over neutral
alumina under an argon atmosphere, using CH2Cl2 as the
eluent. In the 31P{1H} NMR spectrum, 1,2-bis-
(phosphinomethyl)benzene possesses a chemical shift of
−125.5 ppm, consistent with similar reported primary
phosphines (e.g., benzylphosphine, δ = −120.9 ppm).18 BPX
can then be obtained by the solvent-free condensation of two
equivalents of phorone with 1,2-bis(phosphinomethyl)benzene
at 120 °C. We found that the crude reaction yield (determined
by 31P NMR spectroscopy) of BPX was in the range of 5−60%
between runs but under apparently identical reaction
conditions. This might be attributed to trace impurities
between the material used, consistent with a report by Welcher
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Scheme 1. Synthesis of BPX and Its Corresponding Rh and Pd Complexes
a
Table 1. Methoxycarbonylation of trans-4-Octene
b
olefin
yield ester
(%)
sel.
ν(CORh
)
bc
,
ligand
conversion (%)
TON (%)
(cm−1
)
DAdPX
DTBPX
BPX
8
30
85
8
30
85
75
280
800
95
94
94
1941.2
1948.3
1956.8
a
Conditions: 50 bar CO pressure, 105 °C, 20 μmol of Pd(OAc)2, 40
μmol of ligand, 0.2 mmol of CH3SO3H, 3.0 mL of trans-4-octene, 6.0
b
c
mL of MeOH, time = 4 h. Determined by GC. Total olefin
conversion to esters.
addition of 10 equiv of CH3SO3H, consistent with previous
reports.14 It should be noted that conversions may be further
improved if preformed complexes are used.21 All three
complexes gave a high selectivity to the terminal ester,
including the novel Pd-BPX (94%). Pd-BPX showed
significantly higher activity, giving a high conversion of trans-
4-octene to methyl nonanoate (85% yield; turnover number,
TON = 800) in just 4 h compared to Pd-DTBPX under the
same conditions (30% yield, TON = 280). This also suggests
that like Pd-DTBPX, Pd-BPX rapidly isomerizes the trans-4-
octene to its equilibrium mixture but that methoxycarbonyla-
tion is favored only for the terminal Pd-alkyl. Much lower yield
(8%) was obtained when the related ligand DAdPX was used,
consistent with previous reports.13,14
Figure 2. Molecular structure of [(BPX)Pd(O2CCF3)2]. Selected
bond lengths (Å) and angles (deg): Pd1−P1 2.2933(8), Pd1−P2
2.290(1), Pd1−O3 2.2127(2), Pd1−O5 2.103(2), P1−Pd1−P2
100.11(3), P1−Pd1−O3 92.50(6), P1−Pd1−O5 165.29(6), P2−
Pd1−O3 164.28(6), P2−Pd1−O5 89.91(6), O3−Pd1−O5 79.64(8),
C22−P1−C18 105.99(14) and C9−P2−C13 106.25(13).
Isomerizing Methoxycarbonylation. BPX was evaluated
as a ligand in the isomerizing methoxycarbonylation of trans-4-
octene to methyl nonanoate and compared with the structurally
related DTBPX and DAdPX (Table 1). For convenience the
complexes were formed in situ by mixing Pd(OAc)2 with two
equivalents of diphosphine in methanol, followed by the
Encouraged by these results obtained with BPX, the
methoxycarbonylation of a series of olefinic substrates was
examined for Pd-BPX and compared with Pd-DTBPX as a
benchmark (Table 2). With 1-octene, Pd-BPX again gave near-
C
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c
Table 2. Comparison of BPX and DTBPX Ligands in Pd-Catalyzed Methoxycarbonylation of Alkenes
a
b
c
Determined by GC. Total olefin conversion to esters. Conditions: 50 bar CO pressure, 105 °C, 20 μmol of Pd(OAc)2, 40 μmol of ligand, 0.2
mmol of CH3SO3H, 3.0 mL of substrate, 6.0 mL of MeOH, time = 4 h.
