Following palladium catalyzed methoxycarbonylation by hyperpolarized NMR spectroscopy: A: para hydrogen based investigation

Article abstract of DOI:10.1039/c7cy00252a

Pd(OTf)₂(bcope) is shown to react in methanol solution with diphenylacetylene, carbon monoxide and hydrogen to produce the methoxy-carbonylation product methyl 2,3 diphenyl acrylate alongside cis- and trans-stilbene. In situ NMR studies harnessing the parahydrogen induced polarization effect reveal substantially enhanced ¹H NMR signals in both protic and aprotic solvents for a series of reaction intermediates that play a direct role in this homogeneous transformation. Exchange spectroscopy (EXSY) measurements reveal that the corresponding CO adducts are less reactive than their methanol counterparts.

Full text of DOI:10.1039/c7cy00252a

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Following palladium catalyzed
methoxycarbonylation by hyperpolarized NMR
spectroscopy: a parahydrogen based
investigation†
Cite this: DOI: 10.1039/c7cy00252a
Dexin Guan,ab A. Jonathan Holmes,a
ac
a
*
Joaquín López-Serrano
and Simon B. Duckett
PdIJOTf)2IJbcope) is shown to react in methanol solution with diphenylacetylene, carbon monoxide and hy-
drogen to produce the methoxy-carbonylation product methyl 2,3 diphenyl acrylate alongside cis- and
trans-stilbene. In situ NMR studies harnessing the parahydrogen induced polarization effect reveal substan-
tially enhanced 1H NMR signals in both protic and aprotic solvents for a series of reaction intermediates
that play a direct role in this homogeneous transformation. Exchange spectroscopy (EXSY) measurements
reveal that the corresponding CO adducts are less reactive than their methanol counterparts.
Received 10th February 2017,
Accepted 20th April 2017
DOI: 10.1039/c7cy00252a
rsc.li/catalysis
For a number of years, the parahydrogen (p-H2)8 induced
Introduction
polarization (PHIP)9,10 effect has been used to yield mecha-
The alkoxycarbonylation of vinyl acetate reflects an important
step in the production of alkyl lactate that is subsequently
used to prepare a range of hydroxypropionic acid esters. Im-
portantly, esters of this type form biodegradable polymers
and their production therefore reflects a green solution to
waste disposal.1 High molecular weight thermoplastics, and
the production of methylpropanoate,2 reflect other high value
products that are formed on an industrial scale via this reac-
tion, which is most clearly illustrated by Lucite's Alpha pro-
cess, one of the many successes of palladium catalysis. In ad-
dition, the alkoxycarbonylation of 1-alkynes finds a further
role in the formation of unsaturated carboxylic acids that fea-
ture as building blocks in a range of situations.3,4 Conse-
quently, these developments illustrate just how critical early
studies on the mechanism of the methoxycarbonylation of al-
kenes,2,5 and more generally the palladium catalysed hydro-
formylation of alkenes6,7 were for the development of an array
of new clean technologies.3
nistic insight into a range of catalytic hydrogenation and
hydroformylation reactions besides monitoring the formation
of metal hydride based products.11,12 These studies employ
the PHIP effect to increase the 1H NMR signal strengths of re-
action products that contain protons which were originally lo-
cated in a molecule of p-H2 to achieve this goal. The potential
signal improvement on a 400 MHz NMR spectrometer, the
workhorse of many academic and industrial facilities is dra-
matic at 32 000-fold and while it has only been reached for
one system,13 when more widely applied the signal gains
have still allowed the detection of true reaction intermediates
that exist in such low concentrations as to preclude their de-
tection by traditional NMR methods.11,14,15 In addition, as
PHIP enables the analogous detection of scalar coupled
heteronuclei through polarisation transfer, the reliable char-
acterisation of such species is possible16 and when this ap-
proach is coupled with exchange spectroscopy (EXSY) experi-
ments their kinetic significance can be determined.17–20
Workers have also used this method to examine not only a
range of heterogeneous reactions21–25 but some that don't in-
volve a metal centre.26–29 The PHIP approach has therefore
developed substantially from the early starting point of
Weitekamp,30,31 Eisenberg32,33 and Bargon,34,35 as illustrated
in several reviews.11,12
a Centre for Hyperpolarisation in Magnetic Resonance, Department of Chemistry,
Department of Chemistry, University of York, York, YO10 5NY, UK.
E-mail: simon.duckett@york.ac.uk
b Department of Chemistry, South University of Science and Technology of China,
Shenzhen, Guangdong Province, 518055, China
c Instituto de Investigaciones Químicas-Departamento de Química Inorgánica,
Centro de Innovación en Química Avanzada (ORFEO-CINQA), Universidad de
Sevilla-Consejo Superior de Investigaciones Científicas, Calle Américo Vespucio
49, Seville, 41092, Spain
In order to understand the physical basis of PHIP, p-H2
needs to be recognised as simply dihydrogen that exists in
the anti-symmetric magnetic state that is represented by the
36
nuclear wave function Ψ{αβ–βα}
.
It is easy to prepare, and
† Electronic supplementary information (ESI) available: Procedures and NMR
spectra. See DOI: 10.1039/c7cy00252a
when a molecule of it is introduced into a reaction product,
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in high magnetic field, if the two newly formed hydrogen nu-
clei become chemically distinct then, in the absence of relax-
ation, they exist initially in two equally populated, and
coupled, αβ and βα states.8 Consequently when such a reac-
tion is probed by NMR spectroscopy, a non-Boltzmann spin
distribution is created across their NMR addressable energy
levels. Hence, if these nuclei correspond to those of a
dihydride product, with two distinct hydride resonances, 1H
NMR signals result that are of dramatically enhanced ampli-
tude, with anti-phase components that are separated by their
mutual scalar coupling.11,30 In contrast, when a monohydride
complex is formed indirectly from such a dihydride species it
has proven possible to detect its single hydride resonance as
an enhanced-emission signal as a consequence of the related
one-proton-PHIP effect described by Eisenberg.37 Further-
more, when such a reaction is monitored in conjunction with
the only parahydrogen spectroscopy (OPSY) approach it is
possible to harness the magnetic state of the two p-H2 de-
rived protons to readily separate their signals from those of
other protons that arise from what would be referred to, as
normal, or thermally polarised resonances.38,39
(1, where PP′
phosphabicycloij3.3.1]nonyl)ethane)
=
bcope, which is 1,2-P,P′-bisIJ9-
strong
signals
for
[PdIJCHPhCH2Ph)IJbcope)]OTf (2) were detected with
diphenylacetylene and p-H2. The addition of the Lewis base
pyridine (py) enabled the trapping, and hence detection, of
vinyl containing [PdIJCPhCHPh)IJbcope)IJpy)]OTf (3-py) and
the monohydride [PdIJH)IJbcope)IJpy)]OTf (4-py) of Scheme 1.20
These complexes then undergo slow pyridine loss such that
the system maintains hydrogenation activity. A DFT study
considered the role of neutral versus cationic pathways within
this reaction and supported a cationic route that is based
around a monohydride intermediate in accordance with ex-
perimental results.57
Related reactions involving the aqua adduct, [PdIJbcope)-
IJOH2)2]IJOTf)2, with H2 and CO, have also been reported by
Baya, et al.58 They saw the formation of the mixed hydrido
carbonyl complex [[(bcope)Pd]2IJμ-H)IJμ-CO)]ijOTf ] (5), along-
side the dimer [Pd2IJbcope)2IJCO)2]IJOTf)2. In contrast, the
trimer [Pd3IJbcope)3IJμ3-H)2]IJOTf)2 proved to form upon reac-
tion with H2 alone. These products form as a direct result of
the low stability of the corresponding palladium mono-
hydride species.46
In this study, we use PdIJOTf)2IJbcope) (1) to drive the
methoxycarbonylation of diphenylacetylene, and both 13CO
and C6D5–13CC–C6D5 are employed to aid in the characteri-
zation of a series of reaction intermediates that are seen
through the use of p-H2 under mild conditions. A series of
1D-EXSY studies are completed to probe, directly, the reactiv-
ity of these species, with GC/MS measurements being used to
confirm the identity of the organic reaction products. As a
consequence we improve on our understanding of the role
these species play in this important industrial reaction.
We seek here to harness the unique properties of PHIP to
demonstrate that it is possible to extend the range of reac-
tions that are amenable to study using this approach whilst
adding to our understanding of the role that the metal plays
in an important industrial reaction. We have selected the pal-
ladium catalysed methoxy-carbonylation of an alkyne for this
purpose, a reaction that has been widely studied, and hence
provides a good background from which to work.2,40–44 This
study therefore builds on the earlier elegant work of Clegg
and others on the alkoxycarbonylation of alkenes.2,5,45 Such
studies traditionally use a combination of low temperatures
and high pressures,44 in this case in conjunction with a phos-
phine such as 1,2-(CH2PBut2)2C6H4IJdtbpx), to detect a range
of intermediates such as PdIJdtbpx)IJH)IJMeOH)+, PdIJdtbpx)IJH)-
IJCO)+, PdIJdtbpx)IJEt)IJMeOH)+ and PdIJdtbpx)IJCOEt)IJTHF)+, each
Experimental
General conditions and materials
−
with a CF3SO3 counter ion. Van Leeuwen has completed a
All manipulations were carried out under an inert atmo-
sphere, using standard Schlenk or high vacuum techniques.
Solvents were obtained from Fisher and PdCl2 from Acros Or-
ganics. The phosphine ligands were provided by Shell.
[PdIJOTf)2IJbcope)] (1) was prepared according to a literature
methods17 and its identity confirmed by NMR. Ph-13CC-Ph
was prepared as described previously.17 NMR spectra were
collected on Bruker DRX 400, and Avance 500 spectrometers.