quantitative conversion after just 4 h. The independence of
catalytic performance of Pd-BPX from the starting octene
isomer, 4-octene (Table 1) and 1-octene (Table 2, first entry),
respectively, shows that alkene bond isomerization must be
rapid relative to the overall methoxycarbonylation rate. We also
undertook a comparative study of the isomerization of methyl
pent-4-enoate (whose isomers are readily separable by GC) for
Pd-DTBPX and Pd-BPX. Catalysts were formed in situ, and the
reaction was carried out under an atmosphere of argon (as
opposed to CO) to prevent propagation. The catalyst based on
Pd-DTBPX rapidly isomerizes 2400 equiv of methyl pent-4-
enoate to the equilibrium mixture in <15 min, whereas Pd-BPX
was found to isomerize an order of magnitude slower (see SI).
The overall methoxycarbonylation rate is still higher with Pd-
BPX (even with internal alkenes), as the rate-determining step
is not alkene isomerization but methanolysis of the Pd-acyl
species. Indeed, in the methoxycarbonylation of functionalized
internal olefins, such as methyl pent-2-enoate and pent-2-
enenitrile, Pd-BPX also gave a high yield of the linear diester
(93%) and ester-nitrile (92%), with approximately twice the
TON obtained with Pd-DTBPX. Pd-BPX outperformed Pd-
DTBPX with methyl oleate as a substrate, providing a TON of
180 in 4 h, 6 times that of Pd-DTBPX (TON = 30). The
selectivity to the linear diester, 1,19-dimethyl nonadecane-
dioate, determined by GC to be consistent between both Pd-
BPX and Pd-DTBPX, is slightly lower than literature reports
D
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for Pd-DTBPX.25 This is likely due to the use of an in situ
formed Pd catalyst system while applying slightly different
reaction conditions, such as higher substrate/MeOH ratio, a
higher temperature, and the higher CO pressure used in our
experiments compared with earlier reports. Indeed the linear
selectivity has been shown to decrease with increasing reaction
temperature and pressure.13 A comprehensive study of the
selectivity in the methoxycarbonylation of methyl oleate has
been reported by Mecking and co-workers.26 Fortunately, the
linear diester crystallizes from the reaction mixture and can
eventually be obtained in >99% purity. The carbonylation of
fatty acid derivatives, such as methyl oleate, has generated
interest, as it represents a potential route to bioderived
monomers and surfactants.4,21,27,28
We next examined the mexthoxycarbonylation of branched
alkenes (2,3-dimethylbut-1-ene, 2,4,4-trimethylpent-1-ene, and
2,4,4-trimethylpent-2-ene, Table 2). With geminally substituted
2,3-dimethylbut-1-ene, methoxycarbonylation provided exclu-
sively the terminal ester product in 20% yield (methyl 3,4-
dimethylpentanoate), with no internal ester detected. However,
isomerization to the tetrasubstituted alkene (2,3-dimethylbut-2-
ene) occurred as well as extensive acid-catalyzed hydro-
methoxylation of the alkene to the tertiary ether.29 Never-
theless, despite these undesirable side reactions, Pd-BPX was
again substantially more active than Pd-DTBPX. A similar
reactivity pattern was observed for 2,4,4-trimethylpent-1-ene,
where Pd-BPX again gave high selectivity for the terminal ester
(methyl 3,5,5-trimethylhexanoate in 10% yield) in high activity
(TON = 100), with the ether as the main byproduct. Sterically
hindered 2,4,4-trimethylpent-2-ene was also tested. Typically,
trisubstituted alkenes exhibit low reactivity in carbonylation
reactions;30 however, gratifyingly, Pd-BPX also gave superior
yields of the terminal ester (30%), compared to 8% for Pd-
DTBPX. Pd-BPX was also capable of methoxycarbonylation of
cyclic alkenes. It gave moderate conversions in the methox-
ycarbonylation of cyclohexene to give methyl cyclohexanecar-
boxylate (15%), 3 times that of Pd-DTBPX.31 Finally, Pd-BPX
was able to convert styrene (with full substrate conversion) to
the corresponding linear product in 70% selectivity with the
remainder consisting of the branched ester.