The deuterated solvents methanol-d4 and dichloromethane-d2
used in this study were obtained from Sigma Aldrich. The
range of studies on this reaction as a function of phos-
phine40,46 with Claver completing related studies on vinyl ar-
enes.43 The alkoxycarbonylation of alkynes by palladium has
also been examined.47,48 However, it must be borne in mind
that highly active palladium nanoparticles can play
a
role.49–51 We are therefore seeking to detect active reaction
intermediates in this process, which contrasts with the situa-
tion faced in some of the earlier p-H2 based studies of the
hydroformylation reaction where a series of resting states
were detected.52,53
From the perspective of this paper, we harness the known
NMR parameters of these previously reported palladium com-
plexes to support the speciation enunciated here and build
on the fact that PHIP-NMR has already been shown to facili-
tate the detection of a number of related intermediates dur-
ing studies of palladium catalysed hydrogenation of alkynes
under mild conditions. For example when chelating17,54,55 or
monodentate phosphines56 are used as ligands a range of re-
action intermediates are detected. In the case of PdIJOTf)2IJPP′)
Scheme 1 Reported reaction intermediates detected through PHIP-
NMR spectroscopy during the hydrogenation of diphenylacetylene by
1 and p-H2 with the red labels indicating which signals provide an en-
hanced response.20
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reported shifts are temperature and solvent sensitive, all
spectra are calibrated and the chemical shifts quoted refer to
those detected under the conditions indicated. The authentic
methoxycarbonylation product, α-phenyl-cinnamic ester, 6
was prepared independently by the reaction of the corre-
sponding carboxylic acid with methanol.
[PdIJH)2IJμ1-bcope)IJμ2-bcope)]OTf (7). CD3OD, 308 K, 1H
NMR: δ −8.59 (ddt, JPH = 105.4 Hz, 44.2 Hz, 10 Hz, JHH = −10
Hz, Ha), δ −8.61 (ddt, JPH = 105.4 Hz, 44.2 Hz, 10 Hz, JHH
=
−10 Hz, Hb); 31P{1H} NMR: δ 48.9 (m, JPH = 105.4 Hz, 44.2 Hz,
JPP = 29.4 and 12.6 Hz, P trans to Ha), δ 48.2 (m, JPH = 105.4
Hz, 44.2 Hz, JPP = 29.4 and 12.6 Hz, P trans to Hb), 19.2 (t, JPH
= 10 Hz, JPP = 12.6 Hz).
NMR samples
[PdIJCOC(Ph)CHPh)IJbcope)IJCO)]OTf (8). CD2Cl2, 313 K,
1
1H NMR: δ 7.89 (dd, JPH = 22.0 Hz, 3.5 Hz, JCH = 156.2 Hz,
For a typical NMR sample, ca. 1 mg of 1 and 2 mg of
diphenylacetylene (ca. 10 fold excess) were dissolved in 0.6
ml of the specified deuterated solvent in a 5 mm NMR tube,
which was equipped with a Young's valve. When a carbonyla-
tion reaction was being explored, the sample was degassed
and an appropriate mixture of CO and p-H2 added (typically a
1 : 2 ratio, in order to reach a total pressure of 3 bar).
2JCH = 3.5 Hz, 1H, vinyl H); 31P{1H} NMR: δ 36.0, (m, JPH
=
22.2 Hz, P trans to vinyl), δ 43.2, (m, JPH = 3.5 Hz, P cis to
vinyl); 13C{1H} NMR: δ 137.2 (d, JCH = 156.2 Hz, CHPh), δ
137.8 (d, JCH = 3.4 Hz, CPh).
[PdIJMeOOCPhCCHPh)IJbcope)] (9). CD3OD, 333 K, 1H
3
NMR: δ 5.08 (m, JPH = 7.0 Hz, JPH = 7.0 Hz, JCH = 7.5 Hz,
PhCH = 151 Hz). 13C{1H} NMR δ 35.6 (CHPh).
[PdIJcis-PhCHCHPh)IJbcope)IJCO)]IJOTf)2 (10). CD3OD, 333
K, H NMR: δ 4.26 (m, JPH = 13.0 Hz, 3.7 Hz, JCH = 133.5 Hz,
2JCH = 3.3 Hz, PhCHCHPh); 13C{1H} NMR: δ 59.5 (CHPh).
Key NMR data, organic reaction products
1
1
1
CPhIJH)C(Ph)COOMe (6). CDCl3, 298 K, H NMR: δ 7.84
1
2
3
(s, 1H, CHPh, JCH = 156.0 Hz, JCH = 3.5 Hz; JCH = 7.5 Hz), δ
3.82 (s, 3H, OCH3), δ 7.06 (t, 2H, JHH = 7.5 Hz, o-H of PhC), δ
7.18 (t, 2H, JHH = 7.5 Hz, m-H of PhC), δ 7.65 (t, 1H, JHH = 7.5
Hz, p-H of PhC), δ 7.21 (t, 2H, JHH = 7.5 Hz, o-H of PhC), δ
Results and discussion
PdIJOTf)2IJbcope) (1) readily catalyzes the conversion of
diphenylacetylene and H2 in methanol solution into cis- and
trans-stilbene, and diphenylethane.20 The addition of CO to
the H2 feed (1 : 2 ratio) proved to change this product mix,
with the methyl-ester CPhIJH)C(Ph)CO2CD3 (6) of Scheme 2
also being formed according to gas chromatographic analy-
sis. Furthermore, when a series of 31P{1H} NMR spectra are
recorded while this reaction is taking place the signal for 1 at
δ 74.0 is slowly replaced by a δ 51.9 resonance that is due to
[Pd2IJbcope)2IJCO)2]IJOTf)2.
7.38 (t, 2H, JHH = 7.5 Hz, m-H of PhCH), δ 7.52 (t, 1H, JHH
=
=
7.5 Hz p-H of PhCH); 13C{1H} NMR: δ 140.5 (s, 1H, JCH
1
2
156.0 Hz, CHPh), δ 132.6 (s, 1H, JCH = 3.6 Hz, CPh), δ 168.0
3
(s, 1H, JCH = 7.5 Hz, CO), δ 130.6, 129.8 (CPh), δ 129.1, 127.9,
126.7 (CHPh).