to Rh leads to less back-donation from the Rh p/d hybrid
orbitals into the CO π* orbital compared to DTBPX, which
implies BPX coordination could induce a more electrophilic
metal center. To further probe this effect, the basicities of the
diphosphine ligands were studied. Welcher and Day reported
phenyl phosphorinone as having a pKa of 4.61, much lower
than the related simple Et2PPh, with a pKa of 6.25.15 This
suggests that BPX is likely to be less basic than DTBPX. In
order to investigate their relative basicities, equimolar amounts
of DTBPX and BPX were mixed in CDCl3 followed by the
addition of half an equivalent of CH3SO3H per phosphine
1
moiety. H and 31P NMR spectra showed the quantitative
formation of [(DTBPX)(H)2]2+ at 39.6 ppm with the complete
consumption of DTBPX (δ = 27.5 ppm). At the same time the
signal for BPX was still present (δ = 5.7 ppm), though slightly
broadened. This suggests the equilibrium lies toward
[(DTBPX)(H)2]2+ rather than [(BPX)(H)2]2+; see SI Figures
S21 and S22 for NMR spectra. These results suggest that BPX
is indeed a much weaker base in comparison with DTBPX.
For [(DAdPX)Rh(CO)Cl], the 31P{1H} NMR spectrum
(CDCl3) consisted of two broad signals at 53.3 and 29.4 ppm,
respectively. Low-temperature 31P{1H} NMR (−50 °C) was
performed in an attempt to resolve the signals; however, there
was no change in the NMR spectrum. In the IR spectrum, the
lower CO stretching frequency of 1941.2 cm−1 was observed,
suggesting a weaker CO bond and increased back-donation
from the Rh to the CO π* orbital. It was recently shown that
tri(adamantyl)phosphine is significantly more electron donat-
ing than its counterpart tri(tert-butyl)phosphine, which
challenged the common assumption that the adamantyl and
tert-butyl substituents are isoelectronic.35 To probe if this effect
translated to a more basic DAdPX versus DTBPX, we
performed a second analogous competition experiment (see
SI Figure S23). In this case there was an equilibrium between
free DTBPX and DAdPX, fully protonated [(DTBPX)(H)2]2+
and [(DAdPX)(H)2]2+, and partially protonated [(DTBPX)-
H]+ and [(DAdPX)H]+, with ∼60% of the protons residing at
DAdPX and ∼40% at DTBPX. These results suggest that the
pKa values of DAdPX and DTBPX are closer than DTBPX and
BPX, but that DAdPX is slightly more basic than DTBPX.
Mechanistic studies of methyl oleate methoxycarbonylation
using Pd-DTBPX showed that nucleophilic attack of the Pd-
acyl species (methanolysis) is the selectivity- and rate-
determining step of the catalytic cycle.36,37 Computational
studies suggest this alcoholysis step probably does not involve
direct nucleophilic attack of the acyl moiety, but requires
binding of MeOH at the electrophilic Pd center, likely followed
by deprotonation of MeOH by the Pd center.38 Mecking and
co-workers have shown this step may occur through a cluster of
MeOH molecules, which create a lower energy barrier for the
deprotonation.37 Subsequent reductive elimination of the ester
produces a Pd(0) species, which is reprotonated by MeOH to
regenerate Pd−H+ species. These DFT studies suggest the
actual reprotonation of Pd(0), concertedly being generated by
reductive elimination of the methoxy ester, possibly occurs via a
methanol cluster. This is also supported by experimental
studies concerning the methoxylation of Pd-acyl in [P2Pd-CO-
CH2CH3]+ using similar diphosphines.39 A more electron-
deficient Pd-acyl species may better bind MeOH at Pd and
deprotonate MeOH; thus, this may lead to a higher alcoholysis