CPhIJH)C(Ph)COOEt. CDCl3, 298 K, 1H NMR: δ 7.74 (s,
3
3
1H, CHPh, JCH = 7.4 Hz), δ 4.32 (quart, 2H, JCH = 7.13 Hz,
3
OCH2), δ 1.34 (t, 3H, JCH = 7.13 Hz, CH3); 13C{1H}: δ 163.9 (s,
1H, 3JCH = 7.4 Hz, CO).
CPhIJH)C(Ph)COIJOTf). CD2Cl2, 308 K, 1H NMR: δ 7.69
(CHPh).
Detection of CPh(H)C(Ph)COOMe (6) in methanol-d4
solution
Key NMR data, reaction intermediates
The identity of the new organic product, 6, was further con-
1
[PdIJCHPhCH2Ph)IJbcope)]OTf (2). CD2Cl2, 313 K, H NMR:
firmed by comparing appropriate NMR data with those of the
1
δ 4.95 (m, JHH = −4.3 Hz, 11.2 Hz, CHPh), δ 3.10 (dd, CH2Ph,
JHH = −4.3 Hz, 15.0 Hz) and δ 2.93 (dd, CH2Ph, JHH = 11.2,
15.0 Hz); 31P{1H} NMR: δ 32.2 (d, JPP = 90.2 Hz, P trans to al-
kyl), δ 42.1 (d, JPP = 90.2 Hz, P cis to alkyl); 13C{1H} NMR: δ
35.2 (dd, JCP = 16.2 Hz, 5.4 Hz, CH2Ph), δ 62.3 (dd, JCP = 54.0
Hz, 16.2 Hz, CHPh).
authentic compound. The vinyl proton's H NMR signal of 6
appears at δ 7.84, and its carbonyl resonance appears at δ
168.0 and is associated with a 7.5 Hz coupling to the vinyl
proton. Furthermore, when Ph-13CC-Ph is used, the vinyl
proton signal splits again through the addition of 1- and
1
2-bond H–13C couplings of 156 Hz and 3.5 Hz. GC-MS analy-
[PdIJCPhCHPh)IJbcope)IJCD3OD)]OTf (3-CD3OD). CD3OD,
313 K, 1H NMR: δ 6.77 (dd, JPH = 13.6 and 6.8 Hz, 1H,
CHPh); 31P{1H} NMR: δ 21.8 (m, JPH = 13.6 Hz, P trans to vi-
nyl), 39.0 (m, JPH = 6.8 Hz, P cis to vinyl); 13C{1H} NMR: δ
162.1 (CPh), δ 130.1 (CHPh).
sis of this mixture yields a molecular ion at m/z 242 for 6
when it is formed in methanol-d4, which falls to m/z 239
when the reaction is completed in methanol-h4 and increases
further to 252 when d10-Ph-CC-Ph rather than Ph-CC-Ph
[PdIJC(Ph)CHPh)IJbcope)IJCO)]OTf (3-CO). CD3OD, 308 K,
1H NMR: δ 6.90 (br, 1H, PhCH).
[PdHIJbcope)IJCO)]OTf (4-CO). CD2Cl2, 313 K, 1H NMR: δ
−4.64 (dd, JPH = 190.0 Hz, 29.0 Hz, hydride); 31P{1H} NMR: δ
46.0 (d, JPH = 190.0 Hz, P trans to hydride).
1
[[(bcope)Pd]2IJμ-H)IJμ-CO)]OTf (5). CD3OD, 308 K, H NMR:
Scheme 2 Summary of the organic products detected during the
reaction of diphenylacetylene with CO and H2 in the presence of 1 in
methanol-d4; the blue labels reflect atoms that show the PHIP effect.
−5.34 (quint, JPH = 47.2 Hz, 1H, hydride); 31P{1H}: δ −20.9 (d,
JPH = 47.2 Hz).
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is employed. Consequently we deduce that the alcohol is the
source of the esters methyl substituent rather than CO. In a
typical NMR study with 1 at 298 K, the yield of ester proved
to be 70%. However, with 100 bar of CO and 20 bar of H2,
the selectivity of this reaction approaches 99%.