rate if deprotonation is the actual rate-determining step. Hence,
a more electrophilic Pd center in Pd-BPX may account for the
increased activity observed relative to Pd-DTBPX; conversely a
Electronic Properties of BPX. The electronic properties of
BPX were compared to similar diphosphines by measuring the
CO stretching frequency of the analogous [(P−P)Rh(CO)-
Cl] complexes. The complexes were prepared from the Rh
dimer [Rh(CO)2Cl]2 according to Scheme 1. The 31P NMR
spectrum (CDCl3, see Supporting Information Figure S20) of
[(BPX)Rh(CO)Cl] consisted of two doublets of doublets at
51.5 ppm (1JRh−P = 173.7 Hz, 2JP−P = 32.6 Hz, trans to Cl) and
2
13.9 ppm (1JRh−P = 127.0 Hz, JP−P = 32.1 Hz, trans to CO),
which is similar to the 31P NMR spectrum of [(DTBPX)Rh-
(CO)Cl], although in the latter case the respective one-bond
Rh−P coupling constants are slightly smaller at 170.8 and 123.2
Hz, respectively.32 Previous studies have shown correlations
between the magnitude of the one-bond Rh−P coupling
constants and the σ-donation of the trans influencing ligand.33
It is thought that coupling constants of a given metal and ligand
are a good indicator of the relative s characters of the orbitals
involved.34 The slightly larger Rh−P coupling constants
induced by BPX coordination (relative to DTBPX) may
suggest a better overlap between the 5s(Rh) and 3s(P) orbitals.
[(BPX)Rh(CO)Cl] showed a significantly higher CO
stretching frequency (1956.8 cm−1) compared to [(DTBPX)-
Rh(CO)Cl] (1948.3 cm−1). This suggests coordination of BPX
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THF (100 mL) was added dropwise, after which the reaction was
warmed to room temperature and stirred an additional 2 h. The
reaction was quenched by the slow addition of degassed deionized
water (10 mL) followed by the addition of degassed 20% aqueous
NaOH (10 mL). After the addition of MgSO4, the suspension was
filtered and the residue washed with THF (3 × 25 mL). Concentration
of the filtrate in vacuo afforded an oily emulsion, which was passed
through an alumina plug (neutral, activated), eluting with CH2Cl2.
Subsequent removal of solvent in vacuo yielded the product as a
colorless oil (3.45 g, quantitative yield). 1H NMR (400 MHz, CDCl3):
less electrophilic Pd, in the case of Pd-DAdPX, may be the
cause of the reduction in reaction rate. Additionally, the steric
environment created by the bulky BPX (isostructural to
DTBPX) is understood to be crucial in promoting the linear
selective pathway.
CONCLUSIONS
■
Pd-BPX is a highly active and selective catalyst for the
alkoxycarbonylation of a variety of alkenes. The increased
activity could pave the way for industrial applications of this
technology, as higher TONs are necessary to offset the cost of
Pd and ligand. BPX is structurally similar to the benchmark
ligand DTBPX, however, with the P atoms constrained in a six-
membered heterocycle and also containing an electron-
withdrawing ketone group. Both these features are signposts
pointing to electronic differences between BPX and DTBPX
that are responsible for the former’s increased activity, while
maintaining the excellent selectivity to terminal products, and
provide greater insight into the design and development of
future alkoxycarbonylation catalysts.
1
3
δ 7.19−7.10 (4H, m, ArH), 3.03 (4H, dt, JHP = 195.4 Hz and JHH
=
7.2 Hz, PH2), and 2.96−2.91 ppm (4H, m, PCH2). 13C{1H} NMR
(101 MHz, CDCl3): δ 139.3 (ArC), 129.6−129.2 (m, ArC), 126.7
1
4
(ArC), and 18.0 ppm (dd, JCP = 9.9 Hz and JCP = 2.9 Hz, CH2).
31P{1H} NMR (162 MHz, CDCl3): δ −125.5 ppm.
1,2-Bis(4-phosphorinone)xylene, BPX. 1,2-Bis-
(phosphinomethyl)benzene (3.40 g, 20.0 mmol) and phorone (6.25
mL, 40.0 mmol) were mixed in a flask, sealed, and heated to 120 °C
for 22 h. Upon cooling, a viscous yellow oil was formed, which
crystallized on standing. The yellow solid was triturated in MeOH for
16 h, filtered, and recrystallized from boiling MeOH to yield BPX as a
white solid (4.2 g, 47% yield). 1H NMR (400 MHz, CDCl3): δ 7.50−
7.40 (2H, m, ArH), 7.15−7.12 (2H, m, ArH), 3.27−3.22 (4H, m,
PCH2), 2.56−2.52 (4H, m, CH2), 2.39−2.31 (4H, m, CH2), 1.20
(12H, d, 3JPH = 5.4 Hz, CH3), and 1.07 ppm (12H, d, 3JPH = 17.0 Hz,
EXPERIMENTAL SECTION
■
3
CH3). 13C{1H} NMR (101 MHz, CDCl3): δ 210.0 (d, JCP = 1.1 Hz,
General Procedures. Air- or moisture-sensitive reactions were
carried out under an atmosphere of purified argon in either a glovebox
or a vacuum manifold. Compounds were stored in a nitrogen-filled
glovebox. Solvents were dried using an MBraun solvent purification
system, where the solvents were passed through oxygen and moisture
traps under an atmosphere of purified argon. Methanol was
deoxygenated by sparging with argon. CO gas was purified by passing
through oxygen and moisture traps. NMR spectra were recorded on a
Bruker 400 MHz spectrometer. The 1H chemical shifts were
referenced to residual proteo impurities in the NMR solvent used.
13C chemical shifts were referenced to the 13C chemical shift of the
NMR solvent used, whereas 31P chemical shifts were referenced
against a H3PO4 (85% in D2O) external standard. Mass spectra were
recorded on an Agilent G1969A/6210 TOF-MS. Elemental analyses
were determined using an Organic Elemental Analysis Flash 2000
CHNS/O elemental analyzer. In several cases the experimental
elemental analysis values obtained were outside the 0.4% tolerance.
Therefore, NMR spectra of all new compounds have been included
(see SI) to demonstrate the absence of detectable organic
contaminants. Crystallographic data were collected at 110 K on a
Rigaku Saturn CCD area detector with graphite-monochromated Mo
Kα radiation (λ = 0.710 73 Å).
CO), 136.9 (dd, 2JCP = 7.7 Hz, 3JCP = 2.9 Hz, ArCq), 131.3 (d, 3JCP
= 11.2 Hz, ArC), 126.4 (d, 4JCP = 1.8 Hz, ArC), 55.5 (d, 2JCP = 6.4 Hz,
1
2
CH2), 35.4 (d, JCP 19.1 Hz Cq), 31.7 (d, JCP = 25.2 Hz, CH3), 26.4
1
4
(dd, JCP 26.7 Hz and JCP = 8.7 Hz, PCH2), and 25.8 (s, CH3).
31P{1H} NMR (162 MHz, CDCl3): δ 5.6 ppm. IR (KBr): ν
̃
1700 (s)
CO str. HR-MS (+ve ESI): 447.2576 [M + H]+ calcd (C26H41O2P2)
447.2582. Anal. Calcd for C26H40O2P2: C, 69.93; H, 9.03. Found: C,
70.14; H, 8.82.