The signal at around δ 7.6 that is enhanced belongs to the
η3-Ph59 group of the alkyl ligand of 2. The aromatic protons
of the phenyl group of the PdCHPh group therefore couple to
those of the PHIP-enhanced CH proton. In low field, the re-
sult is SABRE type magnetization transfer which is visible
when the sample is interrogated at high field.60
Effect of employing p-H2
Role for 2 in the hyperpolarisation step
A study was then undertaken where the normal-H2 gas used
in these studies was replaced with p-H2. Now, in methanol-
The effect that leads to the detection of PHIP enhanced reso-
nances in 2 plays a vital role in this work. Previous studies
have shown that this complex is formed by the reaction of
[PdIJH)IJbcope)]OTf with Ph-CC-Ph and H2. When such a
study is completed with p-H2, this process leads to the gener-
ation of 2 where two of the alkyl protons originate from this
reagent.20 However, 2 reacts further on the NMR timescale in
a number of ways, forming cis- and trans-stilbene and
[PdIJH)IJbcope)]OTF through reversible β-H transfer. These pro-
cesses act to place p-H2 derived protons in all sites and result
in the hydride signal of [PdIJH)IJbcope)]OTF appearing in emis-
sion through the one-proton PHIP effect throughout these
measurements. When this polarized monohydride precursor
reacts with Ph-CC-Ph, the vinyl complexes 3-CO and 3-CD3-
OD are detected through a hyperpolarized response as exem-
plified by Fig. 1. While 1 has been shown previously to react
with methanol or H2 to yield [PdIJH)IJbcope)]OTF when p-H2 is
used its hydride signal remains unpolarised in the absence
of Ph-CC-Ph;20 the formation of palladium monohydride
cations under such condition is well established.61–63
1
d4, PHIP enhanced H NMR signals are evident as shown in
Fig. 1 at temperatures around 306 K.
These enhanced resonances can be divided into those that
originate from the organic products, cis- and trans-stilbene,
6, and those that are palladium based which include signals
for the previously reported intermediates [PdIJCHPhCH2Ph)-
IJbcope)]OTf (2) and [PdIJCPhCHPh)IJbcope)IJmethanol)]OTf
(3-CD3OD) alongside those for newly seen, [PdIJCPhCHPh)-
IJbcope)IJCO)]OTf, 3-CO. The three alkyl proton signals of 2 are
enhanced through PHIP, whilst those of the vinyl protons of
3-CD3OD and 3-CO, which contain a single p-H2 derived pro-
ton, exhibit the one-proton-PHIP effect described by
Eisenberg.37 In this case, this is a result of their formation
from a monohydride complex, such as [PdIJH)IJbcope)IJCO)]OTf
(4-CO, see later), which is formed after stilbene loss from 2.
The signals for 3-CD3OD and 3-CO are broad, and coalesce
when the temperature is raised, or a lower [CO] is present in
a process that is [CO] dependent.
Route to 6
According to the independent work of Leeuwen40 and Iggo64
the key intermediate that is predicted to be involved in the
formation of 6 is [PdIJCOCPhCHPh)IJbcope)IJmethanol)]OTf
rather than [PdIJCOCPhCHPh)IJbcope)IJCO)]OTf, with rapid
inner sphere nucleophilic attack of the associated alcohol/
alkoxide ligand leading to the ester, alongside the formation
of [[(bcope)Pd]2IJμ-H)IJμ-CO)]OTf (5) which is evident as a weak
signal in the NMR spectrum shown Fig. 2. PHIP therefore al-
lows the detection of a number of predicted species under the
mild reaction conditions described here but fails to show any
direct evidence for [PdIJCOCPhCHPh)IJbcope)IJmethanol)]OTf.
Fig.
1 Two sections of a
1H{31P} NMR spectrum, recorded in
methanol-d4 during the reaction of 1, diphenyl acetylene, CO and
p-H2 showing (a) the one-proton PHIP enhanced signals of 6, 3-CD3-
OD, 3-CO and (b) the PHIP enhanced signals of cis-stilbene and 2, at-
tributed according to the labels of Scheme 1.
Fig. 2 1H NMR spectrum showing the hydride signals for 5 and 7 that
are visible through the PHIP effect.
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Catalyst degradation
In the hydride region of this methanol-d4 sample, signals for
the previously unseen catalyst degradation product
[PdIJH)2IJμ1-bcope)IJμ2-bcope)] (7) are visible, as detailed in
Fig. 2. 7 actually exhibits two overlapping hydride signals at δ
−8.59 and δ −8.61 that appear in a 1 : 1 ratio and are sepa-
rated by less than 10 Hz on a 500 MHz spectrometer (Fig. 2).
These hydride signals are visible for several minutes at 308 K
and can be regenerated by adding fresh p-H2 to the solution.
They also remain visible in a double quantum filtered OPSY-
NMR experiment in accordance with their dihydride origin.
The three 31P NMR signals of this product were located at δ
48.9, 48.2 and 19.2 in the corresponding PHIP enhanced
heteronuclear multiple quantum correlation (HMQC) mea-
surement. We note that the related complex (tBuCOPE)PdIJH)2
has been seen previously in dichloromethane-d2 solution.20
Fig. 3 Upper: Structures of monohydride 4-CO and acyl 8. Lower: (a)
PHIP enhanced 1H NMR signal of 8 (blue atom) and (b) corresponding
NMR signal for 4-CO (blue atom).
Utilisation of dichloromethane-d2 to detect [PdIJCO–
CPhCHPh)IJbcope)IJCO)]OTf (8)
These observations change when dichloromethane-d2 is used
as the solvent instead of methanol. Now, at 298 K PHIP en-
hanced signals are readily seen for 2 and 3-CO. In this case,
Detection of PhHCC(Ph)COIJOTf)
A further, weak singlet is observed as an emission signal at δ
7.69
which
we
attribute
to
the
corresponding
the
δ 3.13 and 2.93 signals of the CH2Ph group of
trifluoromethane sulfonate ester (see ESI†). We note that an
enhanced signal is also seen at δ 9.52 for HOTf in these NMR
spectra and that neither of these signals are seen in metha-
nol-d4 itself, or when a single equivalent of CH3OH is added
to this CD2Cl2 solution as both signals are replaced by an en-
hanced peak at δ 7.84 for 6. The formation of this triflic acid
product is consistent with the promotion of this reaction
with added acid.40
[PdIJCHPhCH2Ph)IJbcope)]OTf (2) are strongly polarised, whilst
that for its CHPh resonance, at δ 4.96, is barely visible. This
dramatic change in relative polarisation level from the 1 : 4
ratio of Fig. 1, in methanol-d4, will be discussed later. We
note that the emission peak for 3-CO appears at δ 6.89, and
is still very broad in appearance, having 17 times lower area
than the δ 3.13 signal of 2. While it sharpens on phosphorus
decoupling, a series of 2D spectra that were recorded to lo-
cate the two expected 31P coupling partners were unsuccess-
1
Detection of CO stabilised [PdIJH)IJbcope)IJCO)]OTf (4-CO)
ful. However, a series of selective H{31P} NMR experiments,
where the 31P decoupling frequency was varied systematically,
suggested the presence of two signals at ca. δ 45 and 30.
Broad features for such species at 298 K are not unexpected
given the high reactivity of these systems.65
The hydride region of this 305 K NMR spectrum is further
complicated by the detection of a new monohydride complex,
as an emission signal, at δ −4.64, with doublet of doublets
multiplicity which simplifies into a singlet on 31P decoupling
(Fig. 3). The corresponding 31P couplings exhibited by this
resonance are 189 Hz and 29 Hz and require a palladium
centre with magnetically inequivalent phosphine ligands that
are trans and cis the hydride ligand respectively. Although
this hydride signal is only observed in the presence of CO,
the use of 13CO failed to change its appearance. However, its
chemical shift and splitting pattern are very similar to that of
[PdIJH)IJdtbpx)IJCO)]OTf reported by Clegg et al.2 and it is there-
fore assigned to [PdIJH)IJbcope)IJCO)]OTf (4-CO). A further and
much weaker hydride signal appears at δ −5.34 after 10 mi-
nutes due to 5.40,58 Complexes of this type have been shown
to exhibit carbonylation activity but are thought to be off-
loop and have been proposed to form via the reaction of
[(bcope)Pd] with 4-CO.40,64
Upon warming to 305 K, these signals all roughly double
in intensity in accordance with an increase in reaction turn-
over. This simple change, however, also allows the NMR sig-
nals for a number of further species to be detected. First, in
the organic region, a weakly polarized doublet of doublets ap-
pears at δ 7.89 as shown in Fig. 3. This emission signal col-
lapses into singlet on 31P decoupling and is therefore metal
based. Phosphorus couplings in this resonance were mea-
sured at 22 Hz and 3.5 Hz, with the associated 31P resonances
occurring at δ 36.0 and δ 43.2 respectively. Furthermore,
when mono-13C labelled Ph-13CC-Ph was used, further 13C
couplings of 156 Hz and 3 Hz were observed on this proton
signal. This new product is therefore assigned to the acyl
intermediate [PdIJCO–CPhCHPh)IJbcope)IJCO)]OTf (8) whose
formation is implicated in the earlier methanol-d4 reaction.
The formulation of 8 as a CO adduct is consistent with re-
sults presented in the literature for related systems.64
Upon warming this sample to 310 K, these two hydride
signals both grow in size, with the difference in relative hy-
dride peak areas falling to 4 : 1. Additionally, in the organic
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1
region of this H NMR spectrum, the enhanced signals of 3-
CO and 8 now appear in a 1 : 1.7 intensity ratio thereby
suggesting an increase in the rate of CO insertion. Further
warming of this sample to 315 K results in the δ 2.93 signal
that is seen for the CHPh group of 2 becoming more strongly
polarised, such that there is a 1 : 8 intensity ratio when com-
pared to the CH2Ph signals and this difference reduces fur-
ther to 1 : 4 at 320 K and 1 : 1.2 at 323 K. Additionally, at 315
K, hyperpolarised cis-stilbene becomes readily visible as a sig-
nal at δ 6.66 with the signals for 3-CO and 8 now appearing
in a 1 : 8 ratio.
has been commented on before.67 Increasing the level of wa-
ter to 5 μl, proved to suppress the signals for both 3-CO and
4-CO whilst further enhancing that for 5 alongside that of the
acid product.