[(BPX)PdCl2]. BPX (0.93 g, 2.09 mmol) and Pd(dba)2 (1.21 g, 2.09
mmol) were suspended in CH2Cl2 (15 mL) and stirred at room
temperature for 8 h. Ethereal HCl (2.0 M, 2.1 mL, 4.19 mmol) was
added, and the mixture stirred for an additional 16 h in air. The
volatiles were removed in vacuo, and the residue was washed with
CH2Cl2 (2 × 20 mL). The combined washings were reduced in
volume, and Et2O was added to induce precipitation. The precipitate
was collected via filtration and washed with Et2O (3 × 10 mL) to yield
a yellow powder (1.04 g, 80% yield). Crystals suitable for single-crystal
X-ray diffraction were obtained by the slow diffusion of hexane into a
1
saturated solution of the complex in CH2Cl2. H NMR (400 MHz,
CDCl3): δ 7.49−7.47 (2H, m, ArH), 7.32−7.30 (2H, m, ArH), 4.78
2
4
(4H, br s, CH2), 3.37 (4H, dd, JHP = 12.4 Hz and JHP = 2.4 Hz,
3
3
Tetrabutyl (1,2-Phenylenebis(methylene))bis(phosphonate).
α,α′-Dibromo-o-xylene (19.5 g, 73.9 mmol) and tri(n-butyl)phosphite
(74.0 g, 80 mL, 296.0 mmol) were heated at 120 °C for 16 h. The
volatiles were distilled over at 120 °C under vacuum, leaving behind a
PCH2), 2.15 (2H, d, JHP = 13.1 Hz, CH2), 2.08 (2H, d, JHP = 11.9
Hz, CH2), and 1.94−1.19 ppm (24H, br s, CH3). 31P{1H} NMR (162
MHz, CDCl3): δ 26.8 ppm. IR (KBr): ν
̃
1700 (s) CO str. Anal.
Calcd for C26H40O2P2PdCl2: C, 50.06; H, 6.46. Found: C, 49.25; H,
5.81. Although these results are outside the range viewed as
establishing analytical purity, they are provided to illustrate the best
values obtained to date.
1
colorless liquid (35.0 g, 97% yield). H NMR (400 MHz, CDCl3): δ
7.25−7.21 (2H, m, ArH), 7.19−7.16 (2H, m, ArH), 3.97−3.85 (8H,
2
m, OCH2), 3.39 (4H, d, JHP = 20.3 Hz PCH2), 1.59−1.52 (8H, m,
3
OCH2CH2), 1.34−1.28 (8H, m, CH2), and 0.88 ppm (12H, t, JHH
=
[(BPX)Pd(O2CCF3)2]. [(BPX)PdCl2] (0.30 g, 0.48 mmol) and
Ag(O2CCF3) (0.21 g, 0.96 mmol) were dissolved in CH2Cl2 (20 mL),
resulting in a yellow solution that turned deep red after a few minutes.
The mixture was stirred for 16 h. The dark brown suspension was
filtered, and the residue was washed with pentane (10 mL). The
organic fractions were combined and evaporated to yield a brown solid
(0.36 g, 98% yield). Crystals suitable for single-crystal X-ray diffraction
were obtained from the slow evaporation of a saturated solution of the
7.4 Hz, CH3). 13C{1H} NMR (101 MHz, CDCl3): δ 131.6 (s, ArC),
2
131.2 (s, ArCq), 127.3 (s, ArC), 66.0 (d, JPC = 3.3 Hz, OCH2), 65.9
(d, 2JPC = 3.4 Hz, OCH2), 32.7 (d, 3JPC = 2.9 Hz, OCH2CH2), 32.6 (d,
3JPC = 3.0 Hz, OCH2CH2), 31.3 (dd, JPC = 137.6 Hz and JPC = 2.0
Hz, CH2), 18.8 (s, CH2), and 13.7 ppm (s, CH3). 31P{1H} NMR (162
MHz, CDCl3): δ 26.8 ppm. HR-MS (+ve ESI): 491.2706 [M + H]+
calcd (C24H45O6P2) 491.2686. Anal. Calcd for C24H44O6P2: C, 58.76;
H, 9.04. Found: C, 58.39; H, 8.66.