Minor reaction products detected in methanol-d4 solution
We now return to the methanol-d4 spectra in order to build
on the earlier observations by incorporating the
dichloromethane-d2 based information. We note, that the
intensity of the resulting PHIP enhanced NMR signals are
again strongly temperature sensitive, and at 304.4 K signals
for 3-CH3OH and 3-CO are visible alongside those of a new
species, 9, at δ 5.08. Upon warming to 306.6 K the signals for
3-CH3OH and 3-CO reduce in size with those of the ester 6
appearing at δ 7.84 alongside that of 9. The hyperpolarised
resonance of 9 also appears in emission, but surprisingly ex-
hibits a triplet multiplicity where JPH = 7.0 Hz, and upon 31P
decoupling it changes into a singlet. Furthermore, when
13CO is used this resonance exhibits a further 7.5 Hz splitting
which is similar in size to that exhibited by 6. When 13CO
Rationalising the change in alkyl proton signal intensities
observed in 2
This difference in intensity is readily apparent in the corre-
sponding 1H-OPSY NMR spectra and confirms that the kineti-
cally dominant addition of parahydrogen places two protons
on the δ 3.10 and δ 2.93 derived sites of 2. Hence the mecha-
nism of formation of 2 must therefore involve a reaction
which shows a level of selectivity for geminal substitution
which reduces as the temperature is raised, presumably be-
cause of alkene rotation in the resulting PdIJH)IJPhCHCHPh)
containing reaction intermediate. We note that similar selec-
tivity has been reported previously by Bargon et al.66
It is noteworthy that warming also results in a fall in the
hydride signal strength of 4-CO with that of [[(bcope)Pd]2IJμ-
H)IJμ-CO)]OTf (5) gaining intensity. The emission signal of 5
now appears clearly with a quintet multiplicity that simplifies
into a singlet on 31P decoupling. It proved possible to dra-
matically increase the size of the hydride signal for 5 by
adding 1 μl of D2O or H2O to this sample and the corre-
sponding acid C(Ph)IJH)C(Ph)CO2H is now produced as a
carbonylation product, revealed through an enhanced signal
at δ 7.96 with signals for 7 being seen, centred on δ −8.65,
and 10 at δ 4.26 (see later). Interesting, the free H2 peak also
proved to be enhanced in these NMR traces, with the corre-
sponding HD signals also appearing in emission as detailed
in Fig. 4. The observation of a hyperpolarised H2 response
1
and C6D5–13CC–C6D5 are used, additional H–13C splittings
of 150.1 and 6.6 Hz are detected and a 13C signal for an al-
kene CH resonance is located at ∼36 ppm. This product
therefore contains 6 as a ligand and we assign it to species 9
of Fig. 5. It is predicted to form via 8 after CO loss and reac-
tion with methanol which leads to ester 6 and DOTf. This
process involves the coordination of 6 to neutral Pd(bcope)
and could proceed in a concerted manner within the coordi-
nation sphere of the metal or within the solvent cage. As the
temperature is raised further the signals for 6 dominate.
In addition, a further weak and polarised emission signal
also becomes visible at δ 4.26 due to 10. 31P decoupling sim-
plifies the appearance of this resonance into a doublet of
doublets by removing JPH couplings of 13.5 and 4.1 Hz, and
when 13C labelled diphenylacetylene is employed additional
2
1JCH and JCH couplings of 133.5 Hz and 3.3 Hz are visible
and a weak alkene CH signal at ∼60 ppm. Now, however, the
addition of 13CO provides no further splitting, although we
note that this product is only detected in the presence of CO
and its intensity is largest when a strong PHIP-signal is seen
for cis stilbene.
When looking at the appearance of these additional 1H
NMR signals, we need to consider the impact of JPP on their
appearance. For conditions where the phosphines are
inequivalent, and JPP > JPH, we will see a virtual triplet
Fig. 4 Three sections of a 1H NMR spectrum recorded during the
palladium catalysed reaction of diphenylacetylene with CO, D2O and
H2 in CD2Cl2 solution are presented. The formation of
C(Ph)IJH)C(Ph)CO2H is indicated by the emission signal at δ 7.96 (left
panel). Hyperpolarised H2 and HD is created (middle panel) alongside
4-CO and 5 (right panel).
Fig. 5 Structures of 9 and 10.
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regardless of the individual JPH values while if JPP < JPH then
a doublet of doublets would be expected. Glueck et al.
reported a number of Pd(0) bis-phosphine trans-stilbene com-
plexes yield 13C signal for the alkene at around 60 ppm, with
the connected 1H signal appearing at ca. 5 ppm but they yield
small JPH couplings of just 2 Hz and JPP couplings of between
20 and 27 Hz.68 Interesting, the corresponding 31P coupling
increases to around 70 Hz in related hexadiene based com-
plexes.69 In the case of related PdIJII) bis phosphine complexes
containing H and MeOH ligands couplings of 16–30 Hz are
typical and so a virtual triplet would be expected for the
alkenic protons in both of these types of product if such large
couplings are retained.2,70
We therefore suggest in 10 that CO binding to the fourth
site occurs, and that the palladium centre is charged, which
means that an out-of-plane cis-stilbene ligand is indicated
(Fig. 5). The signals of 10, alongside those of 2 and
cis-stilbene itself, become more evident as the samples tem-
perature is raised and overtime, the enhanced hydride signal
of 5 appears in the associated NMR spectra. Studies by Parker
et al. have suggested that the alkenes of such species could
be labile and it is therefore not surprising that complexes of
this type are only detected here through a hyperpolarised
response.71
ment with literature predictions40 which predict a role for
[PdIJCOCPhCHPh)IJbcope)IJmethanol)]OTf in the formation
of 6.
Conclusions
In summary, the methoxycarbonylation of diphenylacetylene
has been studied with p-H2 and a number of reaction inter-
mediates have been identified through the use of the PHIP
effect in methanol and dichloromethane solution. This reac-
tion is initiated by the conversion of 1 into the monohydride
complex [PdIJH)IJbcope)IJOTf)] which is readily observed as its
CO adduct, through the hyperpolarised response of
[PdIJH)IJbcope)IJCO)]OTf (4-CO), as detailed in Scheme 3. 4-CO
has been shown to play a direct role in the methoxy-
carbonylation process according to Scheme 3, where the de-
tection of an enhanced NMR signal for the vinyl product 3-
CO is augmented by those of the ester 6. Logically these form
sequentially in agreement with the analogous reactions de-
scribed in the introduction.
6 is most readily formed when methanol intercepts the
acyl product [PdIJCO–CPhCHPh)IJbcope)]OTf, rather than
CO, and leads to [PdIJbcope)] and DOTf. The [PdIJbcope)] that
is formed in this process can then be trapped to form prod-
ucts 5 and 9 according to Scheme 3. 9 is predicted to form by
a simple ligand rearrangement within the coordination
sphere of the metal after MeOD attack leads to 6, but 5 re-
quires a reaction with 4-CO. Baya et al. and others have
suggested that 5 forms via the combination of [PdIJbcope)CO]
and [PdIJH)IJbcope)]+ which would also be expected to be pres-
ent under these conditions.40,58,64
Exchange spectroscopy measurements in dichloromethane
rationalise the failure to detect
[PdIJCOCPhCHPh)IJbcope)IJmethanol)]OTf
A series of 1D exchange spectroscopy (EXSY) experiments
were used to probe magnetization transfer in 8 and 4-CO in
dichloromethane solution at 300 K. When the signal at δ 7.89
for the acyl complex 8 was selectively excited, magnetisation
transfer into the δ 7.69 signal of the triflate ester, alongside
weaker transfer into the δ 4.99 of 2 is readily observed, with
even weaker transfer into the signal for trans-stilbene being
evident. We estimate that the relative rates for these conver-
sions are 0.73 s−1, 0.03 s−1 and 0.006 s−1 respectively. Analysis
of these data requires that the triflate ester reacts to reform
8 with a rate constant of 3.4 s−1. In confirmation of this,
when the signal at δ 7.69 for the triflate ester is probed in a
similar way, rapid transfer into the δ 7.89 signal of 8 is seen
at an identical rate, within error, to that predicted from the
observations based on the excitation of 8.
In addition, signals for the dihydride based deactivation
product 7 are seen in these NMR spectra because of the PHIP
effect. This reflects the fact that its hydride ligands are mag-
netically distinct. As a consequence, 7 must undergo the
In addition, when the hydride signal of 4-CO at δ −4.64 is
selected, magnetization transfer into signals at δ 7.89 (8), δ
7.69 (triflate ester) and δ 4.66 (H2) is seen. The ratio of the
resulting signal intensities for 8 and the triflate ester that re-
sult from magnetisation transfer via 4-CO are 1 : 6.25, and dif-
fer from the predicted ratio of 4.6 : 1 that comes from the ear-
lier rates constants. This observation suggests the existence
of a second, concentration dependent route to the triflate es-
ter involving 4-CO that does not involve 8. Logically, this
involves [PdIJCO–CPhCHPh)IJbcope)IJOTf)], formed by the
competitive trapping of 16-electron [PdIJCO–CPhCHPh)-
IJbcope)]OTf by −OTf rather than CO. This is in direct agree-
Scheme 3 Structures of the reaction intermediates that are detected
through PHIP when hyperpolarized 4-CO converts diphenylacetylene
into 6 and related products.
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reductive elimination of H2 which provides a route to [PdIJμ1-
bcope)IJμ2-bcope)] under conditions where H2 is present. This
neutral species might be expected to undergo phosphine loss
to form [PdIJbcope)] and hence a second route to 5 is possible.
We note that the dihydride [PdIJH)2IJbcope)] would be PHIP ac-
tive because of the second order nature of its hydride li-
gands.72 Hence, as it is not detected, we can conclude that it
is either too reactive to be seen, which is unlikely given the
previous observation of (tBuCOPE)PdIJH)2 or that the contribu-
tion of a dihydride based reaction pathway is minimal in
agreement with earlier literature predictions.57
8 C. R. Bowers and D. P. Weitekamp, Phys. Rev. Lett., 1986, 57,
2645–2648.
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Chem. Soc., 1988, 110, 8564–8566.
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We conclude therefore that PHIP reflects an ideal platform
from which to study this methoxycarbonylation reaction and
thereby establish its potential to follow a wider range of cata-
lytic processes than was previously thought possible. Further-
more, the observation of the enhanced NMR signals for these
reaction intermediates under our mild reaction conditions con-
firms a high level of intermediate turnover during catalysis.
This deduction is based on the fact that hyperpolarised signals
are only expected to retain a visible signal enhancement for 3
T1 periods unless the molecules that provide these signals are
continually replenished. This time will range from 1–10 sec-
onds, according to whether we are dealing with a PdH or CH
based signal, and further confirms the importance of these in-
termediates in the catalytic cycle. By using EXSY methods we
have also established a direct role for 3 and 4 in this process
which is clearly homogeneous in nature.
13 M. S. Anwar, D. Blazina, H. A. Carteret, S. B. Duckett, T. K.
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Acknowledgements
We are grateful for financial support from the China Scholar-
ship Council and the Spanish MEC Consolider Ingenio 2010-
ORFEO-CSD2007-00006 research programme. J. L.-S. acknowl-
edges funding from the Spanish Ministry of Education
(MECD; Subprograma Estatal de Movilidad, Plan Estatal de
Investigación Científica y Técnica y de Innovación 2013-2016).
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