1
4
1
complex in chloroform. H NMR (400 MHz, CDCl3): δ 7.49−7.45
1,2-Bis(phosphinomethyl)benzene. CAUTION: 1,2-bis-
(phosphinomethyl)benzene is pyrophoric! Chlorotrimethylsilane (10.3
mL, 81.5 mmol) was added dropwise to LiAlH4 (3.09 g, 81.5 mmol)
suspended in THF (150 mL) at −78 °C. After complete addition, the
reaction was warmed to room temperature and stirred. After 2 h, the
reaction was cooled to −50 °C and a solution of tetrabutyl (1,2-
phenylenebis(methylene))bis(phosphonate) (10.0 g, 20.4 mmol) in
(2H, m, ArH), 7.41−7.35 (2H, m, ArH), 3.54 (4H, d, 3JHP = 13.1 Hz,
CH2), 3.38 (4H, dd, 2JHP = 12.9 and 4JHP = 3.0 Hz, PCH2), 2.20 (2H,
d, 3JHP = 13.2 Hz, CH2), 2.13 (2H, d, 3JHP = 13.0 Hz, CH2), and 1.9−
1.3 ppm (24H, m, CH3). 13C{1H} NMR (101 MHz, CDCl3): δ 207.0
(t, 3JCP = 3.0 Hz, CO), 132.9 (ArC), 131.2 (ArC), 129.1 (ArC), 54.5
1
(CH2), 41.2 (d, JCP = 18.8 Hz, CH2), 31.8 (CH3), 28.6 (CH3) and
27.0 ppm (d, 1JCP = 19.2 Hz, Cq). 31P{1H} NMR (162 MHz, CDCl3):
F
DOI: 10.1021/acs.organomet.6b00813
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
δ 32.1 ppm. IR (KBr): ν
̃
1700 (s) CO str. Anal. Calcd for
ACKNOWLEDGMENTS
■
C30H40F6O6P2Pd: C, 46.26; H, 5.18. Found: C, 45.86; H, 4.82.
[(DTBPX)Pd(O2CCF3)2]. [(DTBPX)PdCl2] (0.24 g, 0.43 mmol)
and Ag(O2CCF3) (0.19 g, 0.85 mmol) were mixed together in
CH2Cl2. After 16 h the mixture was passed through a glass filter and
reduced in volume (∼5 mL), and the orange complex precipitated
with pentane (3 × 15 mL), before being filtered, washed with Et2O,
and dried in vacuo (0.24 g, 91% yield). 1H NMR (400 MHz, CDCl3):
δ 7.36−7.29 (2H, m, ArH), 7.26−7.22 (2H, m, ArH), 3.37 (4H, dd,
We gratefully acknowledge the Singapore Science & Engineer-
ing Research Council (SERC Grant 092 159 0134) and Agency
for Science Technology & Research (A*STAR) for the funding
of this research. We would like to thank Ng Fu Song for
performing the elemental analyses. We would like to thank Dr.
Matthew Clarke (University of St. Andrews) for helpful
discussions.
4
3
2JHP = 12.9 Hz, JCP = 3.5 Hz, PCH2), and 1.54 ppm (36H, d, JCP
=
14.7 Hz, tBu). 31P{1H} NMR (162 MHz, CDCl3): δ 42.3 ppm (br s).
[(BPX)Rh(CO)Cl]. BPX (0.15 g, 0.33 mmol) and [Rh(CO)2Cl]2
(65.3 mg, 0.17 mmol) were stirred together in CH2Cl2 (10 mL). After
16 h the mixture was reduced in volume to 5 mL, and pentane (15
mL) was added to precipitate the product. The resulting solid was
washed with pentane (3 × 10 mL), and the yellow solid dried in vacuo
(0.13 g, 65% yield). 31P{1H} NMR (162 MHz, CDCl3): δ 51.5 (dd,
1JRhP = 173.7 Hz, 2JPP = 32.6 Hz) and 13.9 ppm (dd, 1JRhP = 127.0 Hz,
2JPP = 32.1 Hz). HR-MS (+ve ESI): 577.1524 [M − Cl]+ calcd
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00813.
̈
Crystallographic details and NMR spectra (PDF)
Crystallographic data for the complexes [(BPX)PdCl2]
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Martin van Meurs: 0000-0002-8816-3458
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DOI: 10.1021/acs.organomet.6b00813
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